Global navigation satellite system

Also called: GNSS, Global navigation satellite constellations · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 12, Ch 4 "Spatial Information Technology"

Meaning

A Global Navigation Satellite System (GNSS) is a group (constellation) of satellites that sends signals so a receiver anywhere on Earth can find its position, plan its navigation and get the exact time. The US GPS, Russia's GLONASS, Europe's Galileo and China's BeiDou are global systems. India's NavIC and Japan's QZSS are regional systems that cover only one part of the world.

GNSS matters because phones, vehicles, ships, aircraft, banks, power grids and phone networks all depend on it. A country that owns its own system does not have to rely on another country's military for location and time.

Explanation

What GNSS gives: positioning, navigation and timing

• Positioning: where the receiver is, given as latitude, longitude and height.
• Navigation: how to get from one place to another.
• Timing: the exact time. Banks, power grids and phone networks use this.
• NCERT treats GPS as one tool of spatial information technology (using computers, satellites and maps to collect and study data about places). The other two tools are GIS and remote sensing. GPS is used to fix exact locations on the ground.

How a receiver finds its position: trilateration

• Trilateration (finding a position from distances to known points, not from angles) is the method every GNSS uses.
• Step 1: the satellite sends a signal.
• The signal says where the satellite is.

• It also says the exact time the signal left.

• Step 2: the receiver works out the distance.

• It notes when the signal arrives.

• Travel time × speed of light = distance to that satellite.

• Step 3: the spheres meet at a point.

• One distance puts the receiver somewhere on a sphere around that satellite.
• Two spheres cross in a circle.

• A third sphere cuts that circle at a point. This gives latitude, longitude and height.

• Step 4: a fourth satellite fixes the clock.

• A phone has a cheap quartz clock that is not exact.
• The fourth distance corrects this receiver clock error.

• So at least 4 satellites are needed for a basic position.

• Why atomic clocks matter:

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

Types: global and regional systems

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

• Global systems cover the whole Earth, so they need many satellites.
• Regional systems cover one region only, so they need fewer satellites.
• GPS, the model global system:
• It is a radio-navigation system based in space. The US Government owns it and the US military runs it [8].
• Its satellites are in 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 tilted 55° to the equator. Each satellite goes round the Earth once every 12 hours [8].
• The design needs at least 24 satellites [8]. The USA promises 24 or more working 95% of the time, and has flown about 31 operational satellites for over a decade [9].

• Each satellite carries backup atomic clocks, and a ground control network tracks the satellites [9].

• NavIC, the model regional system:

• NavIC (Navigation with Indian Constellation) is the working name of IRNSS (Indian Regional Navigation Satellite System). It was renamed in 2016.
• The planned design has 7 satellites. 3 are geostationary (they seem to stay fixed above one point on the equator) at 32.5°E, 83°E and 129.5°E. 4 are in inclined geosynchronous orbit (IGSO) at 29° (a 24-hour orbit that is tilted, so from the ground each satellite traces a figure-of-eight over India) [2].
• All the satellites stay high over the Indian region all the time. That is why 7 are enough, while a global system needs 24 or more.
• Position accuracy is better than 20 m. Timing accuracy is better than 50 nanoseconds (true about 95% of the time) [2].
• It uses the L5 (1176.45 MHz) and S band (2498.028 MHz) signals, plus a new civilian L1 (1575.42 MHz) signal [2]. GPS also uses L1, so most phone chips can already receive it.
• It offers two services. The SPS (Standard Positioning Service) is for all civilians. The RS (Restricted Service) is encrypted and meant for strategic users such as the armed forces [2].
• NavIC works with GPS, GLONASS, Galileo and BeiDou signals [2].

Augmentation: making GNSS more exact

• Augmentation means extra signals that correct GNSS errors and warn users when a signal cannot be trusted.
• Basic GNSS (GPS, NavIC) is accurate to metres. NavIC is better than 20 m.
• SBAS (Satellite-Based Augmentation System) sends corrections through geostationary satellites. India's SBAS is GAGAN, which is accurate to a few metres and gives warnings when a signal is unsafe. It is used by aircraft.
• Ground augmentation uses fixed ground stations such as CORS. It is accurate to centimetres (±3 cm) and is used by surveyors and engineers [7].

Where it occurs

• Worldwide: GPS (USA), GLONASS (Russia), Galileo (EU) and BeiDou (China) cover the whole globe.
• Asia-Oceania: Japan's QZSS covers Japan and the Asia-Oceania region.
• India and its neighbourhood (NavIC):
• It covers India and up to 1,500 km beyond India's borders [2].
• Its geostationary satellites sit over the equator at 32.5°E, 83°E and 129.5°E [2].
• Uses: transport and vehicle tracking, location-based services, surveying, time signals, and safety-of-life alerts such as cyclone and tsunami warnings sent to fishermen at sea [2].
• The IMO (International Maritime Organization), the UN body for shipping, recognised NavIC in 2020, so ships can use it in the Indian Ocean region.
• Current status: in July 2026 the government told Parliament that only 3 satellites give positioning, navigation and timing services. These are IRNSS-1B, IRNSS-1I and NVS-01. At least 4 are needed [4].
• What went wrong:
  • Atomic clocks failed on early satellites.
  • IRNSS-1H failed in 2017, so it could not replace them.
  • NVS-02 (launched 29 January 2025) was left in a 170 × 37,785 km transfer orbit, tilted at 20.8°. The firing signal did not reach a pyro valve (a small valve driven by an explosive charge), most likely because of a loose connector [3].

• Second generation (NVS series):

  • The NVS satellites carry an indigenous rubidium atomic clock (made in India) and the L1 signal.
  • NVS-01 was launched in May 2023 and is working.
  • NVS-03 is ready for launch, and NVS-04 and NVS-05 are at an advanced stage of building [4].

• Indian airspace (GAGAN):

• ISRO and the AAI (Airports Authority of India) built GAGAN [2][5].
• 51 GAGAN-based approach procedures have been designed for Indian runways [5].
• The AAI ran LPV flight trials at Kishangarh Airport, Rajasthan [5].

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

• Across India on the ground (CORS):

• The Survey of India runs more than 1,000 stations, spaced about 60–80 km apart [6][7].
• It is used for land surveys, large-scale mapping, building roads, railways and dams, and precision farming.

Don't confuse with

• GPS: GPS is only one GNSS, the US system. GNSS is the general term for all such systems, including GLONASS, Galileo, BeiDou, NavIC and QZSS.
• SBAS / GAGAN: an SBAS does not find positions by itself. It only corrects GPS signals and sends integrity warnings through geostationary satellites. GAGAN is an SBAS, not a GNSS.
• Regional system (NavIC, QZSS): a regional system covers one region with few satellites (NavIC is designed with 7). A global system covers the whole Earth and needs 24 or more.
• Triangulation: triangulation finds a position from angles. GNSS uses trilateration, which works from distances (travel time × speed of light).

Prelims Hooks

• Trilateration needs at least 4 satellites. Three fix 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].
• Global vs regional: GPS, GLONASS, Galileo and BeiDou (global since 2020) are global. NavIC (India) and QZSS (Japan) are regional.
• NavIC design: 3 geostationary (32.5°E, 83°E, 129.5°E) + 4 IGSO (29°). It covers India + 1,500 km and is accurate to better than 20 m. It uses the L5 and S bands, plus L1 on the NVS satellites [2].
• Working NavIC satellites (July 2026): IRNSS-1B, IRNSS-1I and NVS-01. That is only three, below the minimum of 4 [4].
• Trap: the IMO (a shipping body) recognised NavIC in 2020. GAGAN's aviation certification came from the DGCA (en-route 2013, APV-1 2015) [2][5].

Mains Points

• Strategic autonomy (GS-III, security):
• The risk: GPS is run by a foreign military, so India could lose the service at a key moment in a conflict.
• The answer: NavIC's encrypted Restricted Service gives India its own source of position and time [2].

• The gap: only 3 working satellites against the 4 needed (July 2026). Keeping the service running depends on launching NVS-03 to NVS-05 quickly [4].

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

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

• The NVS-02 pyro-valve fault led to extra backup in valve firing, used on CMS-03 (2 November 2025) [3].

• Infrastructure and disasters (GS-III):

• GAGAN lets small airports with no costly ILS (Instrument Landing System) offer safe landings in poor weather, which helps regional air links [5].
• CORS (about ±3 cm) supports land-record digitisation and the building of roads and railways [6][7].
• NavIC sends cyclone and tsunami alerts to fishermen far out at sea, where mobile networks do not reach. This matters for India's long coastline [2].

Related concepts

• Global Positioning System
• Trilateration
• NavIC
• NVS series satellites
• GAGAN
• Continuously Operating Reference Stations

Read more

• Satellite navigation: GNSS, GPS, NavIC and GAGAN