EOS-05 Satellite: Why ISRO Opted for a Geosynchronous Orbit Instead of Geostationary Orbit

The Indian Space Research Organisation (ISRO) is set to place its latest Earth observation satellite.

EOS-05, in a geosynchronous orbit (GSO) rather than the more commonly used circular geostationary orbit (GEO), ISRO scientists said after two successful orbit-raising manoeuvres over the weekend.

EOS-05, described as India’s first dedicated imaging satellite to operate from a geosynchronous orbit, was launched successfully aboard the GSLV-F17 on Friday. The rocket placed the satellite into a sub-geostationary transfer orbit with a perigee of 171 km and an apogee of around 31,000 km.

The satellite’s onboard Liquid Apogee Motor (LAM) was then fired in a series of manoeuvres to gradually raise and reshape its orbit. Following the operations conducted on Saturday and Sunday, EOS-05 reached an estimated orbit of 20,000 km by 35,786 km.

The satellite is ultimately expected to operate in a geosynchronous orbit at the 85.5° East orbital slot, rather than in a perfectly circular geostationary orbit.

“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 in a geosynchronous rather than a geostationary orbit.

Why the orbit matters

A geostationary orbit is a specific type of geosynchronous orbit. To be classified as geostationary, a satellite must orbit above the equator in a nearly circular path at an altitude of about 35,786 km, allowing it to remain apparently fixed over the same point on Earth.

Such an orbit is particularly useful for applications such as continuous weather monitoring, as the satellite can repeatedly observe the same region without significant changes in its position relative to the ground.

EOS-05, however, will operate in a broader geosynchronous orbit with an elliptical path. The satellite is expected to provide high-value Earth imagery and strategic information, with the Indian Navy among its anticipated users.

Near-perfect GSLV-F17 launch

The EOS-05 mission also marked a major success for the GSLV, which carried the heaviest payload of its class in the launch.

The rocket was originally expected to inject the satellite into a 170 km by 29,000 km orbit. Instead, it achieved a 171 km by 31,000 km orbit, giving the spacecraft additional energy.

“The vehicle has given more energy,” ISRO Chairman V Narayanan said after the launch at the Satish Dhawan Space Centre in Sriharikota.

Spacecraft Director Imteyaz Ahmed described the mission as a precise injection of one of the heaviest and largest Earth observation satellites of its class by the GSLV.

ISRO carried out the first planned orbit manoeuvre on Saturday, firing the satellite’s LAM for 5,406.4 seconds. The manoeuvre placed EOS-05 in an estimated 20,000 km by 31,129 km orbit.

A second manoeuvre was completed successfully on Sunday, with the LAM firing for 338.9 seconds and raising the satellite’s apogee to approximately 35,786 km.

Further manoeuvres are planned to place EOS-05 in its intended geosynchronous orbit at the 85.5° East orbital slot.

Important milestone after earlier setbacks

The successful orbit-raising operations are particularly significant for ISRO following previous difficulties with similar missions.

In August 2021, the GSLV-F10 failed to place the EOS-03 Earth observation satellite into orbit after an issue with the rocket’s cryogenic upper stage. EOS-03 had also been designed to provide near-real-time imaging and rapid revisit capabilities from a geostationary orbit.

Another recent setback involved the NVS-02 navigation satellite, launched aboard GSLV-F15 last year. Its onboard propulsion system failed to fire, preventing the spacecraft from completing the planned orbit-raising manoeuvres.

The successful operation of EOS-05’s onboard engines therefore represents another important achievement for ISRO.

EOS-05 is equipped with multi-band imaging capabilities and is designed to deliver strategic Earth observation data. Once fully operational, it will become India’s first dedicated imaging satellite operating from the geosynchronous orbit, according to ISRO officials.