Satellites and their applications

The first principle

A satellite is useful because it is high: from orbit it can see a large part of the Earth at once and be seen by it, so a single machine can relay a signal, time a signal or look at the ground across a whole country. Every application follows from two choices, which orbit gives the right view and which part of the electromagnetic spectrum carries the right information, and India built its satellites from the start for users on the ground.

The core

Communication: a relay in the sky

• A transponder is the working part. It receives a signal from the ground (the uplink), amplifies it and shifts it to a different frequency, so the strong outgoing signal does not drown the weak incoming one, before sending it down (the downlink).
• Geostationary orbit makes the relay fixed. A satellite about 35,786 km above the equator goes round once a day, so it hangs over one point and a rooftop dish can stay pointed at it; it sees about 42 per cent of the Earth's surface, so three such satellites cover all but the polar regions. The orbit itself is in the orbits and launch vehicles note.
• Frequency decides the trade off. The C band (4 to 8 GHz) passes through rain well and serves television distribution and remote links; the Ku band (12 to 18 GHz) allows small dishes and carries direct to home television; the Ka band (about 20 GHz down and 30 GHz up for satellites) carries far more data but fades in heavy rain. Higher frequency means more bandwidth and smaller antennas, at the cost of more loss in the atmosphere.
• High throughput satellites use many narrow spot beams instead of one wide beam, and beams that do not overlap can reuse the same frequencies, which multiplies capacity; GSAT-11 (2018) and GSAT-20 (2024) are of this kind.

Navigation: timing, not seeing

• A navigation satellite is a precise clock that broadcasts the time and its own position. A receiver measures how long each signal took to arrive, multiplies by the speed of light and so knows its distance from each satellite.
• Four satellites are needed, not three. Three distances would fix a point in space if the receiver had a perfect clock, but a phone has a cheap one, so its clock error is a fourth unknown and a fourth satellite is needed to solve for it. A timing error of one microsecond is a distance error of 300 metres, which is why the satellites carry atomic clocks.
• Relativity must be corrected. Orbiting clocks run slower because of their speed and faster because gravity is weaker; for the American Global Positioning System (GPS) the net gain is about 38 microseconds a day, kilometres of error if ignored.
• Two frequencies remove the ionosphere's error. Charged particles in the upper atmosphere delay a signal by an amount that falls with the square of its frequency, so comparing two frequencies cancels the delay; the Navigation with Indian Constellation (NavIC) uses the L5 and S bands, and the newer NVS satellites add the L1 band used by most phones.

Earth observation and weather

• Passive sensors record energy the Earth sends up. Optical cameras measure reflected sunlight in visible and near infrared bands, showing crops, water and cities, and thermal sensors measure emitted infrared, giving surface temperature; neither sees through cloud.
• Radar supplies its own light. A synthetic aperture radar (SAR) sends microwave pulses and records the echo, so it works at night and through cloud, and longer wavelengths penetrate further into vegetation and soil. By comparing two radar images of the same place, a technique called interferometry, it can measure ground movement of a few centimetres. How images become maps is covered in the geospatial technology note.
• Weather satellites watch from two orbits. Geostationary imagers photograph the same disc of the Earth every half hour or less, which is how a cyclone is tracked, while polar orbiting satellites give finer detail; a sounder reads radiation at many wavelengths to work out temperature and humidity at different heights for forecast models.

Satellite internet

• Distance sets the delay. A signal from a user to a geostationary satellite and down to a ground station travels about 72,000 km each way, so even at the speed of light a question and its answer take nearly half a second, which hurts video calls and online games.
• Low Earth orbit constellations cut the delay by flying at around 500 to 1,200 km, but each satellite then sees a small patch and crosses the sky in minutes, so thousands are needed and users are handed from one satellite to the next.

Dimensions

• Economic. Direct to home television, bank terminals on very small aperture terminals (VSAT) and the timing of power grids and telecom networks all run on satellites, so an outage is felt far beyond the space sector.
• Social. Satellite television in India began as education: the Satellite Instructional Television Experiment (SITE) of 1975 and 1976 reached about 2,400 villages, and EDUSAT (2004) carried the idea to schools.
• Agriculture and environment. Satellites estimate crop area before harvest, map forests and glaciers, and track ocean colour, from which the Indian National Centre for Ocean Information Services (INCOIS) issues potential fishing zone advisories.
• Disaster management. Geostationary imagery underpins cyclone warnings and radar maps floods through cloud; the hazard side lives in the Disaster Management notes.
• Security. Dedicated satellites serve the Navy (GSAT-7, 2013) and the Air Force (GSAT-7A, 2018), NavIC offers an encrypted restricted service, and radar satellites watch borders in all weather; warfare in space is in the drones and emerging warfare note.
• International. The South Asia Satellite (GSAT-9, 2017) gave transponders to neighbours, so satellites are also a tool of diplomacy.

In practice

• The two founding systems. The Indian National Satellite System (INSAT), whose first working satellite flew in 1983, combined communication, broadcasting and weather, and the Indian Remote Sensing (IRS) series began with IRS-1A in 1988.
• Communication today. The GSAT series carries most of India's transponders, and CMS-03 (also called GSAT-7R), launched on LVM3 in November 2025 for the Navy at about 4,400 kg, is the heaviest communication satellite launched from India. NewSpace India Limited (NSIL) now builds satellites on demand from users, starting with GSAT-24 (2022), whose capacity was leased to a direct to home broadcaster.
• NavIC. It was designed with seven satellites, three in geostationary orbit and four in inclined geosynchronous orbit, covering India and about 1,500 km around it with position accuracy better than 20 metres. Its first generation suffered atomic clock failures, IRNSS-1H was lost in 2017 when its heat shield did not separate, and NVS-02 (2025) was stranded in a transfer orbit by a valve failure; by March 2026 too few satellites were transmitting navigation signals for a full position fix, and replacements NVS-03 to NVS-05 are due from late 2026. NVS-01 (2023) carries an Indian made rubidium atomic clock.
• GPS Aided GEO Augmented Navigation (GAGAN), built by the Indian Space Research Organisation (ISRO) and the Airports Authority of India, is not a separate system: it corrects GPS errors and broadcasts the corrections from geostationary satellites so that aircraft can use GPS for approach and landing.
• Earth observation. Cartosat satellites take high resolution pictures for mapping, Resourcesat watches land and crops, Oceansat measures ocean colour and winds, and the Radar Imaging Satellite (RISAT) series and EOS-04 see through cloud. The National Aeronautics and Space Administration (NASA)–ISRO Synthetic Aperture Radar (NISAR), launched in July 2025, is the first satellite to carry radars in both the L band, built by NASA, and the S band, built by ISRO, and it maps the whole land surface every twelve days for earthquakes, glaciers and crops.
• Weather. INSAT-3D (2013) carried India's first atmospheric sounder, and INSAT-3DS (2024), funded by the Ministry of Earth Sciences, continues the series for the India Meteorological Department.
• Strategic expansion. The third phase of the Space Based Surveillance programme, approved in October 2024, plans 52 satellites, many to be built by private industry.
• Satellite internet. The Telecommunications Act 2023 lets the government assign satellite spectrum administratively rather than by auction; global constellations hold Indian licences, but the spectrum price was still undecided in late 2026.

Issues and way forward

• A fragile navigation system. NavIC lost service because clocks failed in orbit and replacement launches failed or were late, which shows that a system is only as strong as its spare satellites; indigenous atomic clocks, a satellite or two kept as spares in orbit and a steady launch schedule are the remedy.
• Too few transponders. India has long leased foreign satellite capacity because its own fleet could not meet demand, which demand driven satellites from NSIL and private operators are meant to fix.
• Launch reliability. Two failures of the Polar Satellite Launch Vehicle (PSLV), in May 2025 and January 2026, cost the country Earth observation satellites; the launcher side is in the orbits and launch vehicles note.
• Use, not just data. India collects far more imagery than ministries and States use, because departments lack technical staff; open data, ready products on platforms such as Bhuvan and trained district staff turn pictures into decisions.
• Crowded orbits and spectrum. Large constellations add debris and compete for frequencies and orbital slots registered with the International Telecommunication Union (ITU), covered in the space policy, commerce and law note.

Common confusions

• GPS and NavIC. GPS is a global system of about 30 satellites in medium Earth orbit, while NavIC is a regional system in geostationary and geosynchronous orbits.
• NavIC and GAGAN. NavIC is an independent navigation system with its own satellites and signals, while GAGAN only corrects and checks GPS signals for aviation.
• Passive and active sensing. A camera records reflected sunlight or emitted heat, while a radar sends its own pulse and records the echo, which is why only radar sees through cloud and at night.
• INSAT and IRS. INSAT satellites are geostationary and serve communication and weather, while IRS satellites fly in low Sun synchronous orbits and image the land.

For the exam

• Keyword bank: transponder, geostationary coverage, C, Ku and Ka bands, high throughput satellite, spot beam, trilateration, atomic clock, relativistic correction, ionospheric delay, NavIC, GAGAN, synthetic aperture radar, interferometry, latency.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction with data: A geostationary satellite sees about 42 per cent of the Earth's surface, and a timing error of one microsecond in a navigation signal misplaces a user by 300 metres, which is why satellites now underpin communication, navigation and forecasting alike.
  • Introduction with a source: Vikram Sarabhai argued that India must be second to none in applying advanced technologies to the real problems of man and society, and the INSAT and IRS series were built on exactly that principle.
  • Conclusion: India's satellites have moved from experiments to infrastructure on which banking, farming, defence and disaster response depend, so the next task is reliability, through spares in orbit, indigenous components and assured launches.
  • Conclusion: Data has value only when it is used, so the gain from NISAR, NavIC and new constellations depends on how well departments turn signals and images into decisions.