Use of GPS in civil aviation demands higher accuracy and reliability in determining position than a mobile phone user would need

These days, anyone who wants to find out exactly where they are can turn to their mobile phones. Phones that pick up signals from orbiting U.S. Global Positioning System (GPS) satellites are now commonplace. The phone uses that information to work out the location and display it on a map.

In a similar fashion, the GPS signals can be used to assist aircraft during take off and land as well as in flying shorter routes to their destination.

But, as there can be hundreds of passengers in a single aircraft, the use of GPS for such purposes in civil aviation demands higher accuracy in determining position than a mobile phone user would need as well as greater reliability in doing so.

One important way to meet the demands of civil aviation has been through what is known as a Satellite-Based Augmentation System (SBAS). Satellites in geostationary orbit, where they match the earth's rotation and therefore remain over the same place on the globe, are used to supplement the GPS signals.

The first such SBAS was the U.S. Wide Area Augmentation System (WAAS) that became operational in 2003. The European Geostationary Navigation Overlay Service (EGNOS) began working in October 2009 but was officially declared available for aviation use only in March this year. The Japanese have a system known by the acronym MSAS.

India is establishing its own system, the 'GPS Aided Geo Augmented Navigation' (GAGAN), a joint effort by the Indian Space Research Organisation and the Airports Authority of India.

The ground segment for GAGAN, which has been put up by the U.S. company Raytheon, has 15 reference stations scattered across the country. Two mission control centres, along with associated uplink stations, have been set up at Kundalahalli in Bangalore. One more control centre and uplink station are to come up at Delhi.

The space component for it will become available after the GAGAN payload on the GSAT-8 communication satellite, which was launched recently, is switched on. This payload was also on the GSAT-4 satellite that was lost when the Geosynchronous Satellite Launch Vehicle (GSLV) failed during launch in April 2010. Two more satellites carrying the same payload are to be launched in the coming years.

The reference stations pick up signals from the orbiting GPS satellites. These measurements are immediately passed on to the mission control centres that then work out the necessary corrections that must be made. Messages carrying those corrections are sent via the uplink stations to the satellites in geostationary orbit that have the GAGAN payload. Those satellites then broadcast the messages. SBAS receivers are able to use those messages and apply the requisite corrections to the GPS signals that they receive, thereby establishing their position with considerable accuracy.

But as with any SBAS, GAGAN needs to do more than simply provide the corrections. Not less important is ensuring the system's integrity. “Integrity is a measure of trust that can be placed in the correctness of the information supplied by the total system,” observed S.V. Kibe, who was at the ISRO Headquarters till his retirement.

It included the ability to provide timely and valid warnings to the users when the navigation system was not performing as required, he noted in article on the GAGAN system published in a recent issue of the specialist magazine Coordinates.

Currently, aircraft from must fly from one place to another along predefined air routes marked with ground-based navigation aids. Planes with SBAS receivers will, on the other hand, be able to take shorter routes, saving both time and fuel.

To help pilots land their aircraft in bad weather and poor visibility, several airports in the country are equipped with ground-based Instrument Landing Systems (ILS). Such ILS equipment is expensive. Consequently, even in airports that have it, only one runway and that too one end of a runway may have the ILS capability.

An SBAS, on the other hand, can provide guidance on both ends of all runways that fall within its coverage area. The U.S. Federal Aviation Administration (FAA) has, for instance, published the approach procedures that aircraft equipped to receive the WAAS signals can use to access 2,300 runways in over 1,200 airports in poor weather conditions.

“WAAS will provide an equivalent level of precision approach service to that of the Category 1 ILS when fully deployed,” according to the FAA. (There are three ILS categories, with those in Category 3 being able to help aircraft land in conditions with the worst visibility.)

When GAGAN becomes operational, it would provide close to Category 1 services across much of India, observed one official associated with the project. In due course, the Indian system would be upgraded and improved to meet Category 1 requirements.

During the technology demonstration phase when GAGAN was tested in 2007 with just eight reference stations and a leased transponder on the Inmarsat 4F1 satellite, the position given by a stationary SBAS receiver during a 24-hour period varied by only two metres to three metres. An ordinary GPS receiver, on the other hand, varied by as much as eight metres to 20 metres during the same period. Moreover, when aircraft fitted with SBAS receivers were flown, the GAGAN was found to provide very good position accuracies.

Once the GSAT-8's GAGAN payload becomes available for use, the full system can be thoroughly tested. However, certification of the system for safety-critical use in aviation will be taken up only only after the second GAGAN-equipped spacecraft becomes operational. The certification will be carried out by the Directorate General of Civil Aviation.

Since all augmentation systems follow common standards laid down by the International Civil Aviation Organisation, aircraft with SBAS receivers can use any of those systems.

India's GAGAN has a reach well beyond the country, from Africa and Middle East on one side to the Bay of Bengal and South-East Asia on the other other. It will therefore fill a gap between Europe's EGNOS and Japan's MSAS systems.

Moreover, as has already happened with GPS receivers, the uses for GAGAN will no doubt go well beyond aviation. Those involved in surveying and map-making will obviously benefit from the better accuracy it provides, as can the transportation sector and marine operations, not to mention recreational applications.

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