GPS and Global Navigation Satellite Systems (GNSS)

A Global Navigation Satellite System (GNSS) is a group of satellites that sends out signals from space so that anyone on Earth with a receiver can find their exact location and the exact time. The Global Positioning System (GPS) of the United States is the best-known GNSS, so people often say "GPS" for all of them.

Today there are four global systems: America's GPS, Russia's GLONASS, Europe's Galileo and China's BeiDou. Two countries, India (NavIC) and Japan (QZSS), run regional systems that cover only their own part of the world.

Why does it exist?

Before satellites, ships and aircraft found their position using stars, maps, compasses and radio beacons on land. These methods were slow and not very accurate, especially over oceans or at night. Satellite navigation gives a position within a few metres, anywhere, at any time, in any weather. Today it runs far more than maps on your phone:

• Transport: ship and aircraft navigation, truck tracking, cab apps, food delivery.
• Farming and fishing: guiding tractors, mapping land, and helping fishermen find fish zones and stay away from sea borders.
• Timing: banks, stock exchanges, power grids and mobile towers all use the exact time from these satellites to stay in step.
• Disaster and defence: guiding rescue teams, missiles and soldiers.

Because so much depends on it, a country that relies only on another country's system is in a weak position if that system is switched off or made less accurate.

Where did it come from?

• 1957: When the Soviet Union launched Sputnik 1, American scientists found they could track the satellite from the small change in its radio signal. They realised the reverse was also possible: a satellite with a known position could help someone on the ground find theirs.
• 1960s: The US Navy built an early satellite system called Transit for its ships and submarines.
• 1973: The US Department of Defense started the GPS project (also called Navstar). GPS reached full operational capability in 1995.
• Selective Availability: For years, the US deliberately made the civilian GPS signal less accurate. This was switched off on 1 May 2000, which made accurate GPS available to everyone and led to the boom in car and phone navigation.
• Russia's GLONASS was completed in 1995, declined after the Soviet Union broke up, and was restored to full strength in 2011.
• Europe's Galileo, the first system built mainly for civilian use, began initial services on 15 December 2016.
• China's BeiDou became a global system when BeiDou-3 was formally put into service on 31 July 2020.
• Japan's QZSS (Michibiki) started official service with four satellites on 1 November 2018.
• India approved IRNSS in 2006, launched the first satellite in 2013, and named the system NavIC in 2016. A frequently cited reason is that during the 1999 Kargil conflict, India could not get the GPS data it needed for the region.

How does satellite navigation work?

Each satellite carries very precise atomic clocks and keeps sending a signal that says, in effect, "I am satellite X, I am at this position, and this is the exact time I sent this message."

1. Your receiver notes the time the signal arrives.
2. Radio signals travel at the speed of light (about 3 lakh km per second). So the delay tells the receiver how far away that satellite is.
3. With the distance from one satellite, you know you are somewhere on a huge imaginary sphere around it. With three satellites, these spheres meet at one point. This method is called trilateration.
4. The receiver's own cheap clock is not exact, so a fourth satellite is needed to correct the clock error. That is why at least four satellites must be in view to get a position (latitude, longitude, height) and the time.

Timing matters enormously. An error of just one millionth of a second in the clock means a distance error of about 300 metres. This is why atomic clocks are the heart of every navigation satellite.

The three segments

Every GNSS has three parts:

• Space segment: the satellites in orbit.
• Control segment: ground stations that track the satellites, correct their clocks and orbits, and send them updates.
• User segment: the receivers in phones, cars, ships, aircraft and so on.

The global and regional systems compared

• GPS (USA): run by the US Space Force. Satellites fly in Medium Earth Orbit (MEO) at about 20,200 km, in six orbital planes tilted about 55°. The design baseline is 24 satellites. As of April 2026, 32 satellites are active.
• GLONASS (Russia): about 24 satellites at about 19,100 km, in three planes tilted about 64.8°. The higher tilt gives better coverage of far northern regions like Russia.
• Galileo (European Union): MEO at about 23,222 km in three planes tilted about 56°. Designed for 24 working satellites plus spares, and run as a civilian system.
• BeiDou (China): a mixed design. BeiDou-3 has 24 satellites in MEO, 3 in geostationary orbit and 3 in inclined geosynchronous orbit. It also offers a short-message service that lets users send text through the satellites.
• QZSS (Japan): a regional system with satellites in high, tilted orbits that stay close to overhead Japan. It mainly improves GPS in Japan's cities and mountains, where tall buildings and hills block signals from low in the sky.
• NavIC (India): a regional system covering India and about 1,500 km around it, designed for 3 geostationary and 4 inclined geosynchronous satellites.

Augmentation systems: making GNSS more accurate

Plain GPS can be off by several metres. That is fine for a car but not for an aircraft landing. So countries run Satellite-Based Augmentation Systems (SBAS). Ground stations at known locations measure the GPS error and send corrections through geostationary satellites. Examples:

• WAAS (USA), EGNOS (Europe), MSAS (Japan).
• GAGAN (GPS Aided GEO Augmented Navigation) of India, built by ISRO and the Airports Authority of India (AAI). The aviation regulator, DGCA, certified GAGAN for en-route flying (RNP 0.1) on 30 December 2013 and for approach guidance (APV-1) on 21 April 2015. Since 1 July 2021, aircraft newly registered in India must carry GAGAN-capable equipment.

India's position and Indian examples

• India runs two systems: NavIC (an independent navigation system) and GAGAN (a correction system that makes GPS more accurate). Do not confuse them.
• In September 2024, the Ministry of Road Transport and Highways amended the National Highways Fee Rules, 2008, to allow satellite-based (GNSS) toll collection. Private vehicles with a working GNSS unit get up to 20 km a day free in each direction, and pay only for the actual distance beyond that.
• NavIC messages are used to warn fishermen at sea about cyclones and high waves.
• India works with the International Committee on GNSS (ICG), set up under the United Nations in December 2005, for which the UN Office for Outer Space Affairs (UNOOSA) is the secretariat. It helps different systems work together without interfering.

Commonly confused concepts

• GNSS vs GPS: GNSS is the general name for all satellite navigation systems. GPS is only the American one.
• Global vs regional systems: GPS, GLONASS, Galileo and BeiDou cover the whole Earth. NavIC and QZSS cover only a region, so they need fewer satellites.
• NavIC vs GAGAN: NavIC gives a position using its own satellites. GAGAN does not give a position by itself; it corrects GPS signals, mainly for aviation.
• Trilateration vs triangulation: trilateration uses distances, which is how GNSS works. Triangulation uses angles.
• Galileo vs BeiDou's special feature: Galileo is run by civilians; BeiDou offers two-way short messages.
• MEO vs GEO for navigation: global systems mostly use MEO so that each satellite sees a big part of the Earth while still moving across the sky; NavIC's geostationary and geosynchronous satellites stay above the Indian region all the time.

Issues, criticism and the way forward

• Dependence and denial: a country using another nation's system can lose accuracy or access during a conflict. This is the main strategic reason for NavIC.
• Jamming and spoofing: GNSS signals are very weak when they reach the Earth. They can be blocked (jamming) or faked (spoofing), sending ships, aircraft or drones off course. Encrypted services, like NavIC's Restricted Service, and backup systems help.
• Low adoption of regional systems: NavIC was slow to reach ordinary users because, until the NVS series, it did not broadcast in the L1 band that most phone chips are built for. Experts have long said that wider use needs L1 signals, low-cost chips and support from phone makers.
• Satellite reliability: a system is only as strong as its working satellites. India's first-generation NavIC suffered imported clock failures and launch problems, which left it below strength for long periods. Indian-made clocks, timely replacement launches and spare satellites are the suggested remedies. The second-generation NVS satellites are meant to replace the old ones. NVS-02, launched on 29 January 2025, never reached its working orbit. A valve on its engine did not open, because the signal to open it never arrived. In August 2026, the government told Parliament that only three NavIC satellites could support navigation services. NavIC needs at least four. ISRO launched NVS-03 on a GSLV-F18 rocket on 30 October 2026 to restore that number. It carries an Indian-made rubidium atomic clock. As of 4 November 2026, it had reached its geosynchronous orbit but still had to finish its checks before starting service.
• Space weather: solar storms can disturb the upper atmosphere and reduce GNSS accuracy for a while.

Concepts to Know

• Atomic clock: an extremely accurate clock that keeps time by counting the natural vibrations of atoms such as rubidium or caesium.
• Medium Earth Orbit (MEO): orbits roughly between 2,000 km and about 36,000 km above the Earth. GPS, GLONASS and Galileo satellites fly here.
• Geostationary orbit: a circular orbit about 35,786 km above the equator where a satellite seems to stand still in the sky.
• Jamming: sending strong noise on the same frequency so that a receiver cannot hear the weak satellite signal.
• Spoofing: sending fake satellite-like signals that trick a receiver into showing the wrong position or time.
• Encrypted signal: a signal locked with a secret code so that only authorised users can read it.