ISRO parks EOS-05 in geosynchronous orbit after a 1,247-second burn

ISRO completed the third and final orbit-raising burn of EOS-05 at 8.57 p.m. IST on 7 September. The satellite's Liquid Apogee Motor fired for 1,247 seconds, a little under 21 minutes. The agency said the estimated orbit after the burn was 34,903 km by 35,884 km and that the spacecraft's health was normal.

That slot is the operational neighbourhood for a geosynchronous imager. EOS-05, also called GISAT-1A, is meant to sit near 85.5 degrees East and look at the same swath of Earth for hours at a time. Polar orbiters sweep the planet in strips. A geosynchronous imager can stare, which is why disaster managers and military users want it.

GSLV-F17 lifted the 2,367-kilogram satellite from the Second Launch Pad at Sriharikota at 2.55 a.m. IST on 4 September and put it into a sub-geosynchronous transfer orbit about 18 minutes later. It was the 19th GSLV flight and the heaviest payload that vehicle has carried. The rocket stood 51.7 metres tall and weighed 420.5 tonnes at liftoff. Its third stage was the indigenous CUS15 cryogenic engine, the same family that failed on GSLV-F10 in 2021. Chairman V. Narayanan called the injection precise. The assured life is seven years. He said he expects more than nine.

Two earlier burns did the heavy lifting. On 5 and 6 September the LAM ran for a combined 5,406.4 seconds, roughly 90 minutes, raising the orbit in steps from a perigee of about 171 km. After the second burn ISRO listed an estimated apogee of 35,786 km and a perigee near 20,000 km. Monday's firing raised the perigee again so the path is close to circular at geosynchronous height.

The mission is a return flight in all but name. GISAT-1, flown as EOS-03 on GSLV-F10 on 12 August 2021, was lost when the cryogenic upper stage underperformed. EOS-05 uses the same instrument family on a later I-2K bus. A working geosynchronous imager gives India a tool it has not had: frequent colour, infrared and, on this design, hyperspectral looks at the same basin, coastline or border without waiting for the next polar pass.

ISRO has said the satellite will serve civilian and strategic users. Floods, cyclones, forest fire and crop stress are the civilian list. The strategic list is not itemised in public notes. A sensor that can dwell over the Indian Ocean and the subcontinent at that altitude is useful to more than one ministry. Officials have also pointed to disaster-management rooms that now depend on a patchwork of polar revisits and commercial imagery when a cyclone stalls on the coast.

Some coverage on Monday described the final orbit as circular geostationary. The numbers ISRO posted are 34,903 by 35,884 km, which is geosynchronous and only slightly eccentric. Engineers had already indicated that the operational path would be a geosynchronous slot rather than a perfect 35,786-kilometre circle. For users the difference is small. For the flight dynamics team it is the orbit they planned.

Commissioning still lies ahead. Solar arrays deployed after separation. Payloads have to be switched on, calibrated and handed to the user agencies. Until those images appear, Monday's result is an orbital fact: the satellite is where it was sent, the engine ran for the time it was asked to run, and the long gap that began with the 2021 failure is closed on the board if not yet in the archive.

GSLV's early nickname on the ISRO campus was unkind. The vehicle has now delivered its heaviest satellite into the transfer orbit it was given and the satellite has finished the job with its own engine. The next public test is a picture from 85.5 degrees East.