Indian Journal of Radio & Space Physics Vol. 36, August 2007, pp. 293-302 GAGAN - The Indian satellite based augmentation system K N Suryanarayana Rao ISRO Satellite Centre, Airport Road, Bangalore 560 017, India Received 2 April 2007; accepted 14 May 2007 Global Positioning System (GPS) from the USA, Global Navigation Satellite System (GLONASS) from the Russian Federation and the proposed GALILEO satellite navigation system from Europe are meant for providing position and timing information for a variety of applications. However, for Safety Critical applications the basic constellations cannot meet the requirements in terms of accuracy, integrity and availability. For this purpose, the basic constellations are augmented by an overlay system. Indian Space Research Organization (ISRO), along with Airport Authority of India (AAI) is implementing the Satellite Based Augmentation System (SBAS) for the Indian region. The project called GAGAN (GPS Aided Geo Augmented Navigation) has a full complement of the SBAS inclusive of ground and onboard segment. The first phase of GAGAN is nearing completion. This paper deals with the basic SBAS concept, GAGAN configuration, implementation and the challenges involved. The roadmap towards the final operational phase is also indicated. Keywords: GAGAN, Global Navigation Satellite System (GNSS), Satellite based augmentation system (SBAS), Indian Master Control Station (INMCC), Indian Reference Station (INRES) PACS No.: 84.40.Ua 1 Introduction Navigation experts worldwide have been discussing for many years about the concept of one navigational system that is available everywhere on the globe, at all the time with extreme accuracy, trusted and easy to use, which overcomes the limitations of the existing conventional navigational aids. The concept is Global Navigation Satellite System (GNSS). Such a system could be used as the sole means of a navigation system and could eventually replace most, if not all, of the costly ground based infrastructures. Satellite navigation and positioning systems represent the most important technological breakthrough in civil aviation navigation, surveillance, and air traffic management since radar was introduced over half a century ago. The GPS, developed by the United States is currently approved for supplemental use in all weather conditions during en-route, terminal air navigation and for non-precision approaches. For the civil aviation community whose requirements are stringent, GPS/GLONASS constellations alone fail to meet such requirements. Thus, the need for augmenting these constellations arises to meet the required navigation performance for aviation use as navigational system covering various phases of the flight. Augmentation This term refers to enhancements to a navigation system. For a navigation system to be declared as usable for civil aviation purposes, it must fulfill the requirements in a given phase of flight. However, a given navigation system considered alone cannot meet all the desirable requirements. It is thus necessary to hybridize two or more systems of navigation in order to obtain suitable performance. The various augmentation options are as follows: (i) Receiver algorithms (RAIM) (ii) Additional sensors (iii) Extra systems (a) GLONASS; (b) GNSS2-Galileo (i) GPS Modernization (ii) Local Area Augmentation Systems (LAAS) (iii) Wide Area Augmentation Systems (WAAS) (a) EGNOS, US WAAS, MSAS The GPS/GLONASS can be augmented in various ways, but the end results vary accordingly. Current GPS and GLONASS constellation does not satisfy the integrity, accuracy and availability requirements for all phases of flight, particularly for the more stringent precision approaches.
294 INDIAN J RADIO & SPACE PHYS, AUGUST 2007 Integrity It is the ability to protect the user from inaccurate information in a timely manner. Integrity is not guaranteed as all GPS satellites are not monitored at all times. In case of any fault, the time-to-alarm is from minutes to hours. The quality of the service is not indicated. Accuracy It stands for the difference between measured and true positions of a vehicle at any given time. Accuracy is not sufficient even with SA off. Vertical accuracy is greater than 10 m. Continuity It stands for the ability to complete an operation without triggering an alarm. Availability It stands for the ability of the system to be used by the user whenever it is required. Thus the basic constellation needs to be augmented for this purpose. 2 Satellite Based Augmentation System (SBAS) The basic functions of an SBAS system are as follows: Ranging It provides additional ranging signals to improve availability, typically via geo-stationary satellites. Integrity channel It provides transmission of GPS and integrity data to navigators. 2.1 Wide area differential (WAD) Wide area differential 1 following: (i) (WAD) provides the Differential correction data to users to improve accuracy (ii) Satellite orbit and clock errors (iii) Differential range corrections (iv) Ionospheric grid computation An SBAS must provide augmentation services with adequate reliability and continuity. 2.2 SBAS concept As shown in Fig. 1 SBAS employs a ranging function to generate GPS-like signals and enable users to use the concerned geo-stationary satellite as one more GPS satellite for ranging purposes. Information of the GPS constellation is transmitted to each user in the real-time via the integrity function of SBAS, while the differential correction function provides ranging error data to each user. The space-based augmentation systems (SBAS) will provide en-route through precision approach navigation services for all aircraft within the covered airspace. 2.3 Major SBAS segments The major segments of an SBAS are shown in Fig. 2. Ground segment It consists of reference stations located at precisely surveyed locations for ranging and integrity monitoring. Master Control Centre This centre collects, estimates and processes the data to generate wide area correction messages and integrity information to the user. Navigation Land Earth Station It up-links the messages to the geo-stationary satellites (GEO) for Fig. 1 SBAS concept
RAO: GAGAN - THE INDIAN SATELLITE BASED AUGMENTATION SYSTEM 295 Fig. 2 Major SBAS segments Fig. 3 SBAS data flow further broadcast and communication links to transfer data collected from the reference station to the master control station. Space segment Space segments are the following: (i) GPS satellites, GLONASS satellites (ii) GEO satellites for data transmission and ranging function (GEO) User segment Similarly, user segments are the following: (i) Signal in Space (SIS) (ii) Receiver capable of receiving and decoding the GPS/GLONASS/GEO broadcast message Figure 3 shows the signal flow between the various SBAS elements. 2.4 SBAS concept of operation The concept of operations of SBAS is described briefly in five steps as follows: The SBAS reference stations are deployed throughout the region of service at pre-surveyed locations to measure pseudoranges and carrier phases on L 1 and L 2 frequencies from all visible satellites. (A semi-codeless technique is used to derive a code measurement on L 2 ). The reference stations send these measurements to SBAS Master Station, which calculate clock and ephemeris corrections for each GPS satellite monitored, ephemeris information for each GEO, and ionospheric vertical delays on a grid. The grid consists of fixed ionospheric grid points (IGPs) at an
296 INDIAN J RADIO & SPACE PHYS, AUGUST 2007 altitude of 350 km above the Earth s surface. Grid spacings are 5 deg 5 deg between 55 S and 55 N and are larger beyond this region. In addition to the corrections, the Master Station calculates error bounds for ionospheric corrections called grid ionospheric vertical errors (GIVEs) at each IGP, and also combined error bounds for clock and ephemeris corrections for each visible satellite, called user differential range errors (UDREs). The Master Station sends these corrections and error bounds to the users through GEO communication satellites with a data rate of 250 bits/s. User avionics apply these corrections to their pseudoranges obtained from GPS measurements, in order to improve the accuracy of their position estimates. They also use the UDREs and GIVEs and other information to calculate error bounds on position error called the Vertical Protection Level (VPL) and Horizontal Protection Level (HPL). For the integrity of the system, these protection levels must bound the position errors with probability greater or equal to 0.9999999 in one hour for en-route through NPA operations and for PA in 150 s. Thus the SBAS signal received at the user looks just like a GPS signal with the exception that the transmitted message modulation rate is 250 bps instead of 50 bps for GPS data stream and the data are FEC error coded. 2.5 Ionospheric corrections in SBAS The ionospheric errors are the most dominant source of errors for GPS users (see Table 1, Appendix 1). The major effects the ionosphere can have on the GPS signal are (i) group delay of the signal modulation, or absolute range error; (ii) carrier phase advance, or relative range error; (iii) Doppler shift, or range rate errors; (iv) Faraday rotation of linearly polarized signals; (v) refraction or bending of the radio wave; (vi) distortion of pulse waveforms; (vii) signal amplitude fading or amplitude scintillation; and (viii) phase scintillation. Due to the above the delays range from a few meters at night to a maximum of 10 or 20 m at about 1400 hrs. The ionosphere is a region of ionized plasma that extends from roughly 50 km to 2000 km above the surface of the earth. Generally, the ionosphere can be divided into several layers in altitude according to electron density, which reaches its peak value at about 350 km in altitude. For 2D ionospheric modeling, the ionosphere is assumed to be concentrated on a spherical shell of infinitesimal thickness located at the altitude of about 350 km above the earth s surface. The ionosphere introduces frequency dependent delays in the signal, which is a function of total electron content (TEC). This dependence can be exploited by dual-frequency receivers to get an accurate measure of the ionospheric activity along that signal path. The master station collects all the ionospheric data collected from all the reference stations. The implementation of the single-layer grid model requires computation of the intersection of the line-ofsight between the GPS receiver and the observed satellite on the ionosphere shell as illustrated in Fig. 4. The intersection point of the GPS signal with the ionospheric shell is defined as ionospheric pierce point (IPP), at which the slant ionospheric delay has an elevation angle of E. The slant delays are converted to equivalent vertical delays at the point Table 1 Standalone GPS error budget 3 Error source Single frequency receiver (C/A), m Dual frequency receiver (P/Y), m Ephemeris data 2.1 2.1 Satellite clock 2.1 2.1 Ionosphere 4.0 1.2 Troposphere 0.7 0.7 Multipath 1.4 1.4 Receiver 0.5 0.5 measurements Vertical error (1-σ) 12.8 8.3 Horizontal error (1-σ) 10.2 6.6 Fig. 4 Ionospheric shell
RAO: GAGAN - THE INDIAN SATELLITE BASED AUGMENTATION SYSTEM 297 where the LOS pierces the shell. This results in a model which is invariant in the vertical direction and varies only with latitude, longitude and time. Using these samples of equivalent vertical delays, a deterministic trend is fit to the ionosphere. This trend is used to predict the delays on a grid of points, called the ionospheric grid points (IGP s). As shown in Fig. 5 the world is divided into imaginary fixed grid of 5 deg 5 deg at a height of 350 km from the earth surface. The master station converts the slant delay observations to vertical delay estimate to the surrounding grid node. The grid points for interpolation, the ionospheric delay value and the GIVE indicator are sent to the user. The GIVE provides a bound on the accuracy of broadcast ionospheric delay. The user determines the ionospheric pierce point (IPP) for a satellite and interpolates using appropriate surrounding grid points to derive the satellite specific vertical ionospheric delay. 3 GPS Aided Geo Augmented Navigation (GAGAN) overview Indian Space Research Organisation (ISRO) along with Airports Authority of India (AAI) has worked out a joint programme to implement the Satellite Based Augmentation System using GPS/GLONASS over Indian airspace. Although meant for civil aviation, the system can be used by a vast majority of users like personal and public vehicles, railways, shipping, surveys, etc. GAGAN Technology Demonstration System (TDS) is a forerunner for the operation of SBAS over the Indian region. The TDS phase of the project implements a minimum set of elements for demonstrating the SBAS proof of concept over the Indian region. The minimum set includes the following: (i) 8 Indian Reference Stations (INRES) (ii) 1 Indian Master Control Centre (INMCC) (iii) 1 Indian Land Up Link Station (iv) Navigation Transponder with L 1 and L 5 functionality (v) Navigation Software (vi) Associated communication links between INRES, INLUS and INMCC (vii) Total Electron Content (TEC) measurement network and associated ionospheric studies The system configuration of GAGAN is shown in Fig. 6. The following paragraphs provide an insight into the implementation of the various major elements. 3.1 Indian Reference Stations (INRES) The INRES collect measurement data and broadcast messages from all GPS and GEO satellites in view and forward to Indian Mission Control Centre (INMCC). Eight INRES stations are established during the TDS phase at Delhi, Bangalore, Ahmedabad, Calcutta, Jammu, Port Blair, Guwahati and Trivandrum as shown in Fig. 7. The sites are identified after carrying out preliminary site survey. Fig. 5 SBAS ionospheric grid points
298 INDIAN J RADIO & SPACE PHYS, AUGUST 2007 Fig. 6 System configuration of GAGAN Fig. 8 GAGAN INRES station at Bangalore Fig. 9 GAGAN INMCC station at Bangalore Fig. 7 Location of the eight Indian reference stations The suitability of the sites was established after carrying out the multipath study, noise survey and elevation profile pattern. Figure 8 shows one of the Typical INRES stations. 3.2 Indian Master Control Station (INMCC) An Indian Master Control Centre (INMCC) is established at Kundalahalli, Bangalore. The measurement data collected every second from each of the INRES receiver chains are transmitted in realtime to the INMCC for correction and integrity processing and generation of SBAS messages with the aid of the navigation software resident. The INMCC comprises of various subsystems like Data Communication Subsystem (DCSS), Correction and Verification Subsystem (C&VS), Operation and Maintenance Subsystem (OMSS) and Service Monitoring Subsystem (SMS). Figure 9 shows the Indian Master Control Centre at GAGAN Project site at Kundalahalli, Bangalore. 3.3 Indian Navigation Land Earth Uplink Station (INLUS) The INLUS receives correction messages from the INMCC, format those messages for GPS compatibility and transmit them to the GEO satellites for broadcast to user platforms. The INLUS is collocated with INMCC at Bangalore. The INLUS also provides GEO satellite ranging information and corrections to the GEO satellite clocks. Message formats and timing will be according to the functional and performance specifications, which are derived from MOPS (Minimum Operation Performance Standard). Figure 10 shows 11-meter dish antenna for up-linking the SBAS messages to GSAT 4.
RAO: GAGAN - THE INDIAN SATELLITE BASED AUGMENTATION SYSTEM 299 3.4 Navigation payload Geo-stationary satellite component consisting of a GPS L 1 and L 5 compatible navigation payload on an Indian satellite positioned at 83 E is part of the GAGAN configuration in the TDS phase. The navigational payload will be flown on GSAT 4, which is scheduled to be launched by April/May 2008. The navigational payload is of indigenous design and is being built in ISRO as per specifications meeting international requirements for signal-in-space. Figure 11 shows the block diagram of the GAGAN navigation payload. The major functions of the geo-stationary payload are to provide the following: (i) C L path using the C-Band uplink for relaying the geo-stationary overlay signal for use by modified GPS receivers and ionospheric correction (ii) A long loop involving INLUS and the C L payload to correct code and carrier phase errors and achieve coherence between them at satellite output point (iii) Adequate short term stability of the transponder signal to ensure accurate operation of the user receivers The navigation transponder being designed and built at Space Application Centre (SAC) is having one of the best features in its class and meets the latest FAA specifications. Figure 12 shows the coverage of GAGAN. 3.5 Communication links The communication links play a vital role in the GAGAN system. Availability of the system will be affected by the poor performance of communication links. All the INRES stations but for Port Blair are linked to INMCC by optical fibre cables. The INRES at Port Blair is linked to INMCC by VSAT link. The data rate is 128 kbits per second. Fig 10 GAGAN INLUS 11-metre dish antenna at Bangalore 4 Ionospheric studies over Indian region An essential and very important component of the SBAS is the ionospheric correction generated and Fig. 11 GAGAN payload block diagram
300 INDIAN J RADIO & SPACE PHYS, AUGUST 2007 Fig. 12 Typical GAGAN coverage broadcasted to the SBAS users through geo-stationary satellite. The low and mid-latitude region is characterised by large temporal and spatial gradients in the ionospheric delay. This coupled with large amplitude scintillation in these regions presents unique problems for the grid based scheme presently used in SBAS. In addition, the SBAS system may also have to deal with large total electron content (TEC) depletion in small localised areas. The above effects result in increased errors when the existing SBAS grid based ionospheric algorithms are used. The successful implementation of the SBAS depends on the ionospheric model over the region. For this study TEC receivers are installed over the region as shown in Fig. 13 below at various airports for data collection. A number of academic and R&D institutions are involved in the process of studying the ionospheric behaviour over Indian region 2 in the context of SBAS in general and GAGAN in particular. 4.1 Initial experimental phase After successful completion of the TDS, redundancies will be provided to the space segment, INMCC, INLUS and the system validation carried out over the entire Indian airspace. Based on the experience of the TDS, additional augmentation will be worked out. Fig. 13 TEC stations 4.2 Final operational phase During this phase, it is expected that the SATNAV programme will become operational. INMCC will be augmented to meet additional requirements. Additional redundancies will be built-in wherever necessary. Acknowledgements Thanks are due to Shri K Anbarasu, GAGAN Project, who has helped in the preparation of the
RAO: GAGAN - THE INDIAN SATELLITE BASED AUGMENTATION SYSTEM 301 manuscript. The encouragement received from Dr S Pal, PMB, GAGAN and Dr. Shankara, Director, ISAC is gratefully acknowledged. References 1 Mishra Pratap & Enge Per, Global Positioning System, Signals, Measurements and Performance (Ganga-Jamuna Press, Lincoln, Mass, USA), 2001, pp.-123-173. 2 Klobuchar J A, P H Doherty, M B El-Arini, Lejeune R, Dehel T, de Paula E R & Rodrigues F S, Ionospheric Issues for a SBAS in the Equatorial Region, Ionospheric Effects Symposium, Alexandria, Virginia, 7-9 May 2002. 3 Parkinson Bradford W & Spilker James J, Global Positioning System: Theory and Applications, Volume I, by (Jr. American Institute of Aeronautics and Astronautics Inc, USA), 1996, pp. 10-17, 478-483, 485-513. A Factors affecting GPS signal Global Positioning System (GPS) is a complex system based on a constellation of satellites transmitting navigational information. There is a potential for failure at any stage of the system, which may cause error in the broadcast navigational information. The pseudorange from the user receiver u to the kth satellite (ρ u k ), is given by ρ u k = (r u k. l u k ) + b u B k + I u k + T u k + ν u k Appendix 1 (1) Also the continuous carrier phase from the user receiver u to the kth satellite (ρ u k ), is given by ϕ u k = (r u k. l u k ) + b u B k + I u k + N u k λ L1 + ξ u k (2) measurements for GPS. The GPS ranging errors are grouped into the following six classes. (i) Ephemeris data Errors in the transmitted location of the satellite (ii) Satellite clock Errors in the transmitted clock, including satellite augmentation (SA) (iii) Ionosphere Errors in the corrections of pseudorange caused by ionospheric effects (iv) Troposphere Errors in the corrections of pseudorange caused by tropospheric effects (v) Multipath Errors caused by reflected signals entering the receiver antenna (vi) Receiver Errors in the receiver's measurement of range caused by thermal noise, software accuracy, and inter-channel biases where ρ k u the pseudorange from the user receiver u to the kth satellite k ϕ u the continuous carrier phase from the user receiver u to the kth satellite l u k the line of sight from the user receiver u to the kth satellite r k u. l k u the calculated range from the user receiver u to the kth satellite b u the user receiver clock offset from GPS time B k the kth satellite clock offset from GPS time I k u the ionospheric delay along the line-of-sight from the user receiver u to the kth satellite T k u the tropospheric delay along the line-of-sight from the user receiver u to the kth satellite N k u the continuous phase cycle ambiguity from the user receiver u to the kth satellite λ L1 the L 1 carrier phase wavelength, 0.1903 m ν k u the pseudorange measurement error ξ k u the carrier phase measurement error As implied by Eqs (1) and (2), a number of factors conspire to corrupt the pseudorange and carrier phase A.1 Ephemeris errors Ephemeris errors result when the GPS message does not transmit the correct satellite location. Because satellite errors reflect a position prediction, they tend to grow with time from the last control station upload. These errors were closely correlated with the satellite clock, as one would expect. Note that these errors are the same for both the P- and C/Acodes. Each satellite has a unique Precision (P) and Coarse Acquisition (CA) codes that distinguish between the different satellites comprising the GPS. A.2 Satellite clock errors Fundamental to GPS is the one-way ranging that ultimately depends on satellite clock predictability. These satellite clock errors affect both the C/A- and P-code users in the same way. This effect is also independent of satellite direction, which is important when the technique of differential corrections is used. All differential stations and users measure an identical satellite clock error. The ability to predict clock behaviour is a measure of clock quality. The GPS uses atomic clocks (cesium and rubidium oscillators), which have stability of about 1 part in 10E13 over a day. If a clock can be predicted
302 INDIAN J RADIO & SPACE PHYS, AUGUST 2007 to this accuracy, its error in a day (~10E5 s) will be about 10E- 8 s or about 3.5 m. A.3 Ionosphere errors Because of free electrons in the ionosphere, GPS signals do not travel at the vacuum speed of light as they transit this region. The modulation on the signal is delayed in proportion to the number of free electrons encountered and is also (to first order) proportional to the inverse of the carrier frequency squared (1/f 2 ). The phase of the radio frequency carrier is advanced by the same amount because of these effects. Carrier-smoothed receivers should take this into account in the design of their filters. The ionosphere is usually reasonably well-behaved and stable in the temperate zones; near the equator or magnetic poles it can fluctuate considerably. A.4 Troposphere errors Another deviation from the vacuum speed of light is caused by the troposphere. Variations in temperature, pressure, and humidity all contribute to variations in the speed of light and radio waves. Both the code and carrier will have the same delays. A.5 Multipath errors Multipath is the error caused by reflected signals entering the front end of the receiver and masking the real correlation peak. These effects tend to be more pronounced in a static receiver near large reflecting surfaces. Monitor or reference stations require special care in siting to avoid unacceptable errors. The first line of defense is to use the combination of antenna cut-off angle and antenna location that minimizes this problem. A second approach is to use so-called "narrow correlator receivers, which tend to minimize the impact of multipath on range tracking accuracy. A.6 Receiver errors Initially most GPS commercial receivers were sequential, in that one or two tracking channels shared the burden of locking on to four or more satellites. With modem chip technology, it is common to place three or more tracking channels on a single inexpensive chip. As the size and cost have shrunk, techniques have improved and 10- or 12-channel receivers are common. Most modem receivers use reconstructed carrier to aid the code tracking loops. Inter-channel bias is minimized with digital sampling and all-digital designs.