Gagan Aircraft Navigation
- With the satellite-based navigation system, the pilot is provided with on-board position for precision and non-precision landing approaches and for en route applications. This will result in the opening up of air connections to a large number of small Airports that lack the conventional full-fledged navigational facilities.
- The basic requirement for a satellite-based navigation system is a constellation of satellites with known orbits, which can be used as reference. Satellite-based navigation system is not new. The US government launched a satellite constellation known as Global Positioning System (GPS) in the 1980s for use by the military. It is also available for civilian use. World over, rail, road and ocean traffic and even individuals have been using it to know their exact position anywhere on the globe and also to chart out the route for their destination.
- The International Civil Aviation Organization (ICAO) has endorsed GPS as the care satellite constellation to provide worldwide seamless navigation for civil aviation.
What is GPS?
- GPS consists of three main segments:
- the satellite constellation
- the ground control network
- the use equipment.
- The satellite constellation is made up of 30 solar-powered satellites, which revolve around the earth in six orbital planes at a radius of about 26, 600 km from the center of the earth. Their main function is to continually broadcast ranging and navigational signals. There are in the form of pseudo random codes (PRC), transmitted as low power radio waves in the L band carrying information on their position in space and time. Each satellite is identified with a unique PRN code and equipped with an atomic clock for precise timing.
- The ground control network consists of six stations across the globe. They constantly monitor the satellites for their health and fine-tune their orbital data, which is transmitted back to them.
- The user equipment is a GPS receiver. It captures the ranging and navigational signals from the satellites in view and computers the user’s position (latitude, longitude and attitude), velocity and time (PVT). Any one with a suitable GPS receiver an individual hiker, a vehicle and road, a ship or an aircraft can receive the signals for navigation purposes.
- The position accuracy of the GPS is about 20 and 30 meters in the horizontal and vertical directions respectively. Though this may be adequate for ocean and road transport navigation, aircraft navigation requires much greater accuracy.
- The ISRO and the AAI signed a MoU to install a space-based augmentation system (SBAS) to render the GPS signal suitable for civil aviation over the Indian airspace. An interesting aspect of the project is the name chosen for this system, which is the same chosen for this system, which is strikingly Indian. IT is called GAGAN (GPS Aided GEO Augmented Navigation).
- As with GPS, SBAS also consists of three segments: The space segments, the ground segment and the user segment.
- The space segment of GAGAN consists of three geosynchronous communication satellites. The first one, GSAT-8, was launched by ISRO on 21 May 2011 from Kourou, French Guiana. The satellite, weighing about 3100 kg, has been positioned in a geostationary orbit at 55-degree east longitude on the Indian Ocean. It carries a dual frequency L1 and L5 navigation payload compatible with the GPS. Since a minimum of three satellites will be added in due course.
- The ground segment consists of 15 Indian Reference Stations (INRESs), an Indian Master Control Center (INMCC) and an Indian Navigation Land Uplink Station (INLUS), all suitably augmented. In the Final Operational Phase, which is currently being carried out, the Reference Stations are located at Ahmedabad, Bangalore, Thiruvananthapuram, Port Blair, Delhi, Kolkata, Guwahati, Jammu, Dibrugarh, Patna, Bhubaneshwar, Nagpur, Goa, Porbandar and Jaisalmer. They are connected to the INMCC at Bangalore. Each station is provided with a minimum of two identical GPs receivers/antennae subsystems to receive GPS signals.
- The INMCC processes the data received from all the 15 INRESs. It will also estimate the integrity and the availability of the GPS satellite and transmits the corrections and confidence parameters to the INLUS.
- The INLUS, also located at Bangalore, format these message consisting of ionospheric, ephemeris and clock drift correction and transmits them to the satellites’navigation payland for broadcasting to the use segment.
- The user segment is a modified GPS receiver installed in the aircraft. It receives these signals and determines the aircraft’s exact location in the sky. The pilot can use this information for the navigation en route and for lending. The pilot can also broadcast this information, along with other aircraft-specific data to other planes and to the air traffic control facilities to obtain seamless navigation service for all phases of flight from takeoff to landing over the Indian airspace.
- GAGAN is capable of better than 7.6 meters accuracy in both vertical and horizontal, and time to alert better than 6.2 seconds, meeting the ICAO standards.
- GAGAN, although being built primarily for civil aviation, can cater to other applications. All the GPS applications could advantageously use the GAGAN signal that will ensure not only accuracy but also integrity. Such applications in future may include Railways and Maritime vessels.
- Individual users in our country can also benefit from GAGAN since the higher positional accuracy through the narrow lanes in both urban and rural areas which otherwise will be difficult.
- GAGAN service is free of charge. Anybody in the coverage area and possessing the commercially available special GPS receivers can get the benefits of GAGAN.
- The experience gained during GAGAN implementation will lead us to the successful completion of the task related to the establishment of the indigenous Indian Regional Navigation Satellite System (IRNSS).
When that happens in a couple of years, our country will have firmly established itself in the field of satellite navigation.