UPSC Prelims 2025 Q94: GAGAN Satellite-Based Augmentation — Explained

GPS-Aided Geo Augmented Navigation (GAGAN) uses a system of ground stations to provide necessary augmentation. Which of the following statements is/are correct in respect of GAGAN?

I. It is designed to provide additional accuracy and integrity.

II. It will allow more uniform and high quality air traffic management.

III. It will provide benefits only in aviation but not in other modes of transportation.

Select the correct answer using the code given below.

  1. I, II and III
  2. II and III only
  3. I only
  4. I and II only

Answer: (d) I and II only

Statement-wise explanation

  • I: correct — GAGAN is India's Satellite-Based Augmentation System (SBAS). It was built jointly by ISRO and the Airports Authority of India (AAI). Its ground stations measure GPS errors and send correction and integrity messages through geostationary satellites. In AAI's words, the purpose is to provide the 'accuracy, continuity, availability, and integrity' that ICAO standards require, so that aircraft can rely on GPS in every phase of flight. Extra accuracy and integrity are what 'augmentation' means.
  • II: correct — GAGAN was built for Indian airspace. It covers the whole region with one certified navigation signal, so aircraft no longer depend only on scattered ground aids. It is certified for en-route navigation (RNP 0.1) and for approaches with vertical guidance (APV 1.0, April 2015). This allows more direct air routes, less fuel use and approach procedures at airports without an Instrument Landing System. The result is more uniform, higher-quality air traffic management.
  • III: incorrect — GAGAN's signal comes from geostationary satellites. Any compatible receiver can use it, not just aircraft. The 2015 PIB release on the launch says GAGAN also benefits agriculture, all modes of transportation, defence, security agencies and disaster management. AAI lists maritime navigation, land transport management and precision farming among its non-aviation uses. The word 'only' makes the statement false.

How to eliminate

Statement III contains the absolute word 'only'. Common sense also rules it out: a correction signal broadcast from a satellite can be received by ships, trains and road vehicles as well as aircraft. So any option that includes III goes. Statement I is simply the definition of 'augmentation': an augmentation system exists to make GPS more accurate and more reliable. That leaves 'I only' or 'I and II'. To choose between them, remember that GAGAN is an aviation SBAS built with the Airports Authority of India. Its main purpose is better navigation and air traffic management over Indian airspace, so II is also true. The fact to remember: GAGAN is an ISRO–AAI SBAS for aviation, but its benefits reach every mode of transport.

Know more

Satellite-Based Augmentation System (SBAS)

On its own, GPS can be off by several metres. Errors come from the ionosphere, satellite clock drift and orbit errors. GPS also cannot quickly warn a pilot when a signal is faulty. An SBAS fixes both problems:

  1. Reference stations at precisely known locations receive GPS signals and measure the errors.
  2. Master control centres calculate corrections and integrity (health) messages.
  3. Uplink stations send these messages to geostationary satellites, which broadcast them on a GPS-like frequency to every user in the coverage area.

GAGAN at a glance

Feature Detail
Built by ISRO and Airports Authority of India
Ground segment 15 Indian Reference Stations (INRES), 2 Indian Master Control Centres (Bengaluru, Delhi), Indian Land Uplink Stations (INLUS)
Space segment GEO payloads on GSAT-8, GSAT-10, GSAT-15
Certification (DGCA) RNP 0.1 en-route (2013–14); APV 1.0 approach (April 2015)
Formal launch 13 July 2015
Distinction 3rd SBAS certified for approach with vertical guidance, after WAAS (USA) and EGNOS (Europe); 1st in the equatorial ionospheric region

Other SBAS systems: WAAS (USA), EGNOS (Europe), MSAS (Japan), SDCM (Russia), BDSBAS (China), KASS (South Korea).

GAGAN vs NavIC. These are often confused. GAGAN does not create positions by itself. It corrects GPS. NavIC (IRNSS) is India's own regional navigation constellation. It sends its own ranging signals and gives positions independently of GPS.

In aviation, GAGAN supports LPV (Localizer Performance with Vertical guidance) approaches. AAI ran the first GAGAN-based LPV flight trials at Kishangarh, Rajasthan, on 28 April 2022.

NCERT Concepts to Revise

  • GAGAN: The question is about this system. Knowing who built it and its design purpose settles the statements.
  • Global Positioning System: GAGAN augments GPS signals, so you need to know GPS's basic limits to see what augmentation adds.
  • Global navigation satellite system: An SBAS works on top of a GNSS. This entry explains the satellite navigation framework GAGAN belongs to.
  • NavIC: NavIC is India's other navigation system and is often confused with GAGAN in options and statements.

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