ISRO and its rockets

1962: it starts with INCOSPAR

In 1962 a committee called INCOSPAR was set up under the Department of Atomic Energy. This is where ISRO's story begins.

1963: the first rocket from Thumba

On 21 November 1963 the first rocket flew from the rocket station at Thumba (TERLS). It was a Nike-Apache, a two-stage sounding rocket: a rocket that goes up, takes measurements and then comes back down, without going into orbit.

1969: ISRO is formed

On 15 August 1969 the Indian Space Research Organisation (ISRO) was formed in place of INCOSPAR. In June 1972 the Space Commission and the Department of Space were set up, and in September 1972 ISRO was brought under this department.

1975: Aryabhata, the first satellite

India's first satellite, Aryabhata, went into space on 19 April 1975 on a rocket of the Soviet Union (USSR). It weighed 360 kg. The satellite was India's, but the rocket belonged to another country.

The Sriharikota centre

In 1969 Sriharikota island in Andhra Pradesh was chosen for a rocket launch centre. On 9 October 1971 a sounding rocket called Rohini-125 flew from there, and the centre began work. On 5 September 2002 it was named Satish Dhawan Space Centre SHAR (SDSC SHAR), after Prof. Satish Dhawan, a former Chairman of ISRO.

1980: SLV-3 puts Rohini in orbit

The first attempt at building India's own rocket was SLV-3. Its first flight, in August 1979, was not fully successful. On 18 July 1980 SLV-3 placed the Rohini (RS-1) satellite in orbit from Sriharikota, and India became the sixth member of the select group of space-faring nations. The Project Director of SLV-3 was Dr A.P.J. Abdul Kalam.

Learn it from a balloon: thrust

Blow up a balloon and let it go: the air rushes out of the back and the balloon shoots forward. A rocket does the same. In the engine, fuel burns to make very hot gas, which rushes out downwards at high speed; in return, the rocket is pushed upwards. This is Newton's third law: every action has an equal and opposite reaction. This push is called thrust.

It works without air

A rocket does not need outside air to push against. The rocket throws out the gas itself, so it works in a vacuum too. It also carries with it the oxidiser it needs to burn its fuel. Fuel and oxidiser together are called propellant.

The speed to stay in orbit

Going up is not enough. To stay in orbit, a satellite has to race sideways around the Earth very fast. At a height of 200 to 400 km this orbital speed is about 7.8 km every second, or about 28,000 km per hour. Moving this fast, the satellite does keep falling, but it falls along the curve of the Earth, so it never reaches the ground.

Why a rocket has stages

Most of a rocket's weight is its propellant. As the propellant burns, the empty tanks become dead weight. So a rocket is built in several stages: when one stage runs out of propellant, its empty tank and engine separate and fall away, and the lighter rocket that is left carries on.

The rocket equation

How much speed each stage adds comes from Tsiolkovsky's rocket equation: Δv = ve × ln(m0 / mf). Here ve is the speed of the escaping gas, m0 is the total mass before the stage burns and mf the mass after it burns. The more dead weight is dropped, the smaller mf and the greater the speed.

Solid

A solid propellant called HTPB. GSLV's S139 core, both S200 boosters of LVM3, the third stage of PSLV (PS3) and all three solid stages of SSLV run on it.

Liquid

Liquids that can be kept in a tank at ordinary temperatures. All four L40H strap-ons of GSLV and its second stage GS2 (Vikas engine) run on UH25 fuel and N2O4 oxidiser. The second stage of PSLV (PS2, Vikas engine) takes UDMH + N2O4, and the fourth stage (PS4) MMH + MON.

Six rockets on one scale

Below, six ISRO rockets stand on one scale, next to a 1.7-metre man. Pick a rocket and its full profile appears below. The bars at the bottom show the mass at lift-off, because height alone does not tell you how powerful a rocket is.

SLV-3 and ASLV

SLV-3 and ASLV were small rockets, all of whose stages used solid propellant. Of ASLV's four flights, those of 1987 and 1988 did not succeed; those of 1992 and May 1994 succeeded.

PSLV: four stages, many versions

PSLV is a four-stage rocket: solid, liquid, solid, liquid. It comes in different versions for different needs: XL has 6 strap-ons, QL 4, DL 2, and CA none at all. PSLV has launched Chandrayaan-1, Mangalyaan, Aditya-L1 and 104 satellites on one flight. Up to PSLV-C62 (12 January 2026) it had flown 64 times. The flights of 1993 (PSLV-D1), 31 August 2017 (PSLV-C39) and 18 May 2025 (PSLV-C61) did not succeed. On PSLV-C62 an anomaly came up towards the end of the third stage (PS3). In February 2026 a national expert committee was looking into its cause.

GSLV, LVM3 and SSLV

GSLV can carry 2,250 kg to geosynchronous transfer orbit (GTO). It has made 20 flights up to GSLV-F18; the flights of 2006 (F02), 2010 (D3 and F06) and 2021 (F10) did not succeed. LVM3 is the heaviest: two large solid S200 boosters, a liquid L110 stage in the middle (two Vikas engines) and a cryogenic C25 on top. Up to LVM3-M6 (24 December 2025) it had made 9 flights, and none is recorded as a failure. SSLV is the newest of these (first flight 7 August 2022); it carries a satellite of about 500 kg to a 500 km orbit.

Planned next: NGLV

The plan ahead: the Union Cabinet approved the development of the next-generation rocket, NGLV (Next Generation Launch Vehicle), in September 2024. The aim is to carry up to 30 tonnes to low Earth orbit (LEO), with a first stage that can be used again (reusable). The plan has three development flights, 96 months and ₹8,240 crore. NGLV has not been built yet, so it is not drawn here.

Higher orbit, slower trip

How high a satellite circles, and in which direction, depends on the job it has to do. Remember one rule: the higher the orbit, the slower and longer each trip round the Earth.

Low Earth orbit (LEO)

Low Earth orbit (LEO) is a few hundred km up. At 400 km a satellite goes once round the Earth in about 92 minutes. In the picture below, see how this orbit seems almost to hug the Earth.

Polar and sun-synchronous orbits

Polar satellites pass over close to the poles. In a sun-synchronous polar orbit (SSPO) a satellite passes over each place at the same local time every day. PSLV can carry 1,750 kg to such an orbit at 600 km.

Geostationary and geosynchronous

Geostationary orbit lies right above the equator, at a height of 35,786 km. There a satellite goes round in exactly the time the Earth takes to turn on its axis: one sidereal day, about 23 hours 56 minutes. So from the ground it seems to stay fixed at one spot in the sky. A satellite whose trip round takes this same time is a geosynchronous satellite; if its orbit is tilted from the equator, its path over the ground makes a figure 8.

First into GTO

A rocket does not take a satellite straight there. It first leaves it in a geosynchronous transfer orbit (GTO): a long oval path whose end nearest the Earth is called the perigee and whose farthest end is called the apogee. For GSLV-F18 the plan was: perigee 170 km, apogee 37,023 km, and a tilt from the equator (inclination) of 20.75°.

An Indian cryogenic stage: CUSP

GSLV's topmost stage is cryogenic. On the early GSLV flights this stage was bought from Russia. The aim of ISRO's Cryogenic Upper Stage Project (CUSP) was to build such a stage in India itself, to take the place of the stage bought from Russia.

The 1992 sanctions

In 1992 the United States imposed sanctions over a deal to get cryogenic engine technology from Russia. ISRO ran a project to build its own cryogenic stage in place of the stage bought from Russia.

2010: two failed flights

The first GSLV flight with an Indian-built cryogenic stage was GSLV-D3, on 15 April 2010, carrying the GSAT-4 satellite. It did not succeed: the cryogenic engine could not burn for more than a second, because its Fuel Booster Turbo Pump stopped abnormally. On 25 December 2010 GSLV-F06, which had a Russian cryogenic stage, also failed.

2014: GSLV-D5 succeeds

On 5 January 2014 GSLV-D5 delivered the GSAT-14 satellite from the second launch pad at Sriharikota. This was GSLV's eighth flight. The flight succeeded, and it carried an Indian-built cryogenic stage.

From CE-7.5 to CE-20

GSLV's cryogenic engine, CE-7.5, gives about 7.5 tonnes of thrust. For the bigger rocket LVM3, India built the far more powerful CE-20 engine, with about 19 to 20 tonnes of thrust. On 16 July 2015 CE-20 was tested at the ISRO Propulsion Complex in Mahendragiri by firing it non-stop for 800 seconds. On 5 June 2017 LVM3 delivered GSAT-19 on its first orbital flight (LVM3-D1).

Very cold

Hydrogen turns to gas above −253 °C, and oxygen above −183 °C. Tanks, pipes and pumps must work properly even in this cold.

Very light hydrogen

Liquid hydrogen is very light, so the same number of kilos needs a big tank.

But the most push

For every kilo of propellant it gives more push than solid or ordinary liquid propellant. That is just what an upper stage needs, where every kilo saved is precious.

The night of 30 October 2026

On the night of 30 October 2026 the GSLV-F18 rocket lifted off from the second launch pad of the Satish Dhawan Space Centre at Sriharikota. It placed NavIC's new satellite NVS-03 in geosynchronous transfer orbit (GTO). The satellite reached its intended orbit. NVS-03 carries an Indian-built atomic clock as well as procured atomic clocks, and it sends signals in the L1 band as well as the L5 and S bands.

20th GSLV, 107th launch

This was GSLV's 20th flight and ISRO's 107th launch from Sriharikota. It was ISRO's third launch of 2026; before it, PSLV-C62 flew on 12 January and GSLV-F17 (with the EOS-05 satellite) on 4 September.

How GSLV-F18 is built

GSLV-F18 was about 50 metres tall and weighed about 420 tonnes at lift-off. It has three stages. First stage (GS1): a solid S139 core in the middle (20.1 m long, 138.2 tonnes of propellant) with four liquid L40H strap-ons around it (each 19.6 m long; 170.6 tonnes of propellant in all four together). Second stage (GS2): a liquid stage with the Vikas engine (42.1 tonnes of propellant). Third stage: the cryogenic CUS15 (14.4 tonnes of liquid hydrogen and liquid oxygen). At the very top, a 3.4-metre metal shell (payload fairing), with the satellite of about 2,250 kg inside.

A flight of about 19 minutes

On a GSLV flight like this, it takes about 19 minutes from launch until the satellite separates: on 29 May 2023 GSLV-F12 released NVS-01 after a flight of about 19 minutes. The animation below shows the nine steps of the flight in the order in which they come.

Planned orbit

GTO: perigee 170 km, apogee 37,023 km, inclination 20.75°. These are the planned figures.

Next step

Now NVS-03 will fire its own engine several times near apogee to raise its orbit and make it round, so that it reaches geosynchronous orbit. This is still to happen.

From IRNSS to NavIC

NavIC used to be called IRNSS (Indian Regional Navigation Satellite System). On 28 April 2016 its seventh satellite, IRNSS-1G, was launched, completing the seven-satellite constellation. That same day Prime Minister Narendra Modi dedicated the system to the nation under the name "NavIC" (Navigation with Indian Constellation). It is built to meet the country's positioning, navigation and timing needs.

Area, accuracy and two services

NavIC is India's own satellite navigation system, which tells you direction and position. It is built for India and the region up to 1,500 km around it, and in the main region it is expected to be accurate to better than 20 metres. It has a Standard Positioning Service for ordinary users and an encrypted Restricted Service for strategic users.

Three bands: L5, S, L1

NavIC's signals come in three frequency bands: L5 (1176.45 MHz), S (2492.028 MHz) and L1 (1575.42 MHz). L1 came with the NVS series: NavIC satellites from 2023 onwards are built to send the standard service signals in all three bands. L1 signals can work together with the signals of other satellite navigation systems (GNSS): they are interoperable.

Distance from a clock

How does it work? Every satellite has a very precise atomic clock, and it keeps announcing: here I am, and this is the time right now. Your phone checks how long the signal took to arrive. Distance = speed of light × time. Light travels about 300,000 km every second, so even an error of one microsecond (a millionth of a second) turns into a mistake of about 300 metres.

Why four satellites

The distances to three satellites give the three unknowns of position (forward-back, left-right, up-down). But the phone's own clock is cheap and can be wrong: that is a fourth unknown. So at least four satellites are needed to tell your position.

Seven satellites, two kinds of orbit

NavIC is designed as seven satellites, plus their stations on the ground (ground stations), which work 24 hours a day, 7 days a week. Three of the satellite places are in geostationary orbit (GEO): right above the equator, at longitudes 32.5°E, 83°E and 129.5°E. The other four are in inclined geosynchronous orbit (IGSO). These two orbits are tilted at 29° (their inclination), they cross the equator at 55°E and 111.75°E, and each orbit has two satellites.

Over India all day

Both kinds of orbit are about 35,786 km up, and one trip round takes as long as one turn of the Earth (about 23 hours 56 minutes). So a geostationary satellite seems fixed at one spot in the sky. An inclined one also stays near its own longitude, but through the day it swings from 29° north to 29° south latitude, and its path over the ground makes a figure 8. In this way all seven stay in the sky over India and the region around it all day, and they stay spread out too: east-west and north-south.

2013 to 2016: the first seven

The first seven IRNSS satellites, IRNSS-1A to IRNSS-1G, were launched on PSLV rockets between July 2013 and April 2016. The first, IRNSS-1A (1 July 2013, PSLV-C22), went into the orbit tilted at 29° that crosses the equator at 55°E. IRNSS-1C went into geostationary orbit at 83°E, and the seventh, IRNSS-1G, at 129.5°E.

The clock trouble

Then came a big problem: three rubidium atomic clocks on IRNSS-1A failed. The cause turned out to be a small part: the feed-through capacitor that carries power (DC supply) to the main part of the clock (its physics package), which got into trouble when its temperature rose too much. The clocks of NavIC and of Europe's Galileo system had problems of a similar kind. Remember, the whole distance sum rests on the clock.

IRNSS-1H and 1I

IRNSS-1H was sent on 31 August 2017 on PSLV-C39 to take over navigation while 1A moved to messaging, but that flight did not succeed. Then on 12 April 2018 IRNSS-1I went up on PSLV-C41 to take 1A's place. Its atomic clocks were refurbished, and the clock fault had been corrected in them. IRNSS-1A was put to sending safety-of-life alerts, so that warnings of cyclones, tsunamis and high waves can reach fishermen. Such messages matter most where ground communication is poor or missing.

The second generation: NVS

To keep the service going, a second-generation series was planned: NVS, five satellites, NVS-01 to NVS-05. The first, NVS-01, went up on 29 May 2023 on GSLV-F12. It is planned to work for 12 years (its mission life), and it carries a rubidium atomic clock made in India by the Space Applications Centre (SAC), Ahmedabad: a technology only a handful of countries have. NVS-02 (29 January 2025, GSLV-F15) was to take IRNSS-1E's place at 111.75°E, but it could not raise its orbit.

Counting the satellites

Here is how the count went. By July 2025, 11 NavIC satellites had been launched: 4 were giving PNT, 4 were used for one-way messages, 1 had been switched off at the end of its service life, and 2 did not reach their intended orbit. In March 2026, 8 of the 11 were working: 3 were sending navigation signals and 5 could send one-way messages. NVS-03, launched on 30 October 2026, is NavIC's 12th satellite.

Vehicles

By 2019, AIS-140 standard vehicle trackers that run on NavIC had been made compulsory in all commercial vehicles, and more than 75 companies were making such trackers. The Indian standard for NavIC in vehicle tracking is IS 16833:2018 (amendment-2).

Mobile phones

3GPP, the body that makes the standards for mobile networks, has accepted NavIC, so that NavIC can be made part of assisted GNSS. Its standard is 3GPP TS 36.171 (Release 16), which is for Assisted NavIC support in LTE mobile networks.

Trains

By July 2025, NavIC had been fitted along with other GNSS in about 8,700 trains, and the target was about 12,000.

At sea

For ships, the IMO (International Maritime Organization) recognises NavIC as part of the World Wide Radio Navigation System (WWRNS). IRNSS-1A has been put to carrying warnings of cyclones, tsunamis and high waves to fishermen.

Disaster warnings

In 2025, early warnings of disasters such as earthquakes, floods, cyclones and landslides were being sent through NavIC under SACHET, a project of NDMA. Besides navigation messages, NavIC's signal can carry text messages too.

Exact time

NavIC's satellites carry atomic clocks, so it gives very precise time too. In March 2026, spreading Indian Standard Time (IST dissemination) was among the uses of NavIC being widened.

NVS-04 and NVS-05

In July 2026, NVS-04 and NVS-05 were in advanced stages of being built.

The March 2026 roadmap

The March 2026 roadmap had three points: complete NavIC's base layer constellation, improve its services, and bring in Indian-made technology, including a space-grade atomic clock, for self-reliance in technology.

Air traffic: still in talks

In March 2026, talks were going on with AAI and the Ministry of Civil Aviation (MoCA) about using NavIC in air traffic management. This is still a plan, not something in use.

Why Sriharikota

Sriharikota is a spindle-shaped island in SPSR Nellore district of Andhra Pradesh. To its west are Pulicat Lake and the Buckingham Canal, and to its east the Bay of Bengal. SDSC SHAR lists the reasons for choosing it: the country's east coast, away from where people live; a good corridor of launch directions for different missions (launch azimuth corridor); closeness to the equator, which helps launches towards the east; and a large empty area for safety.

A boost from the Earth's spin

Why does being close to the equator matter? The Earth spins from west to east. At the equator the ground moves east at about 465 metres per second; towards the poles this speed keeps falling (465 × cos latitude). At Sriharikota (about 13.7° N) it is about 452 m/s. A rocket launched towards the east gets this speed for free right from the start.

The sea to the east

Straight to the east lies the sea, so the flight path goes over the sea, and the spent stages fall into the water.

Two launch pads

There are two launch pads here. The first flight from the First Launch Pad was on 20 September 1993 (PSLV). The Second Launch Pad came into use in 2005; the first flight from it was made by PSLV with the Cartosat-1 satellite on 5 May 2005. GSLV-F18 also flew from the second launch pad.

Planned next: a third pad, a new centre

The plan ahead: the Union Cabinet approved the Third Launch Pad on 16 January 2025, with ₹3,984.86 crore; the target is to build it in 48 months. At Kulasekarapattinam in Tamil Nadu, the Prime Minister laid the foundation stone of a new launch centre for SSLV on 28 February 2024.

Chandrayaan-1

Sent towards the Moon on PSLV-C11, on 22 October 2008.

Mangalyaan (Mars Orbiter Mission)

On PSLV-C25, on 5 November 2013; India's first interplanetary mission. It reached orbit around Mars on 24 September 2014, and ISRO became the fourth space agency to send a craft successfully into orbit around Mars.

One flight, 104 satellites

PSLV-C37 sent 104 satellites together on 15 February 2017: a 714 kg satellite of the Cartosat-2 series and 103 co-passenger satellites, 1,378 kg in all.

Chandrayaan-3

On LVM3-M4, on 14 July 2023 at 2:35 pm, from the second launch pad at Sriharikota. It made a soft landing on the Moon on 23 August 2023.

Aditya-L1

On PSLV-C57, on 2 September 2023. On 6 January 2024 it reached a halo orbit around the Sun-Earth L1 point.

SpaDeX

On PSLV-C60, on 30 December 2024. On 16 January 2025 two spacecraft joined up in space (docking); India became the fourth country in the world to do this.

NISAR

A satellite built jointly with NASA, launched on GSLV-F16 on 30 July 2025.

TV-D1 (getting ready for Gaganyaan)

On 21 October 2023 the crew escape system was tested; the crew module went up to about 17 km.

Gaganyaan: the tests so far

Gaganyaan is India's programme to send humans into space. Several tests to prepare for it have been done: TV-D1 (21 October 2023) tested the crew escape system, and IADT-01 and IADT-02 (Integrated Air Drop Test) dropped a dummy crew module to test the system that slows it down (deceleration system). Work is going on for the next test, TV-D2.

Plan: G1 and the flights after it

Plan: the first uncrewed flight, G1, which will carry a half-humanoid robot called Vyommitra, is targeted for the fourth quarter of 2026. After that, two more uncrewed flights are targeted by 2027, and then the first crewed flight.

Bharatiya Antariksh Station

Bharatiya Antariksh Station (BAS): in September 2024 the Union Cabinet approved its first module, BAS-1. ₹20,193 crore was approved for eight missions, with the target of completing them by December 2028. The full station will have five modules, and the plan is to have it working by 2035. The vision further ahead: an Indian crewed mission on the Moon by 2040.

The other plans

Keep the other plans in mind too: the NGLV rocket (approved September 2024), the third launch pad at Sriharikota (approved 16 January 2025), the SSLV centre at Kulasekarapattinam, and NavIC's NVS-04 and NVS-05 satellites.