PRACE NAUKOWE POLITECHNIKI WARSZAWSKIEJ z. 95 Transport 2013
Jacek Januszewski
Gdynia Maritime University, Poland
COMPATIBILITY AND INTEROPERABILITY OF SATELLITE NAVIGATION SYSTEMS IN DIFFERENT MODES OF TRANSPORT
The manuscript delivered: April 2013
Abstract: Actually (March 2013) more than 60 operational GPS and GLONASS (Satellite Navigation System – SNS), EGNOS, MSAS and WAAS (Satellite Based Augmentation System – SBAS) satellites are in orbit transmitting a variety of signals on multiple frequencies. As nowadays more than one hundred million receivers of all these systems are used in every mode of transportation in the world ķ land, maritime and air ķ the problem of compatibility and interoperability of SNS and SBAS also under development, Galileo & BeiDou and GAGAN, SDCM & QZSS respectively is described in the paper. Three parameters ķ signal in space (frequency carrier), system time and coordinate reference ķ frame (datum) were taken into account in particular. A reply to several questions such as how many systems can be used at the same time and which system must be preferred in individual mode of transport, what does for the user new SNS and SBAS mean is described also. Keywords: compatibility, satellite navigation system, mode of transport
1. INTRODUCTION
Today we can distinct three main modes of transport, land, maritime and area. In each mode one of the most important elements of the safety is continuous knowledge of the current position of the user [13]. Depending on the mode information about this position can be obtained from many different methods, but the method which can be used in all modes, at any moment and any point in the world are global satellite navigation systems (SNS) and additionally in some areas of the world satellite based augmentation system (SBAS). All these systems are known also as GNSS (Global Navigation Satellite System). Each SNS and SBAS found application in one and very often in two or even all three modes of transport. It means that nowadays the many user of transport can make the choice of SNS and, if it is possible, use or no of SBAS. As two next global SNS and at least two next SBAS are under construction [14], [20], [21] and some parameters important for theirs exploitation differ, the problem of compatibility and interoperability of all these systems became for all users of transport essential problem ķ carrier frequency, reference 200 Jacek Januszewski datum and time reference. Nowadays in each mode of transport at least one SNS is used. Both SNS fully operational GPS and GLONASS provide the continuous of the current user’s position but without information about integrity [6], [9], [11], [16]. As the accuracy of this position is sometimes in some regions, for some users insufficient the need of the construction at least one another SNS or SBAS is indispensable. Which SNS or SBAS is recommended depends on the mode of the transport. Many of the differences among GNSS signals can be reconciled within the user’s receivers to produce a melded position /velocity/time (PVT) solution [6]. But the greater and more numerous the different corrections needed, the greater the computational overhead on the device itself, as well as adverse effects on performance, size, weight, power, and cost. We can except that in the near future the various GNSS, used in different modes of transport also, would converge on common standards. Optimizing the alignment of signals and frequencies, time and geodetic coordinate systems, however, are long-term projects, although the sooner progress is made on them, the sooner they become a present reality [5].
2. SATELLITE NAVIGATION AND BASEDಥAUGMENTATION SYSTEMS
At the time of this writing (March 2013) two SNS, American GPS and Russian GLONASS, and three SBAS, European EGNOS (European Geostationary Navigation Overlay System), American WAAS (Wide Area Augmentation System) and Japanese MSAS (Multi Satellite Augmentation System), are operational. Two next SNS, Galileo and BeiDou, in Europe and China, respectively, and two SBAS, GAGAN (GPS Aided Geo Augmented Navigation) and SDCM (System for Differential Corrections and Monitoring) in India and Russia, respectively are under construction. Global SNSs provide the user with a three-dimensional positioning solution by passive ranging using radio signals transmitted by orbiting satellites. Selected parameters of GPS, GLONASS, Galileo and BeiDou (China intends to discontinue use of Compass as the English name for BeiDou) are presented in the table 1. Galileo will be the first to offer global integrity alerts (service Safety of Life). Nowadays on the market is available several hundred different models of SNS and SBAS receivers of one hundred manufacturers at least [5]. Users can obtain their own position by means many receivers: one, two, three or four mentioned above SNSs receivers with or without GPS in differential mode, and/or with or without one, two or three SBASs. The signal of all SNS design must avoid interferences between signals of different satellites. The satellite multiplexing methods exploit the one or the other orthogonality between signals. Code division multiple access (CDMA) guarantees access to different satellites by using orthogonal code sequences, in this case all satellites emit the signals on the same two or more frequencies. Frequency division multiple access (FDMA) exploits the spectral separation of different SNS signals, in this case all satellites use the same Compatibility and interoperability of satellite navigation systems in different … 201 codes. The first method is used or planned in GPS, Galileo and BeiDou systems, the second in GLONASS system (block M) only. Galileo was designed for interoperability with GPS, so it shares some of the carrier frequencies however GLONASS was designed independently of GPS [7], [8], [9], [15], [22].
Table 1 Selected parameters of satellite navigation systems, March 2013 [4], [9], [11], [19], [20], [21]
System Parameter GPS GLONASS Galileo BeiDou
Asia-Pacific region ķ global FOC global FOC global IOC ķ 2014 Operability 2012 since VII/1995 since XII/2011 global FOC ķ 2020 global FOC ķ 2020 Satellite FDMA ķ L1, L2 CDMA CDMA CDMA identification CDMA ķ L3 24 operational + 37 operational ķ 27 operational + Satellite constellation 31 operational 5 ÷ 7 different 5 GEO, 27 MEO, 3 active sphere status 5 IGSO Number of 2 (satellites IIa, 2 (satellites M) carrier IIR, IIRķM) 4 3 3 (satellites K1) frequencies 3 (satellites IIF) GLONASSST GPST – GST – Galileo BDT – BeiDou System time – GLONASS GPS Time System Time Time System Time WGS (World GTRF – Galileo China Geodetic Datum Geodetic System) PZ 90.02 Terrestrial System 2000 ķ84 Reference Frame yes, service Integrity Non Non unknown Safety of Life
Horizontal position a few 2 ÷ 4 5 ÷ 6 10 accuracy [m] 95% depending on service
Augmentation systems supplement SNS with additional ranging signals, a differential corrections service, and integrity alerts. SBAS are designed to serve a large country (e.g. USA) or small continent (e.g. Europe) and broadcast to their users via geostationary satellites. Selected parameters of EGNOS, WAAS, MSAS, GAGAN and SDCM, are presented in the table 2. At middle latitudes (40O ̃ 60O) satellites GEO are visible by the user at angle 40O or lower. It means that in these areas, in urban canyons and mountains regions in particular SBAS cannot be used because its satellites are invisible to users. That’s why Japan decided to create new augmentation system QZSS (Quasi Zenith Satellite System) which will provide positioning services primarily to the users of urban transport, a GPS differential corrections service to a higher resolution than Japanese SBAS, MSAS, in particular. The QZSS constellation will comprise three satellites in separate geosynchronous orbits, 202 Jacek Januszewski inclined to the equator at 45O, there is always at least one satellite over Japan at a high elevation angle, 70O or more [9], [16], [23].
Table 2 Selected parameters of satellite based augmentation systems, March 2013 [4], [9], [11], [20], [21] System Parameter EGNOS WAAS MSAS GAGAN SDCM
GPS, GPS, System aided GLONASS GPS GPS GPS GLONASS in future in future open service since 2009, FOC FOC under under Operability safety of life since 2008 since 2007 construction construction service since 2011
Region Europe USA, Canada Japan India Russia
Number of GEO 1 test signals 2 3 2 2 test signals satellites 1 not active L3 Transmission L1 GPS, L5 L1 GPS L1 GPS L1 GPS GLONASS frequency GPS in future L1 GPS Horizontal position 1 ÷ 2 3 2 a few a few accuracy [m] 95%
3. COMPATIBILITY AND INTEROPERABILITY
In many cases a single GNSS is not enough to guarantee the target user performances, especially in challenging conditions such as urban transport. That’s why the emergence of new SNS or SBAS and modernization of current SNS or SBAS entail discussions on compatibility and interoperability among the different service providers. Compatibility can be defined as the ability of each GNSS to be used separately or together without interfering with each individual system and without adversely affecting navigation warfare and/or other harm to an individual system and/or service. Interoperability refers to the ability of each GNSS having independent control loop to operate jointly with other system without interfering each other on condition that signal frequency ranges, coordinate reference frames and time reference frame coincides as much possible. This ability provides GNSS to be used together to provide better capabilities at the user level than would be achieved by relying solely on the open signals of one system [6], [7], [10], [15], [17], [22]. Compatibility and interoperability of satellite navigation systems in different … 203
A reply to question how many systems can be used at the same time is – all SNS and SBAS but on condition that the receiver is suitable and all these systems are compatible and interoperable, which GNSS must be used as the first – this, which provides integrity information. Additionally the interoperability can be discussed at two different levels, system and signal. In the first case a GPS, GLONASS, Galileo or BeiDou user’s receiver should be able to provide the same navigation solution (coordinates of the position, velocity vector) when used standalone. For transport user of GNSS, the marine user in particular, ideal interoperability, called sometimes interchangeability, can be expressed as navigation with one signal each from four or more systems with no additional receiver cost or complexity [17]. Signal interoperability is achieved when the signal provided by different SNS and SBAS are similar enough to allow a GNSS integrated receiver to use all those signals with minor modification. Finally for GNSS, signal interoperability considers three factors described below in this chapter, carrier frequency, reference datum and system time.
Table 3 Signal in space, frequency carrier in different satellite navigation systems [2], [6], [11], [14], [15] Carrier System and carrier symbol frequency [MHz] GPS GLONASS Galileo BeiDou SBAS L5 ķ satellites E5a ķ L5 ķ satellites WAAS 1176.45 IIF and III in all satellites, B2a K2 and later in future future signals 1 and 2 E5b ķ L3 ķ satellites 1207.14 ķ all satellites, B2b ķ K1 and later signals 3 and 4 L2 ķ all satel- 1227.60 lites current ķ ķ ķ ķ and future L2 ķ all 1242.9375 ķ satellites M ķ ķ ķ ķ 1247.75 and later E6 ķ 1278.75 ķ ķ all satellites, ķ ķ signals 5, 6, 7 L2 ķ all satel- E2ķL1ķE1, all systems 1575.42 lites current ķ all satellites B1C current and and future signals 8, 9, 10 future L1 ķ all 1598.0625 ķ satellites M ķ ķ ķ ķ 1604.25 and later
204 Jacek Januszewski
3.1. CARRIER FREQUENCY
All current and future carrier frequencies used in SNSs and SBASs are presented in the table 3. Each SNS uses or will use three different frequencies at least but one frequency (1176.45 MHz) is (will be) the same in all four SNS, next two (1207.14 MHz and 1575.42 MHz) in three SNS. Nowadays the frequency 1575.42 MHz is common for all SBAS for broadcast GNSS correction, the other frequency 1176.45 MHz will be it in the near future. All five frequencies currently used or planned in three SNS, GPS, Galileo and BeiDou, and in all SBAS (tab.3) are based on the fundamental frequency fO = 10.23 MHz, in the case of 1176.45 MHz, 1207.14 MHz, 1227.60 MHz, 1278.75 MHz and 1575.42 MHz, the factor ( fO) is 115, 118, 120, 125 and 154, respectively. In the case of GLONASS system the signals use FDMA techniques, hence a different carrier frequency per satellite [6], [11], [14], [16]. In the case of SNS radio frequency compatibility (RFC) involves consideration of technical factors such as the protection of user equipment against radio frequency interference from other SNS, effects on receiver noise floor and cross-correlation amongst signals. Almost 80 MEO satellites of four SNS and near 10 GEO satellites of all SBAS can contribute to increase the receiver noise floor, hence affecting the signal to noise ratio at GNSS receivers. That’s why the selection of the same carrier frequency for two or more SNS has a high impact on user’s receiver complexity and cost. Regarding user equipment the standards in what concerns RFC exist only for maritime and aviation transport’s users, IMO (International Maritime Organization) and ICAO (International Civil Aviation Organization) requirements, respectively. For land transport, or road and rail, the largest GNSS market segment, there are no standards [22]. The transponder of Russia SBAS (SDCM) will broadcast GNSS correction on the standard GPS L1 frequency. It means that in the near future all five SBAS mentioned in the table X will use the same transmission frequency (L1). This information is very important and at the same time very useful for all users of area transport. In the case of QZSS to ensure compatibility and interoperability with modernized GPS civil signals, the GPS availability enhancement signals transmitted from QZSS satellites use modernized GPS civil signal as a base, transmitting the L1C/A, L1C, L2C and L5 signals. This minimizes changes to specifications and receiver designs. In addition, L1C and L5 of above signals transmitted by QZSS satellites have interoperability with not only GPS but also Galileo and other GNSS in the future. L1C signal transmitted by the first QZSS Michibiki satellite is the first truly interoperable signal [23].
3.2. REFERENCE DATUM
Although the international civil coordinate reference standard is the International Reference Frame (ITRF), each GNSS has its own reference frame, which depends on the control stations’coordinates hence guaranteeing independence among systems. The reference frame for GPS system is World Geodetic System 1984 (WGS84), its present version is almost identical with the latest version ITRF. The coordinates in GLONASS Compatibility and interoperability of satellite navigation systems in different … 205 system are based on the parameter of the Earth 1990 (PE–90) frame, since September 2007 in version 90.02, also known as Parametry Zemli 1990 (PZ–90.02). The new reference geocentric coordinate system for GLONASS, PZ 90.11 is already coordinated with the ITRF at the centimeter level and shall be introduced soon [2]. Its introduction will make possible to improve the GLONASS accuracy characteristics by 15ķ20% [18]. Galileo system will have its own reference frame GTRF (Galileo Terrestrial Reference Frame) [9], Beidou system adopts the China Geodetic Coordinate System 2000 (CGCS2000) [19]. As currently all SBAS augment GPS system only, the reference frame for all these systems is WGS84 also. The QZSS geodetic coordinate system is known as the Japan satellite navigation Geodetic System (JGS). This coordinate system is defined as the approach to ITRF [23]. Two SNS are said to be interoperable from a reference frame perspective if the difference between frames is below target accuracy. Three reference frames, WGS84, GTRF and ITRF, differ by only a few centimeters (i.e. this difference between WGS84 and GTRF is expected to be within 3 cm), so this is only an issue for high-precision users. Therefore we can say that the problem of compatibility of SNS and SBAS in the case of reference frame (datum) for transport users does not exist. For some transport users, marine (ship) and road (car), in particular, this problem appears not till then when the position obtained from GNSS receiver must be plotted on the chart or introduced to electronic chart. The GNSS position must be determined in the same geodetic datum on which the chart was published meanwhile the majority of the currently used charts are not yet referred to WGS4. As position referred to different datums can differ by several hundred meters or even more the user must have the possibility to choose the right datum in the receiver or know the Satellite Derived Positions notes [15].
3.3. SYSTEM TIME
While most clocks in the world are synchronized to UTC (Universal Time Coordinated), the atomic clocks on the satellites are set to own SNS time. Galileo time system (GST) is based on TAI (Time Atomic International), whereas GPS system time (GPST) and GLONASS system time (GLONASSST) are based on, respetively, the U.S. and Russian versions of UTC. GPST and GST are expected to be within the nanoseconds order of magnitude [8], [12], [15], [16]. In the case of Chinese SNS the time reference is BeiDou Time (BDT), related to UTC through UTC (NTSC – National Time Service Center of Chinese Academy of Science). BDT offset will respect to UTC is controlled within 100 ns (modulo 1 second) [19]. The QZSS time, called QZSST, conforms to UTC and the offset with respect to the GPS system time, GPST, is controlled [23]. The time offset between the difference reference time SNS will be emitted in the navigation message of these systems. Various agreements already specify the time offsets and its provision to the user. The data concerning the offset of GST with respect to TAI and UTC will be included in the Galileo navigation message. UTC can be obtained from GPS receiver, and in the future from Galileo receiver, by adding the integral number of leap seconds and fine UTC/TAI correction information contained in the navigation data. In order to provide an estimate of UTC from GPS, the 206 Jacek Januszewski navigation message broadcast by each GPS satellite includes estimates of the time difference between GPST and UTC(USNO) modulo one second, and its rate. The detailed relations can be found in [12], [16].
3.4. INTERNATIONAL COOPERATION
GNSS parameters, signal design as carrier frequencies, signal structures, navigation messages, codes and modulations, in particular affect directly interoperability and therefore cooperation at international level (countries and organizations) has been conducted from an early stage of development in order to guaranties interoperability. The U.S. Government has engaged European Union and a number of other countries in cooperative activities related to space-based PNT (Positioning, Navigation, Timing) systems. This cooperation is intended to ensure among other things compatibility and interoperability between GPS and other SNS and SBAS, and it can be presented as follows [1], [17], [20], [21]: the GPSķGalileo agreement established four working groups, one group for cooperation on radio frequency compatibility and interoperability, the GPSķGLONASS agreement established two working groups, one for radio frequency compatibility and interoperability and one for technical interoperability between the search-and-rescue capabilities planned for these SNS, the USA - Australia and USA - India cooperation expands upon existing efforts to ensure interoperability between GPS and Australia’s Ground Based Augmentation System (GBAS) and between GPS and India’s SBAS (GAGAN), respectively, USA and Japan have achieved interoperability between GPS and Japan’s MSAS and have also taken steps to ensure interoperability between the next generation GPS constellation (GPS III) and Japan’s QZSS (p.2.2), in 2010 the United and China concluded technical coordination discussions on radio frequency compatibility between China’s Compass and GPS.
Similarly since few years the discussions and different types of agreement between European Union, China, India, Japan and Russia have been achieved.
4. CONCLUSIONS
In each mode of transport new SNS and/or SBAS is for different reasons really recommended, but the use of this (these) next system (s), at the same time as existing already can be useful on condition only that all these systems coexist; the most fundamental requirements for this coexistence are compatibility and interoperability of these systems; Compatibility and interoperability of satellite navigation systems in different … 207
Compatibility of SNSs is recommended because the making use of integrated, two or more SNSs, provides insurance against complete failure of one of SNS; The announcement of Full Operational Capability (FOC) of next SNS means that the operation on more frequencies and correlating a greater number of signals will require more receiver hardware, while handling different types of signal and navigation message will require more receiver software; Nowadays regarding user equipment the standards concerning radio frequency compatibility exist for maritime and aviation transport only, for land transport must be created as soon as possible; As next SNS and SBAS will be compatible and interoperable with currently used GNSS and multi-GNSS fully integrated receivers are already available, in area transport Service of Life (system Galileo) will be very useful for all users, in maritime transport, in coastal navigation and restricted area in particular, the number of satellite visible by ship’s antenna will increase considerably in land transport the greater number of satellites used in user’s position calculation will permit the continuous car navigation in urban canyons. In each cases the final result will be greater safety of transport.
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KOMPATYBILNO I MI DZYOPERACYJNO NAWIGACYJNYCH SYSTEMÓW SATELITARNYCH W RÓ NYCH GA ZIACH TRANSPORTU
Streszczenie: Obecnie (marzec 2013) ponad 60 satelitów operacyjnych nawigacyjnych systemów satelitarnych (SNS) GPS i GLONASS oraz satelitarnych wspomagaj cych (SBAS) EGNOS, MSAS i WAAS znajduje si na orbitach oko o ziemskich transmituj c pakiet sygna ów na ró nych cz stotliwo ciach. Ze wzgl du na fakt, e ju wi cej ni 100 mln u ytkowników wszystkich trzech ga zi transportu, l dowego, morskiego i powietrznego, korzysta dzisiaj z odbiorników w/w systemów, w artykule omówiono problem kompatybilno ci i mi dzyoperacyjno ci SNS i SBAS, równie tych znajduj cych si na etapie budowy, czyli odpowiednio Galileo i BeiDou oraz GAGAN, SDCM i QZSS. Pod uwag wzi to trzy parametry, czyli sygna w przestrzeni (cz stotliwo no na), czas systemu oraz uk ad odniesienia wspó rz dnych okre lanej pozycji. Udzielono równie odpowiedzi na takie pytania jak: z ilu systemów mo na jednocze nie korzysta , który system zalecany jest dla danej ga zi transportu, co oznacza dla u ytkownika pojawienie si nowego SNS i SBAS. S owa kluczowe: kompatybilno , nawigacyjny system satelitarny, ga zie transportu