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GAGAN
1. What is SBAS (Satellite Based Augmentation System)?
Satellite-based augmentation systems (SBAS), such as GAGAN, WAAS, EGNOS and MSAS, complement existing global navigation satellite systems (GNSS). SBAS compensate for certain disadvantages of GNSS in terms of accuracy, integrity, continuity and availability.
Neither the USA's GPS nor Russia's GLONASS meet the operational requirements set by the International Civil Aviation Organization (ICAO) for use during the most critical phases of aircraft flight, in particular landing. To solve it, ICAO decided to standardize several GNSS augmentation systems including SBAS.
The SBAS concept is based on the transmission of differential corrections and integrity messages for navigation satellites that are within sight of a network of reference stations deployed across an entire continent. SBAS messages are broadcast via geostationary satellites able to cover vast areas
2. How does an SBAS (Satellite Based Augmentation System) system work?
SBAS Reference Stations are deployed throughout the region of service at pre-surveyed locations to measure pseudo ranges and carrier phases on L1 and L2 frequencies from all visible satellites.
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 Ionosphere grid points (IGPs) at an altitude of 350km above the earth's surface.
In addition to the corrections, the Master Station calculate error bounds for Ionosphere corrections call grid Ionosphere 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 250bits/s
User avionics apply these corrections to their pseudo ranges 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 must bound the position errors with probability greater or equal to 0.9999999 in one hour for en-route through Non Precision Approach operations and for Precision Approach in 150 seconds.
TOP
3. Where can I find some information on SBAS? Which is the reference documentation on SBAS systems development?
- ICAO Annex10 Volume I (Radio Navigation Aids) 6th Edition July 2006 plus amendment 82 and 83
- RTCA DO 229 C/D
- IS GPS 200 Revision D NAVSTAR GPS Space Segment / Navigation User Interface 07/12/2004
-Details about EGNOS
http://www.esa.int/esaNA/egnos.ht
http://www.esa.int/navigation/egno
http://www.essp-sas.eu/
-Details about of WAAS
http://www.nstb.tc.faa.gov
4. What is GAGAN?
GAGAN is the acronym for GPS Aided GEO Augmented Navigation. The GAGAN uses a system of ground stations to provide necessary augmentations to the GPS standard positioning service(SPS) navigation signal. A network of precisely surveyed ground reference stations (INdian Reference Stations INRES) is strategically positioned across the country to collect GPS satellite data. Using this information, the master control centre (INdian Master Control Centre INMCC) generates messages to correct any signal errors. These correction messages are then uplinked through (INdian Land Uplink Station INLUS) and broadcast through communication satellites (Geostationary) to receivers onboard aircraft using the same frequency as GPS.
The GAGAN is designed to provide the additional accuracy, availability, and integrity necessary to enable users to rely on GPS for all phases of flight, from en route through approach for all qualified airports within the GAGAN service volume. GAGAN will also provide the capability for increased accuracy in position reporting, allowing for more uniform and high-quality Air Traffic Management (ATM). In addition, GAGAN will provide benefits beyond aviation to all modes of transportation, including maritime, highways, and railroads
5. What is the difference between the GPS and GAGAN architecture?
The GPS, GLONASS and Galileo (future) are core constellation of satellites providing signals from space transmitting positioning and timing data. The GAGAN is an augmentation system which augments the GPS constellation by adding accuracy and integrity. This is achieved by sending corrections and integrity information for GPS satellites through geostationary satellites to its users in the pre- defined service volume.
6. What is the status of the GAGAN system in India?
The GAGAN system is being implemented in two phases in India. The first phase called the GAGAN Technology demonstration System (TDS) was intended for the proof of concept over the Indian region. The TDS phase was successfully completed in the year 2005. The implementation of the Final operational phase (FOP) is in progress. The installations of Ground Base Equipments (GBE) are in the final stage. The system is expected to be certified and operational in the year 2013
7. Which satellites are being used to broadcast the GAGAN signal?
GAGAN will have two Indigenous Geostationary satellites broadcasting the SBAS signals. The satellites are identified as GSAT-8 with PRN 127 and GSAT-10 with PRN 128. There is a plan to have an in-orbit spare satellite also.
8. I wish to know more on GAGAN; where may I obtain GAGAN related publications?
A web-site is being planned for GAGAN related information. The details of the proposed site will be made available in the official website of Airports Authority of India, www.aai.aero .
9. Where can I find real time information on GAGAN broadcast status and performances?
A web-site is being planned to display the real time performance of the GAGAN system. The details of the site will be made available in the official website of Airports Authority of India www.aai.aero
TOP
10. What are the GAGAN performances?
The GAGAN, when operational is designed to achieve a performance level of APV1.5 over the Indian land mass and RNP.1 over the oceanic region, within the Indian Flight Information Region(FIR)
11. What is the status of the GAGAN system and what is the current planning?
Same answer as that for question 6
12. In the SBAS operational context, will a common receiver be able to process GAGAN, EGNOS, MSAS and WAAS signals?
To ensure global compatibility in civil aviation, the use of radio navigation aids is governed by the ICAO (International Civil Aviation Organization) Standards and Recommended Practices (SARPs). One of the most important requirements was to ensure that any properly equipped aircraft could benefit from these regional systems by installing a single receiver. RTCA Inc has developed the Minimum Operational Performance Standards (MOPS) for SBAS equipment. The main reference document is DO229 version C/D. This document describes the standards for all GAGAN/EGNOS/WAAS/MSAS receivers. Receivers complying with these requirements, as determined by the appropriate Technical Standard Order (TSO) provide full GAGAN/EGNOS/WAAS/MSAS compatibility. In its full operational configuration, GAGAN is compliant with the RTCA specifications and the ICAO SARPs for use in civil aviation.
13. Is there a list of GAGAN compatible receivers available on the market?
As GAGAN is compliant with RTCA specifications, all SBAS receivers which can receive WAAS/EGNOS/MSAS signals can also receive GAGAN signals.
14. What are the different GAGAN transmission modes?
GAGAN is planned to be operating an open service transmission mode complying to RTCA specifications and ICAO SARPS
15. Is there a map showing the GAGAN service area?
The GAGAN coverage area is given in the figure (Fig1.0), with the Indian Geostationary satellites. The Fig1.1 gives the Indian Flight Information Region(FIR), for which the service is planned. The actual service area can be finalized only after the Signal-in-Space(SIS) availability and the analysis the data collected.
16. What is the GAGAN implementation time schedule?
The system is being implemented in two phases in India. The first phase called the GAGAN Technology demonstration System (TDS) was intended for the proof of concept over the Indian region. The TDS phase was successfully completed in the year 2005. The implementation of the Final operational phase (FOP) is in progress. The installations of Ground Base Equipments (GBE) are in the final stage. The system is expected to be certified and operational in the year 2013.
17. When the GAGAN system changeover will take place from Ground Based Navigation System, will navigation services be interrupted?
There should not be any interruption of service when GAGAN system changeover will take place from Ground Based Navigation System. There will be a period of parallel operation between the two systems before changeover. Also both the systems can co-exist.
18. Can the GAGAN service area be extended?
The GAGAN service area can be extended within the GAGAN coverage area by adding more reference stations. The additional reference stations will be able to collect the GPS measurement data from those areas and can be used for generating accurate Ionosphere and Satellite corrections and integrity parameters
19. Can GAGAN provide an accurate time reference?
The GAGAN network Time (GNT) will be steered to within 50ns from GPS Time and within 20ns from UTC as per the ICAO specifications. Hence GAGAN can provide an accurate time reference within its specifications
20. Is GAGAN signal encrypted or affected by interferences?
The GAGAN signal is intended for civilian safety of life and other navigation applications. The GAGAN signal is not encrypted. The signal may be affected by local RF interferences like the GPS signals
21. Are the WAAS / EGNOS signals valid in the GAGAN coverage area?
The WAAS / EGNOS signals will not give the intended accuracy and integrity in the GAGAN coverage area. This is because the Ionospheric Grid Points (IGP) serviced by WAAS/EGNOS are different from those required for GAGAN coverage area. Also, the GPS satellites visible in the GAGAN coverage will be different from those visible for WAAS/EGNOS. But there will be common area of coverage between the different systems for seamless navigation
22. Why does the GAGAN sometimes not improve the accuracy of GPS?
GAGAN provides the Ionosphere, Satellite Clock and Satellite Ephemeris (ICE) corrections to the users in its coverage area. These corrections are generated by using the GPS measurement data collected from the Reference stations,hence there may be occasions where the GAGAN accuracy may be less than that of GPS due to some local phenomena not observed by the reference stations. But the GAGAN system ensures that all the users in its service volume are always protected within the integrity bounds for a particular level of service. The integrity requirement as specified by ICAO is 1-2x10- 7/app.
23. GPS is being modernized. Will this affect SBAS (e.g. GAGAN) systems?
The introduction of the second civilian frequency (L5) for GPS may change the way the GPS measurement signals getting processed to generate the corrections and integrity information in the master control centre. This may lead to a new SBAS message structure. The change has to evolve after the availability of L5 signals for the entire GPS constellation
24. In which applications could GAGAN be used?
GAGAN even though designed and implemented for civil aviation application has the potential to become the navigational aid for all applications. The expected applications include railways, ships, road navigation etc. The other emerging applications include precise farming and fishing.
25. Where and when can I obtain the GAGAN User Interface Document?
There is no separate User Interface Document for GAGAN. GAGAN system complies to the following:
RTCA DO- 229-C/D Minimum Operational Performance Standards
ICAO Annex10 Volume I (Radio Navigation Aids) 6th Edition July 2006 plus amendment 82 and 83
Current Status of GAGAN
The Final System Acceptance Test (FSAT) was completed during July 2012 and the GAGAN signal in space is evaluated using SBAS receivers at various locations across the country.
GAGAN certification can be completed by the third quarter of 2013.
(Statement released in SEP/2012)
1. What is SBAS (Satellite Based Augmentation System)?
Satellite-based augmentation systems (SBAS), such as GAGAN, WAAS, EGNOS and MSAS, complement existing global navigation satellite systems (GNSS). SBAS compensate for certain disadvantages of GNSS in terms of accuracy, integrity, continuity and availability.
Neither the USA's GPS nor Russia's GLONASS meet the operational requirements set by the International Civil Aviation Organization (ICAO) for use during the most critical phases of aircraft flight, in particular landing. To solve it, ICAO decided to standardize several GNSS augmentation systems including SBAS.
The SBAS concept is based on the transmission of differential corrections and integrity messages for navigation satellites that are within sight of a network of reference stations deployed across an entire continent. SBAS messages are broadcast via geostationary satellites able to cover vast areas
2. How does an SBAS (Satellite Based Augmentation System) system work?
SBAS Reference Stations are deployed throughout the region of service at pre-surveyed locations to measure pseudo ranges and carrier phases on L1 and L2 frequencies from all visible satellites.
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 Ionosphere grid points (IGPs) at an altitude of 350km above the earth's surface.
In addition to the corrections, the Master Station calculate error bounds for Ionosphere corrections call grid Ionosphere 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 250bits/s
User avionics apply these corrections to their pseudo ranges 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 must bound the position errors with probability greater or equal to 0.9999999 in one hour for en-route through Non Precision Approach operations and for Precision Approach in 150 seconds.
TOP
3. Where can I find some information on SBAS? Which is the reference documentation on SBAS systems development?
- ICAO Annex10 Volume I (Radio Navigation Aids) 6th Edition July 2006 plus amendment 82 and 83
- RTCA DO 229 C/D
- IS GPS 200 Revision D NAVSTAR GPS Space Segment / Navigation User Interface 07/12/2004
-Details about EGNOS
http://www.esa.int/esaNA/egnos.ht
http://www.esa.int/navigation/egno
http://www.essp-sas.eu/
-Details about of WAAS
http://www.nstb.tc.faa.gov
4. What is GAGAN?
GAGAN is the acronym for GPS Aided GEO Augmented Navigation. The GAGAN uses a system of ground stations to provide necessary augmentations to the GPS standard positioning service(SPS) navigation signal. A network of precisely surveyed ground reference stations (INdian Reference Stations INRES) is strategically positioned across the country to collect GPS satellite data. Using this information, the master control centre (INdian Master Control Centre INMCC) generates messages to correct any signal errors. These correction messages are then uplinked through (INdian Land Uplink Station INLUS) and broadcast through communication satellites (Geostationary) to receivers onboard aircraft using the same frequency as GPS.
The GAGAN is designed to provide the additional accuracy, availability, and integrity necessary to enable users to rely on GPS for all phases of flight, from en route through approach for all qualified airports within the GAGAN service volume. GAGAN will also provide the capability for increased accuracy in position reporting, allowing for more uniform and high-quality Air Traffic Management (ATM). In addition, GAGAN will provide benefits beyond aviation to all modes of transportation, including maritime, highways, and railroads
5. What is the difference between the GPS and GAGAN architecture?
The GPS, GLONASS and Galileo (future) are core constellation of satellites providing signals from space transmitting positioning and timing data. The GAGAN is an augmentation system which augments the GPS constellation by adding accuracy and integrity. This is achieved by sending corrections and integrity information for GPS satellites through geostationary satellites to its users in the pre- defined service volume.
6. What is the status of the GAGAN system in India?
The GAGAN system is being implemented in two phases in India. The first phase called the GAGAN Technology demonstration System (TDS) was intended for the proof of concept over the Indian region. The TDS phase was successfully completed in the year 2005. The implementation of the Final operational phase (FOP) is in progress. The installations of Ground Base Equipments (GBE) are in the final stage. The system is expected to be certified and operational in the year 2013
7. Which satellites are being used to broadcast the GAGAN signal?
GAGAN will have two Indigenous Geostationary satellites broadcasting the SBAS signals. The satellites are identified as GSAT-8 with PRN 127 and GSAT-10 with PRN 128. There is a plan to have an in-orbit spare satellite also.
8. I wish to know more on GAGAN; where may I obtain GAGAN related publications?
A web-site is being planned for GAGAN related information. The details of the proposed site will be made available in the official website of Airports Authority of India, www.aai.aero .
9. Where can I find real time information on GAGAN broadcast status and performances?
A web-site is being planned to display the real time performance of the GAGAN system. The details of the site will be made available in the official website of Airports Authority of India www.aai.aero
TOP
10. What are the GAGAN performances?
The GAGAN, when operational is designed to achieve a performance level of APV1.5 over the Indian land mass and RNP.1 over the oceanic region, within the Indian Flight Information Region(FIR)
11. What is the status of the GAGAN system and what is the current planning?
Same answer as that for question 6
12. In the SBAS operational context, will a common receiver be able to process GAGAN, EGNOS, MSAS and WAAS signals?
To ensure global compatibility in civil aviation, the use of radio navigation aids is governed by the ICAO (International Civil Aviation Organization) Standards and Recommended Practices (SARPs). One of the most important requirements was to ensure that any properly equipped aircraft could benefit from these regional systems by installing a single receiver. RTCA Inc has developed the Minimum Operational Performance Standards (MOPS) for SBAS equipment. The main reference document is DO229 version C/D. This document describes the standards for all GAGAN/EGNOS/WAAS/MSAS receivers. Receivers complying with these requirements, as determined by the appropriate Technical Standard Order (TSO) provide full GAGAN/EGNOS/WAAS/MSAS compatibility. In its full operational configuration, GAGAN is compliant with the RTCA specifications and the ICAO SARPs for use in civil aviation.
13. Is there a list of GAGAN compatible receivers available on the market?
As GAGAN is compliant with RTCA specifications, all SBAS receivers which can receive WAAS/EGNOS/MSAS signals can also receive GAGAN signals.
14. What are the different GAGAN transmission modes?
GAGAN is planned to be operating an open service transmission mode complying to RTCA specifications and ICAO SARPS
15. Is there a map showing the GAGAN service area?
The GAGAN coverage area is given in the figure (Fig1.0), with the Indian Geostationary satellites. The Fig1.1 gives the Indian Flight Information Region(FIR), for which the service is planned. The actual service area can be finalized only after the Signal-in-Space(SIS) availability and the analysis the data collected.
16. What is the GAGAN implementation time schedule?
The system is being implemented in two phases in India. The first phase called the GAGAN Technology demonstration System (TDS) was intended for the proof of concept over the Indian region. The TDS phase was successfully completed in the year 2005. The implementation of the Final operational phase (FOP) is in progress. The installations of Ground Base Equipments (GBE) are in the final stage. The system is expected to be certified and operational in the year 2013.
17. When the GAGAN system changeover will take place from Ground Based Navigation System, will navigation services be interrupted?
There should not be any interruption of service when GAGAN system changeover will take place from Ground Based Navigation System. There will be a period of parallel operation between the two systems before changeover. Also both the systems can co-exist.
18. Can the GAGAN service area be extended?
The GAGAN service area can be extended within the GAGAN coverage area by adding more reference stations. The additional reference stations will be able to collect the GPS measurement data from those areas and can be used for generating accurate Ionosphere and Satellite corrections and integrity parameters
19. Can GAGAN provide an accurate time reference?
The GAGAN network Time (GNT) will be steered to within 50ns from GPS Time and within 20ns from UTC as per the ICAO specifications. Hence GAGAN can provide an accurate time reference within its specifications
20. Is GAGAN signal encrypted or affected by interferences?
The GAGAN signal is intended for civilian safety of life and other navigation applications. The GAGAN signal is not encrypted. The signal may be affected by local RF interferences like the GPS signals
21. Are the WAAS / EGNOS signals valid in the GAGAN coverage area?
The WAAS / EGNOS signals will not give the intended accuracy and integrity in the GAGAN coverage area. This is because the Ionospheric Grid Points (IGP) serviced by WAAS/EGNOS are different from those required for GAGAN coverage area. Also, the GPS satellites visible in the GAGAN coverage will be different from those visible for WAAS/EGNOS. But there will be common area of coverage between the different systems for seamless navigation
22. Why does the GAGAN sometimes not improve the accuracy of GPS?
GAGAN provides the Ionosphere, Satellite Clock and Satellite Ephemeris (ICE) corrections to the users in its coverage area. These corrections are generated by using the GPS measurement data collected from the Reference stations,hence there may be occasions where the GAGAN accuracy may be less than that of GPS due to some local phenomena not observed by the reference stations. But the GAGAN system ensures that all the users in its service volume are always protected within the integrity bounds for a particular level of service. The integrity requirement as specified by ICAO is 1-2x10- 7/app.
23. GPS is being modernized. Will this affect SBAS (e.g. GAGAN) systems?
The introduction of the second civilian frequency (L5) for GPS may change the way the GPS measurement signals getting processed to generate the corrections and integrity information in the master control centre. This may lead to a new SBAS message structure. The change has to evolve after the availability of L5 signals for the entire GPS constellation
24. In which applications could GAGAN be used?
GAGAN even though designed and implemented for civil aviation application has the potential to become the navigational aid for all applications. The expected applications include railways, ships, road navigation etc. The other emerging applications include precise farming and fishing.
25. Where and when can I obtain the GAGAN User Interface Document?
There is no separate User Interface Document for GAGAN. GAGAN system complies to the following:
RTCA DO- 229-C/D Minimum Operational Performance Standards
ICAO Annex10 Volume I (Radio Navigation Aids) 6th Edition July 2006 plus amendment 82 and 83
Current Status of GAGAN
The Final System Acceptance Test (FSAT) was completed during July 2012 and the GAGAN signal in space is evaluated using SBAS receivers at various locations across the country.
GAGAN certification can be completed by the third quarter of 2013.
(Statement released in SEP/2012)
