Interface Control Document (ICD) for GAGAN Messaging Service(GMS) User Receiver, Click here to download ICD.

FAQ

Q1. What is GAGAN?

GAGAN is an Indian Satellite Based Augmentation System (SBAS). GAGAN stands for GPS Aided GEO Augmented Navigation.

Q2. What is the purpose of GAGAN?

The GAGAN is implemented to provide required accuracy, continuity, availability, and integrity as per ICAO SARPS to enable users/aircrafts to rely on GPS for all phases of flight.

Q3. How does user navigation positioning using satellite work?

The basic principle of navigation positioning is the use of distance measurements at a precise moment in time between a receiver and several navigation satellites whose exact positions in space are known. The satellites emit electromagnetic waves which are propagated through space at the speed of light. It is then possible to calculate the distance separating the satellite from the receiver by determining the time a wave takes to travel from satellite to receiver using a mathematical formula: d = c * t, where 'd' is the distance, 'c' is the speed of light and 't' the time it takes for the wave to travel from satellite to receiver.

Q4. How much should I pay for GAGAN Service?

GAGAN service is free until/unless further notified.

Q5.What services are available from GAGAN?

GAGAN is certified for two different levels of aviation services. I. RNP0.1 Service - for enroute operations II. APV I Service - for precision approach services GAGAN though primarily meant for aviation, will provide benefits beyond aviation to many other user segments such as intelligent transportation, maritime, highways, railways, surveying, telecom industry, personal users of position location applications etc.

Q6. What is IWG?

The Interoperability Working Group (IWG) is formed for the cooperation between SBAS owned or SBAS developing states. Although all satellite-based augmentation systems (SBAS) are regional systems, IWG ensures that all the systems can be integrated into a seamless worldwide navigation network with common understanding and implementation of ICAO SARPS.

Q7. Whether GAGAN comes under IWG?

Yes, GAGAN also comes under IWG. GAGAN is interoperable with other SBAS. The 30th Interoperability Working Group (IWG-30) meeting hosted by China in 2016 has 9 participant countries which have developed or developing SBAS network : India, China, USA, Russia, Europe, South Korea, Japan, Africa, Canada.

Q8. What is GAGAN Architecture ?

The GAGAN system is implemented with 15 Indian Reference Stations (INRES) two Indian Master Control Centres (NMCC), Geostationary satellites and Indian Land Uplink Stations (INLUS).The GPS and GEO satellite data is received and processed at widely dispersed INRES which are strategically located to provide coverage over the required service volume. Data forwarded to INMCC is processed to determine the differential corrections and residual errors for each monitored satellite and for each predetermined ionospheric grid point (IGP). Information from the INMCC is sent to the INLUS and uplinked along with the GEO navigation message to the GAGAN GEO satellite. The GAGAN GEO satellite downlinks this data to the users via two L-band ranging frequencies (L1 and L5), with GPS type modulation. The broadcasted data can be received with GAGAN enabled GPS receivers.

Q9. Who benefits from GAGAN?

Aviation: GAGAN is designed to assist navigation both en-route as well as during landing. The potential benefits will assist air traffic control to cope with increased traffic as well as improving performance while maintaining safety and reducing the infrastructure needed on the ground.

Maritime: GAGAN can be used for navigation operations, traffic management, sea port operations, casualty analysis and offshore exploration and the exploitation of fisheries.

Land transport: GAGAN can be utilized to manage land transport in India, whether it is by road or railways. It will increase both the capacity and the safety of land transport. Companies operating transport services need to know where their vehicles are at all times, as do other public services such as police, ambulance and taxi.

Diverse potential uses: GAGAN can help farmers in aerial crop spraying or precision farming. GAGAN is being used for forest and wildlife resource management system using a receiver designed for specifically this purpose called SXTreo T51.

Q10. Who implemented GAGAN?

The Indian Space Research Organization (ISRO) and Airports Authority of India (AAI) have implemented the GPS Aided Geo Augmented Navigation-GAGAN.

Q11. Who provides GAGAN Services for ANS users for civil aviation purpose?

Airports Authority of India (AAI) provides the GAGANServices for ANS users in the Indian FIR for civil aviation purpose. AAI is public sector under taking in the Ministry Of Civil Aviation, Government of India.

Q12. Is it possible the GAGAN services to make it available beyond Indian Flight Information Regions(FIR)?

Yes. The GAGAN services are available beyond the Indian FIR due to the large foot print coverage area of the GAGAN-GEO satellite signals.It requires an additional Reference stations to be installed to the new locations and data has to be flown to the Bengaluru and Delhi where the processing is done and GAGAN Messages are generated which contains the data for the new locations/areas.

  • Presently GAGAN CVSS is processing only 15 Reference station data.
  • GAGAN CVSS (HW and SW) is having the capability to process the upto 45 reference stations data.

 

Q13. Is GAGAN signal encrypted? How it is protected?

The GAGAN signal is not encrypted. To protect the data, it is FEC encoded (GAGAN message 250bits/sec to 500 symbols/sec) to avoid the decoding or transmission errors.

Q14. Is there a list of SBAS receivers?

Many SBAS receivers are available in the market. A few are listed below. 1. M/s Accord make NexNav series GPS SBAS receiver (certified and MOPS complied) 2. M/s Garmin make GPS-500 SBAS receiver(certified and MOPS complied) 3. M/s Novatel DL-4 SBAS receiver.( Not certified, Not MOPS complied, for SBAS positions only, it can be used for non aviation purpose.)

Q15. Will a receiver be able to process GAGAN, EGNOS, MSAS and WAAS signals?

All SBAS receivers are cable to process the GAGAN, EGNOS, MSAS, WAAS, Augmentation signals for the GPS core constellation. The SBAS receivers certified and compliant with RTCA/DO-229D MOPS are only used for civil aviation purpose.

Q16. What is SBAS?

Satellite Based Augmentation System (SBAS) is a wide area augmentation system that provides augmented accuracy and integrity to a Global Navigation Satellite Systems (GNSS) navigation signal such as GPS. An SBAS provides improved service availability over a wide area and is a more reliable navigation service than GNSS alone. This system uses ground monitoring stations spread across a wide are and provides signals from satellites to support high availability operations from an enroute through to precision approach over a large geographic area.

Q17. 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 calculates 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 calculates 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 greater or equal to 0.9999999 in one hour for en-route through Non Precision Approach operations and for Precision Approach in 150 seconds.

Q18. Which are the SBAS networks in operation or under development? 

There are four SBAS networks in operation today, India’s GPS Aided Geo Augmented Navigation (GAGAN) , the Multifunctional Transport Satellite (MTSAT) Satellite Augmentation System (MSAS), the United States (US) Wide Area Augmentation System (WAAS), and the European Geostationary Navigation Overlay Service (EGNOS), providing coverage geographically to Asia-Pacific, North America and most of Europe . Russian System of Differential Correction and Monitoring (SDCM) is under development. Completion of all five systems will provide near total coverage of the northern hemisphere for approach operations and near worldwide coverage for enroute, terminal and non-precision approach operations.

Q19. What is the status of GAGAN approach procedures ?

GAGAN Approach procedures are under development.

Q20. What are the benefits of SBAS?

There are several benefits of SBAS system :

  • SBAS is designed to enable users to rely on GNSS navigation data for all phases of flight, from en route through category I approach for all qualified airports within an SBAS coverage area.
  • SBAS provides a capability to conduct vertically guided approaches to non-instrumented runways, providing significant improvement to operational safety that was previously un-available.
  • SBAS is not sensitive to temperature fluctuations and has no barometric / temperature limitations. When using Barometric Vertical Navigation (BARO VNAV) a minimum temperature limitation is published for each procedure for which BARO-VNAV minimums are published. This temperature represents the airport temperature below which the use of BARO-VNAV is not authorized to the LNAV/VNAV DA. The pronounced effect of cold temperatures on BARO-VNAV operations means that the approach may not be flown at all using BARO-VNAV when the temperature is below -20 deg Celsius.
  • SBAS is an enabler for FAA Next Generation Transportation System (NEXTGEN) and European Single European Sky Air Traffic Management Research (SESAR).
  • SBAS provides benefits beyond aviation to all modes of transportation, including maritime, highways, and railroads.

 

Q21.What is SBAS interoperability?

Although all SBAS are regional systems, the need to establish adequate co-operation/coordination among SBAS providers is commonly recognized so that their implementation becomes more effective and part of a seamless world-wide navigation system. ICAO SARPs Annex 10 and Aviation Standards support interoperability among SBAS service providers by:

  • Seamless transition between SBAS Service Areas.
  • Evaluating transitions between SBAS and RAIM along with transitions between two SBAS and between SBAS and GBAS.
  • Common interpretation of Standards amongst SBAS Developers.
  • Established a work plan for development of a definition document to support a dual-frequency, multi- constellation user.
  • Currently Limited Global Coverage.
  • Global coverage to be expanded with addition of SDCM, BDSBAS, ASECNA and KASS.
  • Availability of worldwide LPV-200 service expected with addition of a second frequency, extended networks and additional GNSS constellations.
  • Continued support to legacy single frequency users by ensuring backwards compatibility.

Q22.What is SBAS IWG Objectives?

SBAS IWG objectives established to support technical interoperability and cooperation:

Objective 1: Harmonize SBAS modernization plans.

Objective 2: Forum for discussion on SBAS standards.

Objective 3: Harmonize technical improvements for operation and user feedback.

Objective 4: Research and development cooperation on key SBAS technologies.

Objective 5: Support joint SBAS promotion.

Q23. Will SBAS provide a performance comparable to ILS?

Yes. An SBAS LPV approach is designed to provide performance comparable to a Category 1 Instrument Landing System (ILS) approach. In fact, efforts undertaken by the FAA have demonstrated that an SBAS is capable of supporting Height Above Touchdown (HAT) heights down to 200 ft. The level of service provided by an SBAS is contingent on a number of factors:

1. Environmental conditions based on geographic location.

2. The number and density of ground monitoring stations.

3.The approach for monitoring and correcting system errors.

Q24. Can an SBAS avionics receiver be used to fly RNAV approaches?

Yes, an approved SBAS avionics receiver is certified for all of the lines of minima on the RNAV (GPS) approaches (LPV, LP, LNAV/VNAV, and LNAV).

Q25. What is GPS/SBAS LPV, LP, LNAV/VNAV and LNAV approaches with respect to GAGAN SBAS operations?

LPV: is the most desired APV approach. It is similar to LNAV/VNAV except it is much more precise (40m lateral limit), enables descent as low as 200-250 feet above the runway and can only be flown with an approved SBAS Avionics receiver. LPV approaches are operationally equivalent to the legacy instrument landing systems (ILS), but are more economical because no navigation infrastructure is required at the runway.

Localizer Performance (LP): is a non-precision approach (NPA) procedure that uses SBAS precision of LPV for lateral guidance and barometric altimeter for minimum descent altitude (MDA) guidance. These approaches are needed at runways where, due to obstacles or other infrastructure limitations, a vertically guided approach (LPV or LNAV/VNAV) cannot be published. LP approaches can only be flown by aircraft equipped with SBAS Avionics receivers. The MDA for the LP approach is expected to be nominally 300 to 400 feet above the runway.

LNAV / VNAV: approaches use lateral guidance (556m lateral limit) from GPS and/or SBAS and vertical guidance provided by either the barometric altimeter or SBAS. Aircraft that don’t use SBAS for the vertical guidance portion must have a Baro-VNAV system, which are typically part of a flight management system (FMS). When the pilot flies an LNAV / VNAV approach, lateral and vertical guidance is provided to fly a controlled descent and a safer maneuver to the runway. The decision altitudes on these approaches are usually 350 feet above the runway.

GPS NPA (LNAV):refers to a Non-Precision Approach (NPA) procedure which uses GPS and/or SBAS for Lateral Navigation (LNAV). On an LNAV approach, the pilot flies the final approach lateral course, but does not receive vertical guidance for a controlled descent to the runway. Instead, when the aircraft reaches the final approach fix, the pilot descends to a minimum descent altitude using the barometric altimeter. LNAV approaches are less precise (556m lateral limit) and therefore usually do not allow the pilot to descend to as low an altitude above the runway. Typically, LNAV procedures achieve a minimum descent altitude (MDA) of 400 feet height above the runway.

Q26. What comprises a typical SBAS infrastructure??

SBAS infrastructure mainly comprises four elements i.e. Monitoring Station Network, Processing FacilityCenter, Communication Layer & GEO satellites with navigation payloads. Monitoring Station Network is network of precisely surveyed ground reference stations strategically positioned across a geographic coverage region to collect Global Navigation Satellite Systems (GNSS) navigation data. This data is sent to Processing Facility Center in real time using Communication layer. This layer contains the network of Terrestrial as well as Satellite communication links. Processing Facility Centerprocess the data provided by Monitoring Station Networkto generate System User Message to either correct GNSS signal errors or provide alerts to users of potential signal errors. These correction messages are then broadcasted through specialized Navigation Transponders on GEO satellites via GNSS-like signals to an SBAS avionics receiver onboard user aircraft.

Q27. Can GAGAN information be trusted?

GAGAN is a certified SBAS system for safety of life application which satisfies stringent The ICAO SARPS criterion for Integrity. The specification for SBAS requires the system to detect errors in the GNSS or GEO network and notify users within a very small time constraint. Certifying that SBAS is safe for instrument flight rules (IFR) requires proving there is only an extremely small probability that an error exceeding the requirements for accuracy will go undetected. Specifically, the probability is less than 1x10-7, and is equal tonomorethan3secondsofbaddataperyear. For safety reasons, within the specified time constraint, SBAS can do one of two actions: I. Provide a correction to the information that is detected as misleading. If SBAS is able to correct misleading information within the time constraint, there is no lapse in system integrity. II. Notify the user not to use the information.

Q28. What are the future technological developmental activities in SBAS?

Yes, there are plans for technology and performance enhancements in SBAS systems in terms of adding DFMC (Dual Frequency Multi Constellation) capability.:

Use of Dual Frequency (DF) :SBAS providers are making plans for transitioning from L2 to L5to implement new dual frequency L1/L5 service needed to further improve SBAS.availability and continuity. With the adventof new L5 signal in place of L2 signal in 2020 in GPS constellation, a dual-frequency SBAS avionics receiver could use the SBAS corrections message or generate its own ionospheric delay corrections by comparing the L1 and L5 signals.Dual frequency improves availability and continuity, especially during severe solar activity.. The further benefit is to improve robustness against unintentional interference.

Inclusion of additional GNSS constellations :There are also options being considered regarding inclusion of additional GNSS constellations in SBAS such as GLONASS, Galileo and BeiDou (COMPASS) in addition to GPS..Multi-constellation service expands the DF SBAS to include differential corrections for additional GNSS constellations. This adds no new capability, but enables the use of more ranging sources which should improve availability and continuity and provide more margin in scintillation and interference environments

Q29. What is the difference between GPS and GAGAN?

The GPS is 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 improving Accuracy, Availability, Continuity 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.

Q30. Which satellites are being used to broadcast the GAGAN signal?

GAGAN has two Indigenous Geostationary satellites broadcasting the SBAS signals. The satellites are identified as GSAT-8 with PRN 127 and GSAT-10 with PRN 128. Another GEO satellite GSAT-15 (PRN 132)is in orbit and integration with the satellite is in under progress.

Q31. What are the GAGAN performances?

The GAGAN is designed to achieve a performance level of APV 1.0 over the Indian land mass and RNP 0.1 over the oceanic region, within the Indian Flight Information Region (FIR).

 

Q32. I already have GPS. Does GAGAN require a separate antenna? (Frame better question )

No, the GAGAN message is broadcast on the same frequency as the GPS signal. You will just need a GAGAN -capable receiver provided your existing antenna is compatible with the GAGAN equipment. Your current system may be upgradeable. Please contact your manufacturer directly for information on availability, installation and price.

Q33. What approaches can I fly with GAGAN?

Presently GAGAN is designed to provide APV 1.0 approach for landing and RNP 0.1 for enroute operations.

Q34. What is the affect of ionosphere activity on GAGAN performance?

GAGAN collects GPS data at the reference stations. The system is then be able to estimate the signal delay and error due to the ionosphere. This information is then passed to the user as a part of the GAGAN navigation message. In addition, ionospheric data is collected and archived by the GAGAN Test Team for analysis of scintillation and range delay effects by experts in the ionospheric field.

Q35.How does GAGAN know that the correction it sends is valid to my particular location?

The GAGAN supplies two different sets of corrections:

1) Corrected GPS parameters (position, clock etc.) and

2) Ionospheric parameters. The first set of corrections is user position independent - they apply to all users located within the GAGAN service area. The second set of corrections is area specific. GAGAN supplies correction parameters for a number of points (organized in a grid pattern) across the GAGAN service area. The user receiver computes ionospheric corrections for the received GPS signals based on algorithms which use the appropriate grid points for where the user is located. Further, the appropriate grid points may differ for each GPS satellite received and processed by the user receiver as the GPS satellites are located at various positions in the sky relative to the user. The combination of the two sets of corrections allows for significantly increased user position accuracy and confidence anywhere in the GAGAN service area.

Q36. In the SBAS operational context, will a common receiver be able to process GAGAN, EGNOS, MSAS and WAAS signals?

Yes, 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.

Q37. Is there any performance standard defined for SBAS equipments?

Yes, 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.

Q38.What are the different GAGAN transmission modes?

GAGAN operates on an open service transmission mode complying with RTCA specifications and ICAO SARPS.

Q39. 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 available.

 

Q40. Difference between GAGAN Service and 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.

Q41. What’s an MMR?

An MMR is a multi-mode receiver. It would receive the basic GPS signal and the GAGAN signal. It may also include the Ground Based Augmentation System (GBAS) signal at a future date. It may also receive VHF, UHF, VOR or other signals.

Q42. What is the service provider id for GAGAN ?

The service provider id of GAGAN is "3" and it is broadcast in GAGAN message number 17.

Q43. Is GAGAN certified?

Yes

Q44. Who certified GAGAN?

DGCA (Directorate General of Civil Aviation, Government of India)

Now GAGAN SIS is also available from PRN 132.