GAGAN – A visionary approach
In order to provide seamless navigation to the world aviation community by airport service providers, the International Civil Aviation Organization (ICAO) has adopted a new strategy on the implementation of Future Air Navigation System (FANS) and use of non-visual aids for approach, and landing. The solution for the above requirements is Global Navigation Satellite System (GNSS). A stand alone GPS fails to provide the required navigation accuracy for all phases of the flight. Therefore, to achieve the required navigation accuracy, the core constellation i.e. GPS, GLONASS, and Galileo (under planning stage) needs augmentations to fulfill the GNSS, particularly in satellite-based augmentations such as WAAS (USA), EGNOS (Europe), MSAS (Japan), GRAS (Australia) and GAGAN (India). It also includes ICAO’s acceptance of an offer to make the Global Orbiting Navigation Satellites System (GLONASS) available for use by civil aviation. The ICAO Council had earlier accepted a similar offer concerning the GPS. These three satellite constellations (GPS / GLONASS / Galileo) are the key components of the GNSS. This article describes briefly, the different systems including GAGAN (India) and the status of GNSS.
The ICAO strategy
The current strategy for application of non-visual aids to approach and landing was recommended at the Special Communications / Operations Divisional Meeting held at ICAO Headquarters, Montreal in spring, 1995. Delegates to that meeting discussed future ICAO policy in allweather operations and concluded that, for a variety of reasons, it had become impracticable to proceed with implementation of the global Instrument Landing System (ILS) / Micro Wave Landing System (MLS) transition plan. The conclusion was reached in part because of developments in the application of satellite-based technology for approach and landing operations, in addition to the ILS and MLS. In future, the GNSS could be used for all weather approach and landing operations. The ICAO policy allows for continued use of ILS “as long as operationally acceptable and economically beneficial,” and also for implementation of MLS “where operationally required and economically beneficial.” The strategy calls for the validation of GNSS capabilities for use in Category- I operations and for the completion of feasibility studies concerning the use of GNSS in Category II and III operations.
As guidance for air navigation planners, the strategy indicates that “an internationally accepted GNSS with augmentation as required may be available for Category I operations within the 2000 to 2020 time frame.” For Categories II and III, it projects that GNSS-based operations are not to be expected before 2005 to 2020 or beyond. This range of dates, which resulted from discussions at the ICAO GNSS panel meeting, effectively shows the differences between optimistic and conservative estimates of the prospects for GNSS introduction and development in the area.
In principle, the concept of augmentation is not completely new and marker beacons or DME in ILS or DME/P in MLS may be considered as existing examples of augmentation. In the case of GNSS, however,navigation satellites augmentations and their combinations, provide a large variety of functions and performance levels which would allow a great deal of flexibility but which may not be required in the real life environment. Five Air Traffic Service (ATS) provider countries have planned to have their own augmentation system and India is one of them.
Wide Area Augmentation Systems (WAAS) is designed as an augmentation to GPS, which includes integrity broadcasts, differential corrections and additional ranging signals. It provides the accuracy, integrity, availability and continuity required to support all phases of flight through CAT-I Precision Approach (PA). The WAAS consist of one integrated system providing all navigation functionality. The delivery schedule will be accomplished in three phases be delivering an initial operating system and then upgrading the system through preplanned product improvements. Phase- I WAAS will also provide the initial operating system which consists of two WMS’s, 25 WRS’s leased GEOSs and ground uplinks to achieve a primary enroute through NPA capability, as well as enable GPS /WAAS to be used as a supplemental navigation aid for CATI PA. The WAAS project supports the development of standards, certification, facilities and procedures for operational use in the National Air Space (NAS). This includes requirement such as GPS procedures for use by air traffic, unique approach procedure for each location, obstacle clearance requirements, RNP standards, airport surveys, support for training program for civil pilot, flight inspections and revision of FAA regulations and document to reflect satellite navigation use. This project has been implemented in the year 2004 for civilian use restricted to the smaller airport and performance monitoring is being done for reliability of the system.
European Geo-Stationary Satellite Navigation System (EGNOS) is the joint venture of European institutions and space industries to show their strong commitment in the development and system operations. Thanks to interoperability of the different SBAS. European’s EGNOS ensures international cooperation as well as European Independence. The European Tripartite Group composed of the European Space Agency (ESA), the European community and EUROCONTROL, the European organization for safety of air navigation, manages EGNOS. In view of the operational implementation to come, the seven major European Air traffic Service Providers are on the way to from a legal entity. The EGNOS operator and infrastructure group (EOIG), France DGCA, German DFS, Italian ENAV, NAVEP of Portugal, Spanish AENA, Swiss control of Switzerland and NAST of United Kingdom. In addition the CNES (Frabce Space Agency) the Norwegian Mapping Authority and major European air traffic management service providers actively contribute to the development and the future operation of EGNOS. All are part of the collaborator framework of the EGNOS program. The EGNOS service augments the GPS and GLONASS signals. The two satellites system sends a positioning signal to the user. The ranging and monitoring station network acquire, firstly the ranging signal generated by two constellations and GEOS, and secondly atmospheric data. This project is under technical demonstration phase.
The MTSAT (Multi functional Transport Satellites) Satellite based Augmentation Systems (MSAS) is the wide area augmentation system being developed by the Japan Civil Aviation Bureau (JCAB) for civil Aviation. This space based augmentation system will provide enroute through PA navigating services for all the aircraft within Japan airspace. The MSAS employs a ranging function to generate GPS like signals and enable aircraft to use MTSAT as a 25th GPS satellite. The MSAS is similar in function to the WAAS (USA). Information on real time conditions ofthe GPS constellations transmitted to each aircraft via the integrity function of MSAS, while the differential corrections function provides ranging error data to each aircraft. MSAS uses advanced technologies such as satellites orbit ranging and ionospheric and troposphere delay estimation assumption to ensure the reliability of these functions. MSAS has planned for two GEOS. They are: MTSAT-1R and MTSAT–2. MTSAT-1R is in the orbit since year 2004 and MTSAT-2 is expected to be launched in the year 2005. After completion of the certification MSAS will be commissioned and be operational using only MTSAT-1R from 2005 and dual operation will be commissioned in the year 2006 using MTSAT-IR and MTSAT-2.
The Ground-based Regional Augmentation System (GRAS) is a system providing GNSS augmentation service by which the user receives information directly from groundbased transmitters allowingcontinuous reception of the service over a large geographical area of approximately 370 Km (200NM). The ground component may be interconnected in a network. GRAS supports GNSS operations in all the phases of flight including en-route, terminal and instrument approach etc. GRAS should be viewed as complementary to Satellite Based Augmentation System (SBAS) (such as EGNOS, WAAS, GAGAN and MSAS) and Ground Based Augmentation System (GBAS). GRAS is made up of multiple ground stations with overlapping coverage. However, the service provider will have to ensure that the topology of the ground infrastructures meet the operational requirements. The GRAS SARPs (Standards and Recommended Practices) have been submitted to ICAO for acceptance and amendment in the ICAO Annexure-10 volume I
The satellite based navigation system is one of the main components of the satellites based CNS/ ATM system adopted by ICAO for worldwide implementations. Satellite navigation uses two-core constellation of medium orbiting satellites i.e. GPS and GLONASS. The positioning services offered by these two constellations for civilian use including civil aviation falls short of accuracy, integrity availability and continuity requirements of air navigation services. Indian air space in between Europe on the West and Japan on the East occupies a very critical position and, hence there is a need to have a system to bridge the gap between the coverage of EGNOS and MSAS and to facilitate seamless navigation of the aircraft from East to West and vice versa. The expertise available with Indian Space Research organization (ISRO) will be gainfully utilized in implementation of GPS / GLONASS Aided Geo- Stationary Satellite Augmented navigation (GAGAN) to provide the coverage over Indian airspace to the users. Indian augmentation with Indian payloads on GSAT-4 satellites, which are controlled by India, will offer some amount of control and flexibility on the position accuracies available to strategic users.
At present there is no SBAS service provider over Indian airspace. Even if in the future MSAS or EGNOS is in a position to provide SBAS services at a charge over India airspace, tt will be far cheaper and strategically important to have indigenous SBAS in view of the indigenous expertise in satellites technology available with ISRO. India can then become a SBAS service provider in neighboring countries in Asia-Pacific. Since GAGAN is a technology driven project, in which the technology is still evolving, it has been envisaged that DOS / ISRO will be in the right position to drive the project with AAI providing the requisite financial, manpower and the rest of supplementary support for the project. The national plan envisages implementation of a full operational capability SBAS in three phases.
Phase 1: Technology Demonstration Systems (TDS)
This phase requires implementation of a minimum configuration system which would demonstrates the capability of the system to support up to Precision Approach Cat-I over a limited region of the Indian airspace and will serve as proof of concept. The performance objective of this system is to meet the ICAO SARPs requirements. The TDS will broadly consist of adequate number of RIMS, an Indian MCC, NLES, space segment, IONO-TROPO modeling, required communication links necessary software for navigation and communication.
Phase 2: Initial Experimental Phase (IEP)
In this phase, TDS will be expanded to cover the entire Indian airspace and requisite redundancies will be added to the system. SBAS elements not covered in the TDS would be incorporated in the system and their redundancies built up. The system will be available for trial operation and the data collected by AAI during such trail operation would be analyzed by ISRO to further improve the system wherever considered necessary to achieve compliance of ICAO SARPs. The IEP will be completed in a period of one year after TDS.
Phase 3: Final Operational Phase (FOP)
During this phase, GAGAN program would have matured. Extensive tests would be carried out to establish the system stability of various element of the system as a whole. The responsibility and roles of ISRO and AAI would continue to be as the case of initial experimental phase. The system will be extensively used for its evaluation with respect to ICAO SARPs before declaring system operational. The FOP will be completed in a period of one year after IEP.
Finally we wish to add that an SBAS provider only guarantees adequate service in his service volume. In addition to this SBAS broadcast signals will be available anywhere in their Geo-stationary Earth Orbiting Satellites (GEOS) foot print coverage areas. In the case of GAGAN, its footprints overlap those of European Geo-Stationary Navigation Overlay System (EGNOS) and Multi Functional Transport Satellites (MTSAT) Satellite – Based Augmentation System (MSAS). Indian airspace falls on the edge of the footprint of EGNOS & MSAS, GEOS. It means that the level of service over Indian airspace is not defined by EGNOS & MSAS. Consequently degraded signal will be available. GAGAN has been designed taking this into account. The service area of worldwide SBASs is presented in Fig. 1. It is observed from the figure that the EGNOS (A) and WAAS (E) coverage areas are large due to two or more number of GEOS at different locations whereas GAGAN (B) and MSAS (D) have less as they use only one GEOS.
Ionospheric, Tropospheric and Scintillation Modeling
This part of the project, is a challenging task for Indian engineers and scientist to develop the model suitable for GAGAN service area. From the study point of view, the world can be divided into three regions i.e. mid latitudes, the auroral and polar caps, and the equatorial and equatorial anomaly region. The equatorial region extends upto ± 30 deg. Geo-magnetic latitude which is 50% of the earth’s surface. Entire Indian sub continent and many other countries fall in this region.
In the mid latitude regions, SBAS is expected to provide a PA service upto Approach Vertical Guidance (APV) -I & II using a standardized grid i.e. (5°x5°) to communicate ionospheric delay information to single frequency users. This strategy works well in mid latitude regions where the spatial and temporal changes in the structure of the ionosphere are fairly well documented. Unfortunately, in the equatorial ionosphere the spatial and temporal variability is much greater even during quite magnetic conditions. Therefore, the current SBAS Ionospheric Grid Concept may not be applicable in the equatorial region as such. There is a need to collect a new set of data continuously over the equatorial region for year together and develop the necessary Ionospheric and Tropospheric models for GAGAN.
Ionospheric & Tropospheric modeling and scintillation studies in the L-band will be carried out over the entire Indian airspace as an integral part in the TDS Phase. Twenty total electron content (TEC) GPS receivers have been installed at the Center of the 5 deg x 5 deg. ionospheric grid points (IGP) grids over the Indian region. The data from these receivers is being logged into a personal computer and the logged data is being stored in ISRO space application center, Ahmedabad for last one year. This data is disseminated to the universities and Research & Development laboratory for development of the appropriate Ionospheric, Tropospheric and Scintillation model.
Ground Segment for GAGAN
This segment of the project consists of the following elements:
India is playing major role to establish the SBAS service over Indian airspace to provide seamless and universalisation services for user as well as countries adjacent to the Indian boundary. This is being executed by AAI and ISRO jointly. It will also provide the service to any number and type of users with integrity signal over Indian air space. In addition the availability of technical, operational expertise in this area will benefit India. It will provide India the leading role in the Asia Pacific region as SBAS service provider.
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