EOS-05 Mission: Why ISRO Chose Geosynchronous Orbit for Its New Earth Observation Satellite

The Indian Space Research Organisation (ISRO) has successfully raised the orbit of its latest Earth observation satellite, EOS-05, but the spacecraft is expected to operate in a geosynchronous orbit (GSO) rather than a perfectly circular geostationary orbit (GEO).

EOS-05 was launched aboard the GSLV-F17 from the Satish Dhawan Space Centre in Sriharikota on Friday. The rocket injected the satellite into a sub-geostationary transfer orbit with a perigee of 171 km and an apogee of about 31,000 km.

ISRO subsequently carried out two orbit-raising manoeuvres using the satellite’s onboard Liquid Apogee Motor (LAM). The operations on Saturday and Sunday significantly altered the spacecraft’s orbit, taking it closer to its intended operational position.

Following the second manoeuvre, EOS-05 was estimated to be in an orbit of 20,000 km by 35,786 km. Additional manoeuvres are planned before the satellite reaches its final operational orbit at the 85.5° East orbital slot.

Why EOS-05 will use a geosynchronous orbit

A geostationary orbit is a special form of geosynchronous orbit. Satellites in a geostationary orbit travel above the equator in a circular path at an altitude of roughly 35,786 km, allowing them to appear stationary over a fixed point on Earth.

This characteristic makes geostationary satellites particularly useful for applications such as weather monitoring, where continuous observation of the same geographical area is important. EOS-05, however, has been designed to operate in a broader geosynchronous orbit rather than a perfectly circular GEO. ISRO officials said the choice of orbit depends on the specific requirements of a satellite and its mission.

“Each orbit is selected for different purposes. Here we have chosen GSO,” an ISRO scientist associated with the programme said. Another ISRO official confirmed that EOS-05 would operate from a geosynchronous rather than a geostationary orbit.

The satellite is expected to provide high-value Earth imagery and strategic information. The Indian Navy is among the expected users of the data.

GSLV-F17 delivers precise injection

The EOS-05 launch marked a major achievement for the GSLV, which successfully carried one of its heaviest Earth observation payloads. The rocket had been targeted to place EOS-05 in a 170 km by 29,000 km orbit. Instead, it achieved a 171 km by 31,000 km orbit, exceeding the planned apogee by around 2,000 km.

“The vehicle has given more energy,” ISRO Chairman V Narayanan said shortly after the launch. Spacecraft Director Imteyaz Ahmed described the injection as a precise deployment of a large and complex Earth observation satellite. The successful launch was followed by the first orbit manoeuvre on Saturday. ISRO said the satellite’s LAM was fired for 5,406.4 seconds, resulting in an estimated orbit of 20,000 km by 31,129 km.

The second manoeuvre took place on Sunday. The LAM was fired for another 338.9 seconds, raising the satellite’s apogee to 35,786 km.

ISRO said further manoeuvres would be carried out to place EOS-05 in its planned geosynchronous orbit.

Significance for ISRO

The mission is particularly significant following earlier challenges involving GSLV-launched satellites. In August 2021, the GSLV-F10 failed to place the EOS-03 Earth observation satellite into orbit after the cryogenic upper stage did not perform as expected. EOS-03 had been intended to provide near-real-time imaging and rapid revisit capabilities.

More recently, the NVS-02 navigation satellite, launched aboard GSLV-F15, could not complete its planned orbit-raising operations after its onboard propulsion system failed to fire. The successful activation of EOS-05’s propulsion system and the completion of its initial orbit-raising manoeuvres therefore represent an important milestone for ISRO.

EOS-05 features multi-band imaging capabilities and is designed to provide strategic Earth observation data. Once operational, it will become India’s first dedicated imaging satellite to function from a geosynchronous orbit, according to ISRO officials.