Satellite Navigation Systems

Satellite navigation has been tested twice in three years, in 2023 and again in 2025, and neither time from a reference book, because no Prelims reference book carries it. Both questions turned on the same underlying distinction, which is worth stating before anything else: a navigation constellation and an augmentation system are different things, and India operates one of each. Get that straight and most of what follows is detail.

How satellite navigation actually works

A navigation satellite does something narrower than most people assume. It does not track you, locate you, or know anything about you. It broadcasts, continuously and to nobody in particular, a signal carrying two pieces of information: precisely what time it is, according to an atomic clock on board, and precisely where that satellite is in its orbit.

Your receiver listens. It compares the timestamp in the arriving signal against its own clock and works out how long the signal took to travel. Multiply that travel time by the speed of light and you have your distance from that one satellite, which places you somewhere on the surface of an imaginary sphere centred on it. One satellite gives a sphere. Two intersecting spheres give a circle. Three give a pair of points, one of which is usually absurd (out in space, or deep underground) and can be discarded. This is trilateration, and it is geometry, not radio direction-finding.

In practice a receiver needs four satellites rather than three. The fourth is not for position at all. It solves for the receiver's own clock error, because a phone or a car cannot carry an atomic clock, and at the speed of light even a microsecond of clock error becomes roughly 300 metres of position error. The fourth satellite lets the receiver correct its own cheap clock against the satellites' expensive ones. This is why satellite navigation systems are also, and quite seriously, timing systems, used to synchronise power grids, telecom networks and financial transaction records, and why the specifications quote a timing accuracy alongside a position accuracy.

Because the receiver only ever listens and never transmits, the number of users a navigation system can serve is unlimited, and the system has no idea who or how many they are.

Global systems and regional systems: the distinction that gets tested

This is the split the 2023 question was built on, and it is the single most examinable fact on the topic.

A Global Navigation Satellite System (GNSS) covers the entire Earth. Achieving that requires a large constellation in Medium Earth Orbit, typically around 20,000 kilometres up, spread across several orbital planes so that at least four satellites are above the horizon from any point on the planet at any moment. Four systems provide genuinely global coverage:

• GPS, operated by the United States, the oldest and still the default worldwide.
• GLONASS, operated by Russia.
• Galileo, operated by the European Union, notable for being run under civilian rather than military control.
• BeiDou, operated by China.

A regional navigation satellite system deliberately does not attempt global coverage. It serves one part of the world, which lets it work with far fewer satellites. Two such systems exist:

• NavIC, operated by India.
• QZSS (the Quasi-Zenith Satellite System), operated by Japan.

So when a question asks which countries operate their own independent regional satellite navigation system, the answer set is India and Japan. The United States, Russia, China and the EU operate global systems, not regional ones, and the word "regional" is doing all the work in that sentence.

NavIC: India's own constellation

NavIC stands for Navigation with Indian Constellation. It was previously called IRNSS, the Indian Regional Navigation Satellite System, and the name NavIC was given by the Prime Minister when the constellation was completed in April 2016. Both names refer to the same system, which is worth knowing because older material and newer material use different ones.

ISRO designed it as a constellation of seven spacecraft, and the orbital arrangement is unusual enough to be worth understanding rather than memorising. Three satellites sit in geostationary orbit, holding fixed positions above the equator. The other four sit in inclined geosynchronous orbit, which means they return to the same point relative to the ground once a day but, because their orbits are tilted relative to the equator, they trace a figure-of-eight path across the sky rather than hanging still.

That arrangement is a direct consequence of being regional. A global system needs satellites constantly sweeping over the whole planet, so it uses Medium Earth Orbit. A regional system wants its satellites to stay over its own service area permanently, which is exactly what geostationary and geosynchronous orbits deliver. The inclined satellites exist because purely geostationary satellites sit on the equator, which is a poor viewing angle from northern India; the tilted orbits carry satellites high into the northern sky where a receiver can actually see them past buildings and terrain.

Coverage extends over India and a region up to 1,500 kilometres beyond the Indian boundary. This is a genuine limit, not a soft edge: NavIC is not a global system and does not claim to be.

NavIC offers two services:

• Standard Positioning Service (SPS), for civilian users.
• Restricted Service (RS), an encrypted service for strategic users.

Performance, in ISRO's own published figures, is a position accuracy better than 20 metres and a timing accuracy better than 50 nanoseconds over the service area. It transmits in the L5 band (1176.45 MHz) and the S band (2498.028 MHz), and a signal in the L1 band (1575.42 MHz) is being added. The L1 addition matters commercially rather than technically: L1 is the band ordinary consumer GPS chips already listen to, so broadcasting there makes NavIC far easier for ordinary phones to receive without new hardware.

GAGAN: not a constellation at all

GAGAN stands for GPS Aided GEO Augmented Navigation, and the most common error on this topic is treating it as a second Indian constellation. It is not. GAGAN is a Satellite Based Augmentation System (SBAS), which is a different category of thing.

An augmentation system does not provide navigation signals of its own to fix your position from. It improves signals that already exist. Ground reference stations at precisely surveyed locations receive GPS signals and compare the position GPS reports against the position they know they are actually at. The difference is the error in the GPS signal at that moment, caused mostly by the signal bending as it passes through the ionosphere. That correction is relayed up to a geostationary satellite and broadcast back down, so a receiver in the area can apply it and get a much better fix than raw GPS would give.

Three points about GAGAN that questions have turned on:

• It was developed jointly by ISRO and the Airports Authority of India (AAI), not by ISRO alone. The AAI's involvement is the clue to its purpose.
• Its purpose is civil aviation. It was built to meet the navigation requirements of aircraft over Indian airspace, providing the additional accuracy, availability and integrity needed across the phases of a flight. Integrity is the aviation-specific requirement: an aircraft needs not just an accurate position but a timely warning when the signal should no longer be trusted, which raw GPS does not provide.
• It is interoperable with the other regional SBAS systems: the American WAAS, the European EGNOS and the Japanese MSAS. Augmentation systems are regional by nature, because the ionospheric errors they correct are themselves local.

So the clean summary is this. NavIC is India's own navigation constellation, independent of GPS. GAGAN augments GPS and depends on it. They solve different problems and neither replaces the other.

Why a country builds its own system

The strategic argument is straightforward and is the reason regional systems exist at all. A country relying entirely on another country's navigation system is relying on a signal that can be degraded or denied by its operator, and the civilian signal is deliberately less precise than the military one in any case. For guided weapons, for military logistics, and increasingly for critical civilian infrastructure that depends on precise timing, that dependency is a vulnerability. An independent constellation, even a regional one covering only your own neighbourhood, removes it.

Quick revision points

• A receiver needs four satellites, not three: three fix position by trilateration, the fourth corrects the receiver's own clock error. This is why navigation systems are also timing systems.
• Global systems: GPS (USA), GLONASS (Russia), Galileo (EU), BeiDou (China), all in Medium Earth Orbit.
• Regional systems: NavIC (India) and QZSS (Japan). Only these two are regional.
• NavIC, formerly IRNSS, renamed in April 2016: 7 satellites, 3 geostationary plus 4 in inclined geosynchronous orbit. Coverage is India plus 1,500 km beyond the boundary. Two services, SPS (civilian) and RS (restricted, strategic). Accuracy better than 20 m, timing better than 50 ns. Bands L5 and S, with L1 being added so ordinary consumer chips can receive it.
• Regional systems use geostationary and geosynchronous orbits precisely because they want permanent coverage of one area; global systems use Medium Earth Orbit because they need to sweep the whole planet.
• GAGAN is an augmentation system, not a constellation. GPS Aided GEO Augmented Navigation, built jointly by ISRO and the Airports Authority of India, for civil aviation, adding accuracy and integrity. Interoperable with WAAS (US), EGNOS (Europe) and MSAS (Japan).
• The distinction to hold on to: NavIC is independent of GPS; GAGAN corrects GPS and cannot work without it.