Satellite navigation: GNSS, GPS, NavIC and GAGAN

Detail

1. What GNSS is

• GNSS (Global Navigation Satellite System): a group of satellites that sends signals so a receiver on Earth can find three things:
• Positioning: where it is (latitude, longitude, height).
• Navigation: how to get from one place to another.

• Timing: the exact time, used by banks, power grids and phone networks.

• NCERT names GPS only as one tool of spatial information technology (the use of computers, satellites and maps to collect and study data about places), alongside GIS and remote sensing. GPS is used to fix exact locations on the ground.

• There are two kinds of system:
• Global systems cover the whole Earth.
• Regional systems cover one region only, so they need fewer satellites.

System	Owner	Type	Note
GPS	USA	Global	Oldest and most used. Its satellites are in medium earth orbit.
GLONASS	Russia	Global	—
Galileo	European Union	Global	Civilian-run
BeiDou	China	Global	Became global in 2020
NavIC	India	Regional	India plus about 1,500 km around it
QZSS	Japan	Regional	Covers Japan and the Asia-Oceania region

2. GPS: the US system

• GPS (Global Positioning System): a radio-navigation system based in space. The US Government owns it and the US military runs it [8].
• Orbit: medium earth orbit (MEO) at about 20,200 km. MEO lies between low orbits (a few hundred km) and geostationary orbit (about 35,786 km) [8].
• The satellites are spread over 6 orbital planes. Each plane is tilted 55° to the equator [8].

• Each satellite goes round the Earth once every 12 hours [8].

• Size of the constellation:

• The design needs at least 24 satellites [8].
• The USA promises to keep 24 or more working 95% of the time [9].
• In practice the US Space Force has flown about 31 operational satellites for over a decade [9].
• Each satellite carries backup atomic clocks, and a ground control network tracks them [9].

3. Trilateration: how a receiver finds its position

• Trilateration: finding a position from distances to known points. It does not use angles.
• Step by step:
• Each satellite sends a signal that says where the satellite is and the exact time the signal left.
• The receiver notes when the signal arrives. Travel time × speed of light = distance to that satellite.
• One distance puts the receiver somewhere on a sphere around that satellite. Two spheres cross in a circle. A third sphere cuts that circle to a point, which gives latitude, longitude and height.

• A fourth satellite is needed because a phone's cheap quartz clock is not exact. The fourth distance corrects the receiver's clock error.

• Why atomic clocks matter:

• Radio signals travel at about 3,00,000 km per second.
• So a clock error of just 1 microsecond shifts the position by about 300 m.
• This is why very exact atomic clocks are the heart of every GNSS.

4. NavIC: India's regional system

• NavIC (Navigation with Indian Constellation): the operational name of the IRNSS (Indian Regional Navigation Satellite System). It was renamed in 2016.
• Planned design: 7 satellites
• 3 in geostationary orbit (they seem to stay fixed above one point on the equator). They sit at 32.5°E, 83°E and 129.5°E [2].
• 4 in inclined geosynchronous orbit (IGSO): the orbit takes 24 hours, as in geostationary orbit, but is tilted, here at 29° to the equator. Seen from the ground, each satellite traces a figure-of-eight over India [2].

• Why this design: the satellites stay high over the Indian region all the time, so 7 satellites are enough. A global system needs 24 or more.

• Coverage: India and a region up to 1,500 km beyond India's borders [2].

• Accuracy:
• Position accurate to better than 20 m [2].

• Timing accurate to better than 50 nanoseconds (2-sigma: true about 95% of the time) [2].

• Signals (frequency bands):

• L5 at 1176.45 MHz and S band at 2498.028 MHz [2].

• A new civilian L1 signal at 1575.42 MHz [2]. L1 is the band GPS uses, so most phone chips can already receive it.

• Two services [2]:

• SPS (Standard Positioning Service): for all civilians.

• RS (Restricted Service): encrypted, for strategic users such as the armed forces.

• NavIC works with GPS, GLONASS, Galileo and BeiDou signals [2].

• Uses [2]:
• Transport and vehicle tracking
• Location-based services and surveying
• Time signals

• Safety-of-life alerts, such as cyclone and tsunami warnings sent to fishermen at sea

• Milestones:

• IRNSS-1A launched in 2013.
• The IMO (International Maritime Organization), the UN body for shipping, recognised NavIC in 2020. This means ships can use NavIC in the Indian Ocean region.

5. NavIC's problems

• Atomic clocks failed on early satellites. A satellite without a working clock cannot give positions. It can only send messages.
• IRNSS-1H failed in 2017, so it could not replace a satellite with failed clocks.
• Result: the number of working satellites fell. In a reply to Parliament in July 2026, the government said only 3 satellites give positioning, navigation and timing services: IRNSS-1B, IRNSS-1I and NVS-01 [4].
• At least 4 working satellites are needed for basic positioning, because trilateration needs four [4].

6. NVS series: the second generation

• NVS satellites: the second-generation NavIC satellites, built to keep the service running and to add new features [4].
• They carry an indigenous rubidium atomic clock (made in India). This lowers the risk from imported clocks failing.

• They add the L1 signal, so ordinary phones can use NavIC.

• NVS-01 launched in May 2023. It is working [4].

• NVS-02 (launched 29 January 2025) failed:
• The rocket put it into an elliptical transfer orbit of 170 × 37,785 km, tilted at 20.8° [3].
• It could not raise itself into its planned circular orbit [3].
• Cause: the firing signal did not reach the pyro valve (a small explosive-driven valve) on the oxidiser line of its engine. The most likely reason was a loose contact in a connector [3].

• The Apex Committee set by ISRO asked for more backup in such valve systems. These fixes were used on CMS-03 (launched 2 November 2025) [3].

• Next satellites (July 2026): NVS-03 is ready for launch. NVS-04 and NVS-05 are at an advanced stage of building [4]. Five NVS satellites (NVS-01 to 05) are planned in all [4].

7. GAGAN: making GPS precise enough for aircraft

• Augmentation: extra signals that correct GNSS errors and warn users when a signal cannot be trusted.
• SBAS (Satellite-Based Augmentation System): an augmentation system that sends its corrections through geostationary satellites.
• GAGAN (GPS Aided GEO Augmented Navigation): India's SBAS. ISRO and the AAI (Airports Authority of India) built it for Indian airspace [2][5].
• How it works:
• Ground reference stations at known spots receive GPS signals and measure their errors. The biggest error comes from the ionosphere (the charged upper layer of the atmosphere, which slows radio signals).
• A control centre works out corrections and sends them up to geostationary satellites.

• The satellites broadcast the corrections. An aircraft's receiver then knows its position precisely enough to land.

• Certification by the DGCA (Directorate General of Civil Aviation):

• 30 December 2013: for en-route flying (RNP 0.1) [2][5].

• 21 April 2015: for precision approach (APV-1), which guides aircraft down to the runway [2][5].

• It was the first SBAS serving the equatorial region. Near the equator the ionosphere is the most disturbed, so correcting GPS there is the hardest.

• Works with other SBAS systems: the US WAAS, Europe's EGNOS and Japan's MSAS [2][5].
• Use at airports:
• 51 GAGAN-based approach procedures have been designed for runways at Indian airports [5].
• The AAI ran flight trials of LPV approaches at Kishangarh Airport, Rajasthan [5]. LPV (Localiser Performance with Vertical guidance) is a satellite-guided landing that needs no ground radio aids.

• In June 2026, the DGCA flew India's first satellite-based landing approach on a commercial jet, using GAGAN [5].

• Why it matters: small airports with no costly ILS (Instrument Landing System) can still offer safe landings in poor weather.

8. CORS: centimetre-level positions on the ground

• CORS (Continuously Operating Reference Stations): fixed GNSS receivers at exactly known spots. They record signals day and night and send out corrections.
• The Survey of India runs the national CORS network. Union Minister Dr Jitendra Singh launched it as the National Survey Network [6].
• Size and spacing:
• More than 1,000 stations across India [6].

• They are spaced about 60–80 km apart, which gives coverage of the whole country [7].

• Accuracy:

• Real-time positions to about ±3 cm using RTK/NRTK [7]. RTK (Real-Time Kinematic) means the receiver corrects itself live from a nearby station. NRTK is the network version of RTK.

• Methods such as VRS (Virtual Reference Station) can reach about 1 cm, however far the user is from a station [7].

• Uses:

• Land and cadastral surveys (surveys of land ownership)
• Large-scale mapping
• Building roads, railways and dams
• Precision farming and drone surveys

9. Summary: the three layers

Layer	Example	Accuracy	Main users
Basic GNSS	GPS, NavIC	Metres (NavIC better than 20 m)	Phones, vehicles, ships
Satellite augmentation (SBAS)	GAGAN	A few metres, with integrity alerts	Aircraft
Ground augmentation	CORS (RTK/NRTK)	Centimetres (±3 cm)	Surveyors, engineers

Prelims Hooks

• Trilateration needs at least 4 satellites. Three give the position and the fourth corrects the receiver's clock. It uses distances, not angles.
• GPS orbit: medium earth orbit at about 20,200 km, 6 planes at 55°, a 12-hour period. It is not geostationary [8].
• Regional systems: NavIC (India) and QZSS (Japan). Global systems: GPS, GLONASS, Galileo and BeiDou (global since 2020).
• NavIC design: 3 geostationary (32.5°E, 83°E, 129.5°E) + 4 IGSO (29°). It covers India + 1,500 km, with accuracy better than 20 m [2].
• NavIC bands: L5 and S, plus a new L1 on the NVS satellites for phones [2].
• NVS-02 (January 2025) was stuck in a transfer orbit because of a pyro-valve signal fault. Operational NavIC satellites (July 2026): IRNSS-1B, IRNSS-1I and NVS-01, three, which is below the minimum of 4 [3][4].
• GAGAN = SBAS by ISRO + AAI, certified by the DGCA (en-route 2013, APV-1 2015). It is first in the equatorial region and works with WAAS (US), EGNOS (EU) and MSAS (Japan) [2][5].
• CORS is run by the Survey of India (not ISRO). It has more than 1,000 stations and gives about ±3 cm with RTK [6][7].
• Trap: the IMO recognised NavIC in 2020. The IMO is a shipping body, while aircraft approvals come from the DGCA.

Mains Points

• Strategic autonomy (GS-III, security): reliance on GPS, which a foreign military controls, is risky. A country can lose the service at a key moment in a conflict.
• NavIC's encrypted Restricted Service gives India its own source of positioning and timing [2].

• But with only 3 working satellites against the 4 needed (July 2026), service continuity depends on launching NVS-03 to NVS-05 quickly [4].

• Technology lessons (GS-III, science and technology):

• Imported atomic clocks failed → satellites lost their positioning ability → India built its own rubidium clock.

• The NVS-02 valve fault → added backup for valve firing on later missions such as CMS-03 [3].

• Infrastructure (GS-III):

• GAGAN brings precision landing to small regional airports without ILS. 51 procedures have been designed and a commercial jet approach was flown in 2026 [5]. This helps regional air links.

• CORS at about ±3 cm supports land-record digitisation, smart cities and road and rail building [6][7].

• Disaster management (GS-III): NavIC sends cyclone and tsunami alerts to fishermen far out at sea, where mobile networks fail [2]. This matters for India's long coastline.