Tracking Techniques for Inclined Orbit Satellites

TRACKING TECHNIQUES FOR INCUNED ORBIT SATELLITES

KHALID S. KHAN

Andrew Corporation, Richardson, Tx 75081 July 10, 1990

ABSTRACT coincides with the equatorial plane of the planet Earth. The revolution period Some of the international and domestic of an object with respect to earth is communication satellites have been directly proportional to the distance placed in the inclined orbit. This between the object and the planet Earth. decision is receiving wide support The period of rotation of an object in within the telecommunications industry. a geosynchronous orbit matches that of A satellite in an inclined orbit is the earth, and the object looks expected to operate an additional five stationary. years. These additional operating capabilities of a satellite mean more Kepler's laws govern the rotation of an revenue for the owners and less object in an orbit. They also explain the transponder cost for the users. nature of the curve (orbit), the revolution period, and the height of the This paper discusses and furnishes geosynchronous orbit. current information pertinent to satellite maneuvering in the inclined Kepler's first law' is given by the orbit along with various tracking following equation2: techniques needed to access an inclined orbit satellite. It also suggests a c= R (1) hybrid tracking technique for an 1 + E cos 0 inclined orbit satellite. where : = GENERAL c motion of the curve R = parameter of the second order The destruction of the space shuttle in E = eccentricity mid air coupled with the loss of a few 8 = central angle satellites and launching rockets have been blamed for the current Forgeosynchronous orbit, themotionof difficulties of the satellite industry. the curve 'cl equals to the parameters On the other hand, satellite RF of the second order 'R. ' This condition electronics life and reliability have occurs when the eccentricity 'E' of the increased from approximately seven curve equals to zero. years to almost twelve years. COMSAT has proposed a new scheme, called Similarly, Kepler'sthirdlawdealswith COMSAT Maneuver', to fully maximize the the revolution period of an object in return on its investment. This space. This law is mathematically maneuver is being examined as a described below: technique to offset the problems associated with satellite industry. The ,2 = 4x2 x3 (2) IL fundamental principle of this technique : is to conserve fuel and thus extend the where life of a satellite. T = revolution period p = Kepler's constant GEOSYNCHRONOUS ORBIT = 398 ,513.52 km3/sec2 = Most of the Western commercial x Semi-major axis of elliptical communication satellites are located in orbit and the value of a circular orbit called 'Geosynchronous p = Orbit.' The plane of this orbit GM

14.6.1. 0364 CH2831-619010000-0364$1 .OO 0 1990 IEEE where : ...... G = Universal qravitational constant ...... = 6.67 * 10’ km3/kg/sec2 ...... = h ...... M Mass of planet Earth W W ...... = 5.977 * kg cc ...... 0 ...... and 0W ...... v ...... x=r+H W ...... where : 03 ...... r = Radius of the earth c 0.00 = 6,370 km ...... ‘4 ...... = H Altitude of a satellite W ...... moving in a circular orbit != J ...... _J t-W ...... By substituting the values of x in ...... v, equation (2), it becomes as follow: ~ ......

= + H)3 ,2 4r * (r (3) -5.00 lJ -0.50 -I0.50 Similarly, the velocity of an object SATELLITE LONGITUDE(DEGREE) DIURNAL MOVEMENT OF AN INCLINED ORBIT SATELLITE orbiting in geosynchronous circle can OVER A 7 DAY PERIOD be determined by the equation given below: FIGURE 2.0 satellite revolution period as well as v = J gm/a (4) the velocity. This inclined orbit is an

I0’ 7

54 J

10 7 5 1 3

I 10.23 &5 7 10.21 A5 7 ‘0‘23 45 7 IC ALTITUDE h(km) FIGURE I’ RELAllON 3ETWEfN ALTIIUDI. REVOLUllON PERIOD AND ORBITAL VELOCITY OF A SATELLITE

FIGURE 1.0 TIME IN YEAR5

PERFORMANCE PROFILE OF A SYNCHRONOUS ORBIT 5CLAR ARE

FIGURE 3.0 Equations (l), (3), and (4) are the founding pillars of a geosynchronous out growth of the geosynchronous orbit orbit. Figure 1.0 shows graphically where inclination is approximately 0.85’ equations (3) and (4). To attain the per year, and this new orbit is called an revolution period of an approximately ‘Inclined Orbit.’ The orbital motion 24 hours, a suitable height of 22,236 starts small and increases with time. The miles was selected for satellites. inclined orbit tends toward an elliptical shape over a period of time, and the If the eccentricity is gradually major axis of this orbit is toward a increased from zero to one then the north-south direction. orbit tends toward an elliptical shape. This change triggers the changes in the There are several benefits in leaving the

14.6.2. 0365 satellite in the inclined orbit: link with the satellite. The communication between the earth station a. To use as a storage orbit and the inclined orbit satellite has the b. To save the expense of replacing following significant drawbacks: satellites c. To provide back-up to primary a. Degradation in the performance of satellites satellite components and equipment d. To provide additional service and after a period of seven years revenue b. Antenna beamwidth c. Extension of orbital inclination of GRAVITATIONAL DISTURBANCES the satellite d. Geographical location ofthe station Disturbing forces such as the gravitational attraction of the sun and Satellite EIRP (Effective Isotropic moon, the radiation pressure of the Radiated Power) , flux density and the G/T sun, and the earth's gravitational (Gain to Total system Temperature) tend field constantly cause a satellite to to decrease over time. This is also true move from its assigned position. The with the solar power supply. Figure 3.0 activity of keeping the satellite in shows a typical depletion of solar array the allocated position is ' called poweroveraperiodof seven years. This llstationkeeping.l*The stationkeeping reduction of satellite power supply requires the satellite to produce its capabilities has some impact on the own power. The propellant fuel which performancedegradation ofthe satellite provides this power is sufficient for parameters. approximately seven years. For continued operation beyond this period, Generally, smaller antennas with larger one of the following alternatives must beamwidth do not require tracking when be implemented: accessing a geosynchronous satellite. This condition is not true for larger a. Carry more fuel in the spacecraft. antennas which wish to track inclined This proposition will increase the orbit satellites. Thisdrawback requires weight of the spacecraft and may frequent periodic corrections in the increase the cost for launching. earth station antenna look angle. b. Curtail some of the satellite The larger inclination of a satellite stationkeeping activities which poses greater problems forthe accessing will have the least impact on the ground station because the movement of characteristics of the satellite. thesatellite is not in synchronization with the movement of the earth. Please INCLINED ORBIT SATELLITE see equation 4.0. It is financially economical and Other parameters, as such, polarization technically feasible to curtail some of isolation and satellite beam coverage the stationkeeping activities. Most drastically change. domestic satellite vendors have opted for east-west stationkeeping only. To offset these problems a stringent There are two basic reasons for this tracking requirement is needed. decision (a) compliance with the FCC rules and (1)) reduction of up to 96% in Someofthe following international and satellite fuel, which is in limited domestic satellites presently orbitring in supply. The deletion of north-south the inclined orbit are INTELSAT stationkeeping activity approximately satellites at 338.5' E and 177' E, two months prior to the end of the GSTAR 111, EUTELSAT F1 at 16' E t>tc. seven years operational period allows the satellite to drift in a figure I8l SATELLITE TRACKING shape. Figure 2 depicts a week of satellite path3 in an inclined orbit. As the satellites started crowding the geosynchronous orbit, trackingbecame an SATELLITE PARAMETERS essential element of an earth station equipped with fairly large-sized Movement of a satellite in the inclined antennas. The decision to incorporate orbit poses problems to an earth tracking into an earth station is station in establishing a communication contingent upon antenna half-power

14.6.3. 0366 beamwidth, satellite orbit and cost. Step tracking systems equipped with The signal strength of a Trajectory Algorithm (TEA) are being used transmit/receive carrier depends upon to track an inclined orbit satellite. The the alignment of antenna beamwidth with TEA approach takes into consideration respect to the satellite. Therefore, it INTELSAT's eleven parameters along with becomes necessary to implement a some additional parameters. tracking system if the antenna has a narrow beamwidth. Program tracking, an accurate way to follow a satellite at Ku-band frequency, The simplest form of a tracking system is a suitable technique for above could be a mechanical driver that moves mentioned case c which requires the antenna boresight axis in a plane. prediction and computation of all Determination of the required movement physical effects acting on the satellite could be achieved in three ways: !a) with respect to time. Pre-determined receive the satellite signal during satellite positions are furnished by entire communication period, (b) track Telemetry Tracking and Control station to the satellite when it is needed, and the transponder users. (c) beam an earth station toward a satellite based upon pre-determined A simple program tracker is not capable position. of peaking an inclined orbit satellite. A program tracking system must be devised In first category, Monopulse tracking, to take into consideration of parameters an expensive technique, utilizes a real similar to INTELSAT's Ephemeris data. The time nulling process with Automatic Ephemeris data may be different for non- Gain Control. Despite its expense, this INTELSATls satellites. technique could be used by an earth station to track an inclined orbit This solution is well-suited to smaller satellites. antennas. Larger antennas often utilize program in conjugation with step Step tracking can be a suitable tracking. candidate for above mentioned case b. Step tracking is less expensive than HYBRID TRACKING SYSTEM monopulse tracking. This technique utilizes scanning process where the A Hybrid Tracking System (HTS) is a satellite signal power is sampled in concept for accessing inclined orbit and step. geosynchronous satellites. It is a modified version of the conventional A simple step tracker can not take into program tracking system. It is cost account the inclined orbit satellite effective and technically acceptable. positions. The movements due to orbital inclination can be easily approximated The basic principle of operation 0:' the by means of formulae which include the HTS is very simple. Satellite owners, orbital inclination: the latitude of such as INTELSAT, supply their users with the station and the longitude a set of computed data (pre-determined difference between the earth station positions of an inclined orbit satellite) and the satellite. for seven days. An earth station user will enter this information in the HTS THE INTELSAT's approach to computing through a keyboard or remotely. Refer to the positions of an inclined orbit block diagram provided in figure 4.0. satellite has been to sum up all the The HTS will be equipped with software to disturbing effects in three equations refine these data and activate the which take into consideration eleven positioningmechanismoftheantennato parameters, callec' Ephemeris. Some of point it toward the inclined satellite at the significant parameters include mean set times. longitude, drift velocity and its acceleration, longitude and latitude Ephemerisdata, furnishedby INTELSAT, is oscillation-amplification, sine and updated weekly by the TT&C station cosine terms of their rates of change operator. Once this data is entered into in amplitude. These approximate data the HTS, and program tracking is enabled, are obtained from least curve fitting a schedule of antenna coordinates over techniques. This technique has an time is generated. Position command:; are accuracy of +/- 0.01 degree for a sent tothe antenna controller according period of 7 days. to the time schedule, maintaining the

14.6.4. 0367 I 3. Rory Chang and Les Veenstra, INTELSAT Report, March 1989. 4. Electrospace Systems 'Communications SatelliteTracking Techniques forthe 1990's Rev. A February 16, 1989

FIGURE 4.0 antenna's lookanglewith the satellite. Feeding information to the HTS can be handled via a PC-based software package. The only additional requirements then are the PC, the software interface, and the cabling. Program tracking could be incorporated on a single axis drive system with small antennas. HTS can be used along with step-tracking.

CONCLUSION The useful life of a satellite may be extended by shutting off the North- South stationkeeping. The satellite then goes into an inclined orbit, requiring earth stations to have intelligent tracking mechanisms. Usage of an inclined orbit transponder may be cost effective for the users. It can bring additional dollars to the satellite owners. The HTS technique is an adequate method to track a satellite operating in a geosynchronous orbit as well as one operating in an inclined orbit. This unit is economical and accurate. It can be used with small or larger antennas.

ACKNOWLEDGEMENT The author wishes to extend acknowledgement for technical support and guidance that he received from Tom Charlton, Scott Walker, Dale Mowry, Tony Campbell, Barbara Hodge and Doug Pewterbaughof Andrew Corporation.

REFERENCES 1. INTELSAT Document IESS 411 (Rev.1)

2. Miya, K, Satellite Communications Engineering, Lattice Company, Tokyo, 1975.

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