What is ISRO's 'Gagan', which can aid GPS and enable precise navigation of aircraft?

What is ISRO's 'Gagan', which can aid GPS and enable precise navigation of aircraft?

Using a technology known as the Satellite-Based Augmentation System (SBAS), they correct the errors in GPS signals

GPS (Global Positioning System) signals are not always pin-point accurate and are susceptible to factors and disturbances such as ionospheric interference, the location of the moving vehicle, and even the functioning of the GPS satellite, signal availability, etc. Many advanced countries have their own localised solutions to such problems.

Using a technology known as the Satellite-Based Augmentation System (SBAS), they correct the errors in GPS signals and feed the corrected signals to the moving vehicle (GPS user), thus enabling precise navigation.

While the USA, Japan, Russia, China, etc., have had their own systems to perform this corrective action, India just tested its home-grown SBAS to aid the precise operation and landing of a passenger aircraft that uses GPS.

Developed by the Indian Space Research Organization (ISRO) and the Airports Authority of India, this system is known as "GAGAN" (GPS Aided GEO Augmented Navigation).

On Thursday, Indian private airline IndiGo issued a statement saying that they conducted a landing of their ATR 72-600 aircraft at a regional airport (Kishangarh, Rajasthan) using the indigenously-developed "GAGAN" system. This test flight was part of the approval process with the Indian Aviation Regulator (DGCA), which includes training of pilots, validation of approaches, simulator sessions, amongst others.

What is ISRO's 'Gagan', which can aid GPS and enable precise navigation of aircraft?
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Dr. Mylswamy Annadurai, former Director of India's UR Rao Satellite Centre (ISRO), told media that "GAGAN" is a system of satellites and ground stations that provide GPS signal corrections, thus improving position accuracy.

He elaborated that GAGAN's ground segment comprised fifteen ground stations and a Master Control Centre that are spread across the Indian landmass, and the space segment comprised three satellites in geostationary orbit.

The fifteen ground stations are precisely chosen, and their respective GPS locations are well-established and taken as reference locations. At the time of operation, these stations equipped with precise GPS receivers are constantly receiving their respective location data from GPS satellites. Then, the station position as shown by the GPS signal and the original reference locations are compared. The difference, if any, between the two (GPS signal and the original reference location) is detected as errors. All fifteen stations will send their respective signals to the Master Control Centre, where the errors are corrected and the corrected signal is sent to the satellites in GEO orbit. In our case, we have three GEO satellites (Gsat-8, Gsat-10, and Gsat-15) that are equipped with GAGAN transponders. They receive the signal from the ground and broadcast it back in a GPS-like format. Transport vehicles and aircraft equipped with the compatible receiver called SBAS (Satellite-Based Augmentation System) receivers receive this corrected navigation signal with improved accuracy, reliability, and integrity, "he explained.

Simply put, when a stationary object (a ground station) is on the earth's surface, its location in the global reference frame is fixed and does not change. But, when a satellite gives location data about that particular building, it tends to make some minor errors owing to ionospheric disturbances, etc. However, since the pin-point location of the building (ground station) is known, one can identify that the satellite data has an error. This difference/error in satellite data can be calculated by the Master Control Centre and eventually this corrected data can be sent via satellite to the travelling aircraft/vehicle, thus enabling the pilot to know the pin-point location of the craft.

Captain Ashim Mittra, a veteran commercial pilot who serves as Senior VP, Flight Operations at IndiGo, explained to media that "GAGAN" brings the probability of error in GPS data to a bare minimum. According to him, the location data coming from "Gagan" is augmenting and adding more perfection to the existing GPS location. He adds that the accuracy provided by "GAGAN" is as good as that offered by the Ground-based Instrument Landing System (ILS), which guides pilots to make precise landings at major airports. However, while ILS is only available at the busiest airports, it is also very expensive and difficult to maintain.Therefore, in smaller airports (such as those in Tier 2 and Tier 3 Indian cities), where ILS is not available at all, the "GAGAN" system can be used for landing guidance.

As long as a pilot is trained and the aircraft is technologically capable, the GAGAN system can be used in both major and Tier 2 and 3 airports. Sometimes, the ILS faces temporary outages at large airports. In such situations, Gagan can come into play and rule out the possibility of flight operations being interrupted, "Captain Mittra said. However, it must be understood that GAGAN only improves GPS location precision and does not replace GPS.

It must be noted that India's Aviation Sector Regulator issued a mandate for all aircraft registered in India after July 1, 2021, to be fitted with GAGAN equipment. Gagan Satellite Based Augmentation System (SBAS) user equipment is interoperable with all international SBAS systems-WAAS (American), EGNOS (European) & MSAS (Japanese). Once fully adopted in India’s civil aviation sector, GAGAN is expected to modernise the airspace, reduce flight delays, save fuel, and improve flight safety, as pilots using Gagan need not take a circuitous, non-precise approach for landing. Instead, they can precisely land directly into Tier 2 and Tier 3 airports using the accurate data provided by Gagan.

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