EOS-05 satellite: Why Isro chose geosynchronous orbit over circular geostationary orbit

The first earth observation satellite to be launched into a geosynchronous orbit by the Indian Space Research Organisation (Isro), the EOS-05, will be placed in a slightly elliptical GSO orbit instead of a circular GEO orbit known as a geostationary orbit, Isro scientists indicated following orbit-raising maneuvers conducted on Saturday and Sunday.

The EOS-05 satellite, India’s first dedicated imaging satellite from the geostationary orbit, which is meant to provide important strategic data, was successfully launched on Friday by the GSLV F17 rocket and placed in a sub-geostationary transfer orbit (GTO) of 171 km perigee (nearest distance to Earth) and 31,000 km apogee (furthest distance from Earth).

“Each orbit is selected for different purposes. Here we have chosen GSO (geosynchronous orbit),” an Isro scientist associated with the satellite program said. “The satellite will be in a geosynchronous orbit instead of a geostationary orbit,” another Isro official said.

While the geostationary orbit is a subset of the geosynchronous orbit, the GEO orbit is considered optimum for Earth observation as the satellite will be permanently positioned over one location on Earth to collect data in the form of satellite imagery.

“Geostationary orbits are achieved by selecting a geosynchronous orbit that is perfectly circular, described as having an eccentricity of 0 and an inclination of 0 right on the equator, or low enough that it can use propulsion to maintain its position over Earth,” according to Nasa.

A permanent position for an ‘Eye in the Sky’ satellite is useful for weather monitoring since it allows continuous observation of a specific area for weather patterns. One of the users of the EOS-05 is expected to be the Indian Navy for strategic data.

The EOS-05 had a near-perfect launch by the GSLV in its heaviest mission so far. “The vehicle (GSLV F17) was supposed to give a 170 km by 29,000 km orbit, and we have got a 171 km by 31,000 km orbit, which is 1,200 km above the required orbit.

The vehicle has given more energy,” Isro Chairman V Narayanan said soon after the EOS-05 launch at the Satish Dhawan Space Centre at Sriharikota on Friday morning. There has been a “precise injection of the heaviest and biggest earth observation satellite of this class by GSLV,” spacecraft director Imteyaz Ahmed said.

On Saturday, Isro successfully carried out the first planned orbit manoeuvre on the satellite “with a LAM engine burn duration of 5406.4 seconds” to put EOS-05 in a more circular orbit. “The estimated orbit subsequent to the manoeuvre is 20,000 km x 31,129 km.

Further, in the coming days, orbit manoeuvres are planned to eventually place EOS-05 in GSO (85.5°East Orbital Slot)” Isro said. On Sunday, the “second operation to raise the apogee of EOS-05 was completed successfully” with the “LAM engine fired for 338.9 seconds and the estimated apogee after maneuver is 35,786 km,” Isro said.

The GSO of the satellite is currently 20,000 perigee by 35,786 apogee, and the final orbit is expected to be a GSO rather than a GEO, sources said. Isro has had multiple successes in the current EOS-05 mission, starting with the near-perfect performance by the GSLV F17, especially after the failure of the GSLV F10 in the launch of the EOS-03 (GISAT 1 mission) on August 12, 2021, following a subpar performance by the cryogenic final stage of the GSLV rocket on that mission.

Like the EOS-05 mission (GISAT-1A), the EOS-03 mission in August 21 was intended to place an Indian imaging satellite in a geostationary orbit “for providing near real-time imaging with fast revisit capability and real-time monitoring”.

Another success for Isro in the EOS-05 mission is the successful firing of the onboard engines on the spacecraft to allow the satellite to raise its orbit to a more optimal position from the sub-GTO orbit it was placed in on Friday by the GSLV F17.

Last year, the navigation satellite NVS-02 – intended to provide positioning services like GPS – which was launched into GTO by the GSLV F15, had failed after its onboard engines did not fire to facilitate orbit-raising maneuvers.

“Once the EOS 05 is operationalized in orbit, it will become India’s first dedicated imaging satellite from the geo orbit, providing important strategic data for our country,” Isro chairman Narayanan said after the launch.