Global Positioning System

Also called: GPS · Topic: Remote Sensing, GIS and GNSS · NCERT: Class 11, Ch 1 "Geography as a Discipline"; Class 12, Ch 4 "Spatial Information Technology"

Meaning

The Global Positioning System (GPS) is a US radio-navigation system based in space. The US Government owns it and the US military runs it. Its satellites send signals that let a receiver anywhere on Earth find its exact position, its route and the exact time [7].

GPS is the oldest and most used GNSS (Global Navigation Satellite System). Phones, vehicles, ships, banks, power grids and phone networks all depend on it. Because a foreign military controls it, India has built its own regional system, NavIC.

Explanation

What GPS gives and where it sits

• GPS gives three things to a receiver:
• Positioning: where the receiver is (latitude, longitude and height).
• Navigation: how to get from one place to another.

• Timing: the exact time. Banks, power grids and phone networks use it.

• NCERT names GPS as one tool of spatial information technology (the use of 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.

• GPS is a global system because it covers the whole Earth. Regional systems such as NavIC cover only one region, so they need fewer satellites.

The space segment: orbit and constellation

• Orbit: medium earth orbit (MEO) at about 20,200 km [7].
• MEO lies between low orbits (a few hundred km) and geostationary orbit (about 35,786 km) [7].

• So GPS satellites are not geostationary. They move across the sky.

• Orbital planes: the satellites are spread over 6 orbital planes. Each plane is tilted 55° to the equator [7].

• Period: each satellite goes round the Earth once every 12 hours [7].
• Number of satellites:
• The design needs at least 24 satellites [7].
• The USA promises to keep 24 or more working 95% of the time [8].

• In practice, the US Space Force has flown about 31 operational satellites for over a decade [8].

• Clocks and control: each satellite carries backup atomic clocks (very exact clocks), and a ground control network tracks the satellites [8].

How a receiver finds its position: trilateration

• Trilateration means finding a position from distances to known points. It does not use angles.
• Step 1: the signal. Each satellite sends out two facts: where it is, and the exact time the signal left.
• Step 2: the distance. The receiver notes when the signal arrives.

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

• Step 3: the spheres.

• One distance places 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: the fourth satellite.

• A phone has a cheap quartz clock, and that clock is not exact.
• A fourth satellite gives one more distance, which corrects the receiver's clock error.

• So a GPS fix needs at least 4 satellites.

• 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 GPS and of every GNSS.

Making GPS more precise: augmentation

• Augmentation means extra signals that correct GPS errors and warn users when a signal cannot be trusted.
• GAGAN (GPS Aided GEO Augmented Navigation) is India's SBAS (Satellite-Based Augmentation System). An SBAS sends its corrections through geostationary satellites. ISRO and the AAI (Airports Authority of India) built GAGAN [2][4].
• Ground stations at known spots receive GPS signals and measure the 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 then knows its position precisely enough to land.

• CORS (Continuously Operating Reference Stations) are fixed GNSS receivers at exactly known spots on the ground. They correct GNSS positions down to about ±3 cm using RTK/NRTK [6]. RTK (Real-Time Kinematic) means the receiver corrects itself live from a nearby station, and NRTK is the network version.

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

Where it occurs

• Worldwide: GPS covers the whole Earth from medium earth orbit [7]. It is one of four global systems:

System	Owner	Type
GPS	USA	Global (oldest and most used)
GLONASS	Russia	Global
Galileo	European Union	Global (civilian-run)
BeiDou	China	Global since 2020
NavIC	India	Regional (India plus about 1,500 km around it)
QZSS	Japan	Regional (Japan and the Asia-Oceania region)

• India: GAGAN corrects GPS for Indian airspace [2][4].
• The DGCA (Directorate General of Civil Aviation) certified it for en-route flying (RNP 0.1) on 30 December 2013 and for precision approach (APV-1) on 21 April 2015 [2][4].
• 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.
• It works with other SBAS: the US WAAS, Europe's EGNOS and Japan's MSAS [2][4].
• 51 GAGAN-based approach procedures have been designed for runways at Indian airports [4].
• The AAI ran trials of LPV approaches at Kishangarh Airport, Rajasthan [4]. 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 [4].

• India: CORS network. The Survey of India runs it. Union Minister Dr Jitendra Singh launched it [5].

• More than 1,000 stations, spaced about 60–80 km apart, cover the whole country [5][6].

• Uses: land and cadastral surveys (surveys of land ownership), large-scale mapping, building roads, railways and dams, precision farming and drone surveys.

• India's own alternative: NavIC. It covers India and up to 1,500 km beyond India's borders [2]. NavIC works with GPS, GLONASS, Galileo and BeiDou signals [2].

Don't confuse with

• NavIC: GPS is global (at least 24 satellites in MEO at about 20,200 km). NavIC is regional, with a planned 7 satellites: 3 geostationary and 4 in inclined geosynchronous orbit [2][7].
• Geostationary satellites: GPS satellites are in medium earth orbit with a 12-hour period. They are not fixed above one point like geostationary satellites at about 35,786 km [7].
• GAGAN: GAGAN is not a separate navigation system. It is an SBAS that corrects GPS for aircraft [2][4].
• Triangulation: GPS uses trilateration, which works from distances. Triangulation works from angles.

Prelims Hooks

• GPS orbit: medium earth orbit at about 20,200 km, 6 planes tilted at 55°, 12-hour period. It is not geostationary [7].
• Constellation size: the design needs at least 24 satellites, and the USA promises 24 or more working 95% of the time. In practice about 31 operational satellites have flown for over a decade [7][8].
• Trilateration needs at least 4 satellites. Three fix the position and the fourth corrects the receiver's clock error. It uses distances, not angles.
• Clock trap: a 1 microsecond clock error shifts the position by about 300 m. This is why atomic clocks are essential.
• Global vs regional: GPS (USA), GLONASS (Russia), Galileo (EU) and BeiDou (China, global since 2020) are global. NavIC (India) and QZSS (Japan) are regional.
• GAGAN = SBAS for GPS by ISRO + AAI, certified by the DGCA (en-route 2013, APV-1 2015). It is first in the equatorial region [2][4].

Mains Points

• Strategic autonomy (GS-III, security): GPS is owned by the US Government and run by the US military [7].
• A country that relies on it can lose the service at a key moment in a conflict.

• India's answer is NavIC, with an encrypted Restricted Service for strategic users such as the armed forces [2]. But in July 2026 only 3 satellites were giving positioning services, below the 4 needed, so the quick launch of NVS-03 to NVS-05 matters [3].

• Infrastructure (GS-III): GPS plus augmentation turns metre-level positions into usable precision.

• GAGAN allows safe landings at small airports with no costly ILS (Instrument Landing System). This helps regional air links [4].

• CORS at about ±3 cm supports land-record digitisation, smart cities and road and rail building [5][6].

• Economy and daily life (GS-III): the timing service of GPS keeps banks, power grids and phone networks in step. Positioning supports transport, vehicle tracking, surveying and location-based services. So a single outage can affect many sectors at once.

Related concepts

• Global navigation satellite system
• Trilateration
• NavIC
• NVS series satellites
• GAGAN
• Continuously Operating Reference Stations

Read more

• Satellite navigation: GNSS, GPS, NavIC and GAGAN