Contents

Guest editorial Terminals for mobile satellite communications Odd Gutteberg 3 Jens Andenæs 49 Elements of satellite technology Future systems for mobile satellite and communication communications Odd Gutteberg 4 Per Hovstad and Odd Gutteberg 54 Satellite communications Effects of atmosphere on earth-space radio standardisation in Europe propagation Gunnar Stette 22 Odd Gutteberg 63 A review of Norwegian space activities The use of millimetre waves in Georg Rosenberg 26 satellite communications Tord Fredriksen 71 TSAT - a low-cost satellite communication network The effect of interference for an OBP system Terje Pettersen and Petter Chr Amundsen 31 utilising the geostationary orbit Per Hovstad 75 Very Small Aperture Terminal (VSAT) systems - basic principles and design Earth station antenna technology Liv Oddrun Voll and Gunn Kristin Klungsøyr 39 Arnfinn Nyseth and Svein A Skyttemyr 85 Fleet management using INMARSAT-C and GPS - a Norwegian pilot project Arvid Bertheau Johannessen 46 Guest editorial BY ODD GUTTEBERG

Developing the Norwegian The Norwegian telecommunica- telecommunication network has tions industry has been very always been a challenging effort. active since satellite transmis- This is due to this country’s spe- sion was introduced in Norway. cial topography and its scattered Among other things they are population. Furthermore, it has manufacturing the earth stations been difficult to establish reliable in NORSAT-B, and coastal communications with the mer- earth stations and mobile termi- chant fleet, oil rigs, and an nals for the different important Norwegian outpost - INMARSAT standards. Norwe- the Arctic islands of Svalbard gian industry has also developed (Spitzbergen). a new data collection satellite system, called TSAT (Teleme- In the early sixties, it became try via SATellite). evident that communications via satellites opened up new oppor- The last addition to the “family tunities for surmounting these tree” is satellite broadcasting. difficulties. Norwegian Telecom This application is especially understood the possibilities, and attractive for a sparsely popu- as the first country in Europe, lated country like Norway. A Norway established in 1976 a major step was taken when the domestic satellite communica- Norwegian Telecom bought the tion system. This system, called powerful TV satellite “Marco NORSAT-A, utilised leased Polo 2” from Britain in 1992. transponder capacity on the geo- The satellite was renamed stationary INTELSAT-IV satel- “Thor” and moved to the posi- lite for communication between the Norwegian mainland and tion 1 degree west in the geostationary orbit. The purchase of the oil rigs in the North Sea. In 1979 Svalbard was connected to this satellite represents a significant change in Norwegian space this system. The earth station at Svalbard was the first commer- activity. The Norwegian Telecom has now moved from being a cial earth station world-wide, operating with an elevation angle satellite user to a satellite owner and operator - with all the chal- less than 3 degrees. lenging tasks this implies.

The success with the NORSAT-A system has been followed up As the satellite technology improves, the earth terminal by a meshed network, the NORSAT-B system. This system becomes smaller and cheaper, as we have seen in the VSAT utilises Very Small Aperture Terminals (VSAT) for business systems. However, the major challenge for the satellite commu- communications. nity is now to provide a global mobile telephone system with hand-held terminals. This will necessitate studies of new con- As a large shipping nation, Norway caught an early interest in cepts such as satellites in low-earth orbits, inter-satellite links, using satellites for maritime communications. In the seventies multibeam antennas, on-board processing and possibly use of Norwegian Telecom was one of the driving forces behind the higher frequency bands. establishement of the International Maritime Satellite Organisa- tion (INMARSAT), and Norway is at present the third largest shareholder. The first European coastal earth station in the INMARSAT system was built in southern Norway, and put into service in 1982.

3 Elements of satellite technology and communication BY ODD GUTTEBERG

621.396.946 1 Introduction and The satellite control station (TT&C sta- sists of a large antenna and a high power tion) maintains the satellite in orbit. It amplifier. Since it is expensive and com- overview keeps control of the satellite status plicated to generate an equally high More than 35 years ago, on 4 October (Telemetry), orbital information (Track- power in the satellite, the output power 1957, the world’s first artificial satellite ing), and performs orbital attitude from the satellite is generally much was launched by the Soviet Union. The manoeuvres and configures the commu- smaller. Due to this, the receiving earth satellite, which became known as Sput- nication system (Command). stations have to use larger receiving nik-I, opened up a new era of practical antennas; up to 30 metres in diameter, in The communication part of the satellite use of the outer space. The satellite’s the early days of satellite communica- system is simply a radio system with weight was 84 kg and it circled 1400 tions. only one relay station, the satellite. The times around the earth before burning up signals are transmitted on a carrier fre- However, due to larger satellites, i.e. in the atmosphere after 93 days. The quency, f , from an earth station (traffic higher output power, and to better receiv- launch of Sputnik-I was followed by the A station), received in the satellite, ampli- ing technology, the earth stations can use United States’ Explorer-I in January fied, shifted in frequency to f' and trans- smaller and cheaper antennas, e.g. satel- 1958. Even though these satellites were A mitted back to the receiving traffic sta- lite TV broadcasting. primarily not intended for communica- tion. The satellite transmits and receives tions, they demonstrated that this was on radio frequencies mainly in the technically and economically feasible. microwave band, i.e. 3 - 30 GHz. On 2 A brief historical review The use of artificial satellites in earth these frequencies it is feasible to use The first person to suggest the use of the orbits is now a well established and inte- parabolic reflector antennas. Such anten- geostationary orbit for communication grated part of the world’s telecommuni- nas concentrate the radio signals in a satellites, was the English physicist cations network. The evolution of the small cone; thus it is possible to “illumi- Arthur C Clark (born 1917). He pub- satellite technology together with more nate” the whole earth or part of the earth, lished his article “Extra-Terrestrial powerful launchers have made the satel- see figure 2. The illuminated part is Relays” in the Wireless World Magazine lites suitable, not only for long distance called the coverage area, and most of the in October 1945. In this article a satellite communications, but also for national transmitted power from the satellite is system for broadcasting of television, communications, television broadcasting concentrated in this area. The larger the was described. At that time there was a and mobile services. transmitting satellite antenna is, the discussion going on about how to dis- smaller the coverage area becomes. A satellite communication system is tribute television. The present technology divided into two major parts, the earth The up-link signals have only one desti- was not mature for the construction of segment and the space segment. The nation, the orbiting satellite, which reliable and unmanned satellites. In 1945 satellite and its control station form the means that it is required to transmit no-one could predict the rapid develop- space segment, while the earth segment energy only to the satellite. The transmit- ment within electronics, e.g. the inven- comprises the traffic and traffic control ting earth station therefore normally con- tion of the transistor in 1947. stations, see figure 1.

Satellite

down-link

up-link

fA f´B fB f´A

Traffic Satellite control Traffic station A station (TT & C) station B

Figure 1 The main building blocks in a two-way satellite com- Figure 2 A geostationary satellite “illuminating” the coverage munication system area on earth

4 Launch data Date: 10 July 1962 Vehicle: Thor-Delta

The Space Age started with the laun- They were all put into low earth orbits, ching of the first artificial satellite, the height varying between 1,000 and Orbital data Sputnik-I, in 1957. It is worth while 8,000 km. Telstar-I was the most Orbit: Elliptical noticing that the first trans-Atlantic important of these satellites, figure 3. Inclination: 44.8 degrees telephone cable was stretched the This satellite transmitted television same year. In the following years seve- directly from USA to England and Apogee: 5653 km ral satellites were launched, both for France for the first time. The Nordic Perigee: 936 km scientific purposes and for other speci- countries started early to make use of Period: 157.8 min fic applications. Communication expe- satellite communication. In 1964 the riments with passive reflecting satel- Nordic countries built a common lites, Echo-I and -II (1960), were also receive earth station at Råö, outside Satellite data carried out. Gothenburg. It was an experimental earth station which participated in Weight: 77 kg An operational system with passive NASA’s experiments with the Relay satellites was never realised, but the Diameter: 0,9 meter (spherical) satellites. project made essential contributions to No of transponders: 1 the development of the earth station The first satellite in the geostationary Bandwidth: 50 MHz technology. orbit was Syncom-II in 1963. The Output power: 3 Watts The first active telecommunication launch of Syncom-I the same year was satellites were launched in the early a failure. This was an important break- 1960s, Courier (1960), Telstar-I and -II through for commercial satellite com- Figure 3 The Telstar satellite, main data. (1962 and 1963) and Relay-I and -II munication. Several important experi- The satellite was designed and built by Bell (1962 and 1964). ments were carried out. One of them Telephone Laboratories and launched by NASA

INTELSAT I INTELSAT II INTELSAT III INTELSAT IV INTELSAT V INTELSAT VI INTELSAT VII (Early Bird) + IVA 1965 1967 1968 1971 1980 1989 1993

Telephone 240 240 1 200 6 000 12 000 36 000 18 000 channels

Weight in 40 85 150 790 1 040 2 230 1 400 orbit (kg)

No. of satellites 1 3 5 7 + 5 13 6 12

Figure 4 The INTELSAT satellites. Technical development

5 was to examine whether or not geosta- geostationary satellites over the Atlantic Svalbard was connected to the terrestrial tionary satellites could be a part of the and Indian Oceans. The first European public network via this system in 1979 public telephone network. The problem commercial communication satellite, with an earth station at Isfjord Radio, fig- was the time delay of the radio signal. EUTELSAT-I, was launched in 1983. ure 7. The TV broadcasting to Svalbard With a mismatch at the other end of the was established via one of the EUTEL- connection, one would hear oneself half a 3 Applications SAT satellites in 1984. second later. This was thought to be a The most important application of satel- Regional systems can make use of satel- major obstacle for a two-way telephone lite communication has up till now been lites with global, semi-global or spot cov- conversation. Effective echo cancellers inter-continental telephone and TV trans- erage. This can be achieved by larger were then developed. This reduced the missions. This was the first application satellite antennas. The European problem to such a degree that the tele- and is still the most important. Large and EUTELSAT system is a satellite system phone users accepted a connection via expensive earth stations with antennas up with several application areas. The devel- satellite. to 20 m in diameter are employed. The opment of this system started in 1970. In 1965, the first commercial geostation- INTELSAT-A system is an example of The system intended to ary satellite, Early Bird, was placed in a this. - transmit telephony traffic between position over the Atlantic Ocean. This The rapid development in the satellite large national earth stations became the first satellite in the interna- technology and the use of more powerful tional organisation for telecommunica- - exchange TV programmes (Eurovi- launch vehicles has led to the use of tion satellites, INTELSAT. A prelimi- sion) satellite systems in more restricted areas. nary agreement was made in 1964 As an example the Norwegian Telecom - cover the need for special services (as between 11 countries, but the final agree- employs satellites as a part of its domes- business communication) ment was not signed until 1973. This tic telecommunication network and organisation has been the leading one in - distribute TV programmes to cable offers satellite connections between the the development of communication satel- networks. Norwegian mainland and the oil installa- lites. Over 100 countries are members, tions in the North Sea, figures 5 and 6, as and there are almost 600 earth stations in The first satellite system for maritime well as to the Arctic islands of Svalbard 170 countries. INTELSAT has launched mobile communication, MARISAT, (the NORSAT-A system). This system 35 geostationary satellites and only six became operative in 1976. This system is transmits via one of the satellites in the have been unsuccessful. now replaced by the INMARSAT sys- INTELSAT system. tem. The first operational European Figure 4 shows the development of the The main station in the NORSAT system coastal earth station was put into service INTELSAT system. is located at Eik in the south western part at Eik in Norway in 1981. This station The first Nordic INTELSAT earth station of Norway. It was put into operation in will now expand to cover satellite com- was built in 1971 at Tanum in Bohuslän. 1976 and was the first earth station in munication with aeroplanes This earth station communicates with Europe intended for domestic traffic.

INTELSAT-IV

ISFJORD RADIO (Spitzbergen)

Tromsø

Trondheim ca 200 km EIK (main station) Bergen STATFJORD Oslo Stavanger ca 200 km

FRIGG ca 300 km

EKOFISK

Figure 5 The NORSAT-A system started in 1975. The initial sys- Figure 6 NORSAT-A earth station on an oil rig in the North Sea tem used leased capacity on an INTELSAT-IV satellite

6 Figure 7 The earth station at Isfjord Radio, Spitzbergen, 78° Figure 9 Nittedal earth station outside Oslo, Norway North. At this station the maximum elevation angle towards a geostationary satellite is 3.1°

Small aircrafts

Land mobile (cars/persons)

Land mobile Large aircrafts (trucks)

Coastal earth station

Large vessels Small vessels

• Necessary reduction in terminal cost • Increasing number of terminals

Figure 8 Mobile satellite communication system

7 (INMARSAT AERO). The evolution The last application area is broadcast- lites, e.g. the space shuttle, has an points at communication with even ing of television and sound. This is a orbital period of approximately 1.5 smaller units such as trucks, private point-to-multipoint system where we hours. An example of a satellite with a cars and individuals, figure 8. make full use of the advantage of satel- long period of revolution is the moon. lite communication. One satellite in Due to the long distance from earth As the satellites become more power- geostationary orbit will cover 97 % of (ca 385,000 km) the period is about ful it is possible to make simpler earth all households in Norway. Today more 27 days. station equipment. Then the satellite than 100 television programmes are terminals can be located closer to the transmitted via satellite to European user. This is done in the VSAT sys- The orientation of the orbit with countries. Arthur C Clarke’s original tems which are mainly used for busi- respect to the earth’s equatorial plane idea from 1945 has become a reality. ness communication. The Norwegian may also be different, i.e. inclined Figure 10 shows the total involvement Telecom has established an earth sta- orbits. of the Norwegian Telecom in satellite tion at Nittedal, 1987, for European communication. data communication via the EUTEL- The movements of a satellite are de- SAT system, figure 9, as well as a traf- termined by the laws of Newton (1642 fic control station at Eik for business 4 Orbital considerations - 1727) and Kepler (1571 - 1630). communication, the NORSAT-B sys- A satellite’s period of revolution is Applied to satellites Kepler’s laws are: tem. The NORSAT-B system offers determined by the distance from the connections between unmanned earth earth. The farther from earth, the 1 The satellite orbit is an ellipse with stations in Europe. longer the period. The nearest satel- the earth at one focus.

Satellite broadcasting (1984) Satellite business communication (1986)

SVALBARD (1979, 1984) ¥ INTELSAT ¥ INMARSAT ¥ EUTELSAT EIK ¥ TELE-X ¥ NORSAT - A (1976) ¥ THOR OIL RIGS (1976, 1984) ¥ INMARSAT (1982) ¥ NORSAT - B (1986, 1989)

NITTEDAL ¥ EUTELSAT (1986) ¥ TELE-X (1989) ¥ THOR (1992) SHIPS (1982)

Figure 10 The Norwegian satellite involvement; main earth stations and utilized satellites

8 Fc

v v m

Satellite b p

F g r ϕ Apogee Perigee v = velocity of satellite F« F Fc = centrifugal force Earth Fg = gravitational force M c m = mass of satellite b M = mass of earth r = orbital radius aa

r

Figure 12 The elliptical satellite orbit with major and minor semiaxis a and b and eccentricity e = c/a. The semilatus rectum p = h2/µ where h = v ⋅ r (the Figure 11 Forces acting on a satellite in a circular orbit angular momentum per unit mass)

(North Pole) Z Satellite Perigee

Interseeting line between the earth«s N equatorial plane and the ecliptic Vernal equinox Descending (21 March) node ϕ

Sun Earth Y N N ω i equator Sommer solstice Winter solstice Ω (21 June) (21 Dec) Equatorial X Ascending node N Plane The ecliptic Autumnal equinox (the earth«s orbit (21 Sept) around the sun) Figure 13 The geocentric equatorial coordinate system. The X z-axis coincides with the rotational axis of earth. The x,y-plane cuts through the earth’s equator and is called the equatorial plane. The x-axis is the Figure 14 Definition of the x-axis in the geocentric equatorial direction from the centre of earth through the centre coordinate system of sun at the vernal equinox (≈ 21 March), see figure 14

Satellite 3 geosynchronous ∆ orbit VA it rb o ry a n o ti elliptical ta s transfer o e orbit G 120o 120o 200 km low earth orbit N

transfer orbital plane 36 000 km Equatorial inclination Earth plane Earth North Pole

∆ VP

36 000 km

Satellite 1 Satellite 2

120o Figure 16 a) and b) Orbits for launching of geosynchronous satellite. The incli- Figure 15 Three satellites in the geostationary orbit will nation of the transfer orbital plane is equal to or larger than the lati- almost cover the whole earth tude of the launch site

9 2 The radius vector to the satellite where This orbit is called the geostationary sweeps out equal areas in equal h2 orbit (GEO). times. p = semi-latus rectum = µ The geostationary satellite has a 3 The square of the period of revolu- period of revolution a little under 24 c tion is proportional to the cube of e = = eccentricity (o < e < l) hours, due to the fact that the earth the orbit’s semi-major axis. a moves around the sun in one year. a = major semi-axis This means an extra revolution with Considering a circular orbit, the forces h = v ⋅ r = the angular momentum respect to the fixed stars. The period acting on the satellite are shown in per unit mass of a GEO satellite will accordingly be figure 11.  365.25  T = ⋅24⋅60 ⋅60 = 86 164 sec With reference to figure 11, the gravi- The earth is located in F, one of the  365.25 +1 tational force Fg on the satellite is two foci. When the two foci coincide (e given by Newton’s law: = o) the orbit will be a circle. The Using equation (4.6) we can now cal- point closest to earth is called “peri- ⋅ culate the radius of the geostationary = M m gee” and the farthest point is called Fg G 2 orbit: r (4.1) “apogee”. The angle (φ) measured 1/3 from the perigee to the satellite is cal-   T  2  where G is the universal gravitational = µ  led the true anomaly. r   = 42 164 km constant.  2π  The true anomaly, eccentricity and According to Newton’s general law of length of the semi-major axis deter- The satellite’s height above the earth’s motion, the centrifugal force is given mine the satellite’s position in the orbi- surface will be: by tal plane. In order to locate the satel- h = r - Rj = 35 786 km 2 lite in space, we need information = = v about the orbit’s orientation. We intro- Fc ma m where R = 6 378 km = equator radius r (4.2) duce a fixed reference co-ordinate sys- j of the earth. tem called the geocentric equatorial Counterbalancing of forces, F = F , c g co-ordinate system, figures 13 and 14. The satellite’s velocity is: gives This co-ordinate system moves with π = 2 r the earth around the sun, but it does v = 3.075 km/sec M ⋅m v2 T G = m not rotate. r2 r (4.3) The intersection of the equatorial From a satellite in the GEO orbit or the satellite velocity plane and the orbital plane is called approximately 42 % of the earth’s sur- the line of nodes. The ascending node face will be visible. Accordingly, with G ⋅ M v = is the point where the satellite is cros- 3 satellites in the GEO orbit, we will r (4.4) sing the equatorial plane from south to have almost full coverage of the earth, north. The angle between the x-axis except for areas above 81° latitude The period of orbit, T, is given by and ascending node is called the right north and south, figure 15. 2πr T = ascension (RA) of the ascending node v (4.5) (Ω). The angle between the ascending Our problem with the geostationary node and perigee (ω) specifies the ori- satellites is the transmission delay of Inserting for the satellite velocity, entation of the ellipse, and is called the the radio signal between the earth and equation (4.4), the period will be argument of perigee. The inclination the satellite. Since radio waves propa- r3 r3 (i) of the orbital plane is the angle be- gate with the speed of light (300 000 T = 2π = 2π ⋅ µ tween the equatorial plane and the km/s) the time delay will be about G M (4.6) orbital plane. 270 milliseconds to and from the satel- lite. This was in the early days con- where µ = G ⋅ M = 398 600 km3/sek2 The five orbital elements, a, e, Ω, ω, sidered to be a major problem for (Kepler’s constant). and i, completely define the satellite’s speech connections via GEO satellites. This expression is a mathematical sta- orbit around the earth. These ele- However, it showed up that the prob- tement of Kepler’s third law. This ments are time independent. The last lem was much less than anticipated as equation applies even if the orbit is element, ϕ, specifies the satellite’s mentioned previously. elliptical, substituting the radius r with position in its orbit, and is the only the semi-major axis a. time dependent parameter. In the last few years other orbits than the geostationary have been reconsi- An elliptical orbital geometry is shown For communication satellites there is dered with great interest, especially in figure 12. one orbit which is particularly interes- low earth orbits for mobile satellite ting. This is the near circular equato- services, and highly inclined elliptical The equation describing the orbit in rial orbit with period of revolution orbits for broadcasting services, ref polar co-ordinates (r, ϕ) is equal to the rotational period of earth Hovstad/Gutteberg: “Future systems (24 hours). A satellite in this orbit will p for mobile satellite communications” r = follow the earth’s rotation, and for an + ϕ (this issue). 1 ecos (4.7) observer on earth’s surface, the satel- lite will appear to be “fixed in the sky”.

10 θ

V A

∆ V A

Apogee i = inclination

(Ascending node) VS

VA = satellite velocity at apogee VS = synchronous velocity = 3,075 km/s ∆ VA = required velocity increment θ = direction of impulse

Figure 18 Velocity diagram on a plane normal to the line of Figure 17 Multistage launcher Ariane nodes Lattitude 5 Launching figure 16. For the following reasons, the N launch site should be near equator: For a satellite to achieve the geostation- (i)To make maximum use of the surface ary orbit, it must be accelerated to 3.075 velocity of the earth km/s at a height of 35 786 km with zero i inclination. Theoretically, one can place (ii) To achieve minimum inclination. the satellite directly into the geostation- This will give minimum velocity W E Longitude ary orbit in one operation. However, con- increment for the correction of the siderations regarding costs and launching inclination. The minimum inclination i capabilities call for a multistage launch achievable is equal to the latitude of vehicle (2 - 4 stages). The launching of the launch site. the satellite represents a major part of the S total investments in a satellite system. The launch sequence is as follows: The Nodes satellite is placed into low earth orbit (equator crossing) The conventional, and most economical, with an altitude of about 200 km, e.g. by method of launching a satellite is based the space shuttle. There must be two Figure 19 Apparent movement of satellite on the use of the Hohmann transfer orbit, in an inclined synchronous orbit velocity increments; one for injection ∆ into the elliptical transfer orbit ( Vp ) by the perigee kick motor and one for injec- ∆ tion into the geosynchronous orbit ( VA) Despun by the apogee kick motor. The transfer forward Antenna support orbit has an altitude of the perigee and thermal barrier beam apogee corresponding to the transfer and Reflector geosynchronous orbit altitudes, respec- Transmit/receive feed horn and tively. assembly Antenna Alternatively, one can put the satellite deployment directly into the elliptical transfer orbit, Telemetry and and positioning by a multistage launcher, e.g. Ariane, see Forward command antennas solar mechanism figure 17. The idea of using a multistage polar Primary Auxiliary launcher is to get rid of the unnecessary thermal shelf mass, to achieve the sufficient velocity. radiator Despun Correction of the inclination has to be payload done at one of the nodes of the transfer Solar Spinning/ compartment orbit, in order to achieve an equatorial panel despun lock (4) circular synchronous orbit. The velocity extension BAPTA drive (3) AFT Earth sensor (2) increment is depending on the inclination thermal and on the velocity of the satellite. The barrier Spinning lower the satellite velocity, the more eco- section Radial nomical the manoeuvre. The circulariza- thruster (2) Propulsion tion manoeuvre should therefore be car- AFT tank (4) ried out at apogee. This implies that the solar Axial orientation of the transfer orbit has to be panel thruster (2) Apogee motor such that the line between the apogee and perigee is in the equatorial plane, cfr. fig- ure 13. The altitude of the apogee has to Solar be equal to the altitude of GEO orbit. The panel ∆ extension rack (3) magnitude ( VA) and direc-

Figure 20 Exploded view of the THOR satellite

11 tion (θ) of the velocity vector can be calculated from figure 18. If the geo- N Orbit synchronous orbit has some inclina- X tion, the satellite will apparently move Earth in orbital in a figure-of-eight with respect to the plane nodes, figure 19. The maximum excur- roll sion from equator in the north or south direction will be equal to the inclination. X S 6 The spacecraft The main purpose of a communication local vertical satellite is to receive and transmit yaw radio signals within the coverage area on the earth. This means that the spacecraft should be a reliable and stable platform for the communication Sun system. The satellite is generally divid- ed into two main modules: line 1 The communication module (or the “payload”) which includes pitch - repeaters (or transponders) - antennas normal to orbital 2 The service module (or the “bus” or plane (south) “platform”) comprising the following subsystems Y - attitude and orbit control (AOCS) - telemetry, tracking and command Figure 21 Definition of the reference axes, roll, pitch and yaw (TT&C) - power supply.

Figure 20 shows an exploded view of a typical spacecraft.

6.1 Attitude and orbital control Solar subsystem panel The objective of the attitude control Antenna subsystem is to maintain the commu- pointing nication antennas correctly pointed towards towards the earth, and the solar cells earth Despun Reaction correctly pointed towards the sun. The antenna wheels section movements of the satellite about its centre of mass can be described by rotations about the three orthogonal reference axes: roll (x), pitch (y) and Antenna yaw (z), see figure 21. Solar Spinning pointing panel section towards There are two different methods of earth controlling the attitude: spin stabilisa- tion and body-fixed stabilisation (3- axis stabilisation), see figure 22. In the Solar first type, the satellite spins around its Pitch axis panel main axis of symmetry. The rotation is normally about 60 rpm. This will pro- duce an angular momentum in a fixed direction. For a geostationary satellite, the spin axis (pitch) has to be parallel Pitch axis with the earth’s axis of rotation. To maintain the pointing of the communi- Figure 22 a) and b) The two concepts of stabilization cation antennas towards the earth, the

12 Earth

despun communication antenna α ω∆ t1 platform containing the antennas has β to rotate in the opposite direction (de- equator spun). Pitch correction is done by α−β varying the angular velocity of the des- ω∆ t2 pin motor, since the rate of change of angular momentum is proportional to the torque. Yaw and roll corrections are made by thrusters mounted at bolometer appropriate places on the body. A typi- beams cal spin-stabilised satellite is shown in figure 20. ω To keep the gyroscopic stiffness in a Figure 23 Earth contour detection by scanning infrared sensors (bolometers) body-fixed stabilised satellite, one may ω = spin rate of satellite use a momentum wheel inside the 2α = angle between the two bolometer beams satellite. This gives rather moderate β = error angle antenna pointing. Instead, it is usual to ∆t and ∆t = time between the horizon crossings 1 2 use three reaction wheels spinning around the three main axes. While the momentum wheel is spinning at high velocity (about 10 000 rpm), the reac- tion wheels are spinning in both direc- Command tion around zero speed. By varying the Command Command omni antenna Decoder speed and direction of the reaction receiver output wheels, controlling torques can be applied around all axes. When the wheels reach their maximum speed, they must be unloaded by operating

tones thrusters on the satellite’s body.

Ranging A typical body-stabilised satellite is Communication shown in figure 2. For operating the (spot) antenna Telemetry Telemetry torque units, it is necessary to obtain Encoder the attitude of the satellite. This is transmitter input done by sensors on board the space- craft sensing the direction to the sun and earth. As seen from the geostatio- Telemetry nary orbit the sun subtends only 0.5°. omni antenna It is therefore a good source for ob- taining attitude reference. The sun Figure 24 The satellite’s TT&C system sensor consists of photo cells which

Sun in summer N solstice

Earth

Sun in 23,4o autumn Orbital plane and spring equinoxes 23,4o

Sun in S winter solstice

Figure 25 Apparent motion of the sun relative to earth

13 produce an electric current when illu- ching transponders on and off, stee- ratio is about 10 Watts/kg, whereas minated by the sun. ring antennas, firing the apogee boost for the 3-axis stabilised satellite the motor in transfer phase, etc. When ratio is about 50 Watts/kg. This The earth, seen from the satellite, has receiving a command, the command means that if we need a satellite with a much larger view angle (17.3°) and subsystem generates a verification sig- high RF output power, we should its centre cannot directly be mea- nal, which is sent back via the tele- select a body stabilised satellite. The sured. However, using infrared detec- metry link. After checking, an execute disadvantage is that they are more tors (bolometers), one can measure signal is transmitted to the satellite. complex. the edge of the earth, due to the diffe- This prevents inadvertent commands. rence in radiation from the cold sky On-board batteries are needed to pro- As in the telemetry link the omni and the warm earth. Infrared detectors duce power during the launch phase antenna is used during the transfer can be used both day and night. If two and when the satellite enters the sha- phase, while spot-beams are used bolometers are mounted on a spinning dow of the earth (eclipse). This hap- when the satellite is in its geostatio- spacecraft, as shown in figure 23, the pens around the equinoxes, and the nary position. pointing error (β) can be calculated maximum duration is approximately measuring the time between the two The tracking or ranging subsystem 70 minutes. This is due to the fact that horizon crossings (t1 and t2), knowing determines the orbit of the spacecraft. the earth’s equatorial plane is inclined the spin rate (ω) and the angle be- There is a number of techniques that with an angle of 23.4° with the direc- tween the bolometer beams (2α). can be used. One method is to mea- tion to the sun, see figure 25. During sure to pointing of the TT&C earth the summer and winter seasons the On a 3-axis stabilised satellite scan- station antenna towards the satellite, satellite is out of the earth’s shadow, ning mirrors have to be used, since together with the slant range to the figure 26, whereas during the equino- the satellite itself does not spin. satellite, i.e. the range vector. The xes the satellite passes through the A higher degree of pointing accuracy range to the satellite can be measured earth’s shadow once a day for 42 days, may be obtained by using a radio bea- by modulating the command carrier figure 27, i.e. the satellite will experi- con on the earth. In such a system the with multiple low frequency tones ence 84 eclipses per year. satellite antenna is directly locked to (“tone ranging”). These tones are the transmitted radio signal from the demodulated and remodulated on the 6.4 The communication module earth beacon. telemetry carrier and received in the The communication subsystem is the TT&C ground station. By comparing primary system in the satellite. All the phase between the transmitted 6.2 Telemetry, tracking and other functions in the satellite can be and received tones, the range can be command (TT&C) considered as supporting activities for calculated. The highest tone gives the this system. It consists of the receiv- The satellite is supervised and control- best accuracy, but several lower tones ers, transmitters and the communica- led via a dedicated earth station, resolve the ambiguity. tions antennas. One receive/transmit TT&C station, which in turn is connec- chain is called a transponder. A satel- ted to the satellite control centre 6.3 Power supply lite with 5 transponders is shown sche- (SCC). The main tasks of the space- Solar energy is the only external matically in figure 28. The building craft management is to control the energy source for orbiting satellites. blocks are: orbit and attitude of the satellite, moni- The solar radiation intensity is about tor the “health” status, remaining pro- - A wide band receiver and a down 1.4 kW/m2. The sun energy is conver- pulsion fuel and transponder configu- converter ted to electrical energy by means of ration, together with steering of anten- photovoltic cells (solar panels). The - An input multiplexer (IMUX) nas. efficiency of the solar panels is about - 5 channelised sections including the The satellite’s TT&C system is shown 10 - 15 %. On the spin-stabilised satelli- high power amplifiers in figure 24. tes the solar panels form the exterior of the spacecraft’s body, see figures 20 - An output multiplexer (OMUX). The telemetry subsystem collects data and 22. from various sensors on board the The wide band receiver/converter spacecraft, such as fuel tanks pres- One disadvantage of the spin-stabilis- operates in the whole up-link band, sure, voltages/currents in different ed satellite is that only 1/3 of total e.g. in one of the common bands subsystems, sighting from infrared solar array is exposed to the sunlight. and sunlight detectors, temperature, On the body-stabilised satellite the - C-band (5.9 - 6.4 GHz) etc. The information rate is usually solar panels are mounted on two - Ku-band (14 - 14.5 GHz) less than 1 kbit/s. The data are trans- deployable “wings”, as shown in figure mitted on a low-power telemetry car- 2 and 22. The satellite moves around - Ka-band (17.3 - 18.1 GHz) rier using an omnidirectional satellite the earth in 24 hours. For intercepting antenna during the transfer phase maximum solar flux, the solar panel or other bands allocated to satellite and/or a spot beam antenna when the wings have to make one turn per day. communication. satellite is on station. They are driven by two separate The frequency conversion could either motors. The command subsystem is used for be single, as in figure 28, or dual. In remote control of the different func- A measure of efficiency is the ratio of the latter case the first mixer converts tions in the satellite. This may include produced electrical power to the mass the frequency down to an intermediate attitude and orbital manoeuvres, swit- of satellite. For a spinning satellite this frequency (IF). After the channel

14 amplification on IF the signal is con- can be chosen independently of the nel bandwidths may also be unequal. verted up to the down-link frequency, up- and down-link frequencies. The channel amplifier/attenuator sets before the last power amplification. the gain (gain-step) of the transponder The input multiplexer separates the This is often done because it is much in order to control the input back-off of up-link band into individual channels easier to make filters, amplifiers and the TWTAs. This can be done from with a certain bandwidth. For example equalisers at an intermediate fre- the TT&C station by the up-link com- can a 500 MHz wide up-link band be quency. The intermediate frequency mand system. separated into 12 channels with chan- nel bandwidth of 36 MHz. The chan- The last high-power amplifier (HPA) is usually a travelling wave tube ampli- fier (TWTA). This amplifier establis- hes the level of the output power, which may be in the range of 10 - 200 Sun Watts depending on the application. The TWTA operates near saturation. Day The input/output characteristic is Earth shadow shown in figure 29. The output multi- Geostationary orbit plexer combines all the channels Night before signals are transmitted to the earth by the antenna.

6.5 The satellite communication Figure 26 During summer and winter the satellite is always illuminated antenna Antennas are used for receiving and transmitting modulated radio frequent Sun signals from and to the earth. The most commonly used antenna in the Earth shadow microwave frequency band is the para- Geostationary orbit bolic reflector antenna. The antenna is characterised by its radiation pattern Night Day and the maximum gain, that is the antenna’s capability to concentrate the energy in one direction. The design of the satellite antenna is Figure 27 The satellite passes through the earth shadow at the equinoxes once a day depending on required coverage area,

Automatic Travelling Channel Gain Control/ Wave Tube filter Gain Setting amplifier

A TWT

TWT B TWT AB input filter mixer TWT Up-link C Spare Down-link frequency f1 Low Preampli- frequency f2 noise fier D Receiving amplifier antenna Transmit DEC antenna E Local oscillator ∆ ( f=f1-f2) Wide band low Input Channel High power Output noise receiver with multiplexer amplifiers amplifiers multiplexer down converter

Figure 28 A satellite with 5 transponders and single frequency conversion

15 Output power (dBW)

see figure 30. This determines the beam width of the antenna, usually P Saturated output taken as the half-power beam width max (or 3 dB beam width). The 3 dB beam Output back-off width is given by λ θ = K ⋅ (degrees) (6.1) 3dB D where

Input back-off K = 65 - 75, depending on the antenna aperture field distribu- tion λ = wavelength D = diameter of antenna.

Input power (dBW) The gain of the antenna is proportional to its area, and is given by P in 2 at saturation 4π  πD G = η ⋅ A = η λ2  λ  Figure 29 Input/output characteristic of a TWTA (6.2) where η = antenna efficiency (0.5 - 0.8) Coverage area N θ θ = 3dB A = area of antenna

or in dB: 2 equator   πD  G(dB) = 10logη    λ   θ   (6.3)

Sub Obviously the gain and the beam Main lobe o satellite width of a reflector antenna are rela- 17,4 point ted. The gain is approximately given Side lobes by 30000 G = θ 2 3dB (6.4) Antenna radiation pattern where θ is given in degrees. Figure 30 Coverage area of the satellite antenna. For global coverage the required 3dB beam width is 17.4° Should the service area be the whole earth, the required beam width would be 17.4° as seen from the geostatio- nary orbit. This corresponds to a satel- EIRP: 49 dBW 54 dBW lite antenna with 20 dB gain. By in- creasing the size of the satellite anten- na only a small area of the earth is illu- 39 dBW minated. These narrow beams are cal- led “spot beams”. If we should cover an area on earth with a diameter of 300 km, the beam width of the antenna should only be 0.5°, with a correspon- ding gain of 50 dB. This means that we need less power from the HPA in the spot beam to achieve the same flux 45 cm 65 cm 210 cm antenna density on earth as in the global beam. diameter To express the transmitted power of a satellite, the Equivalent Isotropic Radi- Figure 31 The preliminary EIRP contours of the THOR satellite and the correspon- ated Power (EIRP) is used. This is ding receiving antenna diameter for TV reception with good quality simply the product of output power

16 from HPA(Pt ) and satellite antenna nary satellites. In that case, as previ- The maximum latitude coverage of a ε γ gain (Gt): ously mentioned, the satellite will not geostationary satellite ( = 0, = 0) ⋅ move with respect to an earth station, will be EIRP = Pt Gt (6.5) π and the antenna pointing can be fixed. ϕ = − Re max arcsin = 81.3° N or S So instead of plotting coverage con- The pointing of the antenna is defined 2 Rs tours (or gain contours) we can plot by two angles, azimuth (α) and eleva- EIRP contours. An example is given in tion (ε). The azimuth is the angle 7.2 Link analysis figure 31. from true north to the projection of the The purpose of a satellite system is to line to the satellite in the local horizon- A satellite can have more than one provide reliable transmission with a tal plane. The elevation is the angle antenna. Modern satellites employ specified quality of the received signal. above the local horizontal plane to the several antennas, both with global, The transmitted information has to be satellite. hemispheric and spot coverage. modulated on an RF carrier. In analo- Knowing the satellite’s position, i.e. gue systems, where frequency modu- As described, the communication pay- the longitude, and the earth station lation (FM) is the dominating modula- load overall can be characterised by position, i.e. longitude and latitude, we tion method, the signal-to-noise ratio the following parameters: can calculate the earth station anten- (S/N) after the demodulator is a mea- - the coverage area na’s pointing angles. The geometry is sure of signal quality. In digital satel- shown in figure 32. lite links the measure of quality is the - the maximum EIRP bit error rate (BER). The modulation With the terms given in figure 32, the - the input power flux density for satu- method most often used in digital sys- azimuth, elevation and distance to ration of the output high power tem is phase shift keying (PSK). satellite are given by: amplifier  tgγ  In both analogue and digital systems - the channel arrangement or fre- α = arctg ±  there is a unique relationship between  φ  quency plan. sin (7.1) the carrier-to-noise ratio (C/N) and the signal-to-noise (S/N) ratio or the φ γ − Re cos cos R bit error rate (BER). Given the modu- 7 The satellite link ε = arctg s lation method, the performance of a 2 1− (cosφ cosγ ) total link is generally specified in 7.1 The link geometry (7.2) terms of a minimum C/N in a certain In order to communicate with a satel- 1− (cosφ cosγ )2 percentage of time. In order to estab- = lite, it is necessary to know the correct d Rs lish the link quality, we therefore need pointing of the earth station antenna. cosε (7.3) to calculate the carrier power (C) and We shall limit ourselves to geostatio- the noise power (N) at the receiving station.

N E cosβ = cosϕ ¥ cosγ Lattitude of Zenith earth station θ = π/2-ε-β

e R α ε E β Earth Station θ S e O ϕ γ R R Re/cos cos

Rs

β ϕ O

ε d E quator R γ s θ R

Q S Sub Local Longitude of satellite horizontal Satellite earth station plane S point (ERQ)

Figure 32 a) and b) Calculations of azimuth, elevation and distance to a GEO satellite

17 Considering an isotropic antenna, that η = efficiency where is an antenna which radiates with uni- A = physical area k = Bolzmann’s constant form intensity in all directions, the r = 1.38 ⋅ 10-23 W/Hz ⋅ K power flux density (PFD) at a dis- the received power (C) will be: tance, d, will be EIRP We can now define an effective input P = ⋅ = ⋅ = t C PFD Ae 2 Ae (W) noise temperature of a receiver (or a PFD 2 4πd (7.7) 4πd (7.4) network). This is the temperature of a Using the relationship between the noise source located at the input of a where P = the total radiating power. t effective antenna area (A ) and gain noiseless receiver giving the same out- Using a directive antenna with a gain e (G ), equation (6.2), the received put noise power as the noisy receiver. G , the power flux density can be r t power may be written as increased by G : Noise power at the earth station re- t 2  λ  ceiver input has contributions both P ⋅G EIRP C = EIRP⋅  ⋅G = t t =  π  r from the noisy receiver and the noise PFD 2 2 4 d (7.8) 4πd 4πd (7.5) picked up by the antenna. The term in the middle is called the If the receiving antenna has an effec- Hence, the total system noise tempera- free space loss: tive area of ture (TS) is given by  4πd  2 A = η A (7.6) L = T = T + T (7.11) e r o  λ  S A R (7.9) where where In addition to the free space loss, we T = antenna noise temperature have several other sources of attenua- A tion, such as atmospheric absorption, TR = effective noise temperature of Antenna rain attenuation, etc. ref O Gutteberg: the receiver. “Effects of atmosphere on earth-space radio propagation” (this issue). The total noise power (N) is then Feeder LNA given by: All components, active and passive, N = kT B (7.12) produce electrical noise. This is due to S T L T the thermal agitation of electrons. The A R If the received carrier power (C) is thermal noise power (N) increases TO = 290 K measured at the same point, according with temperature (T) and band width to the equations (7.8) and (7.12), the TS (reference point) (B) and is given by C/N ratio will be Figure 33 A typical receiver “front-end” N = kTB (W) (7.10) C 1 G 1 = EIRP⋅ ⋅ r ⋅ N Lo Ts kB (7.13) g Equation (7.13) is called the link bud- (C/N)up get, and the term Gr /Ts is the recei- ver’s “figure-of-merit”. It specifies the PS sensitivity of the receiver, either in the EIRPsat = PS ¥ GS satellite or in the earth station. The reference point for Gr and Ts has to be the same. A typical receiver “front-end” is shown

GS in figure 33. Referred to the low noise amplifier’s input, the overall noise tem- perature is given by T  L −1 T = A + T + T s L  L  0 LNA gd (7.14) GT where L = feeder loss

To = ambient temperature = 290 K PT TLNA = noise temperature of the low noise amplifier.

EIRP = P ¥ G (C/N)down Succeeding stages of the receiver will earth T T not contribute if the gain of the LNA is Figure 34 Basic satellite link sufficiently high.

18 Instead of giving the noise tempera- then In figure 35 is shown the EIRP con- ture of a component, the noise figure C 1 tours for INTELSAT-V A satellite in 1° (F) is often used. This is defined as = west. N 1 + 1 the signal-to-noise ratio at the input ()C / N ()C / N up down (7.19) Using the above developed equations, divided by the signal-to-noise ratio at the corresponding receiving antenna the output: diameters have been calculated, figure Using equation (7.19) we can calculate SNii/ 35. For the calculations the following F = the total C/N ratio, knowing C/N SN/ parameters have been assumed: oo (7.15) ratios on the up- and down-link. C/N = 12 dB (TV reception with If the source noise temperature is at If the (C/N) is more than 10 dB good quality) the standard temperature T = 290 K, up o above (C/N) , the noise contribu- then down B = 27 MHz (receiver noise band- tion from the up-link to the down-link S N 1 N width) F = i ⋅ o = ⋅ o is only a few tenths of a dB. Often it is disregarded in link calculations where F = 1 dB (receiver noise figure) So Ni g kTo B (7.16) there is enough power on the up-link. where Knowing the g = gain of the network. - location of satellite and earth station Using the definition of effective input EIRP: - EIRP and G/T of the satellite 48 dBW 46 dBW noise temperature: 39 dBW kT( + T)B⋅ g T - EIRP of the earth station F = o = 1+ - the required quality (C/N) kTo Bg To (7.17) - the carrier frequencies or - the receiver overall system tempera- T = (F - 1) 290 (7.18) 70 cm 90 cm 210 cm antenna ture T (or noise figure F) diameter A satellite system consists of both an - the equivalent noise bandwidth of up-link and a down-link. Noise on the the receiver (B) up-link will also contribute to the over- Figure 35 EIRP contours for INTELSAT-V-A at 1° west all noise power received at the earth we are now able to calculate the recei- and the corresponding receiving TV antenna station. The basic satellite link is ving station’s figure-of-merit (G/T) or diamters. The different parameters are given in shown in figure 34. receiving station’s antenna diameter. the text The total received noise power at the receiving earth station is given by θ ⋅ ⋅ Nt = Nu g gd + Nd WS IS where down-link N = noise power on the up-link interference u receiving and Nd = noise power on the down-link transmitting patterns g = total transponder gain g = “section gain” on the down- wanted d signal link (free space loss and ≈θ ≈θ transmitting antenna gains). pattern receiving up-link The total noise-to-carrier ratio is then pattern interference N gg + N N = up d down C C N gg = up d + Ndown C C N = up + Ndown ⋅ C / g gd C

C WE IE = C since ⋅ up g gd Figure 36 Simplified interference geometry Ws, Is = wanted and interfering satellite and C = C down WE, IE = wanted and interfering earth station θ = angular separation between satellites

19 stations to be interconnected through the same transponder, multipoint-to- multipoint, shown in figure 37c). The methods for allowing several users to utilise the same transponder are called multiple access techniques. The trans- ponder is a resource which can be characterised by its available power and bandwidth. Efficient use of this common resource is a very important problem in satellite communication. The channels are designed to the users either fixed (pre-assigned) or on point-to-point point-to-multipoint demand (demand assignments). (a) (broadcast mode) There are basically three methods of (b) multiple access: - Frequency Division Multiple Access (FDMA) - Time Division Multiple Access (TDMA) - Code Division Multiple Access (CDMA).

Random Access (RA) may also be uti- lised.

In FDMA, each user is permanently allocated a certain frequency band, out of the total bandwidth of the transpon- der, figure 38a). To reduce the adja- cent channel interference, it is neces- sary to have guard bands between the sub-bands. Frequency drifts of the satellite’s and earth station’s frequency multipoint-to-multipoint (c) converters have also to be taken into consideration. FDMA is the traditional Figure 37 a), b) and c) Satellite network configurations technique due to its simple implemen- tation. However, due to the non-linear T = 30 K (antenna temperature, Keeping the interference from other A characteristics of the transponder, clear sky condition) satellites below a certain value will put figure 29, a certain back-off of the severe limits on the minimum diame- L = 0.5 dB (feeder loss) TWT is necessary for multicarrier ope- ter of the receiving earth station’s ration, in order to control the intermo- f = 11.7 GHz (carrier frequency). antenna. Knowing the radiation pat- dulation products. This reduces the terns for the satellites and earth sta- Corresponding calculations have been total transponder capacity. tions involved, one can calculate the done for the diameters given in figure interference level. The geometry is In TDMA all stations use the same 31. The noise from the up-link has shown in figure 36. carrier frequency, but they are only been disregarded. allowed to transmit in short non-over- In the near future, the interference In the calculations above we have only lapping time slots, figure 38b). Thus, from other satellites may be the criti- considered additive thermal noise. the intermodulation products due to cal factor for the design of earth sta- There are also other sources of degra- the non-linear transponder are avoi- tion antennas. This is already the case dation such as interference from other ded. This means that the high power for certain positions in the geostatio- satellites and radio links, intermodula- amplifier in the transponder can be nary arc over Europe. tion from the unlinear transponder, operated near saturation. Accordingly, etc. The effect of these sources are both the total transponder power and often also treated as additive thermal 8 Satellite networks bandwidth are available. However, the noise and the C/I terms should be TDMA technique obviously requires The simplest form of satellite net- added in equation (7.19): time synchronisation and buffer stor- works is the point-to-point or point-to- age. This leads to rather complex C 1 multipoint configuration shown in = earth stations. 1 + 1 + 1 + figure 37a) and b). So far, we have N ()C / N ()C / N C / I etc. up down only considered these cases. However, In CDMA all users occupy the total (7.20) there is often a need for several earth transponder bandwidth all the time.

20 transponder bandwidth The users can be separated because each channel is multiplied by a unique spreading code. The composite signal is then modulated onto a carrier. The information is recovered by multiply- FDMA ing the demodulated signal with the same spreading code. The receiving earth station is accordingly able to frequency recover the transmitted message by a specific user. No frequency or time co- ordination is needed before accessing guard band the transponder. New users can easily be included. CDMA signals are resis- tant to interfering signals. This pro- perty can be utilised in systems with very small aperture terminals (VSAT), where interference may be received frame length from adjacent satellites. The main dis- advantages are cost and complexity of guard time the receivers. TDMA 9 Literature time 1 Berlin, P. The geostationary appli- cations satellite, Cambridge Uni- synchonizing burst from versity Press, 1988. burst different earth stations

2 Maral, G, Bousquet, M. Satellite communications systems, John Wiley & Sons, 1986.

3 Ha, T T. Digital satellite communi- transponder bandwidth cations, Macmillan Publishing Co., 1986.

4 Evans, B G. Satellite communica- CDMA tion systems, Peter Peregrinus Ltd., 2nd edition, 1991. frequency 5 Pratt, T, Bostian, C W. Satellite communications, John Wiley & coded carrier spectra Sons, 1986. Figure 38 a), b) and c) Basic multiple access methods 6 Dalgleish, D I. An introduction to satellite communications, Peter Peregrinus Ltd., 1989.

7 Feher, K. Digital communications. Satellite/earth station engineering, Prentice-Hall Inc., 1983. 10 Gutteberg, O, Lundstrøm, L-I, 8 ITU/CCIR. Handbook of satellite Andersson, L. Satellittfjernsyn, Uni- communications, ITU, Geneva, versitetsforlaget, 1986. 1988. 11 Gagliardi, R G. Satellite communi- 9 Stette, G. Forelesninger i satellitt- cations, Van Nostrand Reinhold, kommunikasjon, NTH. 2nd edition, 1991.

21 Satellite communications standardisation in Europe

BY GUNNAR STETTE

621.396.946(4) 006 The role of ETSI first time, and in July 1988 the first tion of the environment and cover ele- meeting of the Technical Assembly ments such as: Telecommunications is by nature an was held. internationally oriented activity, and - mechanical protection of personnel there has always been a strong requi- - electrical safety rement for international standardisa- The structure of ETSI tion. For these purposes some very The main product of ETSI is telecom- - electromagnetic compatibility, i.e. successful organisations were created, munications standards. In addition, protection of other radio services CCITT, Comité Consultatif des Télé- where the technology is not mature or (terrestrial and satellite), including graph et Téléphone, CCIR, Comité stable, and where the need for, or EM power emitted by power supply, Consultatif des Radio and ITU, The structure of, a standard is not clear, local oscillator leakage and other International Telecommunications the issue may be studied by the bodies spurious sources. Union, which forms a part of the Uni- of ETSI. In this case the output of the ted Nations system. work is an ETR, ETSI Technical Recommendations could cover ele- Report. ments which deal with quality The organisations mentioned above are global. In addition, a European Most of the standardisation work is - antenna gain receive patterns organisation was created for post and carried out in one of the twelve Tech- - receive polarisation discrimination telecommunications matters, CEPT, nical Committees, TCs, and in the Conference Europeenne des Postes et Technical Sub Committees, STCs. - antenna mechanical capability (poin- Télégraphe. Membership of this orga- The TCs are as follows: ting, polarisation, etc.). nisation was limited to the national NA Network Aspects Telecommunications Operators, the Non-compliance with the recommen- TOs. BT Business Telecommunications dations will not entail refusal of the type approval. CEPT carried out standardisation SPS Signalling, Protocols and Swit- work in many fields, from postal ching stamps to the pan-European cellular The production process TM Transmission and multiplexing mobile telephone system, GSM, which for an ETS was named after a special group of TE Terminal Equipment The preparation of a standard can be a CEPT, Groupe Spécial Mobile. Neit- EE Equipment Engineering very large project. The work is basi- her manufacturers nor telecommuni- cally done within the STCs, where all cations users were accepted as mem- RES Radio Equipment and Systems ETSI members have the opportunity bers of this organisation. SMG Special Mobile Group to be represented. It is further based With the introduction of data systems on voluntary contributions from the PS Paging Systems connected to the telecommunications participants. systems it was impossible to keep all SES Satellite Earth Stations The production process up to an telecommunications activities within ATM Advanced Testing Methods approved European Telecommunica- monopolies. The national networks tions Standard is defined in the Rules are in most countries still entrusted HF Human Factors of Procedures of ETSI. The Draft ETS national monopolies, whereas special will be prepared (if necessary by a networks, terminal equipment and ser- The basis for the work is contributions PT), discussed and agreed in the rele- vices were opened up to competition. from the members of the committees, vant STC, and then submitted for but this is not always sufficient. There There were now many new players in approval by the TC. are also provisions to establish a pro- the field, and these also wanted to ject team (PT) to develop a basic docu- The main steps for the further appro- have a voice in much of the activities ment for further processing in the val process according to the Normal that were carried out by CEPT. CEC, committee system. The PTs can be Procedure is as follows: the Council of the European Commu- paid by ETSI or by external organisati- nity, was not too satisfied with this - Public Enquiry. This will last for ons, mostly the CEC and EFTA, the state of affairs. anything from 17 to 21 weeks. Each European Free Trade Association. ETSI member will then have the During a fact-finding mission on tele- A PT will usually consist of a small opportunity to express their views communications to the United States number of members, typically two to on the proposed standard. in 1986 it became apparent that tele- three, who work concentrated on a communications standards making in - TC Review. The TC will receive the defined Terms of Reference at the Europe had to be substantially reinfor- comments from the Public Enquiry, ETSI Headquarters in Sophia Antipolis ced. In 1987 the CEC published its together with the comments from near Nice. Green Paper Telecommunications. In the Standards Management Depart- it was floated the idea that a European ment of the ETSI Secretariat. There Telecommunications Standards Insti- The structure of ETSs are procedures for resolving dis- tute (ETSI) had to be created. ETSI An ETS (European Telecommunicati- agreements among the members of should have participation from many ons Standard) will normally contain the TC, but it is important to note categories, operators, manufacturers, both requirements and recommendati- that the whole work of ETSI is users, and regulators.In May 1988 the ons. Typically, for a TVRO terminal the based on consensus. Consensus is ETSI General Assembly met for the requirements should deal with protec- here defined as the absence of sus-

22 tained resistance. If consensus can- in the area defined by its terms of refe- SES1, SES4 and SES5 are of “vertical” not be reached, the minority views rence, in line with views expressed by nature, dealing with subjects grouped shall be reported to the Technical the CEC in its “Green Paper on Satel- by types of services and/or their sys- Assembly. lite Communications”: tems aspects. - Voting. The document, as prepared - to allow significant improvements in after the Public Enquiry, is then the satellite communications deve- Relations with other sent out for Voting. It is to be noted lopment of Europe that whereas each ETSI member organisations - to offer new opportunities to the will have the opportunity to express TC-SES is participating actively in the industry. their views during the Public work of CCIR, in particular Study Enquiry, for the Voting process Group 4 (Fixed Satellite Services) and there is only one vote per country. Terms of reference and Task Group TG 4/2, set to prepare The vote of each country is weighed recommendations on VSAT systems as decided by the General Assem- organisation and earth stations. bly. The TC-SES Terms of Reference, as There is a formalised co-operation be- approved by the ETSI 5th Technical tween ETSI and the European Broad- The results of the Voting process is Assembly, are as follows: casting Union (EBU) on the RF part of sent to the National Standards Organi- “The field includes: satellite broadcasting systems and sations, to all the ETSI members and equipment. to the relevant TCs and STCs. - all types of satellite communication services and applications, including TC-SES is actively participating in the There is also a Unified Approval Pro- mobile work of the new “Ad hoc CCIR/CCITT cedure and an Accelerated Unified Experts Group on ISDN/Satellite Mat- Approval Procedure, which can be - all types of earth station equipment, ters”, which has been charged with applied under special circumstances. especially as concerns Radio Fre- the revision, on world-wide basis, of quency Interfaces and Networks those CCITT recommendations which ETSs and CTRs - protocols implemented in earth sta- prove to be insufficiently compatible ETSs are standards, and as such tions for exchange of information, with the implementation of satellite voluntary. In the European context we for connection of network and/or links in the ISDN. have a different type of document, user terminal equipment, control TC-SES is co-operating with CENE- CTR, Common Technical Regulations. and monitoring functions.” LEC, the European Committee for These are mandatory requirements Electrotechnical Standards, on TVRO that have to be met for equipment to The TC-SES is the “primary commit- terminals, the outdoor unit being the be operated within the EC and EFTA tee for co-ordinating the position of responsibility of ETSI. countries. ETSI on the above aspects, vis-a-vis the standardisation of outside bodies, CTRs are often derived from ETSs, but in particular of International Standardi- not all the components of an ETS are TC-SES Programma of sation Organisations (CCIR, CCITT, relevant for a CTR. At the same time IEC, etc.) and of the international Work there could also be other require- satellite organisations (EUTELSAT, The TC-SES part of the present ETSI ments not contained in the ETS that INTELSAT, INMARSAT).” Programma of Work is given in the should be incorporated in the CTR. Annex. It will be noted that the stan- The TC-SES is organised in five Sub The conversion process from ETS to dards are to a large extent based on a Technical Committees, STCs: CTR goes via a Technical Basis for compatibility approach with the main Regulations, TBR, and ETSI is usually SES1: General system requirements emphasis on preventing interference tasked to prepare the TBRs in the rele- for European satellite networks between different systems. vant fields. and earth stations The Work Programma Items DE/SES- As an example, the CEC is now consi- SES2: Radio Frequency (RF) and 2001 and DE/SES2002 have produced dering TBRs and CTRs to be prepared Intermediate Frequency (IF) for Receive Only VSATs and for Trans- for the satellite communications field. equipment mit/Receive VSATs in the 11/14 GHz As a starting point ETSI has been band as prETS 300 157 and prETS SES3: Earth Station application/net- requested to prepare an ETR on this 300 158, respectively. These were work interfaces, and control subject. approved by voting in 1992. and monitoring techniques Draft standards for the C-band, TC-SES Technical Committee SES4: Television and sound pro- DE/SES-2004 and DE/SES-2005, have Satellite Earth Stations gramme equipment been prepared and are ready for Public Enquiry. At the 4th Technical Assembly of ETSI SES5: Earth stations for mobile servi- in Nice, 29 - 30 March 1989, a Techni- ces A draft for the ETS for SNG (Satellite cal Committee on Satellite Earth Stati- News Gathering) station, DE/SES- ons was set up. The basic mission of SES2 and SES3 are of “horizontal” 2003, has been prepared by a Project the new TC was to create European nature, i.e. dealing with subjects grou- Team, and this is also ready for Public Telecommunications Standards ETSs ped by their technology, whereas Enquiry.

23 Standards for control and monitoring that alignment of the European stan- - compatibility with current terrestrial of VSAT networks and VSATs, prETS dards with the global standards cur- mobile networks, in particular with 300 161 and prETS 300 160, were also rently under development in CCITT the GSM approved by voting in 1992. The inter- should be ensured. - system aspects and network mana- connection issues are under prepara- gement issues- the need for Euro- tion by a Project Team. pean standards, and scope and Case study: Low Earth extension of thesestandards. ETS for TVRO stations for the FSS band was prepared under DE/SES- Orbit (LEO) Communica- 4001 and is contained in prETS 300 tions Satellite Systems Future activities 158. A similar document for TVROs ETSI, as a market oriented organisa- operating in the BSS band is ready for There is a growing interest in the tion, must constantly monitor the need Voting. development and deployment of com- for new standards within its field. The munications systems using satellites preparation of the work programma is There are two important continuing in Low Earth Orbits (LEO), in particu- done within the TCs, where the exper- activities. One is to study and monitor lar by the US industry. Also, WARC 92 tise and the contact with the market is radiation limits to/from earth station has allocated new frequencies for present. equipment. The other one is to moni- mobile satellite systems, also for the tor the compatibility of ISDN ETSs systems using LEO satellites. It is apparent from the TC-SES work with satellite links. plan given in the Annex that it focuses The WARC resolved to invite the on the “environmental” aspects of As mentioned above, standards will be organs of the ITU to carry out, as a standardisation, EMC considerations, an important element in the prepara- matter of priority, technical, regulatory etc. This approach was agreed at the tion of TBRs, Technical Basis for and operational studies to permit the early stages of the Committee’s work. Regulations. The CEC has issued a establishment of standards governing One question to be reopened is the mandate to study and investigate in the low orbit satellite systems so as to possible need for standards which detail which parts of currently availa- ensure equitable and standard conditi- involve system design. A possible can- ble standards in the satellite earth sta- ons of access of all countries and to didate would be an open 20/30 GHz tion equipment field match with the guarantee proper world-wide protec- VSAT standard for Europe, similar to essential requirements of the draft tion for existing services and systems the GSM standards for cellular sys- Satellite Equipment Directive. The in the telecommunication network. tems. CTRs shall cover TVROs in FSS and The European Commission has also Of increasing importance in the future BSS bands, data receive-only, VSAT taken an interest in these systems. will also be the work on EMC, and on two-way, and the mobile low data rate The CEC is concerned about the situa- compatibility between the ISDN stan- terminals, connected or not connected tion for European industry, operators dards and satellite links. to the public telecommunications net- and users in this area. Following a let- work. All the ETSs mentioned above ter from the CEC to the Chairman of will form a part of this study. ETSI Technical Assembly on stan- The output of this work, which is dards for such mobile communica- entrusted the TC SES, will be an ETR. tions systems, the TA decided that ETSI would produce an ETSI Techni- Another element of standardisation is cal Report on the matter. This report test specifications. The creation of the should try to set down all of the issues European Single Market of telecom- concerning standardisation, both tech- munications without technical barriers nical, economical and any other fac- to trade implies harmonised European tors. standards and a harmonised European testing and certification framework. The issue is now entrusted to TC SES, and it is being studied by a PT . The The certification authorities and the study should consider some of the testing laboratories in Europe will not problems raised by the use of the LEO be capable of reaching their objectives systems as given below: if, among other things, European stan- dards in the area of test specifications - comparative description of the diffe- do not exist. rent proposed systems The CEC has therefore proposed that - the role of LEO systems in the ETSI through TC SES develop future telecommunication systems ETS/ENs for test specifications of the (PCS, FPLMTS) ETSs mentioned above for the VSAT - protection of other satellite and ter- and TVRO field. The test specifica- restrial services tions for VSAT are of global interest. In order to achieve world wide recog- - network access issues and possibi- nition of tests performed by different lity of interworking with public net- laboratories, the CEC has requested works

24 Annex

List of ETSs under preparation: (As per September 1992)

DE/SES-2001 Ku-Band Receive only VSATs used for data distribution

DE/SES-2002 Ku-Band Transmit/Receive VSATs used for data communications

DE/SES-2003 Satellite News Gathering (SNG) Transportable Stations (14/11-GHz RF/IF Equipment)

DE/SES-2004 Satellite Earth Stations for data communication (C-Band Transmit/Receive VSATs RF/IF Equipment)

DE/SES-2005 Satellite Earth Stations for data communication (C-Band Transmit/Receive VSATs RF/IF Equipment)

DE/SES-3001 Satellite Earth Stations for data communication (C-Band Transmit/Receive VSATs RF/IF Equipment)

DE/SES-3002 The interconnection of Very Small Aperture Terminal (VSAT) systems to the Packet Switched Public Data Networks (PSPDNs)

DE/SES-3003 Standards for Interconnection of VSAT systems to CSPSDNs

DE/SES-3004 Centralised control and monitoring functions for VSAT networks

DE/SES-3005 Control and monitoring function at a VSAT

DE/SES-3007 Standards for interconnection of VSAT systems to ISDN

DE/SES-4001 Standards for interconnection of VSAT systems to ISDN

DE/SES-4002 Transportable Earth Station for Satellite news gathering (general)

DE/SES-4003 ETS on TVRO Earth Stations in the BSS band

DE/SES-5001 Mobile earth stations operating in the 1.5/1.6 GHz bands providing low bit rate Data Communication under Land Mobile Satellite Service (LMSS) and Radio Determination Satellite Service (RDSS)

DE/SES-5002 Mobile earth stations operating in the 10/14 GHz bands providing low bit rate Data Communications under Land Mobile Satellite Service (LMSS)

DE/SES-5003 Mobile earth stations operating in the 1.5/1.6 GHz bands providing low bit rate Data Communications under Maritime Mobile Satellite Service

SI/SES-5-3 Mobile Earth Stations Operating in the 1.5/1.6 GHz Bands providing Voice and High Speed Data Communications under Land Mobile Satellite Service

25 A review of Norwegian space activities

BY GEORG ROSENBERG

629.78(481) Abstract Norway has been a full member of the European Space Agency (ESA) since 1987. This membership is used as an effective tool in strengthening and developing Norwegian industry as well as in covering Norwegian space science and user requirements. The Norwegian Space Centre has a key role in this process. The Space Centre consists of three units: - The head office in Oslo acting as a strategic unit in charge of plans and programmes - Andøya Rocket Range and Tromsø Satellite Station working as operational units with main emphasis in the space science and earth observation areas, respectively. Six basic activity areas have been identified: - Space science, following up and extending Norwegian traditions in cosmic geophysics and astrophysics through participation in ESA’s science programme and through the use of sounding rockets and balloons. - Ground infrastructure, such as Andøya Rocket Range and Tromsø Satellite Station, utilising Norway’s northern location and other advantages. Facilities in the Svalbard area could be of importance in the future. - Satellite Communications is mainly commercial and is by far the largest sector with a large contribution of Inmarsat related products and services. - Satellite navigation and related services in particular based on differential GPS, are also commercialised and are growing rapidly. - Earth observation with emphasis on near real time services based on radar observations, particularly SAR, is being developed for maritime surveillance and environmental monitoring. The launch and operation of ERS-1 has been an important step in this direction. - Industrial development based on ESA’s programmes for space transportation and space station and with spin-offs to the marine and offshore sectors. The total turnover of space related products and services from these areas in Norway is growing rapidly and reached a total of USD 290 M in 1992.

The main objectives for Norwegian in initiating, supporting and co-ordina- Introduction space activities by the year 2000 are to ting space projects and programmes. establish: Around 55 % of the USD 60 M public Norway draws greater benefits from sector funding of Norwegian space its space activities than most other - Norwegian industry and other activities in 1992 is over the Space countries. Our geography as well as space-based business, with an ave- Centre budget, figure 1. In particular our maritime activities imply that we rage rate of growth in national and our participation in the European have extensive requirements for com- international markets of at least 15 % Space Agency (ESA), represents a munication, navigation and earth per annum major part of our space programme. observation services. These require- - A leading international position in ments are being met to an increasing those industrial areas that have The Space Centre responsibilities extent by satellite-based systems. been given priority include: Satellites are also essential in meeting the demand for comprehensive moni- - Considerable industrial growth in - Preparing long term plans for Nor- toring of the global environment. new areas, based on public-sector wegian space activities R&D efforts in space activities - Contributing to the development, These user requirements encourage - Cost-effective space-based systems co-ordination and evaluation of Nor- the development and manufacture of that meet national user require- wegian Space activities high-technology products and servi- ments ces. For this reason, our national - Planning, funding and following up efforts in space activities are of vital - A leading international position in Norwegian participation in ESA pro- importance. Such efforts will enable us those areas of basic science that grammes to exploit the results for industrial have been given priority - Planning, funding and following up growth and development of the natio- - A leading European position in national programmes for the deve- nal infrastructure, and thus help us to those aspects of ground infrastruc- lopment of industrial products, ser- fulfil national goals for economic ture for which we have natural geo- vices and capabilities, for bilateral growth and employment. These activi- graphical or other advantages. co-operation and for the develop- ties are also of decisive importance for ment of applications of earth obser- our space research, and they are vation important means of serving our for- Funding, general strate- eign policy interests. - Supporting and operating the gies and main priorities Andøya Rocket Range and the Norway’s efforts in space are thus These national objectives have been Tromsø Satellite Station. based on user oriented, scientific, established by the Norwegian Space industrial and political considerations. Centre which has a key responsibility

26 Close, committing co-operation be- tween Norwegian industry, users and NAVF (3.5%) research institutions is an essential means of meeting our national objecti- Norwegian Telecom Research (7.6 %) ves. In addition, concentration on par- ticular fields, projects and participating Universities and colleges (11.1 %) organisations is essential. Norwegian Defence Research Establishment (8.1 %) Space activities are of international character, and it is vital that we utilise Others (7.3 %) our status as member of international organisations in order to obtain the Norwegian Space Centre, desired results. The most important of national programmes (6 %) these organisations are INMARSAT, Norwegian Space Centre, INTELSAT, COSPAS/SARSAT, administration (5,3 %) EUTELSAT and EUMETSAT. Norwe- Norwegian Mapping Authority gian membership in the European Space Agency, ESA, is of special ESA programmes (44,3 %) importance in this respect, as it offers Norwegian companies the opportunity Figure 1 Public sector funding of space R&D activities: distribution in per cent (1992) of a total of to participate in the European scienti- USD 60 M fic, technological and industrial net- work, thus promoting internationalisa- tion of Norwegian research and indus- - Auroral Imaging Observatory try. Space science (AURIO) for ESA’s POEM pro- The science research council, NAVF, gramme. This is the largest space is basically responsible for funding Space activities, by nature, have a science project with Norwegian lea- national space science projects while long-term time-scale, but there are dership the Norwegian Space Centre is also opportunities for considerable responsible for our ESA contribution. - Participation in NASA’s Gravity gain on a short-term basis. The areas Threshold experiment (GTHRES) in which Norway has obtained remark- Norwegian space science strategies on IML-1 flown in January 1992. able results internationally, have to a are to great extent been based on our natio- - Utilise the opportunities provided by nal conditions and requirements, con- the ESA programmes firming the cluster theory which was Andøya Rocket Range put forward by professor Michael Por- - Continue the use of Andøya Rocket ter in his book “The Competitive Range for national and co-operative The Andøya Rocket Range is an opera- Advantage of Nations”. projects tional unit under the Norwegian Space Centre. - Develop bi- and multilateral projects In addition to space science five main in areas where the previous points As a consequence of its advantageous sectors for the development of indus- do not meet the scientific require- geographical location and modern, try and services have been selected, ments. highly specialised infrastructure, giving altogether six main areas of Andøya Rocket Range is able to offer activity. The priorities are: Norwegian and overseas organisations - Space physics with emphasis on services for space science and related These areas are: magnetospheric studies and upper activities. These advantages have tra- atmosphere dynamics ditionally been of particular relevance - Basic research in space science for auroral research, but will in the - Astrophysics with emphasis on stu- - Ground infrastructure services for future also be important for ozone dies of dynamical phenomena in which we have natural advantages, research and for measurements of solar and stellar atmospheres and derived products other environmental parameters. - Life science with emphasis on plant - Telecommunications services, and physiology. Andøya Rocket Range is being deve- derived products loped on a commercial basis within - Navigation and positioning services, Main projects are: existing as well as new areas of acti- and derived products vity. An important aspect of this pro- - Participation in eight experiments cess of development is the construc- - Earth observation services, and de- on ESA’s SOHO and Cluster satelli- tion of a new universal launcher which rived products tes, five with hardware will enable larger rockets up to 20 tons - Industrial development based on - Pulsating auroral (PULSAUR) and to be launched. The possibility of ESA’s space transport, space station middle atmosphere (TURBO, establishing a base for launching small and science programmes. RONALD) rocket projects satellites is under study.

27 The recently added benefit of payload - Wind and wave input to meteorolo- new and substantially faster models recovery from the Norwegian Sea is a gical models are under development, table 1. useful and cost-effective service for - Ocean pollution and biomass pro- Completing the system is the satellite many scientific programmes. duction monitoring. based IDUN distribution system An Arctic Lidar Observatory (ALO- which allows the user near real-time MAR) for middle atmospheric re- The geographical areas of interest are access to the processed information, search is being planned in close proxi- primarily those limited by the area figure 2. mity to the rocket range. ALOMAR covered from the Tromsø Satellite Sta- will provide a wealth of new informa- tion with emphasis on the coastal zone Telecommunications tion on the dynamics of the middle and the marginal ice zone of the Arc- The Norwegian Telecom has played a and upper atmosphere in the 10 to 100 tic. A very interesting prospect for the leading role in developing satellite km range. future is the real time use of SAR ima- communications in Norway with the gery for navigation through the North Together with the EISCAT radar sys- Norwegian Space Centre responsible East Passage between the Barents Sea tem which now is being enhanced for our ESA participation. and the Bering Strait which would with a Svalbard facility, this gives a assist operating the shortest shipping Measured in terms of turnover this is powerful set of tools for space science. route between Europe and Japan. by far the most important sector of space activity. Due to a very early start A number of Norwegian research Earth observation in maritime communications, Norway institutes and companies are taking The Norwegian Space Centre is the ranks number two in INMARSAT and part in these activities delivering pro- driving force in developing the field of Norwegian industry has around a 20 % ducts and services and have brought earth observation. The main objective share of the world market for INMAR- Norway in the forefront in the develop- is to develop operational services with SAT ground stations and mobile termi- ment of near real time applications for emphasis on near real time marine nals. This is a purely commercial mar- operational purposes. applications. This implies securing ket which is expected to grow rapidly Norwegian users access to data and as a result of the recent introduction of developing industry and ground infra- Tromsø Satellite Station aeromobile and data transmission ser- structure that can deliver products and vices and due to the new standards B Tromsø Satellite Station became an services for satellites, operators and & M which is being introduced in the operational unit of the Norwegian users. The strategy to achieve these near future. Space Centre on 1st January 1991 and objectives is based on is being developed to operate on a In the business communications area - Participation in ESA programmes commercial basis. Norway opened NORSAT-A in 1976 as the first domestic satellite communica- - Complementary bilateral arrange- The station is suitably located for tion system in Europe serving our off- ments receiving data from satellites in polar shore installations in the North Sea orbit seeing 10 out of 14 passes per - Developing Tromsø Satellite Station and our northernmost communities in day and is in this respect only surpas- and associated infrastructure Svalbard at 78° N. This has been fol- sed by a possible Svalbard facility lowed by NORSAT-B, a meshed type, - Application programmes developing which would see all passes. 2 Mbit/sec, roof top, multipurpose value adding industry A strategically important business area communication system which now - Demonstration projects with user for the satellite station is therefore also is being offered on the European commitment. long-term contracts for operational market. A new concept for a low data services with national and internatio- rate and low cost, VSAT-type system Emphasis is placed on radar and other nal institutions and satellite operators which has been developed on an ESA microwave based sensors, in particu- like ESA. contract, was demonstrated last year. lar SAR. Norway therefore participates The marketing of both these systems As pointed out in the previous section, in ESA’s ERS-1 and ERS-2 program- will benefit from the increasing dere- Tromsø Satellite Station is also an mes and has announced its participa- gulation in Europe. important element in our earth obser- tion in the POEM-1 programme. Nor- vation strategy. Another area of con- In the TV distribution field various way will also be ESA’s source of J-ERS centration is therefore near real-time MAC standards have been implement- SAR-images and is discussing arrange- processing and distribution of data for ed and tested and equipment is being ments for acquiring RADARSAT and operational services, such as monito- offered by Norwegian industry. Seastar data. ring ocean and weather conditions, On the space segment Norwegian natural resources, maritime traffic and The main applications under develop- interests have concentrated on surface ice conditions. ment are: acoustic wave (SAW) based signal pro- A key element in this system is the cessing devices and industry has been - Ice mapping, monitoring and fore- very fast and powerful digital program- space qualified through participation casting mable SAR processor, CESAR, which in ESA programmes. This particular - Ship detection and surveillance has been delivered by Norwegian capability has also resulted in the deli- industry for the ERS-1 SAR processing very of the chirp generators for the - Oil spill detection and monitoring system. A second unit has been orde- radar altimeters on the ERS-1 & 2 - Ocean surface structure monitoring red for the J-ERS-1 processing and satellites.

28 Navigation Generation 1 3 5 The American Global Positioning Sys- tem will be fully operational from 1993. Status Troms¿ Satellite Troms¿ Satellite Under development However, the 100 m accuracy offered Station Station to civilian users is unsatisfactory for ERS-1 SAR J-ERS-1 SAR Real time SAR many purposes. processor processor processor Differential GPS equipment giving Delivered: mid 1991 Delivered: ultimo 1992 In production: 1995 navigation accuracies down to 5 m has been developed and DGPS services Multi Arithmetic 4 1 (- 8) 1, 2, ... > 10 are being offered on a commercial Logic Units basis along the Norwegian coast and in the adjacent ocean areas. Performance 320 M flops 80 (- 640) M flops 5, 10, ... > 50 G flops In addition to this the Norwegian Map- Processing time 7 min 55 sec 30 min (- 2 min) < 30, < 15, ... < 3 sec ping Authority last year installed a 100 km x 100 km complete 11 station DGPS system that full resolution will provide necessary accuracy for offshore and land surveying and for Size Cabinet Work Station Desk Top fast traffic navigation in the air and along the coast. Table 1 Cesar SAR Processor Description The Mapping Authority has also begun the construction of a geodetic laboratory with a VLBI station at Sval- bard. As part of a global network this station will contribute in general to solid earth research and in particular to the global change programme. Space Station and Space Transportation The main objective for Norwegian par- ticipation in the Space Station and Space Transportation programmes is to develop new products, services and markets for Norwegian industry and Troms¿ to secure Norwegian participation in Satellite Station the future exploitation of space. In the IDUN Ariane-5 programme Norwegian NORSAT-B CESAR industry is developing and supplying terminal SAR processor electronic parts as well as mechanical subsystems such as the booster attachment and separation system and the booster separation rockets. In the Columbus programme main emphasis has been on verification, integration and checkout systems, OCEANOR logistics and quality assurance/pro- duct assurance services. These are all areas which benefit from spin-offs to and from Norwegian offshore indus- try. Astronauts have not been a Norwegian priority. Nevertheless, a Norwegian Norwegian Defence candidate was a runner up in the final Research Establishment ESA selection. Nansen Environmental Remote Sensing Center Conclusion Norway is a small country with only 4.2 million inhabitants but with loca- tion, geography and business areas Figure 2 The Norwegian fast delivery SAR image distribution system

29 that benefit from space activities. This can be seen from the total turnover of NOK billion space related products and services 4.0 which have shown a continuous and substantial growth over the past 5 3.5 years and in 1991 reached a total of USD 290 M, figure 3. 3.0

By their very nature space activities 2.5 are global. Norway’s relatively high participation in space related activities 2.0 therefore makes us more globally ori- ented at a time in history where we all 1.5 must learn how to co-operate better. We consider this to be an advantage 1.0 and based on our past success we look to the future with controlled optimism. 0.5

0 1990 1991 1992 1993 1994 1995 1996

Imports Products Services

Figure 3 Total turnover of space-related products and services in Norway (calcula- ted figures and prognoses)

30 TSAT - a low-cost satellite communication network

BY TERJE PETTERSEN AND PETTER CHR AMUNDSEN

Abstract 621.396.946 A closed satellite communication network designed specifically for low data rate communication (300 - 9,600 bps) at Ku-band is described. The low data rate design of the TSAT system allows the transponder load and power consumption to be scaled in proportion with the data rate, resulting in an optimised utilisation of the geostationary orbit. Solutions to the normal problem areas in communication at low data rates (frequency drift, phase noise and interference) are described. A cost/performance comparison between traditional VSAT systems and the TSAT system is given, together with a technical description of the TSAT system.

1 Introduction drift, phase noise, and interference The TSAT design allows a Hub com- requirements not met by small con- parable in size and complexity to tradi- The Very Small Aperture Terminal ventional antennas. The difficulties ari- tional VSATs (Very Small Aperture (VSAT) systems have over the last sing from these problem areas is dis- Terminals). Together with the low- years developed rapidly within satel- cussed in section 4, together with cost Remote Terminals, TSAT requi- lite communication. Together with general solutions to these problem res low investments in earth station various global systems for mobile areas. How these problem areas have equipment. The efficient utilisation of communication, this development has been solved in the TSAT (Telemetry satellite power and bandwidth results made satellites an attractive medium and data transfer via SATellite) sys- in very low operational costs. In figure to a large number of users, based on tem, which is under development by 1 the following features can be observ- cost/performance advantages. the Norwegian company Normarc a/s ed: Current VSAT systems are normally (1)-(5) is discussed in section 5. - Low data rate: For 300 to 1 200 bps, only cost-effective for large networks Trellis encoded 4 FSK is used. with a high utilisation of the available 2 Main TSAT features and Above 2 400 bps, BPSK with For- channel capacity, since the necessary ward Error Correction will be used. investments and operational costs are applications quite significant. Current VSAT tech- TSAT is a low-cost full duplex closed - Small earth stations: The Hub Sta- nology sets a lower limit on the offer- star network, dedicated to low-rate tion, with an antenna diameter of 1.2 ed data rate. Reducing the system data transfer via satellite. Flexible or 1.8 m is comparable in size to the cost, and offering data rates tailored to interface options allow solutions to Remote Terminals in traditional the users’ requirements are key issues most low-rate data collection demands. VSAT systems, while the TSAT in developing satellite communication An arbitrary number of Remote Termi- Remote Terminal antennas are as systems to a broader range of applica- nals are controlled by one common small as 55 or 90 cm in diameter tions. These systems are expected to Hub Station. (figure 2). introduce satellite communication to a large number of new users, removing the image of satellite communication as exotic and expensive solutions to “ordinary” communication needs. Environmental, ecological and secu- rity surveillance and process control and monitoring, are typical examples of applications with requirements that are not solved in an optimised way by today’s VSAT systems. The transmis- sion data rate demand is in most cases as low as some few hundred bits per second, and the main traffic direction is from the Remote Terminals to the Hub Station. In order to make such systems economically attractive to the user, and to represent an optimised utilisation of the geostationary orbit, some main demands should be fulfil- led: small and low cost earth stations, low power consumption, and low satel- lite transponder load with respect to bandwidth and power. Figure 1 Closed TSAT network. The figure highlights the main characteristics of the TSAT system: A straightforward down-scaling of - low data rates (300 - 2,400 bps) today’s VSAT systems with respect to - small antennas (RT dia. 55 or 90 cm, Hub dia 1.2 or 1.8 m antenna sizes, power amplifiers and - Hub controls the network via the outbound link bandwidth reduction, is not possible - Remote Terminals share one or several inbound links due to three main problem areas inhe- - any Ku-band communication transponder can be used rent in the satellite link: frequency - operates independently of any other system

31 - The outbound link, indicated by the 3 Low data rate satellite The predominant traffic direction for solid arrows, is running continuously, these applications is from the Remote addressing and commanding the communication systems Terminals to the Hub Station, where the Remote Terminals. The following section will identify appli- data can be stored or distributed. Trans- cation areas requiring low data rate com- mission of control data to the Remote - The Remote Terminals share one or munication. In section 3.2 and 3.3 we Terminal is normally also required. several inbound channels in a Time discuss the current low data rate satellite Division Multiplexed mode, either in a These applications are characterised by: communication system INMARSAT-C polled sequence or by random access. and the Spread Spectrum Systems, before - Few characters per message (status The Hub Station has the ability to comparing current VSAT systems with reporting, measurement values) receive several inbound carriers, in the proposed low data rate concept such parallel, allowing a single Remote Ter- - Relaxed response time requirements as the TSAT system. minal, or a small group of Remote Ter- (10 sec. to a few hours) minals, to use one dedicated carrier if 3.1 Applications - Often static low data rate traffic, domi- necessary. nated by traffic from the Remote Ter- Current VSAT systems are unable to - Any conventional Ku-band communi- minals to the Hub Station. offer cost-effective solutions in many cation transponder can be used. applications that require low data rate - Closed network, since the system can communication. These applications Other potential applications with low be dedicated to one single user, and include SCADA (Supervisory Control data rates include: this user does not have to pay for, or And Data Acquisition) applications such - Low-rate data broadcasting, with con- worry about, other control or com- as: tinuous monitoring of the remote ter- mand earth stations. - Surveillance, e.g. environmental, eco- minals logical, or security - General low data rate communication, Possible applications include: - Data collection of e.g. meteorological, such as telex, telefax etc. in areas with - General remote instrument control and geomechanical, or seismological data poorly developed infrastructure data collection - Process or system monitoring and con- - Transaction oriented applications at - Collection and distribution of data for trol, e.g. hydropower, oil and gas low data rates, e.g. credit card verifica- environmental, ecological and security industry tion, Automatic Teller Machine. surveillance - Road traffic control. - Remote process monitoring and con- The communication demands for many trol. data collection applications can be satis- fied by the use of satellite networks with data rates in the range 300 - 1,200 bps. Networks for these applications may range from large systems covering a whole continent, to systems with a few Remote Terminals. Current VSAT sys- tems offer a capacity much higher than this, leading to unnecessary, and often unacceptable, investments and opera- tional costs. In order to make satellite communication systems cost-effective and attractive for most low data rate applications, some main demands must be fulfilled: - Small and low cost earth stations - Low satellite transponder load with respect to bandwidth and power, i.e. an optimised utilisation of the geostation- ary orbit - Low power consumption, allowing battery operation - Operation under severe environmental conditions.

Figure 2 TSAT Remote Terminal, consisting of: - antenna (55 cm dia. shown, 90 cm optional) - RF Front End between antenna and horn - Main Unit integrated with mounting structure

32 Before comparing traditional VSAT ventional Ku-band satellite transpon- inbound link. Table 1 shows that the systems to low data rate systems, der is assumed. inbound link requires a small fraction some comments will be given regar- of the power needed for the outbound In traditional VSAT systems, the pre- ding existing global systems operating link. The additional transponder load dominant message direction is from at low data rates, using INMARSAT-C of a new inbound channel is not signi- the Hub to the RT. The transponder as an example, and we also make ficant. load for the outbound link is therefore some comments on the Spread Spec- significant due to the high data rate The use of several inbound frequen- trum systems which traditionally have and the relatively small RT antenna cies will be a flexible, cost-effective offered solutions to the problem areas diameter. This implies that a large way of increasing the network capa- related with low data rate satellite numbers of VSATs must share the city. This feature also allows servicing communication. same outbound carrier, in order to the same traffic type with optimised 3.2 INMARSAT-C bring the space segment cost down to protocols on dedicated frequencies, reasonable levels compared with the e.g. random access on one frequency INMARSAT-C is a global mobile com- earth station costs. The low data rate and polling on another frequency. munication system operating at L- system on the other hand, requires a The low data rates, and the possible band, offering data communication at very small fraction of the transponder flexibility in communication protocols, 600 bps. The INMARSAT-C terminals capacity. With a small Hub, compar- imply that cost-effective digital signal are relatively low-cost, and can often able to a traditional VSAT in size, com- processors (DSPs) and microproces- withstand severe weather conditions. plexity, and cost, a closed star network sors can be used, together with mode- However, the INMARSAT-C concept can be economical even for communi- rate complexity modem and data pro- as a global system requires the use of cation with a few distant sites. tocol software. complex control stations leading to a The limiting channel for SCADA appli- relatively expensive data transmission. cations, however, is normally the A dedicated VSAT low data rate net- work can therefore compete cost-effec- tively even at low traffic levels.

3.3 Spread spectrum systems Typical Parameters TSAT VSAT Spread spectrum techniques allow -5 simple solutions to the problem areas Eb/No for BER 10 7 dB 7 dB for low data rate satellite communica- Rain & impl. margin 6 dB 6 dB tion mentioned above (6). In these sys- tems, the data is spread over a large Hub antenna diameter 1.2 or 1.8 m 3 - 10 m frequency band with a low power den- sity. The total transmitted and recei- RT (VSAT) antenna diameter 55 or 90 cm 1.2 - 3 m ved power is, however, approximately the same as in conventional modula- Data rate 0.3 - 2.4 kbps 9.6 - 512 kbps tion schemes. Some current low data Nature of traffic Mostly static Random rate VSAT systems utilising spread Main traffic dir. RT → Hub Hub → RT spectrum with code division multiple access (CDMA) have been developed, such as Qualcomm’s “Omnitrack”, and Transponder load example the eSAT system which is under deve- lopment by Schrack, Austria and Hub antenna diameter 1.2m 5 m Space Engineering, Italy. RT (VSAT) antenna diameter 55 cm 1.8 m Spread spectrum systems can be cost- Data rate 300 bps 64 kbps effective for a large number of Remote Terminals with a relatively high utilisa- No of power limited carriers tion of the available network capacity. Hub → RT 5 000 240 An optimum use of the geostationary RT → Hub 32 000 600 orbit for medium and small systems is impossible, due to the extensive use of bandwidth inherently required by Relative transponder cost spread spectrum techniques. Hub → RT 5 % 100 % 3.4 Comparison between tradi- RT → Hub 0.8 % 40 % tional VSATs and TSAT In table 1, some main characteristics Table 1 Comparison between TSAT and VSAT systems for an optimised low data rate collec- The table shows that the TSAT parameters are scaled proportionally to the data tion system such as the TSAT system, rate. The TSAT Hub size, complexity and cost is comparable to traditional are compared with a traditional VSAT VSAT terminals. The transponder cost is given relative to the VSAT system out- data distribution star network. A con- bound link cost

33 4 Problem areas and The relative phase noise versus carrier axis cross- and co-polar radiation den- recovery loop bandwidth is shown in sity from earth station antennas. To solutions for low data figure 3 (1), (5). The phase noise from avoid unacceptable interference into rate satellite communi- a typical satellite, and the transmit/- the VSAT channel from adjacent satel- receive chain, are integrated to give lites, robust modulation and coding cation systems total phase noise, with the transmis- schemes, and antennas with low front- There are three main problem areas to sion data rate as parameter. Two to side-lobe ratios, must be used. In overcome in order to obtain low data phase noise contributions can be iden- traditional VSATs it is therefore recog- rate systems with the performance tified in figure 3: nised that for earth station antennas sketched above: smaller than 1 metre in diameter at - Phase fluctuations tracked by the Ku-band, spread spectrum modulation - Frequency drift loop. The contribution is inversely is necessary. As discussed in section proportional to the loop bandwidth - Phase noise 3.3, spread spectrum solutions will - Additive thermal noise which contri- only be cost-effective for large net- - Interference requirements. butes proportionally with the loop works. bandwidth. 4.1 Frequency drift To achieve cost-effective low data rate systems, small low-cost antennas with To obtain a minimum channel separa- For acceptable performance, the total satisfactory specifications must be tion, the transmit frequencies must be phase error relative to the carrier used. Such antennas are already com- stable. This will normally put require- should be below -15 dBc for binary mercially available (see section ments on the RTs’ local oscillators phase shift keying (BPSK), and 5.4.2.1). which are not compatible with low- -25 dBc for quadrature phase shift key- cost solutions. The implementation of ing (QPSK) (7). From figure 3 we see compensation schemes to stabilise the that for data rates below 2 400 bps, 5 The TSAT system transmitted frequency enables the use phase shift keying should be avoided. In this section, the TSAT system con- of low-cost oscillators. Various frequency shift keying techni- cept is further described, focusing on ques have been compared by DELAB, the central parts of the system design. 4.2 Phase noise and modulation Norway, concluding with Trellis scheme Coded 4 FSK as the best compromise 5.1 Transmission system between bandwidth, energy/bit and Coherent modulation schemes such as The basic functions in the Remote Ter- bit error rate requirements (4),(5). quadrature phase shift keying minal are shown in the simplified (QPSK), is extensively used in digital 4.3 Interference requirements - block diagram in figure 4. The conver- satellite communication. For very low antenna performance ter chain design in the Hub transmis- data rate systems, coherent modula- sion system is identical to the design tion methods should be avoided, due The ITU regulations and various space in the Remote Terminal, with the to the impact from system phase segment owners’ requirements, exception of utilising a more stable noise. restrict the allowable boresight and off reference oscillator (10-9).The unique properties of the TSAT transmission concept are: Total phase error relative carrier [dB] - The reference oscillator in the Hub, 0 with a stability of 10-9, is the fre- 0.3 kbps quency reference for the Hub and all RTs -5 2.4 kbps - The Hub continuously transmits a 9.6 kbps -10 carrier with modulated data that acts as a system pilot, frequency-loc- BPSK limit king the Remote Terminal transmis- -15 sion OGu 16 64 kbps - The use of a low-cost reference -20 oscillator in the Remote Terminal (stability of 10-5). QPSK limit -25 This low-cost concept ensures that the transmitted frequency stability com- -30 plies with optimised transponder utili- 101 102 103 104 sation. The receive chain forms a fre- Carrier recovery loop bandwidth [Hz] quency locked loop, and the transmit carrier is locked to the received car- Figure 3 Relative phase noise versus carrier recovery loop bandwidth. The data rier. The Hub Station detects and rate is given as parameter. The acceptable phase noise levels for BPSK transmits the satellite frequency drift and QPSK are indicated, showing that coherent modulation should be to the Remote Terminals, which cor- avoided for data rates below 2,400 bps. rect the transmit synthesizers as

34 Link budget for INTELSAT VA and EUTELSAT II-SMS necessary, keeping the transmit fre- quency stable within a few Hertz. Common Parameters The TSAT system offers the following Hub station antenna 1.2 m modulation methods versus transmis- Eb/No 9 dB sion data-rates: BER 10-8 - Trellis encoded 4 FSK for 300, 600 Rain attenuation 0.5 dB 1% value or 1 200 bps Pointing loss 0.5 dB - BPSK modulation (FEC optional) Implementation margin 1.5 dB from 2 400 bps. INTELSAT VA - F12 Output back off 1.8 dB 5.2 Satellite and geostationary Transponder bandwidth 241 MHz orbit utilisation RT/Hub location 46 dBW satellite EIRP contour The TSAT network can use any Ku- band communication transponder: Data Rate 300 bps 1200 bps Up-link: 14.0 - 14.5 GHz Channel separation 2 kHz 8 kHz Down-link: 10.95 - 11.7 GHz or 55 cm 90 cm 55 cm 90 cm 12.5 - 12.75 GHz RT antenna The system fulfils ITU recommendati- Outbound link ons and satellite providers’ require- EIRP, Hub 40 dBW 36 dBW 46 dBW 42 dBW ments for fixed satellite services. Two examples of link budgets for the RF-power, Hub 26 dBm 21 dBm 32 dBm 28 dBm INTELSAT VA - F12, 241 MHz trans- No of power ponder and the EUTELSAT II-SMS limited carriers 5 000 13 000 1 250 3 200 transponder are given in table 2. The output back-off (OBO) is as speci- Inbound link fied by the Norwegian Telecom for EIRP, RT 32 dBW 32 dBW 38 dBW 38 dBW their intended use of an INTELSAT V RF-power, RT 24 dBm 20 dBm 30 dBm 26 dBm transponder (1.8 dB), and by EUTEL- No of power SAT for the SMS-transponder limited carriers 32 000 32 000 8 000 8 000 (4.2 dB). For the INTELSAT V - F12 satellite, the 46 dBW contour is used, EUTELSAT II - SMS giving a coverage area of Scandinavia with approximately 1 dB margin. For Output back off 4.2 dB the EUTELSAT II satellite, the Transponder bandwidth 36 MHz 44 dBW contour is used, giving a cove- RT/Hub location 44 dBW satellite EIRP contour rage area of Western Europe up to the northern parts of Norway and Sweden. Data Rate 300 bps 1200 bps As can be seen from the table by com- Channel separation 2 kHz 8 kHz paring frequency bandwidth load and carrier power, the transponders are RT-antenna 55 cm 90 cm 55 cm 90 cm power limited. The power load is expressed by the ratio of the TSAT Outbound link carrier to the total satellite power. One 41 dBW 37 dBW 47 dBW 43 dBW TSAT system uses only one outbound EIRP, Hub carrier, and a large number of Remote RF-power, Hub 27 dBm 22 dBm 33 dBm 28 dBm Terminals can share one or several No of power inbound carriers. The annual satellite limited carriers 2 800 7 200 700 1 800 cost should therefore be negligible even for users with a very limited Inbound link number of Remote Terminals. EIRP, RT 34 dBW 34 dBW 40 dBW 40 dBW RF-power, RT 26 dBm 21 dBm 32 dBm 27 dBm 5.3 Transmission system No of power elements limited carriers 15 400 15 400 3 900 3 900 5.3.1 Introduction The overall design criteria for the Table 2 Link budget for INTELSAT VA and EUTELSAT II-SMS transmission system are: The table shows that for the TSAT system, the transponders are power limited. A TSAT net- work utilizes only one outbound link, and therefore almost a negligible part of the transpon- - Low-cost Remote Terminal der is used for each TSAT network, resulting in low operational costs

35 Rx carrier Frequency locked loop Modem User & interface 10.95 - 11.7 GHz CCU 12.5 - 12.75 GHz

Rx control Numerical Tx control Controlled Tx carrier Synthesisers Ref.Osc. transistors. The integration of mecha- 14.0 - 14.5 GHz 10 ppm nical and microwave design enables cost effective solutions. Figure 4 TSAT Remote Terminal Concept 5.3.2.3 Main Unit The transmitted frequency is frequency locked to the received frequency, enabling the use of low-cost local oscillators with a low stability (e.g. The design includes high-perfor- 10 ppm) mance, low-cost digital Phased Lock Loop (PLL) circuits. - Low satellite transponder load, reception at Ku-band. The synthesi- The modem is digitally implemented giving low operational costs zing procedure is developed at the in a Digital Signal Processor (DSP). In Norwegian Telecom Research. the DSP, both carrier- and clock-reco- - Reliable operation under all relevant very is performed, together with opti- operating conditions. The antenna is of the off-set Dual Sha- mised filtering, adding/check of CRC ped Gregorian type. This construction and optimal FEC (Forward Error Cor- 5.3.2 Remote Terminal principle allows the radiation pattern rection). The modem software is deve- to be shaped with low side-lobe levels. Low power consumption is a critical loped by SINTEF DELAB, Norway Inherent in the synthesizing proce- factor for applications in remote areas. (4). dure is also the control on cross-polar The TSAT Remote Terminal is design- levels. As opposed to most low-cost The Remote Terminal Communication ed with this in mind. Three operatio- Ku-band antennas, these antennas Control Unit (CCU) receives and ana- nal modes are offered, enabling the have therefore the required perfor- lyses the data from the transmission user to reduce the power consumption mance for both reception and trans- system, and performs the requested to an absolute minimum: mission, (8)-(10). actions. The CCU also includes the 1 Transmit and receive 13 W user interface hardware and software. To protect the antenna and electric 2 Receive 7 W equipment in special severe environ- 5.3.3 Hub Station ments (high mountain territories, 3 Sleep (pre-set wake-up) 0 W The Hub Station forms an integral part exposed coastal areas etc.), a special of a closed cost-effective total system, enclosure with flexible antenna ran- Figure 5 shows a simplified block dia- with the Hub Station installed close to dom canvas will be available. gram for the Remote Terminal. The the users’ data collection centre or main modules are commented on 5.3.2.2 RF Front End control centre. below. The integrated RF Front End consists The converter chain design in the Hub 5.3.2.1 Antenna of: transmission system is identical to the design in the Remote Terminal, with Figure 2 shows the Remote Terminal - OMT (Ortho Mode Transducer) the exception of utilising a more stable antenna, where the electronic equip- - HPC (High Power upConverter) reference oscillator(stability of 10-9), ment is integrated into the mechanical and will not be further commented. structure. The shown antenna has a - LNC (Low Noise downConverter). The other Hub modules are briefly 55 cm aperture diameter, but is also commented below. available with 90 cm diameter. The The OMT is combined with the RF antenna is die cast in large quantities Front End housing. The HPA RF The Hub antenna is 1.8 m or 1.2 m in by Fibo-Støp, Norway for satellite TV power output is 1 Watt, utilising FET diameter, and of the same type as the

Ref. Osc. 10 ppm

Serial communication

O ∆ Modem RS232, RS422 Antenna M Synthesizer Σ Σ Synthesizer & RS485 T Control Optional Analog Data input Collection Digital Module I/O

RF Front End Main Unit Figure 5 Remote Terminal block diagram. The block diagram shows the main modules in the Remote Terminal, divided into the RF Front End and Main Unit. An optional digital/analogue interface module is also included

36 Outbound channel

Frame 1 Frame 2 Frame 3 Frame K

Remote Terminal antennas. The Hub SYNC. Main Unit may include several demo- dulators, corresponding to the number Slot 1 Slot 2 Slot i Slot L of inbound links. The Hub Communication Control Unit (CCU) sends and receives data be- tween the transmission system and the Network Supervisory Terminal U.W. Freq.dev. Data CRC Flush bits (PC) at the Hub Station, performing the overall network control. Additional equipment is added when several inbound channels are required. Command Ack. Data field field field 5.4 Communication protocols and network management The Remote Terminals share one or several satellite channels (inbound links) with a transmission delay of Inbound channel approx. 260 ms. With the outbound link, the Hub can control several inbound links (on sepa- rate frequencies), where the access Slot 1 Slot i Slot M protocol may be separately optimised to the specific traffic and user require- ments for each channel. The TSAT network is controlled by the network control and management U.W. RT Data CRC Flush bits system in the Supervisory Terminal. The network operator at the Supervi- sory Terminal may reconfigure the Figure 6 The broadcast TDM outbound link controls and commands the Remote Terminals. The network parameters, and access net- Remote Terminals share the inbound link(s) according to a modified Slotted Aloha protocol work status and statistics, through the network management interface in the Supervisory Terminal. The Supervi- The TDM outbound link consists of a Slotted Aloha is implemented as the sory Terminal is the gateway between frame divided into slots and subslots basic access protocol. Slotted Aloha is the TSAT system and the user’s com- with data fields. The frames are broad- a random access protocol allowing a puter system, including the gateway to cast to all the Remote Terminals. The reasonable channel utilisation. The public or private networks, if required. start of a frame is identified by a uni- simplicity in the basic protocol eases que data packet in the first slot. This implementation and reduces system 5.4.1 Communication protocol slot contains management information complexity. Simple modifications of The TSAT system is capable of offer- for controlling the Remote Terminal the protocol, e.g. assigning different ing an efficient use of the available transmit frequency, and must there- priorities or traffic types to various channel capacity for applications with fore be received before the RT can slots in the frame, may let the user in a varying requirements. The network is transmit on the inbound channel. The simple manner optimise the network therefore designed with a large degree slot also contains other management- performance. The flexible design of flexibility. The network access met- related information, such as log-on fre- allows more complex protocols to be hods and protocols can then be tailor- quency, network configuration, and implemented if required. ed to meet specific user requirements protocol parameters. A data packet, The slotted Aloha protocol allows the regarding network traffic, response transmitted in one slot, consists of Remote Terminals to transmit data time, number of Remote Terminals, fields containing unique word, fre- packets at the beginning of the first etc. quency deviation, data, CRC, and flush available slot. (Information about the bits. The key design issues for the inbound slot status is broadcast by the Hub.) link access protocol is flexibility and By letting the slot structure on the out- The length of the slot is configurable, simplicity. The varying applications for bound link control and synchronise dependent on the application type TSAT demand optimisation of the the inbound link slots, each slot on the assigned to the inbound channel. The access protocol to meet specific user inbound link is referred relatively to data packet on the inbound link con- requirements. The basic TSAT access the frame start on the outbound link. sists of a unique word, RT address, protocol is a modified Slotted Aloha These features enable the TSAT sys- packet length, data, CRC, and flush random access protocol (9). The tem to be tailored to meet various user bits. The slot access is controlled by frame structure of the outbound and requirements, maximising the channel the values in an array where the N inbound link is shown in figure 6. utilisation. array elements correspond to the N

37 slots in each frame. Each RT is confi- A complete demonstration network 7 Matyas, R. Effect of Noisy Referen- gured with initial values in this array, has been in operation since autumn ces on Coherent Detection of FSK with one value for each slot determi- 1992 through the INTELSAT VA satel- Signals, IEEE COM-26, No. 6, June ning the slot access. The number of lite. From winter 1993, three pilot sys- 1978. allowable values affect the degree of tems are in operation, collecting data traffic control. in customer networks. The TSAT 2000 8 Bjøntegaard, G, Pettersen, T. An system will be commercially available Offset Dual-Reflector Antenna sha- 5.4.2 Network control from autumn 1993. ped from Near-Field Measure- The network control and management ments of the Feed-Horn: Calculati- system evaluates the statistics collec- ons and Measurements, IEEE ted from the network units, such as 7 Acknowledgements Trans. AP-31 no. 8, November the retransmission level and response 1983. The TSAT development and experi- delay, and performs the necessary mental verification is sponsored by actions to prevent the network from 9 Pettersen, T. Dual Shaped Offset European Space Agency, Norwegian reaching congestion and instability. Reflector Antennas. Documentation Space Centre and Norwegian Tele- The user can configure network and of experience and calculated and com. protocol parameters, enabling the sys- measured results, TF-report 35/87. tem to meet the requirements of inte- Normarc has co-operated with the sub- Norwegian Telecom Research., rest. contractors SINTEF DELAB (digital June 1987. 5.5 User interface modem and OMT development)), Scandpower A/S (communication pro- 10 Skyttemyr, S. Radiation Pattern The Standard Remote Terminal is tocols feasibility study and design), and Gain Measurements on shaped serial communication ports. Other and Informasjonskontroll A/S (com- Offset Dual-Reflector 90 cm interfaces are optional user interface. munication protocols design and Antenna, Norwegian Telecom An optional data collection module, implementation). Research Measurement Report, allows digital and analogue I/O to be November 1988. interfaced directly with the Remote Terminal. References 11 Tanenbaum, A S. Computer Net- works, Prentice-Hall Inc., 1981. The Hub Station includes the Supervi- 1 Pettersen, T. Satellite Data Collec- sory Terminal containing the Network tion System at Ku-band, European Control and Management system. The Microwave Conference, 1990. Supervisory Terminal may be interfa- ced to the user’s computer system, or 2 Pettersen, T. Closed low-rate sat- to external private or public networks. com network with optimised utilisa- Serial communication ports are stan- tion of the geostationary orbit, Tele- dard, with other interfaces being optio- com '91, Technical Symposium. nal (X.25, X.21, TCP/IP etc.). The TSAT system may be used as a 3 Amundsen, P C, Pettersen, T. The direct replacement of existing TSAT System - A Novel Low-cost modems connected to terrestrial lines. closed Satcom network for Low The TSAT system may also be confi- Data Rate Communication, 9th. gured to include the direct interface Int. Conf. on Digital Satellite Com- and control of the connected equip- munications, Copenhagen, May ment and instruments, resulting in a 1992. simplified total solution. 4 Huseby, K, Lie, A. Software and 6 TSAT system status functional description of a digital MODEM for the TSAT satellite A feasibility study of the TSAT concept data collection network, SINTEF- was completed in December 1990 (5), DELAB Report, STF 40 F91140, with a laboratory demonstration pro- Jan. 1992. ving the expected performance of all critical modules. The bit error rate and 5 Normarc a/s, supported by frequency tracking performance were DELAB and Scandpower A/S. demonstrated to be within specifica- Design and development of a satel- tions, with nominal thermal noise and lite data collection system at Ku- phase noise added. band, Final Report to ESTEC Con- A full duplex TSAT link was demon- tract No. 8709/90/NL/RE. strated on a Hub and RT laboratory model in December 1991, confirming 6 Alper J R et al. The Book on VSATs, the expected performance of the com- Gilat Communication Ltd. plete transmission hardware and the digital modem software.

38 Very Small Aperture Terminal (VSAT) systems - basic principles and design

BY LIV ODDRUN VOLL AND GUNN KRISTIN KLUNGSØYR

1 Introduction this requirement. Accompanied by 2 Specifications 621.396.946 various systems for mobile communi- The telecommunications industry is cation, this development has made 2.1 VSAT constantly developing to meet the satellites an attractive medium to a changing needs of the users. In order to get a more precise defini- large number of users based on tion of VSAT systems the European Through new technologies telecom- cost/performance advantages. Telecommunication Standards Insti- munications service providers can: tute (ETSI) has proposed the follo- VSAT networks are characterised by a - Expand the network coverage to wing specifications of the transmit and large population of small and inexpen- new areas receive terminals (2): sive earth stations (VSATs) at the cus- - Improve the quality of basic commu- tomer’s premises. They communicate - Operating in the exclusive part of nications services through relatively small antennas with the Ku-band allocated to the Fixed a central large earth station called the Satellite Services (FSS), 14.00 to - Reduce the costs of services to allow hub station (figure 1). The hub station 14.25 GHz (earth-to-space), 12.50 to more users includes a Network Management Sys- 12.75 GHz (space-to-earth), and in - Add new services to increase the tem (NMS) which is responsible for the shared parts of the Ku-band, value of telecommunications servi- the monitoring and control of remote allocated to the FSS and FS (Fixed ces to users. VSATs. The communication with the Services), 14.25 to 14.50 GHz (earth- terrestrial network is also via the hub to-space) and 10.70 to 11.70 GHz An important part of the recent deve- node. (space-to-earth) lopment in telecommunications is the The VSATs operate as part of a satel- - In these frequency bands linear introduction of VSAT systems. lite network used for the distribution polarisation is normally used and Very small aperture terminal (VSAT) and/or exchange of data between the system operates through satelli- systems have developed rapidly over users. It is difficult to give a precise tes with 3 degree spacing the last years, and have been a major definition of a VSAT system because - Designed for unattended operation part of the recent development within of the lack of standardisation. A VSAT the satellite communication industry. is usually defined as a terminal with an - Limited to reception and transmis- VSAT networks have been a success antenna with diameter 2.4 m or less, sion of baseband digital signals mainly because they address a topo- which is likely to provide digital servi- - The information bitrate transmitted logy that appears to be ideally suited ces of 2 Mbps or less (1). Such servi- towards the satellite shall be limited to satellite communication - point-to- ces are data distribution, data network- to 2.048 Mbps multipoint. Traditional terrestrial net- ing, voice services and digitally com- works always had trouble addressing pressed videoconferencing services.

Video Training

Video Reciver

Video Uplink Teller for Training Stations VSAT Indoor Unit

SAC U Cash & P Base C Band

Platform Teller System Alarm Environmental System Control SNA/SDLC

ATM Typical Branch Office Data Center

Figure 1 Example of a VSAT network in a Baking Environment

39 - Antenna diameter not exceeding 3.8 multiple access), and CDMA (code m or equivalent corresponding aper- division multiple access), but TDMA ture. is the most common. The inbound channel (remote VSAT to hub) often The equipment characterised compri- use slotted Aloha which is a form of ses both the “outdoor unit” and the Random Access (RA). “indoor unit”. The outdoor unit is usu- In mesh topology there is direct com- ally composed of the antenna subsys- munication between the remote VSAT tem and the associated power ampli- terminals. This minimises the time fier and Low Noise Converter (LNC). delay which is critical concerning The indoor unit is composed of the speech services. The internal signal- remaining part of the communication ling network will have a star topology, chain, including the cable between the because the signalling processor is indoor and outdoor units. located in the central node, which is VSAT HUB This standard does not contain the often referred to as the DAMA STAR: HUB ⇔ VSAT VSAT network hub station. (demand assignment multiple access). The access method used in a mesh 2.2 Hub station network is typically Frequency Divi- sion Multiple Access (FDMA). The entire network is organised by the hub station via the network mana- gement system (NMS). The operator of the network management system is 3 Evolution responsible for the following essential Since their introduction VSATs of this functions: kind have followed an evolution through which three distinct genera- - Monitoring and controlling the net- tions can be identified. The first gene- work ration of VSATs demonstrated, in the - Configuring the network late 70s and early 80s, the feasibility of transmit and receive data communica- - Troubleshooting the network tion systems. The second generation - Charging. introduced the reduction of antenna size due to higher EIRP (effective iso- VSAT HUB VSAT Communication between the NMS tropically radiated power) Ku-band and network components is continu- satellite channels, and the advent of STAR : Doble hop (VSAT ⇔ VSAT) ally maintained. The NMS regularly basic network management systems. polls the nodes of the network to VSATs of the third generation (deve- obtain normal activity statistics, infor- loped since 1987) are characterised by mation about system failures and error designs taking into account the need recovery. for open and standard architectures. Many of these VSATs operate in swit- The VSAT systems present two kinds ched networks based on architectures of topologies: star topology and mesh corresponding to the standards of the topology (figure 2). telecommunications industry such as The star topology is the traditional X.25. VSAT network topology. The commu- The earliest VSAT systems had a nication links are between the hub and STAR topology and they started in the the remote terminals. This topology is USA during the late 70s, largely in pri- well suited for data broadcasting or vate corporate networks containing data collection. The only way to com- thousands of sites. Some 85 per cent municate between the remote termi- of the world’s VSATs are located in the nals is via the hub station (double USA, and about 90 per cent of US hop). This makes it impossible to offer VSATs are in private dedicated hub speech services between the termi- VSAT HUB VSAT networks operated for only one corpo- nals, because the time delay in the rate user (3). In Europe this private ⇔ double hop (500 ms) is too severe. MESH : Direct connectivity (VSAT VSAT) VSAT solution will not be the rule Traffic channels The connectivity on the space seg- since regulation and other issues will Signalling channels ment is provided by digital carriers in drive most customers to utilise a sha- both directions, organised with vari- red VSAT hub operated by a VSAT ous access schemes. The access tech- Service Provider. The average number Figure 2 VSAT system topologies: star topology and mesh niques used in a star network can be of VSATs per hub might, in the USA, topology. Mesh topology offers direct connecti- both FDMA (frequency division multi- approach 800, but internationally the vity betwen the remote VSAT terminals ple access), TDMA (time division number is closer to 100 sites per hub.

40 For some years also meshed VSAT 4.1.1 Network configuration Transponder capacity for user data systems have become available. An traffic is shared among user stations in NORSAT-B consists of one main earth application is for example telephone FDMA (Frequency Division Multiple station (MS) and a network of user services in areas with insufficient ter- Access). stations (US) sited at the users’ premi- restrial networks. Circuits are design- ses. The main station is located at Eik NORSAT-B is designed for using a Ku- ed on demand, allow-ing for an effici- earth station outside Stavanger. All band transponder, that is 14 GHz up- ent use of the space capacity. More establishment of connections, monito- link and 11/12 GHz down-link. Today recently, high rate (typically 2 Mbps) ring and control of the network and a transponder in INTELSAT VA (359° E) VSAT services were introduced (for the transponder is done from this sta- is used. This transponder covers Nor- example the NORSAT-B system). tion. It also takes care of all charging thern and Central Europe (figure 3). What these systems have in common information. Satellites from EUTELSAT and the is that they require more powerful next generation of INTELSAT satelli- VSAT stations and a high down-link From a signalling point of view, it is a tes (INTELSAT VII, 1994) give better power. Consequently only a small star network. That is, signalling be- European coverage. Most likely NOR- number of carriers (channels) can be tween two user stations will always go SAT-B will be transferred to INTEL- supported by a satellite transponder via the main station. Concerning traf- SAT VII when this satellite is ready for and transmission costs are correspon- fic, it is a meshed network with direct use. dingly high. US to US connectivity. That is, the traf- fic itself is conveyed directly between The available access techniques allow 4.1.2 Available bitrates and user stations. In this way single-hop for inherent flexibility. TDMA (time connection types traffic, which minimises time delay, is division multiple access) gives the offered. Today the available bitrates are N * 64 best flexibility, but at the price of high kbps, where N is 1, 6, 12, or 32. That earth station costs. When using Signalling information between the is, NORSAT-B offers digital connec- FDMA/SCPC (single carrier per chan- main station and the user stations is tions from 64 kbps to 2 Mbps. The nel) flexibility is limited and earth sta- exchanged by using dedicated signal- next generation of NORSAT-B termi- tion costs are lowered but remain still ling channels. The main station is nals will probably include N = 2, 3, 4, on the high side. In other words, high continuously broadcasting on a Broad- 5, 10, 15, and 20 as well. rate meshed VSAT communications casting Signalling Channel, while the are currently handicapped by the need user stations share the capacity of a The type of connections possible in to operate powerful earth stations and Common Signalling Channel. NORSAT-B are point-to-point, point-to- by relatively high transmission costs. 4 VSAT systems in Norway As examples of VSAT systems two of the VSAT systems in Norway are de- scribed: NORSAT-B and NORSAT PLUS. NORSAT-B, which is a well established system, is given a fairly thorough description. NORSAT PLUS, -3dB -5dB which is a conventional VSAT system, is just briefly introduced.

4.1 NORSAT-B In 1976 Norway became the first coun- try in Europe to use satellites in its domestic telecommunications net- work. This first system, NORSAT-A, was originally established to handle the telecommunications traffic to the oil installations on the Norwegian con- tinental shelf and the Arctic islands of Svalbard. As the next step NORSAT-B was deve- loped in Norway by EB Nera in co-ope- ration with Norwegian Telecom. This satellite system became operative in 1990. In addition to the areas served by NORSAT-A, NORSAT-B was plan- ned to provide business communica- tion on the Norwegian mainland. Lately further expansion to a complete European market has been considered. Figure 3 Coverage of the Intelsat VA transponder used for NORSAT-B

41 multipoint and multipoint-to-multipoint tion is impossible due to network conges- The Standard S stations are “customer (conference). Both duplex and simplex tion, an alarm will be given. built”. They can be adapted to the needs connections can be offered, and transmit of the individual customer. Possible vari- and receive bitrates can be chosen inde- 4.1.4 Charging ants may be stations with duplicated pendently. equipment to fulfil stringent require- For fixed connections you pay a fixed ments on communications reliability, sta- 4.1.3 Establishment of connections price a year. This price depends on band- tions with non-standard sizes of antenna width used. For both switched and pre- NORSAT-B allows connections to be or transmitters, or stations with addi- booked connections costs are charged established in three different ways: tional channel units. only for the call duration. Price per - Fixed circuits: This corresponds to minute depends on bandwidth used. 4.1.6 Applications leasing fixed lines. The capacity in the If specified, charging information is satellite is permanently reserved, and NORSAT-B was one of the first high available on a per call basis. This will be cannot be used by others, irrespective speed switched systems available. High transmitted after disconnection. of whether or not one chooses to trans- speed connections (2 Mbps) are used for mit information all the time. bulk data transfer. Examples are trans- 4.1.5 User stations mission of pictures from the ERS-1 satel- - Prebooked circuits: Some time in Two standard types of user stations are lite from Tromsø Satellite Station to FFI advance the customer makes an order defined, referred to as Standard A and at Kjeller and transmission of environ- for a circuit to be set up between speci- Standard B. Any user station which can- mental data from Finnmarksvidda to fied user stations. This order is fed into not be classified according to these stan- Kjeller (NORSAR). This represents large the main station, which then estab- dards is assigned to a third group called amounts of data to be transferred (figure lishes the circuit at the desired time. Standard S (specially built stations). 4). - Switched circuits: The circuit is con- The Standard A station uses an offset 2 Mbps connections are also used for nected and disconnected on demand, at type antenna of 3.3 metres diameter. The remote printing of newspapers. In this request from the user. The necessary station offers all available bitrates: today way the newspaper can be printed simul- information concerning the circuit can they are 64 kbps, 384 kbps, 768 kbps and taneously at several places, reducing both be pre-stored in the user station, or fed 2.048 Mbps. transportation costs and distribution time. into it from a manual keyboard. The Standard B station is the smallest Another application is video conferenc- Prebooked and switched connections will one. The antenna diameter is 1.8 metres. ing. The conferencing is not limited to compete for the same network resources. The station offers only 64 kbps connec- two parts only (max five parts). The If establishment of a prebooked connec- tions. video conference market has been expected to grow rapidly for years, but the growth is still slow. NORSAT-B is also used as back-up for terrestrial circuits when a high degree of reliability is imperative. This increases reliability because NORSAT-B terminals are installed at the users’ premises and the network is independent of other telecommunications networks (figure 5). Since NORSAT-B offers single hop traf- fic (transmission delay of 250 ms), speech transmission is of acceptable quality. In addition to fax and other lower rate data transmissions, it can therefore be used for ordinary telephone connections.

NORSAT-B’s advantage is its flexibility. The user is allowed to set up a variety of different network topologies ranging from simplex point-to-point to full duplex multipoint-to-multipoint. Bitrates can be chosen from 64 kbps to 2 Mbps independently for transmitting and receiving. This can be utilised in compa- nies spread over a large geographical area with need for a wide range of com- munication facilities. The

Figure 4 Examples of NORSAT-B applications

42 Access control station

NORSAT-B #@!?© NORSAT-B user station user station

Traffic Traffic concen- Modem Modem concen- trator trator Back-up Back-up switch switch

Figure 5 NORSAT-B used as back-up for terrestrial circuits

VSAT 1

UPC

UPC Dial Backup

Data SAC 1 NetView PC Concentrator

NetView Data NMS Application VSAT 2 Concentrator SAC N

UPC Host Data Concentrator UPC Dial Backup Color Data NMS Dial Graphics Concentrator Gateway Backup Workstations

High-Speed High-Speed Host LAN NMS LAN

Figure 6 NORSAT PLUS Network

43 same system can be used for data - Data Concentrators (DC) - DCs pro- - Integration of these networks with a transmission of different bitrates, vide device (host ports, modems, larger variety of Customer Premises video conferences, distance education, printers, etc.) connectivity in 8 ports Equipment (CPE), and more advan- telephone and fax services, etc. building blocks into the network. ced terrestrial networks including DCs operate protocols at user selec- fibre optic networks, newer swit- Another example of integration of ser- table port speeds up to 64 kbps. DCs ching equipment and ISDN. vices can be in telemedicine, where support protocols, including IBM both video conferences, distance edu- SNA (System Network Architec- Today, integration of the VSAT net- cation, transmission of data and pictu- ture) and X.25. With the unique works with the terrestrial common res with different resolutions are need- “plug and play” architecture for pro- carrier network is via gateways gene- ed. tocol support, additional protocols rally located at the hub station. In the NORSAT-B’s main drawbacks are the can be easily implemented future, the VSAT networks will be high price level and the limited cove- interfacing with the ISDN terrestrial - Network Management System rage area (see figure 3). The coverage network and multiple gateway interfa- (NMS) - the NMS provides com- area problem will probably be reduced ces may become more important with plete control and monitoring facili- when Intelsat VII comes into operation greater use of full mesh network archi- ties for network operation. in late 1994. tecture. It is important that in the selection of link and network layer The hub station components are con- The price level can be reduced when protocol standards for ISDN, satellite nected via a high-speed LAN. new and cheaper user stations are networks will be considered with available. Such stations are under 4.2.2 Remote Sites respect to unique properties of these development. A large part of the total networks, such as time delay and Each network location is equipped costs is due to the satellite transpon- broadcast capabilities. with a VSAT where all communication der, and can therefore be reduced if devices are located indoors with the the number of users increases. In addition to future trends in the exception of the satellite dish and the VSAT ground networks one can outdoor unit (ODU). Each VSAT may expect new technology to be impor- contain up to two Universal Protocol 4.2 NORSAT PLUS tant in the space segment area. New Cards (UPC). VSAT UPCs support communication satellites will incorpo- NORSAT PLUS is a two-way VSAT one or more protocols (up to four), rate the following features, which will star-type network which will be put including SNA and X.25, allowing con- have a significant impact on future into operation autumn 1992. It consists nectivity for a wide variety of devices. VSAT networks: of a hub station, multiple remote sites, 4.2.3 Satellite Access Methods and a network management system - Amplifiers with higher output power with colour graphics user interfaces NORSAT PLUS uses the Adaptive - Use of spot beams and scanning (figure 6). The hub is located at Nitte- Assignment Time Division Multiple beams dal Earth Station and is operated at a Access (AA/TDMA) method and the 24 hours/day basis. The system offers Permanent Assignment Time Division - On-board processing data transmissions up to 64 kbps and Multiple Access (PA/TDMA) method - Intersatellite links. supports IBM SNA and X.25 protocols for inbound (remote to hub) transmis- in standard configuration. The system sion, and continuous time division These features will permit higher is manufactured by GTE Spacenet. It multiplexed (TDM) for outbound (hub capacity VSAT networks with lower is operating in Ku-band with 1.2 metre to remote) transmission. Both cost earth stations and greater flexibi- antennas and will make use of INTEL- inbound and outbound carriers offer lity. The use of intersatellite links may SAT V space segment. NORSAT PLUS data transmission at bitrates up to 64 provide direct connectivity and inte- is dedicated to business data commu- kbps. gration into other networks without nications and primary application is requiring terrestrial connections. expected to be typically transaction Direct integration with mobile net- oriented database enquiry and selec- 5 Future trends works may also be possible by this tive data broadcasting from a central Dr Golding at Hughes Network Sys- method. database to groups of users. tems has stated that future trends in VSAT networks will be driven by the VSAT systems have not grown as 4.2.1 Hub Station following goals (4): rapidly in the rest of the world as in The hub station resides at the central - Lowering costs of the VSAT termi- the USA. This is mainly because of the network facility to provide host con- nals, hub stations and installation of regulatory environments. In order to nectivity into the network. Hub station these networks make the situation in Western Europe components include: approach the situation in the USA, the - Providing a greater range of service, European Community intends to - Hub Radio Frequency (RF) Equip- including voice and compressed extend the applications of the gene- ment video services rally agreed principles of Community - Satellite Access Controllers (SAC) - - Providing networks that are more telecommunications policy to satellite SACs provide logic for processing user friendly and flexible in terms of communications. This will implicate transmission and receipt of data via operations, administration and main- liberalisation of earth segment and ter- satellite tenance minals.

44 After the recent development in Eas- References tern Europe this has been considered to be a new and very promising mar- 1 Pelton, J N. International VSAT ket for VSAT systems because of the Applications and ISDN. IEEE Com- lack of terrestrial networks. ESA munications Magazine, 60-61, May (European Space Agency) has studied 1989. the market opportunities for VSAT networks in Eastern Europe. They 2 ETSI. Satellite Earth Stations conclude that the market for tradi- (SES); Transmit/receive VSATs tional business VSAT (star topology) used for data communications ope- is limited, but for the so-called “uncon- rating in the FSS 11/12/14-GHz- ventional” VSAT systems (mesh net- bands. (DE/SES-2002), ETS 300 works), which offer telephony, the 159, February 1992. market appears to be very promising (5). 3 Mesch, R G. Shared VSAT Hub Expectations. VSAT ’91 Conference & European Satellite Users Show, Luxembourg, 5-7 November 1991.

4 Golding, L S. Future Trends in VSAT Networks. IEEE Communi- cations Magazine, 58-59, May 1989.

5 Pinglier, A. Marked opportunities for VSAT networks in Eastern Europe. In: Proceedings of The 8th European Satellite Communicati- ons Conference, London, December 1991, 129-139.

45 Fleet management using INMARSAT-C and GPS - a Norwegian pilot project

BY ARVID BERTHEAU JOHANNESSEN

621.396.946 Introduction INMARSAT-C As the terminals grow smaller and become more portable, satellite com- In the transport industry, communica- A major trend in satellite communica- munication has become more attrac- tion with the land vehicles of the trans- tion is the evolution towards small, tive to land mobile users. As opposed port fleet has until recently been done portable terminals. INMARSAT, the to terrestrial cellular telephone sys- through HF/VHF radio, cellular tele- International Maritime Satellite Orga- tems, the INMARSAT satellite sys- phone or payphones along the road. nisation, has a central role in this evol- tems offer global coverage and are For navigation, road maps have been ution (1). INMARSAT is an internatio- independent of the specific systems of the only choice. These methods, al- nal inter-governmental partnership for the different countries. This is also though well proven, all have major mobile satellite services, owned by 67 interesting when the growing traffic limitations and disadvantages. The ve- signatories. There is one signatory in towards Eastern Europe is taken into hicles are often difficult or impossible each country. USA, represented by consideration. A cellular system like to reach, because of the limited cove- COMSAT, is the largest shareholder GSM will most likely have its strength rage of these communication systems. with 24.6 %. Norway, represented by in high traffic areas, but the satellite Norwegian Telecom, has a share of The introduction of satellite communi- systems are independent of the local approximately 10.7 %. cation systems like INMARSAT-C and infrastructure. the satellite navigation system GPS The work-horse of INMARSAT has (Global Positioning System), offers a until now been INMARSAT-A, a mari- INMARSAT-C is a new system very wide range of new possibilities. This time communication system for tele- well suited for land mobile use. The article briefly presents these systems phony and telex services. This system system uses L-band (1.5/1.6 GHz) and describes some applications rela- has been operating since 1982, and through four satellites in the geostatio- ted to them. Emphasis is given on a today there are more than 20 000 ship nary orbit. It is a system for transmis- Norwegian pilot project, known as the terminals, known as ship earth stati- sion and reception of low rate data ”Sties Project“, where a fleet manage- ons, in use, and approximately 30 with a 600 bit/s bit rate. From the ter- ment system using INMARSAT-C and coast earth stations are established restrial networks, it is available GPS was tested in a real-life transport around the world as gateways to the through telex, X.25, X.400 or through environment. terrestrial networks (2). a normal telephone connection, using a modem. The system offers global coverage divided into four satellite regions: - Atlantic Ocean Region East (AOR-E) - Atlantic Ocean Region West (AOR-W) 600 bits/sec data - Indian Ocean Region (IOR) - Pacific Ocean Region (POR).

Each of these satellite regions is hand- Coast Earth Station led by several coast earth stations. access control and The INMARSAT-C mobile earth sta- signalling equipment tion is small and compact and is opera- ted through a standard laptop PC and dedicated software. Among the manu- Store-and-forward facturers of INMARSAT-C mobile message switch earth stations are Norwegian ABB Nera and Danish Thrane & Thrane.

Rescue Public switched Using INMARSAT-C, the driver is able co-ordination telecommunications networks to communicate with the headquar- Telex, voice-band data, teletex, Electronics unit centre email, packet-switched data network ters, other vehicles or the receiver of the cargo anytime and anywhere through text messages. The headquar- Leased lines Computers Telex Navigation ters may send broadcast messages to Printer Micros to mainframes network system the entire transport fleet or part of the fleet. In addition, the system can auto- matically transmit reports from the vehicle, containing data like the cargo temperature or the vehicle position. Figure 1 The INMARSAT-C system

46 GPS (Global Positioning packages, such as position reports, User needs and require- possible. For position reporting, both System) maritime and land mobile formats are ments For the determination of the vehicle defined. The format for land mobile For a typical transport company, we position, GPS (Global Positioning Sys- position reporting consists of 1 to 3 can imagine several parties totally tem) is the system by choice. GPS is packets, each 15 bytes long, and con- dependent on communication (6). expected to be fully operational during tains: The position (longitude and lati- Among these are the transport com- 1992 - 1993 and operates through 24 tude) in degrees, minutes and 1/100 pany headquarters, the drivers, the satellites (21 operational plus 3 back- minutes and the time of the position. receivers of the cargo and external up) in an approximately 20,200 km In addition, short messages may be partners like insurance companies. height orbit (3). By receiving the sig- included in this format. These will by Some typical situations when the head- nal of three satellites, the position in preference be MEMs (Macro Enco- quarters need to obtain contact with two dimensions is calculated with an ded Messages), which are predefined the drivers are when accuracy of 50 - 100 m. By receiving numbered messages. 255 numbers are - plans are changed signals from four satellites, the posi- allocated for these messages and tion in three dimensions is calculated. some of them are already defined by - rerouting the vehicles GPS receivers are manufactured by a INMARSAT, while some are left for - exchanging customs information number of manufacturers. The size of custom definition. Another protocol is the receivers is still decreasing, and called Polling. This is a protocol enab- - the customer asks for his goods the prices are still falling. The INMAR- ling the fleet operator to poll the mo- - address of delivery is changed SAT-C terminals are equipped with a biles in order to request reports from serial interface so that a GPS receiver them, or to send messages to them. - delay occurs. may be connected for automatic or Instead of the mobile sending periodi- Today, the headquarters are depen- manual transmission of positions. cally reports to the fleet operator, the dent on the drivers to call them, and operator may request this when neces- The above mentioned position accu- problems appear when the driver does sary. racy should be sufficient for the moni- not call or is unable to get through. A toring of transport fleets. For naviga- Typical polling commands may be: mailbox service, or store-and-forward tion, a higher accuracy is required. system like INMARSAT-C, omits the - Send report immediately. (This There are several possibilities for need for the parties to be constantly command also includes information obtaining such an increased accuracy. stand-by. Fast change of plans can be on what type of report is expected) One possibility is differential GPS. By executed on a very short notice. establishing fixed reference stations, - Start reporting on a regular basis Today, the vehicles are traced through the difference between the position border stations, forwarding agents or - Stop reporting received by GPS and the actual known truck stops. Tracing the vehicles this position of the reference station can be - Receive text message. way is time consuming and expensive. calculated and transmitted as a correc- tion signal to the mobile users opera- Message Terminals Cargo Temperature ting within a distance of the reference station. Differential GPS offers an accuracy of 1 - 5 metres even for moving objects. In Norway, Statens Kartverk has established such a sys- tem called SATREF. INMARSAT-C terminals with integra- ted GPS receiver are also available. Maps/Vehicle positions The two systems are very well suited for integration as the GPS receive fre- quency slot is allocated between the INMARSAT-C transmit and receive slots (approx. 1.5 GHz). Communication protocols in INMARSAT-C INMARSAT-C is prepared to handle several types of information, and diffe- Local Area Network rent communication protocols are de- fined (4), (5). In addition to the mes- sage protocol, protocols for a better utilisation of the transmission capacity Telex X.25 X.400 Telephone/ are defined. A protocol for data repor- Modem ting is defined. This protocol makes low cost transmission of short data Figure 2 Typical configuration at the headquarter

47 The insurance company needs contact ware originally delivered with the Concluding remarks with the vehicles when damage, acci- INMARSAT-C terminals. The Sties project has shown that dents or delays occur. In the case of INMARSAT-C, combined with GPS, accidents, accurate information about The GPS receivers were set for perio- provides the transport industry com- course of events and position is nee- dic transmission of positions. At Sties munication possibilities no other sys- ded for assistance or rescue actions. head office, a PC equipped with a tem can offer. In the USA a report large colour display was running map For small companies one or more PCs released by the Government Accoun- software, where the received positions or workstations are sufficient for the ting Office in May 1991 reveals that were displayed for each vehicle. Re- handling of messages and acquisition effective Intelligent Vehicle/Highway ceived messages were routed through of data, like positions or cargo tempe- Systems (IVHS) could reduce travel Sties’ local area network to internal rature. For larger companies, a confi- time in congested areas by up to 50 %, addresses or to a common mailbox. guration like this is suggested: drop petrol consumption by up to 10 % and reduce automobile pollution by up A Local Area Network (LAN) is con- The experiences gained by this project to 15 % (7). Research on IVHS is done nected to the telex network, have been very useful. Feedback has on a large scale. These systems are X.25/X.400 network or the telephone been given by drivers and transport not limited to communication as pre- network through a modem. Messages agents, the most important user group sented in this paper, but are intended will be routed to different users in the at the head office. The software user as a total control system for road traf- company based on the message interface is very important, and all fic. This is fleet management in the addresses. The position data for each unnecessary functions were removed. broadest sense. Satellite communica- vehicle may be routed to a dedicated A simple user interface for the trans- tion and navigation offered by INMAR- PC/workstation running suitable soft- mission and reception of messages SAT-C and GPS, should be very well ware, like a map monitoring program, remains. This user interface of the suited as part of such systems. and specific data like cargo tempera- software was one of the things that ture and status reports for each vehi- had to be adjusted during the project. cle may be routed to and saved on a It occurred that too many functions dedicated PC/workstation. We can were hidden for the driver. When also imagine access to databases con- something went wrong, like a mes- References taining customer data, geographical sage did not seem to get through, the data, ferry timetables, customs infor- driver wanted to know a little more 1 Haga, O J. Mobil satellittkommuni- mation, dynamic information about about what actually went wrong. The kasjon, perspektiver for 90-årene. road accidents and possible rerouting. software user interface was adjusted, Telektronikk, 3/4, 231-236, 1991. Such information may be available and the new version included indicati- both to the vehicle and to the head- ons like signal level on a scale 1 - 5. 2 Berzins, G et al. INMARSAT: quarters. The drivers also gained experience Worldwide mobile satellite servi- making them able to utilise the system ces on seas, in air and on land. better. Problems like low elevation Space Technology, Vol 10, No 4, The Sties project angle, (especially in Norway) can be 231-237, 1990. In the spring of 1991 a fleet manage- omitted when parking the vehicle in ment test project was started in Nor- the right direction and on the right 3 Teunissen, P J G. De geodetische way. Participants were the transport spot. lijn. De Ingenieur, nr 6/7, 24-30, company Sties Termo Transport A.S, 1991. Norwegian Telecom and the software Also, on the hardware side useful developing company Euro Traffic A/S. experiences were gained. The cockpit 4 INMARSAT. Standard-C System Sties Termo Transport A.S is one of in a vehicle is not too big, and no definition manual. Release 1.3 July the leading transport companies in space is redundant. PCs with hard dri- 1989. Norway, owning and making use of ves are not made for the rough treat- more than 250 special vehicles for ment and temperature fluctuation offe- 5 Ekman, A. A new international transport of frozen and refrigerated red “on the road”. Different equipment position reporting service for mari- cargo. They operate all over Europe, configurations were tried, and it time and land mobile users. Paper including Eastern Europe. seems like the PC is the most fragile presented at the U.S Institute of part. However, PCs specially designed Navigation Satellite Division 2nd In 5 trucks, INMARSAT-C terminals, for mobile use are manufactured and international technical meeting, laptop PCs and GPS receivers were currently brought on the market. The ION GPS-89, September 1989. installed. Some of the trucks were also drivers participating in the Sties pro- equipped with logging facilities for ject all had a positive attitude towards 6 Beltran, J. Lakseeksporten og infor- cargo temperature. Norwegian Tele- new technology and operating PCs. masjonsflyten, satellittkommunika- com provided the equipment and Sties Some were a bit more sceptic to functi- sjon med mobile enheter. Internt provided the vehicles and drivers. A ons like the position reporting, which notat Sties/Televerket, februar software package was custom develo- might have some “big brother watches 1991. ped by Euro Traffic. This is terminal you” effect. However, this kind of con- software for easy operation of the trol is also appreciated, taking into 7 Manuta, L. Intelligent Vehicles, INMARSAT-C terminal more suited consideration the security aspects. Smart Satellites. Satellite Commu- for these specific needs than the soft- nications, February 1992, 21-24.

48 Terminals for mobile satellite communications BY JENS ANDENÆS

Introduction INMARSAT C, as well as control sta- under the brand name Saturn, and in the 621.396.946 tions for INMARSAT A, INMARSAT C following their main characteristics and ABB Nera’s involvement with mobile and INMARSAT Aero. The majority of their applications are described. satellite communication started in 1968 the deliveries are export and are in use all (then Nera A/S) with a system study over the world. Saturn 3s90 report. The product development began in 1975 and the first generation ship earth For the mobile terminal market we are This is the latest version of our series of station was type approved and put on the today manufacturing stations for INMARSAT A stations. The equipment market 1st September 1978. The market Inmarsat A and Inmarsat C that are used comprises an antenna unit, electronic expanded rather slowly in the first years both in the maritime sector and for appli- unit, teleprinter, and a telephone set. since the equipment was costly in both cations on land. The antenna unit (figure 1) is enclosed in purchase and usage, and the shipping The market has been characterised by a a 1.5 m glass fibre radome with a total companies had adapted their way of run- continuous technical development with weight of 95 kilos. The antenna element ning the business to unreliable communi- price competition and falling prices. This is a 1 m parabolic dish mounted on a sta- cation using HF radio between the shore has made it necessary to bring out new bilised pedestal. The antenna needs to be based offices and the ships. From the product generations approximately every stabilised regarding several different mid-80s the market started to expand second year to take advantage of new and motions, roll and pitch, sudden changes more rapidly, and with the expansion of more cost effective technology, and to in course, and change in the direction to the INMARSAT charter to include aero- develop new services as they become the satellite due to the ship moving on nautical and land mobile communication, available in the system. the surface of the earth. The roll and new market segments opened. pitch stabilisation is a passive system that with the aid of two gyro wheels mounted ABB Nera is today the largest supplier of Mobile stations directly on the antenna system, forces the INMARSAT stations for the earth seg- ABB Nera is currently manufacturing antenna to point in the same direction ment of the system. This includes mobile many different mobile satellite terminals even if the ship rolls and pitches up to 30 stations for INMARSAT A and for INMARSAT A and INMARSAT C degrees. Sudden course changes are com- systems. The stations are all marketed pensated for immediately by using infor- mation from the ship gyro compass. The last system is a slow “step track” that gives the antenna small deviations in ele- vation and azimuth, measures the corre- sponding small changes in received sig- nal strength and moves the antenna con- tinuously to the direction giving maxi- mum signal strength. At power-up the antenna will, if there is no signal present, scan by itself over the hemisphere and acquire the satellite automatically. If the power to the system fails, the pointing data is stored and when power is resumed, the antenna will go to the last pointing direction to re-acquire the satel- lite. The RF equipment consists of two sub- units mounted on the stabilised

Figure 1 Saturn 3s90 stabilised antenna system with the radome removed Figure 2 Saturn 3s90 Electronics Unit

49 The electronics unit (figure 2) comprises Also under development is an enhance- modulators, demodulators, audio proces- ment of the service to full duplex sor, control processor, high stability fre- 64 kbits that will be available late this quency reference, interfaces for the tele- year. This service may be used for data phone and teleprinter and power supply. transmission of large data files in both directions, video conferencing using The telephone interface is standard two- compressed video, or it could be used for wire with the same levels as in the terres- multi channel operation using digitally trial network, to allow the interconnec- coded telephone, for instance six simul- tion to standard equipment without spe- taneous voice channels coded at 9.6 cial adapters. Call set-up is done in the kbit/s. same way directly from the keypad on the telephone set. Since the station looks Saturn CompacT like an ordinary branch line, it may be directly connected to fax machines, voice This station has been developed to meet band data modems, or to a PABX if it is the rising demand for transportable sta- required to use the station from several tions that was created when it became different outlets. allowed to use stations on land. When the need for communication arises sponta- The station is equipped with two separate neously, e.g. an earthquake, there is an telephone extensions with its own unique urgent need for equipment that is easy to Figure 3 Saturn CompacT in operation numbers in the INMARSAT system. If a transport, quick to put into operation, and telephone set is connected to one exten- above all is easy to use (figure 3). To sat- sion, and the fax machine (or a data isfy this demand Saturn CompacT was modem) connected to the other, one has developed simultaneously with Saturn direct in-dialling to the desired extension, 3s90, and the equipment is based on the it is possible for instance to send fax to same modules. the ship automatically at any time with no need for attendance to the equipment All electronics including the antenna is on board. Group 3 fax may be sent to and integrated into a custom made rugged from ships at 9600 bps. The same speed polythene case measuring 70 x 60 x 30 is attainable with voice band data cm, and with a weight of 34 kg. The suit- modems. case is equipped with handles so it may be carried by one or two persons, and The prime users of this equipment are wheels for easy transportation (figure 4). generally large ships of all categories, but also trawlers and luxury yachts constitute The antenna element is a sectioned 90 cm a major user group. During the last years parabola that is packed inside the lid dur- with liberalisation, a large number of ing transport. Under the cover in the these stations have been placed on land main section there are two assemblies as fixed installations in areas where inter- mounted on shock absorbers, a water- national communication lines are not proof cooling tunnel, and the electronics available, or difficult to obtain. unit. The RF units and the power supply is mounted on the cooling tunnel, with For users with the need of higher data heat sinks protruding into the air flow rates a new option has been developed generated by a thermostatically con- that offers 56 or 64 kbits data transmis- trolled fan. Figure 4 Saturn CompacT ready for operation sion from the terminal to shore. In the return direction there is at the same time The electronics unit is almost identical to a normal analogue voice channel capable the corresponding unit for Saturn 3s90. antenna system to minimise the loss in of 9.6 kbit/s. This service is especially The only difference being a slightly dif- the antenna cable. The units comprise attractive for the oil industry for instance ferent configuration, and the removal of duplex filters, low noise amplifier, down- sea-bed surveying, or on drilling ships the outer panels since the unit is inte- converter, up-converter and a power generating large amounts of data that grated into a different enclosure. amplifier with an output power of 25 needs to be analysed on shore. Other Watts. All connectors to external equipment are potential applications may be real time available at the exterior of the suitcase The connections to the electronic unit compressed video transmission to experts protected by a small panel so that the lid below decks are via a single screened on shore in conjunction with on-board can be closed and the equipment left out special cable containing a coax cable and services and repair, or it could be used in all weather conditions after the equip- a main cable. The coax cable feeds all for video remote supervision of ment has been powered up and the necessary control signals between the unmanned installations. antenna has been aligned with the satel- antenna unit and the electronics unit, lite. In this condition the such as receive IF, transmit IF, frequency reference for up- and down-converters and supervisory and control signals in serial form.

50 equipment is rain and splash proof and The service may also be used for TV antenna element is omnidirectional, there will operate in a temperature range of news flashes by coding the video to an is no need for any stabilisation or track- -25° C to +55° C (the unit has in extreme acceptable quality (e.g. 768 or 384 ing system in contrast to INMARSAT A cases been operated down to -45° C). kbit/s), storing the data on disk and terminals. The only requirement is that retransmit the data at 64 kbit/s. A two there is unobstructed sight to the satellite. The power supply is universal, and will minute’s news flash coded at 384 kbit/s The RF part comprises filters, low noise accept input voltages from 90 to 264 will be transmitted in 12 minutes, which amplifier, up and down converters and a VAC without the need for any manual is a large improvement from using a power amplifier with an output power of voltage setting in order to cope with the voice band modem that would take 1 25 Watts. different voltage standards used around hour and 20 minutes for transmitting the the world. If it is to be operated in places The connection to the indoor mounted same amount of data. The cost of the with no available source of energy, then electronics unit is via a single standard transmission would also be less than a solar panels and battery together with a coaxial cable carrying receive IF, trans- quarter of the cost using a voice channel DC/AC inverter may be used, or a small mit IF and power supply. and a modem. portable power generator. The functional parts of the electronics Saturn C unit are modulator, demodulator, control Readying the station for operation is a processor, power supply and interface simple task, taking no more than 5 min- Saturn C is our first generation mobile circuits for external connected equip- utes by one person. The antenna is terminal in the new Inmarsat C system ment. All the electronics are integrated assembled on a universal joint mounted for low data rate message switched ser- into three replaceable sub-units, and the on a small post, using the suitcase as a vices. It is primarily developed for mar- complete assembled unit measures 28 x stable base. Aligning the antenna with itime use, and is fully compliant to the 7.5 x 26 cm, and weighs only 3.7 kg. the satellite is easily done with the aid of requirements of GMDSS (Global Mar- There are no controls on the electronic a compass and a map showing the point- itime Distress and Safety System). The unit, so it may be mounted anywhere ing direction for the satellite overlaid a equipment comprises four main parts; indoors. The unit is DC powered and will global map. The received signal strength antenna unit, electronic unit, PC, and accept voltages in the range 10 - 33 V. is indicated both on an LCD display, and printer (figure 5). The antenna unit is an Optional external power supplies are as the pitch of a beeping tone that may be integrated unit where the antenna and the available for AC or combined AC/DC switched on for antenna alignment. The RF equipment are enclosed in a water- with automatic change-over if the main beam width of the antenna is approxi- proof encapsulation, measuring 42 cm supply fails. mately 14°, so the pointing accuracy is high and 12 cm in diameter and with a quite modest. total weight of 3 kg. The antenna element The system is operated from a PC that is an omnidirectional quadrifilar helix, may be a note-book PC, or an ordinary The Saturn CompacT offers the same covering the entire hemisphere down to desk-top type. For maritime applica- basic services as the Saturn 390, i.e. two 15° below the horizon. Since the telephone outlets and telex. The tele- phone lines may be connected directly to fax machines, voice band data modems or PABXs. Users that have the need for encryption may use commercially avail- able equipment for encrypted telephony, telex, data and fax. Prime users for this equipment has been international organisations such as the Red Cross and various UN organisations operating in areas hit by disaster, or on peace keeping missions. Other users are embassies, corporations operating in countries with poor infrastructure, and news agencies and broadcasters (e.g. Peter Arnett from CNN in Baghdad dur- ing the Gulf war). The 56 or 64 kbit/s services are of partic- ular interest to broadcasters. With new coding standards it is possible to obtain 7.5 and 15 kHz high quality audio con- nections, from anywhere on the earth, to the studio without having to arrange spe- cial lines to the location.

Figure 5 Main parts of Saturn C maritime version

51 tions where the station constitutes a part The system is a store and forward sys- Satellite communication has always been of the ship equipment, the PC must com- tem, where the messages are first com- difficult in the far northern regions with ply with GMDSS regarding temperature pleted by the user before the transmission geostationary satellite systems, since the range, humidity and vibration, and for starts. The message is then sent to the satellite is very low on the horizon, and these applications a note-book PC is used selected coast earth station that checks sea reflections interfere with the main that has been type approved together the message for any transmission error signal. Saturn C may be used in a special with Saturn C by INMARSAT. If the ter- before it is retransmitted via the terres- configuration, using two antenna units minal is to be used on land, this is not trial network to the subscriber. If the placed at different heights together with relevant, and in this case other PCs may transmission contained any errors, these one electronic unit. If the signal fails be used since the electronic unit has a blocks of data are automatically retrans- with the current antenna, it will automati- large internal memory that will store any mitted. When the message has been cally switch to the other, and thus enable incoming message for later retrieval if delivered to the subscriber, the system reliable operation further north than the PC is busy with other tasks. will send a confirmation back to the would otherwise be possible. sender if specified. The basic service is The operator interface is based on pull- Although Saturn C is primarily designed telex, but other services are becoming down menus, and pop-up windows for maritime use, it can also be used for operational such as X.25, data modem arranged in a logic and easy-to-use fash- land mobile applications such as fleet connection or leased line for special ion. There are built-in dynamic help management of trucks. If the terminal is applications. screens, that may be called upon via a connected to a GPS receiver it is possible function key, and continuous updated to have the truck’s position plotted auto- status messages are displayed at the bot- As a part of GMDSS, the equipment has matically on electronic charts, and keep tom of the screen to give the operator a a capability called enhanced group call track of the trucks. By interfacing the good feel for the status of the system at (EGC). This function permits addressing equipment to other sensors measuring all times. To ease the transmission of messages to a ship or a defined group of container temperature, door locks, etc., messages to recurring numbers, lists are ships based on geographic position, one may have continuous knowledge of available where the numbers may be nationality or whether they belong to a the truck or the cargo condition. stored and recalled when needed. All predefined group. This makes it possible In addition to the above mentioned appli- incoming and outgoing messages are to transmit gale warnings, that are cations, the terminal may be used for logged in a separate log-file containing received only by the ships that are in the fixed installations in areas with in- the most important data of the call, and affected area. Likewise, it is possible to adequate communication, where a mes- its current status (if it is sent, or if it has alert all ships within a certain radius for sage switched system may solve the been received and confirmed by the other assistance in case of accidents. This communication needs in a cost efficient part, etc.). The list of coast earth stations requires that the equipment is continu- way. Since the messages may be sent in the system is automatically being ously updated about its position, which between the terminals, an electronic mail updated by the system, and may be dis- is normally done by connecting the ter- network may be set up that operates played with information on the available minal to a GPS receiver via its NMEA totally independent of the terrestrial net- services of each one. 0183 interface, or it may be done manu- work. ally on the PC. The terminal also has a facility for transmitting emergency mes- Saturn C Portable sages, including the position of the ship, at the touch of a button that may be This equipment has been targeted to remote from the equipment. The emer- users that travel to areas with poor com- gency message will automatically con- munication, but need smaller and less tain the ship’s position in addition to the costly equipment and require text based type of emergency that may be entered communication. The design philosophy manually. has been similar to Saturn CompacT; all necessary equipment is integrated in a rugged shock-proof suitcase measuring The system has, besides its messaging 51 x 41 x 21 cm, with a total weight of service, special protocols for polling and about 20 kg including antenna, electron- data reporting. A typical application for a ics, batteries with charger, PC, and ship might be to send regular position printer (figure 6). reports. The terminal is then set up to automatically send position reports to a The antenna is a two-element flat micro predefined address at regular intervals, strip patch antenna, developed at the e.g. once per day. The polling and data Norwegian Telecom Research. The reporting facility is particularly suited for antenna has a gain of about 11 dB telemetry applications and control of enabling the output power from the HPA equipment in inaccessible locations, such to be reduced from 25 Watts to 2 Watts, as buoys collecting environmental data or with a corresponding 70 % reduction in on land with e.g. river water level moni- power consumption during transmission. toring. The equipment

Figure 6 Saturn C Portable ready for use

52 may be operated without taking the the equipment only when data is to be com and NTNF realised that satellite antenna out of the suitcase, since the transmitted. This reduces the size and communication was the way to go for lid is transparent to the signals. The the cost of solar panels and batteries future maritime communication, and lid may be locked in any position, and considerably. Saturn C was especially supported the initial developments. it is equipped with an elevation indica- designed with low power consumption There was a large home market in the tion that facilitates the alignment with in mind and is thus very suitable for shipping industry, and we had a back- the satellite. Total time needed for set- these applications. ground in both developing and marke- ting up equipment for operation is less ting communication equipment to the than a minute. The beam width of the Saturn B and Saturn M shipping industry. When the develop- antenna is 65° in elevation, and 35° in ment projects were established, it was These are next generation products azimuth, which makes the acquisition with a dedicated team with pioneering that have been under development for of the signal a trivial matter. spirit and the success has come over a year and will be put into produc- through a continuous strive to utilise If the equipment is to be used in a tion next year. available technology to reduce cost semi-permanent or permanent loca- Saturn B is a follow up of Saturn 3s90, and to develop new services and in tion, the antenna may be remote from and will be developed in both mari- this way maintaining the competitive the other equipment and placed out- time and transportable versions. This edge. doors on a small adjustable foot that is is a satellite terminal where all the ser- also packed in the suitcase. The stan- With the current projects well under- vices are digitised to expand the capa- dard length of the supplied cable is way for next generation digitised city in the system. The antenna sys- 5 m which is adequate in most cases, equipment, the basis for a continuous tem remains basically the same, and is but the antenna may be remote up to strong position for ABB Nera in the a stabilised 90 - 100 cm dish. The ser- 100 m using suitable coax. INMARSAT market in the 90s has vices are high quality voice coded at been made. Power is always a problem with port- 16 kbit/s, fax and data at 9.6 kbit/s able equipment that is to be used all plus optional high speed data at around the world, and Saturn C is 64 kbit/s. The primary users will be therefore delivered with a universal the same as for Saturn 3s90 and power supply that accepts voltages in Saturn CompacT. the range of 90 to 264 VAC, or from Saturn M is currently being developed external 12 V battery. The built-in in two variants; maritime and portable. power supply also supplies the PC and The basic services are medium quality printer with appropriate power. voice coded at approximately 4.1 Saturn C Scada kbit/s, fax and data at 2.4 kbit/s. The antenna size including radome will be Saturn C Scada is not a single configu- 60 cm for the maritime version. This ration but covers several different vari- opens up new user groups that are ants dependent on the applications. prevented from using the current Fixed installations on land may use generation equipment due to its size. the same directive antenna unit deve- In the maritime sector it can be placed loped for the Saturn C Portable while on smaller yachts and fishing vessels, applications at sea, for instance on thus greatly expanding the potential buoys, will use the omnidirectional market size. A portable variant will maritime antenna. The electronics unit have a weight of 9 kg. is in both cases the same as is used for The electronics unit for the stations is the maritime applications but with the extremely compact, with the measures software adapted to SCADA applica- of 31 x 21 x 7 cm. An important factor tions using the polling and data repor- for the reduction in size is an in-house ting protocols. These protocols per- development of ASICs with approxi- forms the transmission of small data mately 600,000 transistors performing packets at a much lower user charge most of the critical modem functions. and shorter transmission delay com- pared to the normal message proto- cols. This is of great significance for Reflections systems that transmits data maybe ABB Nera’s results and position in the several times a day for a prolonged INMARSAT field are relatively rare in time. Norwegian electronics industry, espe- The equipment is normally connected cially in a field with strong internatio- to a dedicated controller instead of the nal competition from Japan and the PC controlling both data loggers, sen- USA. There are no single factor that is sors and the transmission. Controllers the cause of the success, but rather a are available that conserve the power set of factors that all contributed to the used by the equipment by switching results. The foundation was made off the power when idle and activate early when both the Norwegian Tele-

53 Future systems for mobile satellite communications

BY PER HOVSTAD AND ODD GUTTEBERG

621.396.946 “Mobile satellite communication servi- mobile services. The company’s - The coastal earth stations (CES), ces provide communications on the income is made up from hiring out which connects the satellite trans- move - however you go (by sea, by land, transponder capacity in the satellites. mission to the national and internati- by air), wherever you go (spanning the onal telecommunication network INMARSAT has 64 member countries, world) - with anyone, anywhere and with one signatory per country. The - The Network co-ordinating stations anytime” Norwegian Telecom represents Nor- (NCS), which handles the channel (Jai Singh, INMARSAT, 1991) way, which is the third largest signa- allocation in a coverage area tory with a share of around 13 %. USA - Mobile earth station (MES), i.e. the 1 Introduction is the largest with 25 %. Denmark and user terminal. The first satellite system for mobile Sweden have only 2 % and 1 %, respec- communication (for ships), MARISAT, tively. Altogether, Europe owns more The Norwegian coastal earth station is was put into operation in 1976. This than 50 %. located at Eik, Rogaland. It operates system was succeeded by the INMAR- At present, INMARSAT has 10 satelli- towards the Indian Ocean as well as SAT system in 1979. The 1990s will tes in the geostationary orbit. They towards the Atlantic East. most probably revolutionise the cover four areas, see figure 1: mobile telecommunications services, Several different terminal standards particularly for personal communica- - Atlantic Ocean - East (AOR-E) are employed in the INMARSAT sys- tion. In this field, satellites will play an tem, see figure 2. INMARSAT-A has - Atlantic Ocean - West (AOR-W) important part because of their unique been operative since 1982. It is a relati- properties: a potensial large coverage - Indian Ocean (IOR) vely large terminal (ca. 1 metre para- area and fast establishment of the ser- bolic antenna) which is used for telep- - Pacific Ocean (POR). vice. hony (FM), telex and data. INMAR- SAT-B, which is the successor, is 2 INMARSAT The INMARSAT system is made up of mainly like INMARSAT-A, but uses four main parts: digital modulation. This will give a bet- INMARSAT is an international com- ter utilisation of the space segment pany owned by the member states. - The space segment, comprising the and thereby lower prices. INMARSAT- The aim is to specify, establish and satellites with their control stations M is a smaller and cheaper terminal operate communication satellites for (TT&C) and operational control which uses a lower bit rate for the telephone channel. Aero is a special POR version for the use in aeroplanes. 178o E INMARSAT-C is a low-speed (600 bit/s) data message system based on “store-and-forward technique”. The terminals are equipped with small omnidirectional antennas. The INMARSAT system today has around 20,000 terminals. The number of terminals is expected to increase beyond 70,000 as early as 1993. The development is pointing towards com- munication with ever smaller units, Eik earth station (CES) like cars and people. This is the rea- AOR-E son why INMARSAT expects a drama- IOR tic increase in the number of termi- 12 700 km nals, more than a million in the year 2000, figure 3.

3 The role of satellites in 35 700 km future telephony o 64.5 E The end user wants required informa- IOR o tion transferred to a receiver in the 55.5 W most practical way, with highest possi- AOR-W ble quality and at lowest possible price. The way in which this is done is irrelevant to the end user. Satellite communication is only one means of 15.5o W meeting user requirements, and it is AOR-E justified only in those cases where this form of communication can compete Figure 1 INMARSAT coverage area with other methods.

54 St.A St.B St.C St.M Aero

Antenna 0,9 m 0,9 m 15x15 cm 30 cm - Type Parabol Parabol Omni lin.array array - Steering Steerable Steerable Stationary Az steering electronic

Telephony FM 16 kb/s - 4,8 kb/s 9,6 kb/s One evolutionary trend seen today is Telex/Data 9,6 kb/s 9,6 kb/s 600 b/s 2,4 kb/s 10,5 kb/s that the wish for greater accessibility, particularly for telephony, leads to a Cost/terminal NOK 175 000 - NOK 35 000 - ca NOK 1,5 mill demand for smaller, mobile terminals. The land mobile telephone network Charge/min NOK 43 (telephony) - - - NOK 43 (telephony) offers connections via terminals inte- grated in a handset of pocket format. NOK 23 (telex) NOK 20 (telex) Earthbound cordless networks will be characterised by a relatively small Figure 2 INMARSAT services coverage area per base station and the possibility of high capacity through land mobile networks, either because Whereas land mobile networks are frequency reuse. it is technical impossible (oceans, inac- preferable where the demand for traf- On the one hand, satellite networks cessible/deserted areas) or economi- fic is large, satellite systems will have will be able to offer connections within cally uninteresting (low traffic inten- their strength in areas with a lower a much larger coverage area and sity). Figure 4 shows the expected glo- traffic intensity. It therefore seems thereby also possibilities for a more bal coverage area in the year 2000 for probable that both systems will find flexible utilisation of the capacity with- land mobile services. their places in the future mobile com- in a given geographical area. On the munication concept, and that they will Satellite networks will be well suited other hand, satellite networks will be able to be complementary to one as a temporary solution since it can be have a lower maximum throughput another, rather than fighting for the established in a relatively short time, with limited possibilities for frequency same market. without major investments on the reuse. Price per transmission channel ground. The large coverage also will also be much higher for satellite makes satellite networks attractive for 4 Requirements for a land networks. areas where earthbound networks will mobile satellite system Development of a network for perso- not be developed for the above rea- In order to satisfy the user a set of nal telephony is in its infancy, and will sons. For long distance connections requirements has to be made to the require large investments and a lot of (international) satellite networks will satellite system. In its turn this will time before it is operational with a sig- be able to compete with land mobile determine the design of the system: nificant penetration in the market. In networks because they can establish a this future network there will be many direct connection , or a connection 1)User terminals should be mobile, areas not relevant for developing by with far fewer relay points. reasonably priced, small and light-

Market

73,000 Number of terminals 18,000 1,500 1 Mill

A, C, M, B, A, C, A, C, M, B, Services A Aero, Navigation Aero Aero Paging, P

Year 1982 1991 1993 2000

Figure 3 Development of INMARSAT services

55 weight. Terminals without directive antennas are also desirable. 2)The system should have good acces- sibility; low probability of blocked calls and little loss of calls 3)The quality of the connections should be comparable to that of the present stationary ground network.

If the system is to be realised with handheld terminals there will, for safety reasons, be an upper limit for Figure 4 Coverage areas of land mobile services (yellow) in year 2000 allowed radiated power. Battery capa- city and requirements to active time, will also limit the output power. From these criteria the output power should be in the area of 1 - 2 Watts. Margin (dB) 5 a) A handheld terminal should not be 20 dependent on pointing the antenna to the satellite. This means that the ter- minal must be equipped with an appro- ximately omnidirectional antenna, i.e. 0 dB antenna gain. An omnidirectional antenna will have a fairly large noise 80% 95% 98% temperature due to the thermal noise from the earth.

10 Supposing a system noise temperature of 240° K the specifications for a “typi- cal” user terminal will be:

EIRPup = 0 - 3 dBW -1 G/Tdown = -24 dBK estimated The system’s availability is an impor- o tant parameter. Blockage of the signal ∆ because of trees, buildings, etc., will 0 for a large part of the time make com- o o o o munication difficult. Figure 5 shows 0 30 60 90 necessary blockage margin for diffe- Elevation angle rent availabilities as a function of the 1.5 GHz blockage margin ∆ 20 GHz rain attenuation elevation angle. In order to operate with different availabilities at 95% availability with a reasonable margin, the eleva- tion angle for a mobile satellite system ο 20 GHz rain attenuation should exceed 30° according to this at 98% availability model. 5 b) The time delay introduced a geostatio- nary satellite connection is approx. 1/4 second each way. For telephone 1.5 GHz connections this is experienced by many people as a major reduction in quality. For other types of communica- tion this is also undesirable, since spe- 1.5 GHz cial solutions often must be employed for taking care of this time delay. 5 Techniques to meet user requirements Traditional communication satellites Figure 5 Blockage margin for different availabilities as a function of elevation angle, figure 5 a). consist of transparent transponders The model is based on satellite measurements and measurements with helicopter. The mea- where the signals are amplified and suring set-up is shown in figure 5 b) (Ref. University of Texas) shifted in frequency before they are

56 5o transmitted back to earth. Usually 0o there are input and output multi- plexers, comprising mechanical swit- ches for connecting redundant circuits in the case of faults, and to a certain degree enable cross connections be- tween different up- and down-links. In the following we will present some relevant techniques, which to a certain extent may be used to improve and increase the efficiency of the traditio- nal satellite connection.

5.1 Satellite orbits Figure 6 Polar coverage for different elevation angles. Satellites located at 26° West, 63° East and 180° East The fact that a satellite in the geostati- onary orbit, as seen from the earth, is fixed, has of course great advantages for many applications, but this does not exclude other satellite orbits from Tundra having advantages. A disadvantage of geostationary satellites is the fact that they render poor coverage of the nor- thern and southern areas, see figure 6. Molnya In addition to the geostationary orbit, there are mainly two types of orbits which are relevant candidates for tele- communication purposes: low earth N orbits and elliptical orbits, see figure 7. Orbiting satellites have to avoid the earth’s atmosphere, in order to pre- Inclination=63.4o vent friction and damage from colli- ding with particles in the atmosphere. Geostationary These effects are of significance up to (circular) a few hundred kilometres, i.e. the height of the satellite orbits should be greater. Equatorial plane Low circular Around the earth at equator there is an area of strong radiation; the van- Allen belts. These belts consist of elec- trically charged particles, mainly pro- tons and electrons, and are located approx. 0.2 to 7 earth radii above the equator. The high energy protons are at a maximum level around a distance S from the earth of 5,000 km, while the electrons have two maxima, one around 5,000 km and one around Figure 7 Different satellite orbits 25,000 km. These two maxima are that the power from the solar panel is their apogee for a period of time. By denoted the inner and the outer van- reduced by up to half a per cent per using several satellites in the same Allen belts. revolution in the transfer orbit. orbit, one can make sure that one Moving into an area where the satel- satellite is at any time visible within Even if it may be possible to design lite is exposed to particle collisions, the area acceptable for communica- satellites that are better protected may damage the satellite and reduce tion. against particle collisions, it is clear its performance and lifetime. The that satellite orbits should avoid at satellite will be particularly at risk By choosing wanted inclination, cove- least the inner van-Allen belt. when moving through the inner van- rage areas with a very high elevation Allen belt, where it will be exposed to 5.2 Elliptic orbits angle can be realised, wherever it may collisions with particles with an energy be wanted. This is an attractive qua- of several hundred million electron- One result of using satellites in ellipti- lity, particularly for mobile communi- Volts. For satellites injected into geo- cal orbits is the fact that the satellites cation and for communication in the stationary orbit it has been estimated become quasi geostationary around northern areas. The fact that the satel-

57 Figure 8 8 a) The Molniya orbit. The track is shown I in figure 8 a). The view-angles towards the satellite for 8 hours around apogee A are shown in figure 8 b) for different locations S

these, and the problems of particle col- lisions are therefore considerably 2 hours reduced. As for MOLNIY, the inclina- tion is 63.4°. The radial velocity of the satellite is less in the utilised part of the TUN- DRA orbit than in the MOLNIYA orbit (an even number of satellites is Orbital heigth = 39100/1250 km I = Isfjord Radio, Spitzbergen assumed). Thus the TUNDRA orbits Orbital period = 12 hour S = Southern Spain will give fewer problems with doppler inclination = 63,4o A = Near apogee shift than MOLNIYA. The radial velo- city , which causes the doppler shift, 0oN 8 b) will be reduced from 2,600 m/s to 600 m/s with a constellation of three satel- lites. The ground track of a TUNDRA orbit Southern Spain is shown in figure 9, as well as azi- muth and elevation angles for the same three earth station locations as for MOLNIYA. Revolution period of the satellites is 24 hours.If three satel- 270oW 90oE lites are employed, even here, Europe Near apogee 80o will experience satellites also in TUN- 70o DRA orbits with an elevation angle of 60o o at least 50°, for the duration of the 50 elevation angle satellite’s life. For transmitting speech and data the high elevation angle will be particu- Isfjord/Spitzbergen larly attractive for mobile communica- o tion, while the special coverage areas 180 S are interesting for all types of commu- nication. However, the distance out to lites are not over equator makes it pos- the use of both MOLNIYA and TUN- the satellite is comparable to that of sible to realise single hop connections DRA orbits for mobile communication geostationary satellites, and nothing is to areas on the same hemisphere that and sound broadcasting. gained with respect to free space atte- otherwise would be inaccessible. One The MOLNIYA orbit has a perigee nuation. could for instance imagine a direct height of 1,250 km and an apogee connection between Oslo and Alaska Before systems for satellites in ellipti- height of 39,100 km with an inclination with the help of satellites in elliptic cal orbits may be put into operation, of 63.4°. Figure 8 shows the ground orbits. systems based on low earth orbit satel- track (track of sub-satellite point) of a lites will also be available. Therefore, Because of irregularities in the earth’s MOLNIYA orbit where the apogee to what extent elliptical orbits will be gravitational field, an orbit with an point is located at 15° E. Furthermore, used for speech/data transmission is inclination of 63.4° is the only one azimuth and elevation angles for three uncertain, due to the advantage of low which does not lead to a drift of the different locations of earth stations earth orbit satellites with respect to perigee. Satellite in orbits using this are shown. If three satellites in a time delay and free space attenuation. inclination will therefore consume MOLNIYA orbit are used, with an However, in order to realise regio- little fuel for orbital corrections. For angular spacing of 120°, the whole of nal/national systems, elliptical orbits communication in Scandinavia this is a Europe would see the satellite with a may be an alternative to low earth very favourable inclination, and the minimum elevation of 50° (Spitzber- orbits. use of elliptical orbits should therefore gen included) for eight hours around be better suited to Scandinavia than apogee. Because of the revolution many other areas. period being 12 hours, it will pass over 5.3 Low earth orbits the northern hemisphere twice per In particular there are two orbits Low earth orbits have a radius consi- day, separated 180°. Such a system which are most interesting: The MOL- derably smaller than the geostationary could for example be utilised both in NIYA orbit with an revolution period one. Motorola’s planned system IRI- Scandinavia and in Alaska, see figure 8. of 12 hours, and TUNDRA with a 24 DIUM has a distance from the earth of hours revolution period. The LOOPUS A disadvantage of the MOLNIYA approx. 765 km, in contrast to the geo- study in Germany which looked into orbits is the fact that they cross the stationary orbit distance of 36000 km. concepts for mobile communication, van-Allen belts twice per revolution. When the distance to the satellite is concluded with a solution consisting of The TUNDRA orbits with perigee and reduced so drastically, great gains are satellites in MOLNIYA orbits, while apogee heights of 24,500 and 47,100 achieved in the link budgets: The dif- ESA’s ARCHIMEDES study assessed km respectively, is passing outside ference in free space attenuation to a

58 Figure 9 The Tundra orbit. The track is shown 9 a) in figure 9 a). The view-angles towards I the satellite for 8 hours around apogee are shown in figure 9 b) for different A locations S

geostationary satellite and to an IRI- DIUM satellite will from Oslo be approx. 33.5 dB. This means that if you transmit with 0.1 W to an IRI- DIUM satellite, you would have to use 220 W towards a geostationary satel- lite in order to reach the same flux 2 hours density at the satellite.

For speech communication the time Orbital height = 47 100/24 500 km I = Isfjord Radio, Spitzbergen delay of 1/4 second for a geostatio- Orbital period = 24 hours S = Southern Spain nary hop is by many people conside- inclination = 63,4o A = Near apogee red a major reduction in quality. In the low earth orbit concepts this problem 9 b) 0oN is avoided because the distance to the satellite is negligible. The low orbiting height makes launching less expen- Southern Spain sive, and puts fewer requirements on rockets and launching facilities. Since the satellite is so close to earth it will only be able to cover a small area, and only for a short period of a time (in IRIDIUM, where the revolution 270oW 90oE period for the satellite is 100 minutes, Near apogee 80o each satellite will cover an area for 9 70o 60o minutes at the most). In order to 50o elevation angle obtain continual coverage of an area, a large number of satellites is necessary. IRIDIUM is a global system consisting of 77 satellites in 7 polar orbits, see Isfjord/Spitzbergen figure 10. Polar orbits (90° inclination) do not necessarily provide the best 180oS coverage for Norway. A simulation of a system consisting of 77 satellites at a eaten up by the lower antenna gain, enable low transmitting power from distance of 1,000 km shows that the but one is still left with a 10 - 20 dB net the terminal, it is necessary to use inclination giving the highest elevation gain per link in relation to geostatio- several antenna beams in the satellite, in Norway is around 70°, see figure 11. nary satellites. Omnidirectional anten- see figure 13. Such a concept also ena- nas are also necessary for realising bles frequency reuse. One disadvan- Another possible low earth orbit con- handheld terminals. tage is that the payload increases in cept is shown in figure 12a. A circular complexity. orbit is assumed with an orbital height Hence, low earth orbiting satellites are of 1,000 km and an inclination of 63.4°. 7, 19, 37 or more beams may be utili- well suited for systems with a large This orbit will have a revolution period sed in such a cell structure, depending (global) coverage area and for termi- of approx. 1 hour 46 minutes. The on the beam width and the minimum nals with approximately omnidirectio- coverage area for a given position is elevation angle. IRIDIUM uses 37 nal antennas, i.e. mobile terminals. shown with elevation angles of 0°, 10° beams. The terminals will also profit on the and 30°. With 72 satellites in 6 planes, lower power consumption with these areas as far as Spitzbergen will be 5.5 Inter-satellite link orbital constellations. covered with an elevation of minimum Due to the restrictions of weight and 10°, and all “civilised” areas of the The fact that the time delay is avoided, volume of the satellite, the capacity globe with an elevation angle of mini- makes low earth orbit satellites attrac- and the number of beams one can or mum 25°. Figure 12b shows the cove- tive for all applications where this is an wants to realise in one and same satel- rage radius as a function of minimum inconvenience. In order to increase lite, is limited. If one wishes to incre- acceptable elevation angle for orbital the quality of speech communication, ase the performance of a system bey- heights of 500, 1,000 and 1,500 km. and to enter into competition with opti- ond this, one have to use more satelli- cal fibres in long distance telephony, The fact that the satellites move tes. Today, communication via satellite low earth orbit satellites may be a pos- across the sky creates problems and utilises up- and down-link in the same sible way to go. However, it seems like increases the cost for systems based satellite. This limits the advantage of a this application is still a few years into on earth stations with directive anten- multi-satellite constellation considera- the future. nas. For terminals with approximately bly, as each satellite can be regarded , omnidirectional antennas, however, 5.4 Multi-beam antenna isolated from the other satellites in the this is not a problem. Certainly, some system. By establishing links directly of the gain in the link budget in rela- To cover an area on the earth with a between the satellites (ISL), it will be tion to geostationary satellites will be high flux density and at the same time possible to have access to the capacity

59 0o 12 a) elevation 10o 30o

Orbital heigth = 1000 km Orbital period = 106 min Inclination = 63,4o Figure 10 The Iridium System Figure 12 a) Coverage areas for different elevation angles for a given satellite 77 low earth orbiting satellites. Inclination = position. The track of a low earth orbit 90° and orbital height = 765 km (Number of satellites is later reduced to 66) 12 b) Elevation (o) Coverage radius (km) 60 4 000 Troms¿ Trondheim 50 3 000 Orbit heigth Oslo 40 1500 km

2 000 1000 km 30 500 km 77 satellites i 1000 km height 20 1 000

90 80 70 60 Inclination (o)

Figure 11 The elevation angle for 50 % of the time as a o o o o function of inclination, assuming 77 satellites 0 30 60 90 with an orbital height of 1,000 km (ref. Ellen Elevation angle Lystad, NTH, 1992: Orbital Program: CEBAN Figure 12 b) Coverage radius as a function of elevation angles for different orbital heights

(bandwidth, effect and coverage area) the terminals will take care of the up- OBP is a good technique in a mobile for all the satellites in the system, wit- and down-link. According to Motorola, satellite system where multi-beam hout having to be connected via an this system is meant to render a global satellites and ISLs are utilisied. Today extra earth station (double hop). mobile telephone coverage in the last the technique is not implemented in part of the 1990s. The first satellite in any satellite, but several companies As ISLs have no problem with the the system is to be launched in 1994. are considering to include this in atmosphere, such links are usually future satellites. assumed realised at high microwave ESA is working with ISLs in their rese- frequencies (23 or 60 GHz) or in the arch programmes and will probably, in OBP is a relatively new technique, optical area. some of their satellites in the future. even if the idea is well-known (“switch- board in the sky”). Because of the As far as known, ISLs are today not 5.6 On-board processing many advantages that OBP brings implemented in any network of satelli- When using on-board processing along, perhaps the most important tes, but as switching in satellites be- (OBP), all up-links in the satellite are being the great simplification of the comes widespread, it seems natural regenerated and switched in an ordi- earth segment and the effective and that this technique will be used, not nary switch-matrix, before the signals flexible utilisation of the space seg- only for switching between beams are distributed to the various down- ment, it is most probable that OBP, in towards the earth, but also for swit- links. In this way a full separation of some form or other, will be seen ching beams between satellites. up- and down-link is achieved, and implemented in even more space seg- In the IRIDIUM concept ISLs are used functions like bit rate changes and ments in the future. between the 66 low earth orbiting more down-links per up-link etc., can satellites. The satellite that is nearest be implemented in the satellite.

60 θ

El

00 12 10 00 12 10

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

22 21 23 20 9 24 37 8 10 25 19 2 11 36 7 3 26 18 1 12 35 6 4 27 17 5 13 34 16 14 28 33 15 29 32 30 31

Figure 13 Coverage of a multibeam

00 12 10 12 00 antenna (θ = beam width, El = mini- 00 12 10

1 1 1 1 1 1 1 1 1 1 1 1 mum elevation angle in coverage area) 1 1 1 1

5.7 Small satellites Modern geostationary communication Figure 14 Different network concepts satellites usually weigh more than one and a half ton and cost more than produced at approx. NOK 10 mill., and suitable also for other special purpo- NOK 1,000 mill. From the moment the Motorola’s production of the 386 kg ses. National/regional tailor-made sys- decision is made to build a new satel- IRIDIUM satellite is estimated to cost tems, often based on satellites in a lite until it is launched, a time of five to around NOK 100 mill., including non-geostationary orbit, could be reali- ten years typically elapses. Because of launch. sed without any major organisation or the large investment per launch it is large investments. important to minimise the risks of the It is obvious that the transponder capa- launch. This result in using well tested city that may be put into such a small technology in the design. The long satellite is limited (one IRIDIUM satel- time span from planning to launching lite can handle max. 6,400 simultane- 6 Network concepts of a satellite leads to new geostatio- ous two-way telephone channels with In traditional mobile satellite net- nary satellites being launched with the mobile units), but the possibilities works, most of the traffic is between a ten to fifteen years old technology in of implementing new technology and mobile terminal and a terminal (telep- the equipment on board. more sophisticated solutions compen- hone) in the public network. With the sate for some of this. In the low earth market potential that seems to be pre- In order to realise satellites with more orbit concept where each satellite sent for land mobile satellite terminals, up-to-date technology in the payload, covers a small area for a fraction of new systems have to be designed also the time from idea to launch, and the time, the need for capacity is limited, for communication between two risk involved with each launch, must and it is here that the flexibility of mobile units. A considerable amount be considerably reduced. One way of small satellites has its full effect due to of traffic is expected to be of this kind. achieving this is to use small satellites. the inexpensive launch. These may be produced at a fraction There are mainly two network con- of the cost of a large satellite, and The simple and inexpensive produc- cepts which may be used for connec- much faster. A 50 kg satellite can be tion and launch make small satellites ting two mobile terminals:

61 Number Weigt of sat. Orbits Heigth (km) (kg)

IRIDIUM 77 7 polar 765 380

ARIES 48 4 polar 1000 125

GLOBALSTAR 24/48 8 incl. 52o 1390 230

ELLIPSO 24 4 incl. 63o 2900x425 meters of the system are still to be ODYSSEY 12 3 incl. 55o 10 600 1125 decided. The system is planned to be operational in 1998. Project 21 Orbits not decided For 30 years ago the first “real” tele- (INMARSAT) communication satellite - the Telstar - was launched. At that time it represen- Figure 15 Planned satellite systems for mobile telephony ted a tremendous technological advance. It should be noted that this satellite was orbiting in what we today call a “low earth orbit”. The wheel has - Telephony, paging, come to a full circle! localisation and data services

- Weight < 0,5 kg 00 12 10 - Volume < 0.5 litre

- Output power < 1 watt 1 1 1 1 1 1 1 1 - Price < NOK 7000

Figure 16 Preliminary specifications of an INMARSAT-P terminal

- The traditional method is connec- - A more efficient method for this tion in the ground network, as type of communication is to use a shown in figure 14 (bottom). All traf- satellite network, as shown in figure fic from mobile terminals is directed 14 (top). Traffic is switched directly to a gateway station. This station to the other mobile unit, either in establishes further connection the same satellite, or in another through the ordinary telecommuni- satellite via inter-satellite links. Traf- cation network. If it is a mobile-to- fic to terminals in the public net- mobile link, a second gateway-to- work is routed via the most conveni- mobile link must also be established ently located gateway station with in one of the following ways: respect to the terminal in the public network. The method has obvious ⋅ from the same gateway to the advantages by its efficient utilisation same beam of satellite capacity and the minimi- ⋅ from another gateway to the same sation of satellite hops and other satellite, but in another beam connections. At the same time the method requires new satellite types, ⋅ from another gateway to another which may involve a higher risk and satellite. great developing costs for an initial Such a network concept has the period. advantage that large gateway sta- tions can compensate for the small mobile terminals, in order to satisfy 7 “Project 21” the overall requirements to reliabi- During the last years several suggesti- lity. Such a concept may also be rea- ons have emerged for concepts using lised with traditional satellites at a satellites in low earth orbits for mobile low risk. For communication with a communication. A summary of some terminal in the ground network this of the planned systems is shown in concept will work satisfactorily. For figure 15. mobile-to-mobile links the concept will occupy unnecessary capacity INMARSAT is also planning a satellite and double hops are inevitable. In system for hand portable terminals. the worst cases triple hops may This system has the preliminary name occur if the gateway stations, of Project 21 - “a vision for the 21st cen- which there may be many hun- tury”. The main specifications for the dreds, are connected to the public handheld terminal are given in figure network on a too low level in net- 16. A discussion of strategy is cur- works hierarchy. rently taking place, and the main para-

62 Effects of atmosphere on earth-space radio propagation

BY ODD GUTTEBERG

1 Topic The time varying parameters are the height (h) for water vapour and oxy- 621.396.946 additional loss (L ) and the earth sta- gen, see figures 1 and 2. The growing demand for transmission a tion system noise temperature (T). capacity has forced the satellite tele- According to CCIR [4] the equivalent Earth station antennas without trac- communication systems to use fre- heights are; king will also have a decrease in gain quencies above 10 GHz. Systems are (G) due to variations in angle of arri- h = 6 km for oxygen for frequen- now operating at 11/14 GHz, and the o val. cies below 50 GHz utilisation of 20/30 GHz frequency band is being planned. At these fre- The major tropospheric propagation hw = 2.2 - 2.5 km for water vapour in quencies the radio waves will suffer factors that will influence satellite com- the frequency range 10 - 20 degradation through the non-ionised munications are; GHz part of the atmosphere, i.e. the tropos- - absorption due to atmospheric phere. Particularly dominant is the Specific attenuation (dB/km) gases attenuation and depolarisation due to 40 rain and wet snow. In the high-latitude - radio ray bending due to the de- Pressure : 1Atm part of the Nordic countries, additional crease in refractive index with 20 Surface temp : 20oC Water vapour problems arise due to very low eleva- height 10 tion angle to geostationary satellites. concentration :7.5 g/m3 - scattering due to atmospheric turbu- All these factors contribute strongly to lence 5 the design criteria for satellite sys- - attenuation due to rain tems, such as 2 O - depolarisation due to rain and snow 2 - localisation of earth stations 1 - radio noise due to attenuation in - r.f power requirements gases and rain. 0.5 - antenna sizes Basic considerations and reviews are - capacity requirements, etc. 0.2 found in [1-4]. In the following chap- ters measurement of the different and the technical solutions to be used. 0.1 effects will be presented, bearing in Our main goal in designing satellite mind the system planners’ need for 0.05 systems, is that all offered services prediction models. H2O must have the quality and reliability 0.02 the customers want. 3 Atmospheric absorption 0.01 This paper deals with the different Radio wave absorption is due to gase- aspects of slant path propagation prob- ous constituents in the atmosphere, 0.005 lems encountered in high latitude re- primarily water vapour and oxygen. 3 510 20 50 100 200 350 gions, i.e. elevation angles less than Frequency (GHz) In order to calculate the total slant 20 degrees, and frequencies above path attenuation, we need the specific Figure 1 Specific attenuation due to oxygen and water 10 GHz. attenuation (γ), and the equivalent vapour in the atmosphere [1] 2 Introduction to satellite Normally a satellite transmission link is down-link limited. Accordingly, the important equation in designing satel- lite systems is: Leo  C  G 1 1 = EIRP + + + + L + L   o a N down T k B h ≈ o 6 km (1) L ew h ≈ θ w 2.3 km-2.5 km where EIRP = Satellite effective isotropic radiated power G/T = Earth station “figure-of-merit” k = Bolzmann’s konstant R ≈8500 km B = Transmission bandwidth eff Lo = Free space loss L = Additional loss a Figure 2 Parameters for calculating the total slant path atmospheric absorption θ = elevation angle All terms expressed in dB. h = equivalent height of water vapour (hw) and oxygen (ho) Reff = effective earth radius = 8,500 km

63 Total atmospheric attenuation (dB)

4

20 GHz 3

Elevation angle correction (∆θ) Variation in ∆θ 12 GHz 0.5o ±0.05 Calculated, 2 standard atmosphere (ρ=7.5 g/m3, T=15oC) o Elevation angle ± 0.4 correction ∆θ 0.04

Measured, using satellite signal 1 ± 0.3o 0.03

Variation in ∆θ

0.2o ±0.02 0 5 10 15 Elevation angle (deg)

Figure 3 Total atmospheric attenuation as a function of 0.1o ±0.01 elevation angle for a standard atmosphere (15° C tempe- 01o 2o 3o rature, 7.5 g/m3 water vapour density). Measured value Geometric elevation angle (θ) at 3° elevation angle is indicated o Calculated curves using 500 radio soundings at Spitzbergen (78 N) The total attenuation can then be cal- CCIR data for polar air culated; Figure 4 Average ray bending in polar areas γ ⋅ γ ⋅ A = o Leo + w Lew (dB) (2) where the equivalent path length is found from geometric considerations, Elevation (deg) figure 2: = 2h Le 4 Horizon contour Svea 2 θ + 2h + θ o o sin R sin (16.71 E,77.90 N) eff (3)

o Using this approach, the atmospheric 0.2 attenuation for 12 and 20 GHz has been calculated as a function of eleva- 3 Satellite position tion angle, figure 3. A standard atmos- (10oE) phere is assumed, i.e. ground tempe- Geostationary orbit rature of 15° C and water vapour den- sity of 7.5 g/m3.

As can be seen, the attenuation for 12 2 and 20 GHz are less than 1 dB for ele- 180 200 vation angles down to about 3° and Azimuth (deg) 15°, respectively. At very low elevation angles, the atmospheric absorption Figure 5 Horizon contour and geostationary orbit for Svea, Spitzbergen (16,71° E, will become a critical design factor. 77,90° N)

From C/N-measurements of 12 GHz This is quite close to the calculated have been found [5]. The ray bending satellite signals at Spitzbergen (78° N, value, which indicates that the model has been calculated, using the method 3° elevation angle), the atmospheric used for predicting the atmospheric described in ref. [6]. The results are absorption has been estimated. The absorption is reasonably good. shown in figure 4, together with data measurements were performed with from CCIR [4]. two satellites (OTS and EUTELSAT-I, F-2), and two receiving antennas (3 m 4 Angle of arrival As can be seen, there is good correla- and 7.6 m in diameter). At elevation tion between the calculated angle devi- The decrease of refractive index with angles less than 5°, defocusing loss ations and the CCIR data. height, causes bending of radio waves. and the decrease in antenna gain due This means that the apparent elevation to wave front incoherence will be of The normal refraction effect has been angle to a satellite will be greater than importance. For the actual antenna successfully utilised when receiving the geometric elevation angle. At very sizes and elevation angle, these effects the satellite transmissions from low elevation angles, the correction are estimated to 0.7 - 0.9 dB [4]. EUTELSAT-1 (10° E) in Svea at Spitz- will be of the same order of magnitude Taking this into account, the satellite bergen (77.9° N). Figure 5 shows the as the elevation angle. signal measurements showed an local horizon contour, and the geosta- atmospheric absorption of 1.1 - 1.3 dB From 500 radio soundings at Spitzber- tionary orbit. The satellite is approxi- at 3° elevation, figure 3. gen (1960), the refraction profiles mately 0.2° below the local horizon.

64 Antenna gain reduction (dB) 1.0 D/λ=600 (ex. 6m, 30 GHZ)

Precentage of abscissa value is exceeded D/λ=300 99.999 (ex. 7.5m, 12 GHZ) 99.99 99.9 99 Isfjord radio, 1986 Elevation angle: 3.2o 0.5 90

50 D/λ=100 (ex. 1.5m, 20 GHZ) 10

1

0.1 0.01 0.05 0.1 Pointing error (deg) -0.06 -0.04 -0.02 0 0.02 0.04 0.06 Deviation in angle of arrival (deg) D = antenna diameter λ = wavelength Figure 6 Cumulative distribution of angle of arrival measured at Isfjord Radio, 1986. Elevation angle 3.2°, frequency 11.5 GHz, mean ground tempe- Figure 7 Reduction in antenna gain due to variation in angle rature 4° C of arrival

Precentage of time abscissa value is exceeded 100 100 100 4 GHz, 1.7o elevation 4 GHz, 1.7o elevation Isfjord Radio 3.2o elevation 11.8 GHz, 3.2o elevation 11.8 GHz, 3.2o elevation

11.8 GHz 10 10 10 11.6 GHz 11.6 GHz, diversity

July-Aug 1979 1 1 1

July 1982 July-Aug 1980 June 1980 0.1 0.1 0.1 July 1980 Aug 1980 July-Aug 1978

Precentage of time abscissa value is exceeded 0.01 0.01 0.01

Diversity July 1982 0.001 0.001 0.001

0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Scintillation [dB] Scintillation [dB] Scintillation [dB]

Figure 8 Measured scintillation dis- Figure 9 Measured scintillation dis- Figure 10 Measured scintillation distributions tributions 4 and 11.8 GHz. Isfjord tributions 4 and 11.8 GHz. Isfjord 11.8 GHz (1980 and 1982) and diversity Radio, Spitzbergen, 1979 Radio, Spitzbergen, 1978 and 1980 (1982)

65 However, when taking into account The turbulence activity is increasing Due to turbulence activity, the receiv- the ray bending, the satellite will be with rising temperature in the atmos- ed signal will consist of a direct signal seen above the local horizon. phere. One should therefore, to a cer- plus a scattered signal. If we assume tain extent, expect a dependence be- that the scattered field consists of Normally, the refractive index decays tween scintillations and the ground several components with random exponentially with height. Atmosphe- temperature. It has been shown that amplitude and phase, then the resul- ric turbulence will, however, cause such a relationship exists [11]. ting signal distribution could be de- random fluctuations about the average scribed by a Rice-distribution, i.e. a value of the refractive index. This From statistical measurements at constant vector plus a Raleigh distribu- results in random fluctuations in the Spitzbergen (ε = 3.2°, f = 11.8 GHz), ted vector. apparent elevation angle to the satel- the following equation has been esta- lite. This effect is important for earth blished: In the measurements made at Spitz- stations operating at very low eleva- bergen, the scintillations were found A = 0.27 ⋅ T + 6 (dB) (4) tion angle without antenna tracking sci g to follow Rice-distributions with diffe- system. rent values of the power ratio (K) of where the random components to the steady Measurements of fluctuations in angle A = fading level for 99.99 % of a component as given in table 1 [9]. of arrival have been performed at sci month Spitzbergen during summer 1986 [7] According to the theory for turbulent using the telemetry beacon of EUTEL- Tg = monthly mean ground tempe- scatter, the standard deviation of the SAT-I, F-1 (13° E). rature scintillation amplitude is given by [4]: To obtain better accuracy, two 3- metres antennas were used, one poin- ting slightly above and the other Worst Worst summer Average winter below the nominal direction to the day month month satellite. The cumulative distribution of fluctua- tions in elevation angle is shown in K - 10 dB - 13 dB - 20 dB figure 6. As can be seen, the devia- tions in angle of arrival is less than ±0.02° in 95 % of the time. The distri- Table 1 bution is approximately Gaussian. The slow variation due to satellite move- ment was filtered out. July/Aug July/Aug Figure 7 shows the reduction in Measuring period 1979 1978 1980 antenna gain as a function of the poin- ting error. A fluctuation in the angle of arrival of 0.02° reduces the gain 0.8 Frequency (GHz) 11.8 4 4 11.8 dB for a 6 metre antenna at 30 GHz or o 0.2 dB for a 7.5 metre antenna at Elevation angle ( ) 3.2 1.7 3.1 3.2 12 GHz. Antenna diameter (m) 3.0 4.5 4.5 3.0 G(R) 0.923 0.954 0.940 0.923

5 Tropospheric scintillations Table 2 Amplitude scintillations are generated by refractive index fluctuations in the lower part of the troposphere (<1 km). Frequencies 11.8 / 4 11.8 / 4* The fluctuations are caused by high humidity gradients, and temperature Elevations 3.2 / 1.7 3.2 / 3.1 inversion layers. For very low elevation paths to high Theoretical latitude regions with scarce amount of rain, e.g. polar areas, tropospheric scaling factor 0.97 0.55 amplitude scintillations are the domi- nating propagation effect. Average measured Long term measurements of this effect scaling factor 1.11 0.59 have been performed at Spitzbergen both at 4 GHz (1.7° and 3.1° elevation Table 3 angle) [5] ,and 12 GHz (3.2° elevation *There are no simultaneous measurements of 4 and 11.8 GHz at 3° elevation angle) [8, 9]. Examples of measured angle. Since the average temperature for July/August 1978 and 1980 are approxi- cumulative distributions are given in mately the same, the cumulative scintillation distribution for these periods are figures 8, 9 and 10. compared.

66 γ ⋅ b 11 = a R (dB/km) (6) When knowing the cumulative distri- 7  1  12 1 bution of rainfall rate in one point, we σ( f ,θ, D) ~ ( f ) 12 ⋅   ⋅[]GR( ) 2 sinθ  where a and b are parameters depen- are able to calculate the attenuation ding on frequency and temperature distribution by using equation (6) and (5) [1]. Some values for a and b are given figure 11. In Norway, rain intensity where in table 4. data from tipping-bucket gauges are f = frequency (GHz) available in more than 50 places and Since the rain intensity will vary along θ = elevation angle (degrees) for several years, This information has the path, the attenuation A(t) is obtai- been used to identify regions of diffe- G(R) = antenna aperture averaging ned by integrating the specific attenua- rent rainfall rate statistics, see figure factor [4]. tion γ over the total path length l: 12. l For the experiments at Spitzbergen, A(t) = ∫ γdx The described prediction model has we have the technical data as shown in been used to calculate the 12 GHz rain o (7) table 2. attenuation distribution for; where The cumulative distributions of scintil- - Zone A, elevation angle 11.5° γ = f[R(x,t)] lations for the different measuring (Tromsø) periods are shown in figure 8 and 9. Knowledge of the rainfall rate distribu- - Zone A, elevation angle 18° Knowing one distribution, equation tion in one point is generally not (Trondheim) (5) can be used to scale this distribu- enough for calculating the attenuation tion to other frequencies and elevation distribution. To be able to predict the angles. Equivalent path length (km) rain attenuation, we introduce the con- A comparison of the theoretical sca- cept of “equivalent path length”, which 20 ling factors, calculated from equation is defined by; (5), and the average scaling factors A (dB) = γ ⋅ l obtained from the measured distributi- eq ons in figure 8 and 9, are given in table 3. ε o or =13 The measured values are in accor- A( p) 15 ε=16o l = dance with the calculated values. This eq γ ( ) Kirkenes, ε=10o means that the turbulence theory can R( p) (8) be used with good accuracy for scaling where A(p) and R(p) are the attenua- a measured scintillation distribution to tion and rain rate exceeded for p per- other elevation angles or frequencies. centage of time (equal-probability 10 To reduce the fading margin required values). for satellite links operating at very low The equivalent path length is found elevation angle, space diversity may from simultaneous statistical measure- be employed. Site diversity measure- ments of rain attenuation and rainfall ments have been performed at Spitz- 5 o rate. This has been done at 12 GHz Kjeller, ε=22 bergen with a lateral separation of both at Kjeller [10], and Kirkenes 1150 metres [9]. The single site and [11], with elevation angles 22° and joint distributions are shown in figure 10°, respectively. The measured equi- 10. As can be seen, a substantial valent path lengths are presented in improvement is achieved. At a 3.2 dB figure 11. Linear interpolation has fading level, the availability improves 2 10 100 been used for calculating the equiva- from 99 % to 99.9 %, due to site diver- Rain rate (mm/hour) lent lengths for intermediate values of sity. The diversity gain is 2 dB at 0.1 % the elevation angle. Figure 11 Equivalent path length through rain of time. For smaller percentage of time, even larger diversity gain is expected.

6 Rain attenuation Frequency Horisontal pol Vertical pol (GHz) Rainfall plays a major role in satellite communication, especially for systems abab operating above 10 GHz. A radio wave propagating through rain will be atte- 12 0.0188 1.217 0.0168 1.200 nuated due to absorption, and scatte- 15 0.0367 1.154 0.0335 1.128 ring of energy by the water drops. The attenuation in dB per km (γ) is related 20 0.0751 1.099 0.0691 1.065 to the rainfall rate (R) in mm/h. For 30 0.187 1.021 0.167 1.000 practical applications this relationship can be written; Table 4

67 Kirkenes

A Troms¿ - Zone C, elevation angle 21° (Bergen).

The predicted and measured distribu- tion are compared in figure 13 and 14. As can be seen, the deviations are Bod¿ quite small, and within what is expec- Rain climatic zones ted due to year-to-year variations of the rainfall rate. Zone A B C As the equivalent path length is inde- B % pendent of frequency, attenuation dis- tributions for other frequencies can be 1 2 3 7 predicted. Figure 15 shows the calcu- 0.1 7 12 20 lated cumulative rain attenuation dis- Trondheim 0.05 11 16 25 tribution for 20 GHz in Zone A, eleva- tion angle 20°. The predicted attenua- 0.01 22 30 36 tion is compared with attenuation results from the Helsinki 20 GHz radi- B Rainfall intensity exceeded (mm/h) A ometer [12]. Helsinki has approxima- tely the same rain intensity climate as C zone A. There is a reasonable agree- ment between the distributions. Oslo BergenOGu 6 The good correlation obtained be- tween the calculated and measured rain attenuation distributions should demonstrate the usefulness of the de- Stavanger B scribed prediction procedure.

Figure 12 Rain climatic zones for Norway

Precentage of time abscissa value is exceeded Precentage of time abscissa value is exceeded 1 1

Kirkenes (May-Oct 79)

0.1 0.1 Calculated (Zone A, ε=11.5o) Bergen (May-Sept 79)

Trondheim (May-July 79)

Troms¿ (May-Aug 82) 0.01 0.01 Kirkenes Calculated (May-Oct 80) (Zone A, ε=18o)

Calculated (Zone C, ε=21o)

0.001 0.001 02468101214 02468101214 Attenuation (dB) Attenuation (dB)

Figure 13 Cumulative distributions of rain attenuation measured in Figure 14 Cumulative distributions of rain attenuation measured in Kirkenes (ε = 10.5°) and Tromsø (ε = 11.5°). Calculated Trondheim (ε = 18°) and Bergen (ε = 21°). Calculated dis- distribution for zone A (ε = 11.5°) tributions for zone A (ε = 18°) and zone C (ε = 21°)

68 Precentage of time abscissa value is exceeded Reduction in earth station G/T 8

1 T =25oK 7 r

6 Calculated Zone A ε=20o f=20 GHz 5 o 0.1 Tr=75 K

4 o Tr=125 K

Helsinki radiometer (1985-87) 3 ε = 20o

0.01 f = 20 GHz 2

1

0246810121416 0 12345678 Attenuation (dB) Down link absorptive rain attenuation (dB) Figure 15 Cumulative distribution of 20 GHz attenuation measured Figure 16 Reduction in earth station G/T as a function of the down with radiometer in Helsinki (zone A, e = 20°). Calculated link attenuation. Tr = receiver noise temperature. Clear sky distribution for zone A (e = 20°) including 0.8 dB gaseous noise temperature of antenna Tao = 25° K absorption

The degradation of the down-link car- 7 Noise Temperature rier to thermal noise ratio (dB) will be 9 Snowfall Attenuation Any absorbing medium, such as at- the sum of the reduction in G/T (dB), Dry snow has little effect on frequen- mospheric gases and rain, will in addi- and the down-link attenuation (dB). cies below 30 GHz. Sleet or wet snow tion to attenuating a radio signal pro- can, however, cause larger attenuation duce thermal noise power radiation. than the equivalent rainfall rate [10]. This noise emission is directly related 8 Cross polarisation Along the North Atlantic coast of Nor- to the intensity of absorption. If we To increase the channel capacity, way, the intensity of wet snowfall is consider the atmosphere as an absorb- orthogonal polarisations can be known to be high. Measurements at ing medium with an effective tempera- employed. However, due to atmosphe- 12 GHz in Tromsø (70° N) showed ture of T and a loss factor of L, the ric effects, there will be a transfer of m that the “winter attenuation” was contribution to the earth station anten- energy from one polarisation to anoth- much larger than “summer attenua- na noise temperature T is given by; er. This will cause interference in dual a tion” for the year 1982. Attenuation up polarised satellite links. Depolarisa- T = (1 - 1/L)T (9) to 15 dB due to sleet were experienced a m tions are mainly caused by rain and [11]. snow along the path. Tm varies between 260 - 280° K depen- ding on the atmospheric conditions. Cross polarisation discrimination 10 Conclusions (XPD) has been measured at Kjeller Prediction models for gaseous absorp- An increase in antenna noise tempera- and Spitzbergen. Figure 17 shows tion, tropospheric scintillations and ture will give a corresponding decre- equi-probability plots of the XPD, and rainfall attenuation have been estab- ase in the earth station figure of merit the copolar attenuation [8-10]. The lished. The described methods have (G/T). As the clear sky system noise depolarisations measured at Kjeller successfully been used when design- temperature decreases due to better are caused by rainfall, whereas the ing commercial 11/14 GHz satellite performance of low noise front-ends depolarisations measured at Isfjord systems in Norway, including Spitz- and antennas, the decrease in G/T and Spitzbergen, are believed to be bergen. could be larger than the attenuation due to sleet, snow and/or atmospheric itself. To a certain extent, these models can turbulence. be utilised for planning of satellite sys- An example of the degradation in G/T tems in the 20/30 GHz band. Through as a function of down-link absorptive Due to the short measuring period at the Norwegian participation in the pro- rain attenuation is given in figure 16. Kjeller (two summer months), there is pagation experiments with the OLYM- a discrepancy between the predicted The following values are assumed: PUS satellite, more reliable data of the XPD (CCIR), which is based on long- atmospheric influence on satellite - Gaseous absorption: 0.2 dB term statistics, and the measured links will be established. XPD. - Antenna ground interception factor: As the attenuation increases, different 3.5 % (NORSAT-B antennas) Snow and atmospheric turbulence techniques have to be used in order to - Effective absorbing medium tempe- appear to be a less polarising medium keep the fading margins to a mini- rature: 280° K. than rain. mum. These techniques include site diversity, power control, spot beams in This corresponds to a clear sky the satellite and possibly adaptive allo- antenna noise temperature of 25° K. cation of system capacity.

69 11 References Crosspolarisation (dB) 1 Ippolito, L J. Radio wave Propaga- 32 tion in Satellite Communications, Kjeller, June - July 1980 Van Nostrand Reinhold Co. Inc, 30 New York, 1986. Isfjord Radio, April - Aug 1979 28 2 Davies, P G, Norbury, J R. Review of slant path propagation mecha- 26 nism and their relevance to system performance, IEE Proceedings, Vol 24 130, Part F, No 7, Dec 1983. XPD=27.3 - 7.6 log A 22 3 Davies, P G, Norbury, J R. Review of propagation characteristics and 20 prediction for satellite links at fre- quencies of 10-40 GHz, IEE Procee- 18 dings, Vol 133, Part F, No 4, July 1986. 16 XPD=32 - 20 log A 4 CCIR Recommendations and 14 (CCIR) reports, Vol V. Propagation in non- ionized media, ITU, Geneva 1986. 12

5 Osen, O. Propagation Effects in 10 High Latitudes, Symphonie Sympo- 1 2 3 4 5 6 7 8 910 20 sium, Berlin, 1980. Attenation (dB)

6 Schulkin, M: Average Radio-Ray Figure 17 Crosspolarisation discrimination (XPD) as a function of co-polar attenu- Refraction in the Lower Atmos- ation (A) phere, Proceedings IRE, May 1952, 554-561.

7 Erring, B. Variasjoner i innfallsvin- kel for satelittkommunikasjon ved lave elevasjonsvinkler, Hovedopp- gave, NTH, Trondheim 1987.

8 Gutteberg, O. Measurements of tropospheric fading and crosspola- rization in the Arctic using Orbital Test Satellite, IEE Conference Publication 1, No 195, Part 2, 71-75, IEE 2nd International Conference on Antennas and Propagation, York 1981.

9 Gutteberg, O. Measurements of atmospheric effects on satellite links at very low elevation angle, AGARD Conference Proceedings, No 346, 5-1 to 5-12, 1983.

10 Gutteberg, O. Eksperimenter med OTS-satellitten, Telektronikk 4/1981.

11 Gutteberg, O. Low elevation propa- gation in high latitude regions, TF report No 7/83, Norwegian Tele- com Research, 1983.

12 Kokkila T. Private communications

70 The use of millimetre waves in satellite communications

BY TORD FREDRIKSEN

Introduction transmitter and receiver. In satellite Obviously, frequencies near the atte- 621.396.946 communications the medium along nuation peaks should not be used in This paper presents the most impor- most of the path is close to free space, earth-space communications. How- tant issues concerning the use of milli- which is non-discriminative against ever, frequencies in the absorption metre waves in satellite communicati- any particular frequency and therefore band around 60 GHz can be used in ons. Present satellite communication very easy to describe. Matters are inter-satellite links. This would enti- uses microwave frequencies in the C more complicated along the part of the rely eliminate any interference prob- and Ku bands, that is the frequencies wave path that is in the earth’s atmos- lems with terrestrial systems. A pos- 4/6 GHz and 11/14 GHz. The reason phere, as we shall see. sible use is in links between earth sen- for using these frequencies is that sing satellites and relay satellites for they are not reflected in the atmos- Keeping matters simple we will in the downloading data to the earth. phere like lower frequencies as VHF. following only deal with the most 4/6 GHz was the first band to be used important propagation variable, name- 1.2 Attenuation by hydro- and as it has limited capacity, systems ly attenuation. meteors operating at 11/14 GHz were As the electromagnetic wave (i.e. the deployed. If the volume of satellite car- Rain is the major attenuation pheno- signal) travels through the atmos- ried traffic continues to increase over mena and its effect increases with fre- phere, it is attenuated by gaseous the next years (as is likely), we may quency. absorption and absorption and scatte- find ourselves in the situation where ring in water in different forms: Figure 2 shows attenuation increasing all these bands are completely filled. clouds, fog, snow, hail, rain and ice. with frequency and with rain rate. This is the reason why higher frequen- This constitutes a frequency discrimi- (Typical rain rates for Norway are cies are considered. Millimetre waves nating and time varying channel in given in (1), fig 12.) have wavelengths shorter than one which the amount and distribution of For a system to stay in service, i.e. centimetre, thus the frequency is over water is the important variable. The maintain acceptable C/N at a given 30 GHz. The bands of interest are exact effects of atmospheric disturban- rain rate, it has to be designed with a 20/30 GHz (even though the fre- ces are not completely known, but link margin greater than the corres- quency is too low in a strict sense) and today, attenuation phenomena are ponding fade depth. One fade level up to approximately 60 GHz (higher modelled with some accuracy. distribution is given in (2), figure 3 for frequencies are not of practical inte- In designing an earth-space communi- 11.6 GHz and 50 GHz (at Lario, Italy). rest yet). Experiments are currently cation system, estimation of expected Note the extreme fade levels at 50 carried out at 20/30 and 40/50 GHz in propagation conditions is of prime Europe to study the behaviour of milli- importance. Part of the specification of metre waves in earth-space communi- a communication system is the maxi- Specific attenuation (dB/km) cation. mum allowed outage time, i.e. portion 100 The frequencies up to ca 300 GHz of time that the link is not functioning. have been assigned to different servi- Dependable propagation data are the 50 ces by ITU (International Telecommu- key to give reliable figures on this nications Union, a body in the United outage time. Nations system), and whereas the C 20 and Ku band offer bandwidths of 1 1.1 Absorption in gases GHz each, the bands at 20/30 and 10 upwards offer several GHz each. This Gaseous absorption is mainly due to 5 means that bandwidth limitation does oxygen and water vapour. As shown in O not have to be a critical design factor. figure 1, attenuation generally increa- 2 ses with frequency and exhibits peaks 2 First part of this article will discuss the at resonance frequencies of the O and use of millimetre waves in terms of 1) 2 H O molecules. Below 10 GHz, the 1 the problem of absorption in the 2 specific attenuation is relatively con- atmosphere, 2) the advantages of stant. Above 10 GHz, one must avoid 0.5 small wavelengths and 3) the techno- the peaks and thus use the “frequency logical challenges. Section 4 briefly windows” between 22.2, 60 and 118 GHz. describes ways of overcoming the 0.2 obstacle of attenuation. Finally, in sec- The O2 absorption is relatively con- tion 5, an outline of a proposed mobile stant over time, depending only on air 0.1 satellite communication system based pressure variations. The H2O absorp- H O on millimetre waves is given. The pro- tion, however, is time-varying in pro- 0.05 2 posal abandons the familiar idea of the portion with changes in water vapour geostationary satellite. density. This variation itself applies 0.02 equally to all frequencies and is there- 1 Propagation fore no issue in this context. However, the attenuation levels are higher at 110102 The obvious basis of all radio commu- higher frequencies. This is an impor- Frequency (GHz) nication is that electromagnetic waves tant limiting phenomena in using milli- propagate through the space between metre waves. Figure 1 Specific attenuation due to atmospheric gases

71 GHz, reaching 100 dB for a small frac- obtained by changing the frequency Specific attenuation (dB/km) -4 tion (10 ) of time. from f1 to f2. (It is of course closely 100 related to antenna gain and path loss.) For instance, to have an availability Realising this gain can be done by 150 mm/h greater than 99.9 % at this site (outage 50 time less than 0.1 % or 9 hours in one 100 mm/h i) reducing antenna area while main- year), one needs a minimum margin of 50 mm/h taining EIRP 20 3 dB for an 11.6 GHz link and mini- 25 mm/h mum 22 dB for a 50 GHz link. The lat- ii) reducing power consumption while ter is clearly not feasible due to un- maintaining EIRP 10 acceptable increase in required EIRP iii)increasing EIRP and thus impro- and/or receiver G/T (figure of merit). 5 mm/h ving the fade margin. 5 As apparent from figure 3, (2), the required excess margin (horizontal 2 1.25 mm/h distance between the two curves) i) and ii) give the advantages of smal- increases dramatically with increased ler and less power consuming earth availability demands. stations and satellites, thereby redu- 1 cing launch cost or facilitating more 0.25 mm/h This somewhat frightening characte- complex satellites. Earth stations can ristic of using higher frequencies can 0.5 become mobile. ii) generally implies be dealt with in two ways: narrower beams, thereby facilitating 1 Accepting higher outage rates frequency reuse. 0.2 2 Applying methods such as site As we have seen in section 1, this does diversity, frequency diversity, adap- 0.1 not come without a penalty in the form tive coding or adaptive power con- of more and deeper fade time. The trol at the cost of station complexity 0.05 penalty balances the advantage at and mobility. some fade intensity probability. Refer- ring to figure 3, we have a frequency 0.02 We will come back to this in sections 4 increase advantage of and 5. 0.01 Gf = 20 log(50/11.7) = 12.7 dB 1 2 5 10 20 50 100 200 500 1000 2 The principal advantage Frequency (GHz) of high frequency: The point at which the horizontal dis- tance between the 50 and 11.7 GHz Figure 2 Specific attenuation due to rain “frequency gain” curves equals this value is at approxi- High frequency allows narrow beams, mately P(out) = 3 . 10-3. If we accept i.e. concentrating the power in small outage probabilities higher than this areas. Given a transmitter-receiver we have a net gain/advantage in incre- radio system, Friis equation of recei- asing the frequency. Conversely, if we Probability ved power (in free space) is demand lower outage probabilities we 2 have to compensate by increased link  1  P = P A A margin or shared resource techni- -2 "low availability" r t t r  λ  10 R ques. This balance points divides two ≈ . -3 sets of systems: P(out)limit 3 10 saying that decreased wavelength ∆A=12.7 dB increases received power, other para- - low availability systems meters held constant. Pr and Pt are 10-3 - high availability systems. "high availability" received and transmitted power, Ar and At are the effective antenna areas, l is wavelength and R is the distance. High availability systems will gene- Comparing the received power at two rally be integrated in the terrestrial

-4 different frequencies, other parame- communications network and there- 10 ters held constant: fore under strict requirements regar- 2 ding quality of service. Such systems 50 GHz 1 2 11.6 GHz P ( λ )  f  are also known as network oriented r1 = 1 =  1  systems. Low availability systems will 2   Pr2 1 f 2 not serve as an integral part of the ter- 10-5 ( λ ) 20 40 60 80 100 120 2 restrial system, they will therefore be inherently user oriented. It is likely Attenuation (dB) This can be viewed as a gain that users will own and operate their f 20log 2 own earth stations. Interfacing with the earth net might well be possible Figure 3 Cumulative attenuation statistics at 11.6 GHz f1 (dB) and 50 GHz, Lario, Italy. Note the extreme attenuation anyway. values at 50 GHz

72 3 Technology 4 Fade combatting A third technique is power control, in which emitted power from the satellite The use of millimetre wave compo- techniques is adjusted according to propagation nents requires elaborate HW develop- This section discusses methods to conditions. A sensible way of using ment. Such components are not in counteract the deep fades likely at this would be in a satellite serving a large scale commercial use (at pre- high frequencies. The goal is to main- large region (Europe) by many narrow sent). This means that most parts have tain high availability despite the fades. beams. The output power margin to be custom built. These techniques are most likely to be could be continuously assigned to the An inherent property of all common used in network oriented systems. worst-off beams. The excess power transistor technologies is that noise The following presents four such tech- that can be made available in the satel- factor rises and gain falls with increa- niques. lite is of course limited, say to 10 dB in sed frequency. This has a negative one beam. Consequently, this is proba- impact on receiver G/T. Silicon transi- Site diversity is a technique where two bly not a good method at high fre- stors are for instance not applicable at or more earth stations are used to quencies (see again figures 2 and 3), millimetre wave frequencies. GaAs receive the signal. The distance be- but maybe it is feasible at 20/30 GHz. transistors are better. In particular, tween the stations can be for instance Again, considerable overhead has to AlGaAs HEMTs (High Electron Mobi- 15 km. This technique requires as a be added in terms of link monitoring lity Transistors) have been produced minimum a set of duplicate stations and shared resource administration. with very promising characteristics. with an earth connection between Adaptive coding is another shared Figures like NF = 0.8 dB and gain = them and equipment for co-ordination resource technique. The backup 8.7 dB at 63 GHz have been reported. of the stations. The simplest imple- resource is time or frequency which is At present, different experiments are mentation is just choosing the strong- allotted to the bad-off earth station in conducted (in the US and Japan) to est signal. More sophisticated signal the form of extended time frames in obtain high quality devices at high fre- processing techniques make use of time division systems or extra band- quencies. the correlation of the signals to in- width in frequency division systems. crease the legibility. In the limiting Thus this technology is somewhat The extra capacity is put to use by case of statistically independent sta- immature, and one cannot expect increased redundancy (excessive) tions, the probability of simultaneous large-scale production with high yield coding. Implementation in FDMA sys- outage is P = P . P , P and P in the near future. Large-scale produc- syst 1 2 1 2 tems would be difficult because of being the probabilities of outage at sta- tion of completely integrated millime- required shifts of carrier frequencies, tions 1 and 2, respectively. This is a tre wave circuits (wave guides, filters, whereas TDMA systems would major improvement, consider for mixers, amplifiers on one substrate) is require simple time slot shifts. instance that P = 0.01 % required necessary to produce cheap user ter- syst means P = 0.0001 = >P = P = 0.01 = 1%. Except for site diversity, the four tech- minals. syst 1 2 However, statistical independence is a niques discussed here all utilise some Another challenge is increased accu- strong assumption and is seldom justi- system-common resource to relieve racy on all dimensions. Acceptable fied. Depending on the geographical the areas experiencing unpleasant mil- dimension error is a function of wave- separation, the assumption might hold limetre wave propagation conditions. length, and as this is lowered, accu- in the case of high intensity rain, i.e. As such they are based on systems racy/precision must be increased. deep fades, because this normally consisting of many large stations. This applies in particular to antenna occurs in showers of limited geograp- They add considerable complexity to production. hical extension. In the case of low the system. The possible benefits intensity rain it is not likely to hold must be subject to further study to A third problem is the DC-to-RF power because this type of rain normally quantify improvement or gain. If these conversion. This is generally more effi- covers a large region. These depen- techniques can be applied success- cient at low frequencies. Other losses dencies have to be studied further to fully, it will be possible to design the (e.g. in mixers) also increase with fre- achieve data for system design. (normally operating) millimetre wave quency. One can compensate for this part of the system as a low-availability by having larger solar arrays, but this Another technique is frequency diver- system, i.e. with outage times in the is of course not an attractive solution sity, where a complete backup trans- order of 1 %. because of higher launch mass. mitter and receiver exist at a lower fre- quency. This presupposes a large So, it must be clear that realising the number of stations so that the capacity 5 A low availability possible merits of millimetre wave sys- of the backup system is small compa- system for mobile tems is not straightforward. One red to the overall capacity. (Or else it might risk that higher losses and would be better only to use the communications lower component efficiency cancels backup.) Again, statistically indepen- This section will briefly describe a mil- out the nice features and that one is dent stations with large spacing is limetre wave satellite system for left with a net disadvantage in applying desirable. Satellite complexity will mobile communications proposed by higher frequencies. Keeping this in increase due to the need for steerable Valdoni et al. (3). This is an example mind and putting effort into solving antennas (for the backup) and due to of a low-availability system that these problems, one should be able to the inclusion of a fade monitoring and exploits the nice features of high fre- construct systems with lower power capacity assignment system as well as quency at the cost of worsened propa- consumption than today’s have. the backup system itself. gation conditions.

73 Operating at 40/45 GHz it will offer by electronically steerable antennas. References full duplex 64 kbit/s (ISDN 2B+D) Second, Doppler shift can be in the with bit error rate 10-6 and 99 % availa- order of several hundred kHz. This is 1 Gutteberg, O. Effects of atmos- bility. Earth station antennas will be mainly caused by satellite, not earth phere on earth-space radio propa- phased arrays of approximate size station, movement and has to be com- gation, Telektronikk, 4/92, 63-70. 22 x 22 cm (Rx) and 6 x 6 cm (Tx), i.e. pensated in the satellite or earth sta- suitable for integration in cars. The tion. All this means that both the satel- 2 Carassa, F. Application of millime- idea is to supplement terrestrial cellu- lite and the earth stations increase in ter waves to satellite systems, Alta lar systems by offering 1) good cove- complexity. This has also been the Frequenza, vol LVIII N.5-6, Sept- rage in suburban and rural areas and case with terrestrial cellular systems. Dec 1989. 2) higher bit rates. The main advan- Today’s mobile stations and base sta- tage of a satellite mobile system over a tions have very advanced network 3 Valdoni, F et al. A new millimetre terrestrial one is that once launched, it access procedures and signal proces- wave satellite system for land serves the complete planned coverage sing. In moving to mobile satellite sys- mobile communications, European area, including scarcely populated tems, one must expect the complica- Transactions on Telecommunica- areas. The terrestrial system, on the tion level to increase further with func- tions and Related Technologies, other hand, is built up gradually, one tions like electronic antenna tracking, ETT vol I-N.5, Sept-Oct 1990. cell at a time, and it is not economical dynamic beam shaping, new handover to deploy stations in many rural areas. procedures, dynamic “bandwidth” allo- 4 Hovstad, P, Gutteberg, O. Future cation and frequency shift compensa- systems for mobile satellite com- A good space segment would consist tion. munications, Telektronikk, 4/92, of three satellites in highly inclined 54-62. orbits. Because millimetre waves are Conclusion seriously impeded/stopped by obsta- The shift upwards in frequency is cles in the path, the elevation angle necessitated by the congestion of should be as high as possible, in order bands in present use. But, into the bar- to reduce the probability of obstacles gain, millimetre waves have the merit in the path. Also, high antenna gain of narrow beams and/or small anten- eliminates multipath propagation and nas. On the other hand, they experi- thus the receiver depends only on the ence difficult propagation conditions direct path. Ideally then, the satellite and the component technology is in its should be in zenith. A highly inclined adolescence. This may favour systems elliptical orbit like the Molniya orbit completely different from today’s, for ((4), figures 7 and 8) gives a quasi sta- instance mobile satellite systems in tionary satellite near the apogeum for inclined low earth orbits or elliptical 8 hours per revolution of 12 hours. orbits. As with all technology changes, Using three satellites properly phased it is not just a simple case of “interpola- in three orbits give continuous cove- ting” on, or “adjusting”, contemporary rage from near zenith elevations (>60 systems. We have to expect or even deg.) all over Europe. I repeat here pave the way for completely new sys- the orbital parameters: tems. T = 12 hours inclination = 63.4 degrees Hmax/Hmin = 39,100/1,250 km H handover = 23,500 km visibility = 8 hours.

This will give very high elevation angles, reducing the blocking prob- lems significantly. Besides, attenua- tion in rain and gases, which is propor- tional to the path length through the atmosphere, is also reduced when elevation increases. But this orbital constellation entails new difficulties compared to geostatio- nary systems. First, there is the zoo- ming effects: As the distance to the satellite changes, cell size (coverage area per beam) changes with a factor of 1.7 from handover to apogeum. This might necessitate dynamic beam sha- ping in the satellite, which is possible

74 The effect of interference for an OBP system utilising the geostationary orbit

BY PER HOVSTAD

621.396.946 Abstract 629.78 A method is described to calculate the required increase in earth station antenna diameter for a system based on high-gain satel- lite receive spots and low uplink EIRPs in an environment with interference from other satellite systems. The system is compar- able to other systems on the downlink, and interference is therefore assumed to affect primarily the uplink. Correspondingly, cal- culations are performed only for the uplink. It is shown that in an environment with interference, earth station design will be determined by the need to suppress this interfe- rence, by proper transponder planning and choice of position in the geostationary arc. However, it can be seen that the effects of interference can be greatly reduced.

tions, choice of frequency) is com- Required OBP earth 1 = 1 + 1 pared to the interference free station characteristics in C / N + I C / N C / I (1) case, and gives a measure for the impact of interference. an environment with Where I is the sum of all interfe- interference ring sources: I = Σ I (2) The ESA OBP (On-Board Processing) i i 1 OBP quality system is designed to allow direct 2) Establish the characteristics of the communication between small, inex- various interfering sources. requirements pensive earth stations in a mesh type The quality requirements aimed to 3) Establish the ground and space network. In order to achieve this goal, meet are those given in CCIR Rec. segment characteristics as well as ESA intends to use a payload with 614: wanted signal characteristics. multiple high gain spot beams comb- BER (3) ined with a regenerative digital base- 4) Determine C/N as a function of tot band switch. This last feature in order EIRP and G/T for uplink in the > 1 ⋅ 10-7 for less than 10 % to obtain a large coverage area case with no interfering sources. of any month (Europe) in a flexible way. 5) Required uplink EIRP and down- > 1 ⋅ 10-6 for less than 2 % Terminal data rates will be 64 kbit/s - link G/T to meet the quality speci- of any month 2 Mbit/s with access on a multi fre- fications are found for the interfe- > 1 ⋅ 10-3 for less than 0.2 % quency TDMA system. 2 Mbit/s on rence free case. Assuming HPA of any month the uplink, and 33 Mbit/s on the power, receiver noise figure and downlink. antenna characteristics, this is con- In link budget calculations, the up- and verted to antenna diameter. Because of the high satellite G/T, and downlinks are independent of each Assuming that the same antenna is the low earth station EIRP, the system other because of the regenerative used in both directions, the largest is believed to be sensitive to interfe- nature of the payload. In [2], A10-12, it diameter will apply. rence from other systems on the is shown that with a 0.3 dB link mar- uplink. This paper describes a simpli- 6) The critical rain attenuation is gin, each link can be designed inde- fied method to determine the effect of found as the case where the qua- pendently to meet the quality require- interference upon the earth station lity requirements lead to the lar- ments, still meeting the overall perfor- design of an OBP system operating in gest terminal antennas. This value mance objectives. the FSS Ku-band (up 14.0 - 14.5 GHz, is used in the following calcula- With the modem and multi-carrier down 12.5 - 12.75 GHz). tions. demodulator characteristics from [2], The procedure to determine the influ- 7) Assumptions on interfering sys- Table A10.2-1, CCIR Rec. 614 can be ence that interference will have on ter- tems, their orbital position and rewritten: minal design is as follows: spectral power distributions are (E /N ) (4) 1) Specify the quality requirements made. b 0 up for the system, given by the requi- < 14.0 dB for less than 10 % 8) Uplink C/I as a function of assum- red C/N+I. Interference is assum- of any month ed interference characteristics and ed uncorrelated to wanted signal, terminal EIRP is calculated. < 13.2 dB for less than 2 % but not necessarily with uniform of any month spectral power distribution. It is 9) Up-link C/N + I is calculated from assumed to affect the uplink much (1) as a function of terminal EIRP < 9.0 dB for less than 0.2 % more than the downlink, since and satellite G/T. of any month while the downlink has power flux 10) Necessary uplink C/N + I from the (E /N ) densities comparable to that of b 0 down quality requirements then determi- other systems, the uplink is cha- < 13.8 dB for less than 10 % nes terminal EIRP, and thus racterised by an unusually high of any month antenna diameter, when assuming gain satellite antenna and corre- HPA power and receiver noise < 12.9 dB for less than 2 % spondingly low uplink EIRPs. figure unchanged. of any month Interference calculations are there- fore performed only for the uplink. 11) Antenna diameter for various inter- < 8.1 dB for less than 0.2 % C/N + I is given from the equation: ference scenarios (orbital posi- of any month

75 (EIRP) + G dBWK-1 GS ( ) T SAT 70

Zone 4 14.5 GHz, Uncoded QPSK 2.176 Mbits-1

65 The necessary 0.3 link margin on each of the coding gain (assuming trellis 8- link is not included. psk), BER = 1 ⋅ 10-3 becomes the most critical on the downlink. The corre- Treating the OBP channel interfe- sponding C/N is 15.4 and 10.3 dB on rence and noise sources as “equiva- the up- and downlink, respectively. lent” gaussin noise sources, together The chosen design criteria are ([2]): -7 with the assumption that usable band- 60 10 width is 1.2 times the symbol rate ([2], [EIRP + (G/T) ] (7) A10-6), leads to: gs sat = 56.8 dBWK-1 10-6 (C/N+I)up (5)

< 16.2 dB for less than 10 % [EIRPsat + (G/T)gs(clear sky)] of any month -1 55 10-3 = 65.8 dBWK < 15.4 dB for less than 2 % of any month 2 C/N and C/I calculations < 11.2 dB for less than 0.2 % On a linear form, (6) can be rewritten: Clear Sky 10-3 of any month (C/N+I) (C / N) (θ ,θ ) 50 down x t r < 16.0 dB for less than 10 % EIRP (θ )⋅(G / T) (θ ) = x t x r of any month ⋅ ⋅ ⋅ ⋅ ⋅ 0.01 0.10.2 1.0 2.0 10.0 Lx Attatmx k BxOBP Marginx Attrainx < 15.1 dB for less than 2 % % of Worst Month (8) of any month Similarly C/I can be written: Figure 1 ([1] figure 3.3.6.1-3) < 10.3 dB for less than 0.2 % Required uplink (EIRP)gs + (G/T)sat to meet a specific of any month BER for a given percentage of time (worst month) ( ) (θ θ ) C / I up t , r In an interference free environment, EIRP (θ )⋅(G / T) (θ )⋅T C/N+I equals C/N, and is given by = up t sat r sat G -1 ⋅ ⋅ (EIRP) + dBWK Iup Marginup Attrainup SAT ( T ) GS(Clear Sky) (C/N)x = [EIRPx + (G/T)x] - Lx (6) = θ ⋅ 75 EIRPup ( t ) Bup - Att - 10log(kB ) atmx x (9) Zone 4 - Marginx - Attrainx 12.7 GHz, Uncoded QPSK where I is given by: -1 32.768 Mbits Where System Noise Figure = 3dB 10-7 x I = ∑ I = ∑ I + ∑ I 70 -1 - denotes either up or down i j k Mbits i j k EIRP - Transmit station Effective -6 EIRP (θ )⋅ DEG ⋅ B ⋅G 10 Isotropic Radiated Power = upj Is upj upOBP rscj Mbits-1 I j (dBw) BupIj (G/T) - Receive station, figure of EIRP (0û)⋅ DEG ⋅ B I = upk upk upOBP 10-3 merit (dB/K) k ⋅ 65 BupIk XPDupk Mbits-1 L - Free space attenuation (10) (dB) In a linear scale, the parameters are as Att - Atmospheric absorption atm follows: (dB) EIRP 60 k - Bolzmann’s konstant x - effective power transmitted towards 1.38 ⋅ 10-23 (J/K) the receiver. On uplink this is assu- B - Bandwidth (Hz) med to be boresight EIRP, on down- link actual EIRP, depending on earth Att - Rain attenuation (dB) rain station position in downlink beam

In [2], the effect of rain attenuation is G/Tx 55 -3 Clear Sky 10 discussed, and the required [EIRPx + - receive station figure of merit (G/T)x (clear sky)] to achieve a specific (linear). On uplink this is assumed to 0.01 0.10.2 1.0 2.0 10.0 BER in this interference free environ- be actual G/T, depending on earth % of Worst Month ment is given as a percentage of time, station position in uplink beam, on figures 1 and 2. It can be noted that in downlink boresight G/T is assumed Figure 2 ([1] figure 3.3.6.1-4) this particular case, the BER = 1 ⋅ 10-6 is the most critical one for uplink and Required downlink (ERP)sat + (G/T)gs(clear sky) to meet a specific BER for a given percentage of time (worst month) downlink. However, with the addition

76 Interference from satellite Interference from satellite systems system transmitting cross OBP System on adjacent orbital positions, transmitting polar towards same orbital co-polar position

δ Orbital separation 1 δ Orbital separation W

OBP Payload Interfering satellite 1 Interfering satellite w

θ θ xi dnw θ ci1 θ ciw θ θ um θ uv uv θ θ dnx dn1

θ dm Interfering earth station θ 1i θ (co orbital position, wi cross polar)

OBP Uplink boresight i OBP Uplink Interfering earth station 1 traffic θ Interfering earth station w n1 (adjacent satellite 1, co-polar) terminal m θ (adjacent satellite w, co-polar) nw Adjacent satellite 1 Adjacent satellite w downlink boresight downlink boresight

Adjacent satellite 1 Adjacent satellite w Cross polar downlink OBP Downlink uplink boresight uplink boresight boresight traffic terminal n

OBP Downlink boresight j

Figure 3 Interference model for calculating required OBP traffic terminal characteristics

θ Tx suppressed sufficiently to be negli- - noise temperature of receiving sys- - the angle between the boresight of gible. Likewise it is assumed that it is tem an antenna and the direction of the possible to make the necessary pre- received/transmitted signal. cautions to avoid adjacent channel ∑ I j interference from the same orbital j Equations (5), (8) and (9) in (1) then position (same satellite owner). - total received co-polar interference give the required boresight characte- ristics for the ground station: ∑ Ik Interference from terrestrial systems k EIRPgs (0û) could be a problem that would affect - total received cross polar interfe- terminal design, especially in cases of  A + B  rence = (C / N + I) +10 ⋅log up up  low interference from satellite sys- up  A ⋅ B  tems. Calculations should therefore be EIRP (θ )  up up  upj/k Is performed to determine its impact on - effective interfering uplink power in (11) required terminal characteristics. direction of OBP payload 3 Interference characteristics However, this kind of interference is DEGupj/k not dealt with here. In [7], maximum - degradation factor giving the incre- 3.1 Interfering sources terrestrial system EIRP is set to ase of spectral power density at OBP +55 dBW, and in the case of EIRP uplink frequency compared to that of Interfering sources can be satellite > +45 dBW, the antenna boresight white noise interference in same systems operating either in the same should be more than 1.5° from the channel bandwidth orbital position, or in an adjacent orbi- geostationary orbit. Typical radio-relay tal position as well as terrestrial sys- systems use antennas 1.8 m, or larger, B upOBP tems (e.g. radio-relay systems). Inter- and have a bandwidth of at least 10 - wanted signal uplink bandwidth ference can be both co- and cross- times that of the OBP system, this Grscj polar, and from systems operating on would mean an antenna suppression of - satellite co-polar receive gain in direc- the same frequency, or at an adjacent at least 20 - 25 dB combined with a tion of interfering source j channel. spreading effect of spectral power den- sity due to the larger bandwidth. It is XPD In this study, it is assumed that both upk also understood that the Ku-band - cross polar gain of interfering signal cross polar and adjacent channel inter- uplink frequencies are not commonly ference from satellite systems opera- B used for radio-relay systems, some upIj/k ting on adjacent orbital positions is - interfering signal j/k bandwidth

77 bands are not at all assigned for terres- ation and free space attenuation is + = f 0 B/2 ( ) trial systems. This leads to the therefore assumed to be the same for EIRP ∫ − Pfdf f 0 B/2 (15) assumption that terrestrial interfe- all signals on the up- and downlink, Equation (14) can thus be rewritten: rence is probably not a problem on the respectively. uplink. On the downlink, interference 2  f − f  Pf( ) = EIRP⋅ ⋅cos2  0 ⋅ π from terrestrial systems will be a pro- 3.3 Cross polar interference B  B  blem only to a limited number of ter- Cross polar interference is generated (16) minals on certain geographical sites, both in the transmitting and the recei- and is therefore not considered in this − B 〈 − 〈 + B ving antennas, as well as in the depola- for f 0 f 0 f f 0 study. 2 2 risation on the link. Total XPD on the EIRP/B is the power density when The interference considered to affect uplink is given by the expression: assuming the signal uniformly distri- the OBP system, is therefore: δ θ ⋅δ θ XPDup = Gtgsx( ) Grsc( ) (12) buted over the bandwidth. The rest δ θ ⋅δ θ - Systems operating on an orthogonal + Gtgsc( ) Grsx( ) therefore can be regarded a correc- δ θ ⋅ polarisation in the same frequency + Gtgsc( ) DEPOLup tion factor, or a degradation factor ⋅δ θ band towards the same orbital posi- Grsc( ) (linear) compared to white noise. In decibel tion scale this degradation factor with Where x/c means cross- and co-polar the above assumptions becomes: - Systems operating on the same pola- antenna gains, respectively, and t/r in risation in the same frequency band DEG ( f ) transmit/receive direction. Cross- TV towards an adjacent satellite. − polar interference is assumed to come  2  f f   =−10 ⋅log 2⋅cos  0 ⋅ π  from systems operating on the same   This interference model for calculat-  B  (17) orbital position, and ground station ing the OBP traffic terminal require- antennas are therefore represented DEG (f) is shown in figure 4. It can ments, is shown in figure 3. TV with boresight values. Gtgsx is assumed be seen that for signals more than ⋅ to be 30 dB below Gtgsc (WARC), 0.25 B from the centre frequency, the 3.2 Propagation considerations -3 ⋅ θ hence 10 Grsc( ). spectral power of the interfering TV On the uplink, the interfering stations signal becomes lower than the “white The depolarisation (DEPOL) is the will typically not have the same rain- noise EIRP”. For a 27 MHz TV signal cross polar to co-polar ratio at the fade pattern as the OBP traffic termi- this corresponds to f ± more than receive end of the link with no cross- 0 nal. The physical separation will give 10 MHz. polar component at the transmit end. uncorrelated fading between the vari- From “Radio Regulations” it is assum- 3.4.2 Digital TDM ous stations. To give a correct picture ed to be: of the uplink conditions, it is therefore Digital single carrier TDMA systems DEPOL necessary to calculate the (C/I)up will typically use QPSK. The corres- − 1 []30⋅log f −40⋅log() cos φ −20⋅log Att taking into account the independent = 10 10 el rain ponding power spectral distribution is and possibly different rain-fade pat- given by: (13) terns. However, for the sake of simpli- PTDMA ( f ) city, and because it is believed to be of Where f is frequency in GHz, φ eleva- 2 el  ⋅ π ⋅ − ⋅  little effect, the worst case is used in tion angle sin( 2 ( f 0 f ) Tb ) = P ( f )⋅   this study. Rain attenuation is conse- TDMA 0  ⋅ π ⋅( − )⋅  DEPOL(12.5 GHz, 25°, 1.1 dB) 2 f 0 f Tb quently set to zero on the interfering   = 33.8 dB = 418 ⋅ 10-6 1 1 uplinks. (On 20/30 GHz where rain for f − i ⋅ 〈 f − f 〈 f − i ⋅ ,i〉1 0 ⋅ 0 0 ⋅ attenuation is higher, this may not be 4 Tb 4 Tb correct, but this study is concerned 3.4 Interference spectral distri- (18) only with Ku-band.) On the downlink, bution 1/T is the bitrate which is twice the all signals will have more or less the b 3.4.1 TV-signals same attenuation pattern since they symbol rate. PTDMA(fo) is power are all received by the same earth sta- “Typical” analogue FM TV signals are spectral density at centre frequency. tion, and the rain attenuation is assu- assumed to have a cosine square med to be the same for all incoming power distribution over the channel Before transmission, this signal is usu- signals. bandwidth. ally filtered in some kind of square  − π  root cosine filter, with a frequency The minimal acceptable [EIRPgs = ⋅ 2 f 0 f ⋅ ⋅ P( f ) Pf( 0 ) cos  2  response R(f,a): +(G/T)sat], and the critical C/N + I  B 2  inserted in equation (6), will give the + (14) − 〉 1 a corresponding critical rain attenuation. 0 f 0 f B B 4Tb − 〈 − 〈 +   π  − − On the uplink, all earth stations capa- for f 0 f 0 f f 0 ( ) = 2 − − ( − ) 1 a 〈 − 〈1 a 2 2 Rf,a cos  ( f 0 f 1 a ) f 0 f   4a  4Tb 4Tb ble of interfering with the OBP pay- − 1 − 〈1 a load, are within the coverage of the Where P(f0) is the power spectral den- f 0 f 4Tb small satellite spot beam. On the sity (W/H z) at centre frequency f0, downlink, all interfering satellites will and B is RF signal bandwidth. The sig- (19) be in an orbital position not far from nal EIRP is given by: the OBP payload. Atmospheric attenu-

78 5 a is the roll-off factor, typically in the 3.5 Interference power levels range of 0.4 - 0.5. R(f,a) for a = 0.4 is Interfering systems are assumed to shown in figure 5. Transmitted power operate in compliance with the CCIR distribution is: maximum power levels [8] giving P(f) = P (f) ⋅ R(f) (20) EIRP at centre frequency: TDMA up DEGTV θ (dB) The total signal EIRP is found in a EIRPup/40 kHz( ) = 39 - 25 similar way as in (15), and (18) can be ⋅ log(θ) dBW/40 kHz (24) rewritten:

i ⋅ 1 Total EIRPup is given by: = EIRP ⋅ 2Tb ⋅ 2 P( f ) + x( f ) i 1 f 0 i /4Tb EIRP = EIRP - 10 2Tb ∫ x( f )2 df up up/40 kHz − ⋅ B f 0 i /4Tb log(40 kHz/ up) - DEG (25) -15 -0.5 f -f 0.5 (21) 0 Thus giving: π B Where x(f) = [(sin (2 (fo -f)Tb))/ π ⋅ ⋅ θ ⋅ θ (2 (fo - f)Tb)] R(f). EIRP/(i 1/2Tb) EIRPup( ) = 39 - 25 log( ) - 10 Figure 4 Degradation factor for TV signal relative to is the “corresponding” white noise ⋅ white noise in the same channel bandwidth, with the same log(40 kHz/Bup) - DEG (26) density over the signal bandwidth EIRP i ⋅ 1/2Tb. The degradation factor then Where B and DEG is given by the becomes: up interfering signal characteristics. 1 DEG ( f ) Multicarrier systems are assumed to TDMA have the same degradation factor as     TDMA systems when calculating total a=0.4  i ⋅ 1  EIRP per carrier. = ⋅ 2Tb ⋅ 2 10 log + x( f )  f 0 i /4Tb  2   ∫ x( f ) df  3.6 Interfering systems in the  −   f 0 i /4Tb  orbital arc (22) R(f) The OBP payload is planned for (linear) DEGTDMA(f) for a = 0.4 and 0.5 are launch some time in the late nineties. shown in figure 5. It can be seen that It is difficult to predict the exact inter- TDMA signals have more evenly ference scenario at that time, especi- spread spectrum than analogue TV ally the uplink interference within a signals. For frequencies more than rather narrow frequency band, and 0.27 times signal bandwidth off the small geographical area which is the centre frequency, power density be- interference of concern to the OBP 0 comes less than that of white noise payload. Through proper frequency -Rs/2f Rs/2 with the same EIRP. planning and choice of spot beam con- 0 figuration, the interference level may Figure 5 Square root raised cosine response with roll-off be considerably reduced. The interfe- 3.4.3 Multicarrier factor a = 0.5 rence scenario is expected to vary a For interference from multicarrier great deal along the orbital arc, and by It is, however, not possible to envisage type of systems, a scenario of proper choice of orbital location of the the interference scenario from these 64 kbit/s SMS type of carriers with OBP payload, the technical require- estimates with the required certainty, identical boresight EIRP is envisaged. ments for the earth stations could be to perform a calculation of OBP earth From [2] it is assumed that transpon- reduced. station requirements as a function of ders with this type of traffic can be Figure 7 gives an estimate of the utili- payload orbital position. This overview assumed to have an evenly spread sation of the geostationary orbit over of the utilisation of the geostationary spectral density because of the consi- Europe in early 1997 for the 14.0 - orbit therefore is only used for infor- derably larger bandwidth of the OBP 14.5 GHz uplink band. It can be seen mation, while calculations are perform- signals. Thus, spectral density is given that satellites are concentrated in ed for specific interference configura- by: three major areas; the INTELSAT tions. EIRP AOR positions in approx. 18 - 35 P( f ) = P = (linear) degrees west, the INTELSAT IOR 4 Ground station BW (23) positions in 57 - 66 degrees east, and the EUTELSAT positions in 3 - 22 characteristics Where EIRP is the boresight EIRP per degrees east. Between these three carrier, and BW is the carrier separa- 4.1 Ground station antennas groups of satellites, there will be two tion. areas with considerably lower satellite All antennas are supposed to perform density. like the WARC receive satellite anten-

79 5

a=0.4 a=0.5 5

DEGTDMA (dB)

-5 -1f -f 1 -1 1 0 f0-f B B Figure 6 Degradation factor for TDMA signal, relative to white noise in th same channel bandwidth, with the same EIRP

nas [4], when no other information is 2 T = T (atmosphere, galactic  π ⋅ f ⋅ D t t available (e.g. satellite antenna cove- G(0) = η ⋅   noise, ...) = T (f,ø ) rage diagrams): c (29) t elevation ≈ [Tt(12.5 GHz, 17°) 15 K] 2* = co-polar linear boresight gain, − ⋅ θ 1.2 []θ D antenna diameter (m) T = 290 K G (θ ) = G (0û)⋅10 0 0 c c η antenna efficiency θ = T = rain temperature Gc ( ) Gc (0û) (linear) 0<η<1 R ** 1 ⋅− − ⋅ θ 10 []17.5 25 log()θ Total system noise is given by: θ = ⋅ 0 Gc ( ) Gc (0û) 10 θ = ⋅ c ⋅ 180 0 1.16 f ⋅ D π ⋅ (27) (30) Tgs = TA Attfeed (33)

θ = 3 dB beamwidth, assuming + (1 - Attfeed)T0 + TR *for o〈 〈1.3 aperture distribution θ Att = feed and antenna losses in a 0 0.5 + 0.5 ⋅ (1 - r2)2 feed θ linear scale ** for 〉1.3 θ 0 4.2 Up-link EIRP TR = (FR - 1)T0 = receiver noise temperature Up-link boresight EIRP is given by:  2 * − − ⋅ θ FR = receiver noise figure, linear  3 1.2 ()θ  EIRP (31) θ = ⋅  0  up Gx ( ) Gc (0û) 10 − ** Thus, giving ground station G/T: G (θ ) = G (0û)⋅10 3.3  G (0)⋅ P  x c = ⋅  tgsc up  *** 10 log (G / T)gs (34) − − ⋅ θ  Att  []4 4 log()θ −  feedtx  G (θ ) = G (0û)⋅10 0 1 x c = 10 ⋅log(D2 ⋅ M)(dBW)  G (0)  gstx = 10 ⋅log gs  (28) ⋅  Att feedrx Tgs  P = Ground station HPA θ up = ⋅ 2 ⋅ −1 * for 0〈 〈0.5 power (W) 10 log(Dgsrx N)(dBK ) θ 0 θ Attfeed tx = feed and antenna tx loss, ** for 0.5〈 〈1.67 in linear scale. 4.4 Ground station antenna θ 0 diameter θ ***for 〉1.67 The effect of interference on the OBP θ 0 4.3 Ground station G/T terminals is that EIRPup has to be Receive terminal noise is calculated increased compared to the interfe- where: from the model shown in figure 8. rence free cases. This can either be θ Gc( ) = co-polar linear absolute Antenna receive noise temperature is done by increasing the antenna diame- antenna gain given as: ter, or the HPA power. G (θ) = cross-polar linear absolute Traffic terminals are characterised by x T = [(1 - P ) ⋅ T + P ⋅ T + (32) antenna gain A S t S 0 the antenna diameter required to meet (1 - 1/Attrain down)TR] (K) the quality requirements in CCIR Rec. 614. HPA power is in this study kept PS = portion of sidelobes towards fixed. Up- and downlink calculations ground (typically 0.05) will give different antenna diameters.

80 INTELSAT VI/VII

INTELSAT VI/VII Uplink 2x500 MHZ, European uplinks unknown 60OE INTELSAT VI/VII

INTELSAT VI/VII

50OE

TÜRKSAT II Uplink from Turkey no interference expected 40OE

30OE EUTELSAT II (?) Eurobean 2x500 MHz

TÜRKSAT I Uplink from Turkey 20OE no interference expected DFS 2 (?)/ Uplink unknown, spotbean KEPLER 1 (?) Germany and environs

DFS 1 (?) Uplink unknown, spotbean Germany and ervirons EUTELSAT II (?) eurobean 2x500 MHz 20OE ASTRA

EUTELSAT II

EUTELSAT II (III?) Eurobean 2x500 MHz O 10 E EUTELSAT II (III?)

EUTELSAT II (III?)

Telecom: EUTELSAT II (?) Uplink unknown, spotbean TELECOM 2 C/ France and ervirons EUTELSAT II (?) EUTELSAT II: Eurobean 2x500 MHz OE INTELSAT VI/VII Uplink 2x500 MHz, European uplink unknown

TELECOM 2B Uplink unknown, spotbean France and ervirons TELECOM 2A Uplink unknown, spotbean 10OW France and ervirons

INTELSAT K Eurobean, European uplinks unknown 20OW INTELSAT VI/VII

INTELSAT VI/VII Uplink 2x500 MHz, European uplinks unknown INTELSAT VI/VII

30OW HISPASAT/ INTELSAT (VI/VII?) Assuming all uplink taken by either of two INTELSAT (VI/VII) Uplink 2x500 MHz, European uplinks unknown ORION Uplink 2x500 MHz, W. Europe+ spotbeans

40OW

3x72 MHz from Europe (?) PANAMSAT eurobean pol. & frequency unknown 14.0 14.5

Figure 7 Possible utilisation of the geostationary orbit over Europe in early 1997 for the 14.0 - 14.5 GHz uplink band

81 The largest one will give the minimal C/(N+I)up = 15.4 dB f0 XPD TDMA acceptable diameter. = f + 36 MHz Cross polar Att = 1.1 dB Rain attenua- 0 OBP rainup TDMA interfe- Equation (31) can be rewritten to give tion correspon- rence centre fre- the required uplink antenna diameter: ding to critical quency, (72 MHz C/(N+I) 1 transp.)  1 1 EIRP  2 D = ⋅10 10 up (m) Margin = 0.3 dB gstx   DEG = 3 dB Degradation fac- M (35) TV L = 207.1 dB tor relative to Where required EIRP is given by up up white noise with equation (11). L = 206.0 dB down same EIRP in Similarly, the downlink antenna dia- Attatmup = 0.3 dB same channel meter can be found from equation (34) bandwidth. (see Att = 0.2 dB to be: atmdown chap. 3.4) 1 5.2 OBP payload assumptions DEG = 1.5 dB  1 1 ()G / T  2 TDMA D = ⋅10 10 gs gsrx   G/T = 9 dBK-1 Edge of coverage G = 42 dB Satellite boresight  M  (36) rsc b G/T receive gain, Where required (G/T) is given by gs interfering sour- the clear sky interference free condi- EIRP = 44.7 dBW Down-link bore- ces in boresight. tions in (7). sight EIRP per car- rier This means that off axis EIRP is deter- To characterise the OBP traffic termi- mined by: nal requirements, minimum antenna Grsc eoc = 36 dB Edge of coverage θ diameter should be determined for all receive gain EIRPup ( ) (37) the interference cases found appli- Att = 1.5 dB Receive feed loss = 64.3 - 25 ⋅ log(θ) (dBW) cable. Space segment characteristics, feedrx for TV and all other ground station characte- Ts = (Grsc eoc)/(Attfeedrx G/T) ristics, should be kept the same (linear) = 70.1 - 25 ⋅ log(θ) (dBW) throughout the study. = 355 K Satellite noise tempe- for TDMA rature ⋅ θ 5 Assumptions for calcu- = 39.5 - 25 log( ) (dBW) per 5.3 OBP terminal assumptions carrier for multicarrier lating OBP terminal = 54.5 - 25 ⋅ log(θ) (dBW) total n = 0.65 Antenna efficiency characteristics within OBP uplink (100 % fill) P = 12 W HPA power Assumptions are mainly those given in [2]. up Multicarrier systems may typically use Att = 1 dB feedtx small earth stations with a limited 5.1 OBP signal and propagation Att characteristics feedrx = 1 dB number of carriers per station. In [2] a typical SMS carrier is seen to have a f up = 14.5 GHz 5.4 Interference characteristics boresight EIRP of 52 dBW per 64 kbit/s carrier. If it is assumed that fdown = 12.5 GHz δ Grsc = 0 dB Interfering sources in an 1.8 metre antenna with a radiation n = 1.2 n = bandwidth/- boresight diagram, as shown in figure 9, is used symbol rate for these transmissions, the off axis δG = 0 dB rsx EIRP will be: BupOBP = 1.2 MHz 2 Mbit/s QPSK XPD = 26.2 dB θ EIRPup( ) (38) B = 27 MHz TV signal bandwidth up TV = 29 - 25 ⋅ log(θ) (dBW) per Bup TDMA = 72 MHz 120 Mbit/s carrier for multicarrier QPSK, n = 1.2 = 44 - 25 ⋅ log(θ) (dBW) total Bup multi = 64 kHz Channel separa- within OBP uplink (100 % fill) tion SCPC system This latter estimate of multicarrier f = f Adjacent satellite 0 adj 0 OBP earth station off axis EIRP appears to rain interference at , Att be more realistic, calculations should r centre frequency T also be performed with these values, f0 XPD TV giving approximately 10 dB lower = fO OBP + 18 MHz Cross polar TV interference levels than maximum per- Receiver interference cen- missible level from CCIR. Feed loss FR tre frequency, (Assuming stag- Since it is shown that CCIR limits may gered transpon- give unrealistic high off axis EIRPs for Figure 8 Model for calculating ground station noise tem- ders/36 MHz multicarrier, this may also be the case perature transp.) for TV and TDMA. Calculations for

82 Radiation level [dB] 0

20 these kinds of interference should also be performed with off axis EIRP 10 dB down.

52 - 10 log (D/λ) -25 log θ In the calculations, it is assumed that the edge of coverage will be at the θ 32 - 25 log log -6 dB contour of the satellite receive 42 - 10 log (D/λ) -23 log θ beam. For some constellations, this 40 might be a little bit low. Calculations =23 - 25 log θ for f=14 GHz D=1.8 m therefore should be performed also with the -3 dB contour as the edge of coverage.

6 Results

60 It is not possible to give an estimate of the uplink interference scenario from within each of the narrow OBP spot beams. Calculations have therefore been performed by investigating the effect of one single interference contri- bution. The results from these calcula- tions are given in table 1 and figure 10. 01020304050 It should be noted, however, that these results are achieved by using Azimuth angle (Degrees) the method described in this article. Figure 9 Measured azimuth wide angle radiation pattern for 1.8 m shaped offset antenna. Reference curves for antenna characteristics are shown for com- It can be seen that the OBP system is parison very sensitive to cross polar interfe- rence. It is therefore essential that the OBP operator has full control of the I+W A+W I+P A+P utilisation of the orthogonal polarisa- tion in the same frequency band for No int. 0.6 0.4 0.6 0.4 the same orbital position, e.g. by using TV 3¡ 5.2 3.3 1.7 1.1 both polarisations in the same band TDMA 3¡ 5.2 3.3 1.7 1.1 for OBP purposes. Multi 3¡ 4.8 3.0 1.5 0.9 For adjacent satellite interference, it TV 6¡ 2.4 1.5 1.0 0.6 can be seen that without any precau- TDMA 6¡ 2.4 1.6 1.0 0.6 tions taken, interfering satellites have Multi 6¡ 2.0 1.3 0.9 0.6 to be at least 9° away in order to meet the design goal of OBP antennas smal- TV 9¡ 1.4 0.9 0.7 0.5 ler than 2 metres, using solid state TDMA 9¡ 1.5 0.9 0.7 0.5 amplifiers. By shrinking the coverage Multi 9¡ 1.3 0.9 0.7 0.5 area from the -6 to the -3 dB contour, required antenna size can be reduced TV XPD 0¡ 6.4 4.5 - - by approximately 35 %. Off axis EIRP TDMA XPD 0¡ 3.3 2.4 - - of the interfering earth stations will Multi XPD 0¡ 3.7 2.7 - - directly affect the OBP terminals. This is something that is beyond the OBP Table 1 Required OBP terminal antenna diameter as a result of one single inter- operator’s control, and relying on fering uplink signal. Adjacent satellite interference is assumed to be co- lower off axis EIRPs than specified by polar while interference towards the same orbital position is cross polar CCIR may therefore not be recom- Initial coverage assumptions (I): mended. - edge of coverage at -6 dB contour - interfering sources in boresight The OBP system is seen to be sensi- tive to adjacent satellite interference. Worst case off-axis EIRP’s (W) Since the probability for this interfe- - max. permissible according to CCIR rence will vary a lot along the geosta- Alternative coverage assumptions (A): tionary arc, as may be seen from - edge of coverage at -3 dB contour figure 7, the choice of orbital position - interfering sources at -1 dB contour should if possible be in an area with More probable off-axis EIRP’s (P): few adjacent satellites, thus reducing - off-axis EIRP’s 10 dB down from W the risk of interference.

83 Antenna diameter (m) 8 (16) 3o orbital 6o orbital 9o orbital 0o orbital separation separation separation separation 7 (14) Cross polar interference 6 (12)

5 (10)

4 (8)

3 (6)

2 (4)

1 (2)

0 no TV TDMA multi- TV TDMA multi- TV TDMA multi- TV TDMA multi- inter- carrier carrier carrier carrier ference

I+W Initial coverage assumptions (I): I+P More probable off-axis EIRP's (P) - edge of coverage at -6 dB contour - off-axis EIRP's 10 dB down from W - interfering sources in boresight Worst case off-axis EIRP's (W) - max permissible according to CCIR

A+W Alternative coverage assumptions (A): A+P - edge of coverage at -3 dB contour - interfering sources at -1 dB contour

Figure 10 Required OBP terminal uplink antenna diameter (m) to meet the interference from one satellite interferer. Antenna diameters are cal- culated assuming a 12 W SSPA HPA, numbers in parentheses for a 3 W HPA

5 Rudge et al. The Handbook of 7 Conclusions 8 References Antenna Design, IEE (Peter Pere- The method described in this article grinus), 1982. 1 EUTELSAT III Phase 1 Study shows that without any precautions Report, chap. 3, Network and trans- taken, the effect of one single interfe- 6 CCIR Rec. 614, Volume IV. mission aspects. rer may be the need to increase OBP terminal antenna diameter by up to 10 7 CCIR Rec. 406-6, Volume IV and 2 EUTELSAT III Phase 1 Study times, probably making the system IX. Report, Annex 10, On-Board Pro- commercially uninteresting. It can be cessing Link Budgets and Interfe- seen that there are a number of possi- 8 CCIR Rec. 524-3, Volume IV. rence Constraints. bilities to reduce this interference down to an acceptable level. It is, how- 3 EUTELSAT III Phase 1 Study ever, evident that for this kind of sys- Report, chap. 5, Satellite and tem, interference should be one of the launch vehicle. major concerns when determining sys- tem characteristics. 4 Radio Regulations, Appendix 30A, (Orb 88).

84 Earth station antenna technology

BY ARNFINN NYSETH AND SVEIN A SKYTTEMYR

1 Introduction The synthesis is based upon Geo- 2.3 Production of earth station 621.396.67 metrical Optics (GO), and when using antennas Norwegian Telecom Research (NTR) this method the diffraction effects are has for many years been engaged in The first offset dual-reflector antenna not included. The radiation pattern developing different earth station was built in 1981, and a few years later including these effects can be found antennas, in co-operation with other production started at Raufoss. Their by a Physical Optics (PO) analysis. research institutes and Norwegian production technique was based on industry. Figure 1 shows the GO rays in a 3.3 m composite materials on an aluminium antenna. It is possible to see the shap- honeycomb structure. The technique The applications are business commu- ing of the aperture distribution by the fulfilled the requirements and Raufoss nications (VSAT), satellite TV up-link varying distances between the rays. produced several 3.3 m and 1.8 m and receive only, and mobile satellite communications. In the early 1980s a synthesis method for offset dual-reflector antennas was developed, and 3.3 m and 1.8 m anten- nas were produced. Later smaller antennas of the same type have been used for satellite TV reception. These antennas have been successfully pro- duced and marketed by the Norwe- gian company Fibo-Støp. In the last 3 years NTR has developed flat plate antennas for mobile satellite communications. Some of these anten- nas will be manufactured by ABB NERA.

2 Shaped offset dual- reflector antennas 2.1 Introduction Since 1980 Norwegian Telecom Re- search (NTR) has constructed and built many different shaped offset dual-reflector antennas (1). The uni- que synthesis method developed at NTR for construction of such anten- nas, was presented at the ICAP confe- rence in 1981 by G. Bjøntegaard and T. Pettersen (2) and later published in (3) and (4).

2.2 Synthesis and analysis method A good description of the synthesis method can be found in (2), (3) and (4), and we will therefore not describe it in detail. The goal of the method is to obtain a prescribed aperture distri- bution, giving the wanted radiation pattern with high gain, low sidelobes, or a combination with rather high gain and quite low sidelobes. This goal can be obtained by shaping the sub- and main-reflectors, and in this way let them have small deviations from the original confocal system (with parabo- lic and elliptical or hyperbolic surfaces of revolution). The method also gives a low cross polarisation. Figure 1 3.3 m antenna with GO ray-path

85 antennas up to 1987, but the production method was too expensive to produce antennas in large numbers. Figure 2 shows a 1.8 m antenna on a transportable earth station. In 1988 the production was transferred to EB-NERA (now ABB-NERA). The reflectors are produced at Ticon in Dram- men. They use polyester with glass fibre as material, and are able to produce the antennas cheaper. At the moment, they only produce 3.3 m antennas. These antennas are intended for Ku-band (11/14 GHz), but with a modification of the feed horn they can be used at other frequency bands (12/18 GHz or 4/6 GHz). With a modified sub reflector, the 1.8 m antennas are also used at Ka-band (20/30 GHz). The large antennas (D > 1.5 m), are only produced in a limited number, but smaller antennas are mass-produced for the consumer and professional market at Fibo-Støp, Holmestrand. At the moment they are producing three different types of antennas in large numbers (55 cm, 90 cm and 1.2 m antennas). The 55 cm and 90 cm antennas have Figure 2 1.8 m antenna on transportable earth station been produced for some years now, and they have obtained a good reputation both in Norway and in other European countries. They are shaped for high gain, and their efficiencies are higher than 80 %. The 55 cm antenna has also obtained a price for good design. Figure 3 shows this antenna. The antenna is sold through several large and small companies selling equipment for the satellite-TV receive market. Last year, the production of a new 1.2 m antenna started. This antenna is intended for both the receive-only consumer mar- ket and for the business communication market in the 11/14 GHz band. It is shaped to have rather low sidelobes and still having high gain. Like the other antennas produced at Fibo-Støp, this antenna will be sold at a low price. 3 Microstrip array antennas 3.1 Introduction In 1989 Norwegian Telecom Research (NTR) performed a study of array anten- nas in communication systems. The study report (5)concludes that antennas for mobile satellite terminals

Figure 3 Fibo-støp 55 cm antenna

86 are one of the most promising applica- 3.3 INMARSAT-M Receive Transmit tions for array antennas. NTR is now developing antennas for band band As a consequence of the results from this INMARSAT-M, a system which will be study, we started construction of antenna operational in 1993. This system will use Axial ratio, elements for L band (1.5 - 1.6 GHz)(6). terminals with antenna gains in the asimuth 1.8 dB 5.8 dB The elements are modified square region 13-16 dBi. Array antennas with a (± 10o) patches. As can be seen from figure 5, small number of elements are a natural two of the corners of the elements are cut choice for these terminals. The small size off. The goal is to obtain circular polari- of the antennas gives only small losses, Axial ratio, sation with only one feed point. In order and the antennas may be manufactured elevation 3.5 dB 4.5 dB to improve the axial ratio, the elements with low cost and low weight. Table 2 (± 30o) are sequentially rotated and phase shifted gives an overview of the main specifica- when used in arrays (7). This technique tions for the INMARSAT-M system (8). Gain ≈ 11.5 dBi ≈ 11.5 dBi gives almost perfect circular polarisation Table 3 gives the most important perfor- in the boresight direction of the antenna, mance parameters for the first prototype and very good circular polarisation in an Return loss < -17.5 dB < -14.0 dB antenna for a portable INMARSAT-M angular area around the boresight. In terminal. A photo of the antenna is addition, reflections to the input port can- Table 1 Antenna for a portable INMARSAT-C terminal. shown in figure 5. This antenna will be cel each other, improving the VSWR per- Measurements used in ABB-NERA`s Saturn-M Portable formance of the antenna. terminal. 3.2 INMARSAT-C NTR has also developed an experimental INMARSAT-M antenna for land mobile The first antenna which was developed applications. This antenna has been used was an antenna with higher gain for for experiments with different tracking INMARSAT-C terminals. The ordinary systems, both mechanical and electronic C antenna is “nearly” omnidirectional. A photo of the prototype is shown in fig- NTR developed a new antenna with ure 6. The electronic system is based on approximately 11 dBi gain. This antenna digital phase shifters, making it possible is now being manufactured by ABB to shift the antenna beam in discrete NERA, and is used in the Saturn-C steps. This work is aimed at developing Portable terminal. A short description of Specifications low cost antenna systems for land mobile the performance of the C antenna is applications. Using mechanical steering given in table 1. A photo of the antenna is a costly way of realizing tracking sys- Minimum Landmobile is shown in figure 4. tems, while the electronic steering has G/T -12 dBK the potential for reducing the cost of the Maritime outdoor unit. -10 dBK

Max EIRP Landmobile 25 dBW Maritime 27 dBW

Channel 8 kbit/s bit rate

Bandwidth 1525 - 1559 MHz (RX) 1626.5 - 1660.5 MHz (TX)

Polarization Right hand circularly polarized, axial ratio less than 2 dB on-axis Figure 4 Antenna for a portable Figure 5 Antenna for a portable INMARSAT-C terminal INMARSAT-M terminal Table 2 INMARSAT-M specifications

87 References Receive Transmit band band 1 Kildal, P S, Pettersen, T, Lier, E, Aas, J A. Reflectors and feeds in Axial ratio, Norway. Telektronikk, 2-3, 1988. Also published in: IEEE ant. and asimuth < 0.5 dB < 1.0 dB ± o propagat. soc. newsletter, April ( 10 ) 1988.

Axial ratio, 2 Bjøntegaard, G, Pettersen, T. elevation < 0.5 dB < 1.0 dB A shaped offset dual reflector (± 30o) antenna with high gain and low side- lobe levels. ICAP81, IEE conf. publ. Gain ≈ 14 dBi ≈ 14 dBi 195, 163-167, 1981. 3 Bjøntegaard, G, Pettersen, T. An off- Return loss < -27 dB < -28 dB set dual-reflector antenna shaped from near-field measurements of the Side lobe < -17 dB < -17 dB feed-horn: Theoretical calculations level and measurements. IEEE trans. ant. and propagat., AP-31, 973-977, Nov. Table 3 Antenna for a portable INMARSAT-M terminal. 1983. Measurements 4 Bjøntegaard, G, Pettersen, T, Skytte- myr, S A. Design of dual shaped off- set reflector antennas. 9th QMC ant. sym. proc., London University, 1983. Also appearing in Clarricoats and C Parini (ed.). Recent advances in antenna theory and design. Microwave exhibitions and publish- ers limited, 1989.

5 Nordbotten, A, Nyseth, A, Skytte- myr, S. Array antennas in communi- cation system. NTR report 33, 1989. (In Norwegian).

6 Skyttemyr, S A. Transmission line model for microstrip antenna. NTR report 32, 1992. (In Norwegian). Figure 6 Experimental antenna system for land mobile INMARSAT-M terminal 7 Teshirogi, T, Tanaka, M, Chujo, W. Wideband circular polarised array antenna with sequential rotations and phase shift of elements. Proceedings of ISAP '85, 117-120, 1985.

8 Inmarsat-M System Definition Man- ual. Issue 3.0, Module 2.

88