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Sky is the limit

Print edition : Sep 12, 2008 T+T-
How the satellite-aided navigation system works, an illustration.-BY SPECIAL ARRANGEMENT

How the satellite-aided navigation system works, an illustration.-BY SPECIAL ARRANGEMENT

The Rs.774-crore GAGAN project will improve airport and airspace access in all weather conditions.

FOR smooth flow of air traffic, India is planning to introduce its own augmented satellite-based navigation system. Demonstration of the futuristic Global Positioning System (GPS)-aided Geo Augmented Navigation System, or GAGAN, was in mid-2006 and initial countrywide trials were done last year.

GAGAN will meet the civil aviation industrys emerging needs in communication, navigation and surveillance (CNS) and air traffic management (ATM). Primarily designed as a modern positioning aid for pilots, its immediate effect would be greater efficiency and safety for aircraft uniformly across the 110 airports managed by the Airports Authority of India (AAI). This will mean smooth communication with the ground control, neat landings, timely warnings and fewer collisions.

AAI is entrusted with the responsibility of providing air navigational services (ANS), infrastructure and air traffic services (ATS) over the designated airspaces. Besides, the sovereign airspace of India, ATS are also provided over a 3.8 million sq km area in the Arabian Sea, Bay of Bengal and the Indian Ocean in accordance with the International Civil Aviation Organisation (ICAO) rules governing the designation of air space over high seas. Indias total airspace of approximately 6 million sq km has been divided into four Flight Information Regions (FIRs) Mumbai, Delhi, Kolkata, and Chennai and a sub-FIR in Guwahati for better and efficient airspace management.

The ambitious Rs.774 crore GAGAN project is being implemented in three phases through 2008 by the AAI with the help of the Indian Space Research Organisations (ISRO) technology and space support. For now, GAGAN is a regional augmentation based on the United States militarys GPS and enables seamless air traffic between different regions. GAGAN has eight reference stations including Delhi, Guwahati, Kolkata, Ahmedabad, Thiruvananthapuram, Bangalore, Jammu and Port Blair, and a master control centre at Bangalore.

In the coming decades, if it comes up with its own orbiting constellation of navigation satellites, GAGAN will catapult India into the privileged club of satellite-navigation haves. Right now the U.S. and the Russian military satellite system GLONASS rule this space; Europe is trying to have its own 30-satellite Galileo system.

The initiative in the form of GAGAN will result in significant reduction in delays and congestion. We are not going to lag behind any country in the world, says AAI Chairman K. Ramalingam.

Ever since GPS was conceived as a military system, it has provided two types of positioning service Standard Positioning System (SPS) and Precise Positioning System (PPS); that is, coarse positioning and precise positioning. The latter can only be achieved using classified receivers that have information about the finest encryption of the signal. The existing GPS provides an accuracy of 100-150 metres in aircraft positioning using GPS receivers. It cannot meet accuracy requirements of less than 7.6 m in positioning required for aircraft landing at the airports in Indian airspace.

The uncertainties in position accuracies available through the core GPS are due to the ionospheric delays, satellite ephemeris and clock errors. In order to provide enhanced accuracies with integrity and continuous availability for aviation use, it is essential to have an augmentation system capable of collecting data in two frequencies over the service area, separating errors at the master control centre and communicating a corrected message to the aviation user in the same frequency as that of the core GPS. To achieve this, a satellite-based augmentation system (SBAS) consisting of a geostationary space segment for the core constellation, a ground segment consisting of reference stations, a master control centre and an uplinking earth station are required. The reference stations collect dual frequency data, which are then communicated to the master control centre which separates the errors; the corrected navigation message is sent to the navigation transponder onboard the geostationary satellite, which translates it to the user in the GPS civil frequency.

GAGAN is an Indian SBAS to provide navigation system for all phases of flight over the Indian airspace and in the adjoining areas. It is applicable to safety-of-life operations, and meets the performance requirements of international civil aviation regulatory bodies. The final, operational phase of GAGAN is likely to be completed by May 2011.

As part of the programme, a network of 18 total electron content (TEC) monitoring stations were installed at various locations in India to study and analyse the behaviour of the ionosphere over the Indian region. GAGANs technology demonstration system (TDS) signal in space provides a three-metre accuracy as against the requirement of 7.6 metres. Flight inspection of the GAGAN signal is being carried out at Kozhikode, Hyderabad and Bangalore airports and the results have been satisfactory so far.

GAGAN, after its final operational phase completion, will be compatible with other SBAS systems such as the Wide Area Augmentation System (WAAS), the European Geostationary Navigation Overlay Service (EGNOS), and the Multi-functional Satellite Augmentation System (MSAS) and will provide seamless air navigation services across regional boundaries. While the ground segment consists of eight reference stations and a master control centre, which will have subsystems such as data communication network, SBAS correction and verification system, operations and maintenance system, performance monitoring display and payload simulator, Indian land uplinking stations will have dish antenna assembly. The space segment will consist of one geonavigation transponder.

GAGAN will increase safety by using a three-dimensional approach operation with course guidance to the runway, which will reduce the risk of controlled flight into terrain (an accident whereby an airworthy aircraft, under pilot control, inadvertently flies into terrain, an obstacle, or water).

GAGAN will improve airport and airspace access in all weather conditions, and the ability to meet environmental and obstacle clearance constraints. It will enhance reliability and reduce delays by defining more precise terminal area procedures that feature parallel routes and environmentally optimised airspace corridors.

Flight management systems (FMS) will then be poised to save operators time and money by managing climb, descent and engine performance profiles. The FMS will improve the efficiency and flexibility by increasing the use of operator-preferred trajectories. It will reduce workload and improve productivity of air traffic controllers and provide coverage of oceanic areas that terrestrial systems are not able to cover. It will provide uniform levels of navigation performance over the entire airspace.

GAGAN will offer high position accuracies over a wide geographical area like the Indian airspace. These position accuracies will be simultaneously available to 80 civilian and more than 200 non-civilian airports and airfields and will facilitate an increase in the number of airports to 500 as planned. These position accuracies can be further enhanced with ground-based augmentation system.

Terrestrial navigation aids such as distance measuring equipment and instrument landing system serve a single runway, and separate instrumentation is required as the number of runways are increased. Since this SBAS serves an entire geographical area, the government can save the maintenance costs associated with older, more expensive ground-based navigation aids. All runways at an airport can be served by a single SBAS.

Introduction of satellite-based navigation will help in the gradual phasing out of terrestrial navigation aids. SBAS will also assist in upper airspace management while better position accuracies will enable closer spacing of aeroplanes.