Indian Space and Research Organization (ISRO) has successfully completed the Post-Mission Disposal (PMD) operation of the communication satellite GSAT-12. The satellite carrying 12 extended C band transponders was launched on July 15, 2011. It was located at 83° E longitude till March 2021.
After the launch of its replacement satellite CMS-01 in 2020, it was later relocated to 47.96° E longitude. The satellite served for more than a decade.
The Geosynchronous Earth Orbital regime is one of the most populated and highly utilised regions. Internationally accepted space debris mitigation guidelines by the UN and IADC recommend disposing of an object away from the GEO region at its end-of-life.
The recommended practice is to re-orbit the object to a nearly circular “graveyard” orbit sufficiently above the GEO region so that the orbit would not decay back into the GEO-protected zone under the influence of perturbation forces, such as non-uniformity of Earth shape, Sun’s Moon’s gravity, solar radiation pressure, etc., within another 100 years.
Hence, the final disposal orbit must meet specific criteria on the minimum raise in perigee altitude depending on the object’s reflectivity, mass, shape, and size. The graveyard orbit also needs to be nearly circular (very low orbital eccentricity, less than or equal to 0.003).
For GSAT-12, the required minimum raise in perigee altitude was estimated to be 261 km. As a result of meticulous operations management by the Master Control Facility (MCF), the available fuel of GSAT-12 was more than sufficient to meet this goal. A series of seven manoeuvres were carried out to raise the orbit above the GEO altitude of 35786 km.
The first manoeuvre on March 16, 2023, was a shorter burn to circularize the orbit, followed by another six burns, typically of 150 seconds duration. These burns were performed alternately at the perigee and apogee points, 12 hours apart, to raise the respective altitude in steps of approximately 116 km.
This strategy ensured that the intermediate orbits also remained almost circular. After completing the seventh burn on 19th March 19, the satellite reached a super-synchronous circular orbit of nearly 400 km above the GEO altitude.
Space Debris Mitigation guidelines also recommend passivation/removing all energy sources, fluidic and electrical, to minimise the risk of any post-mission accidental break-up. Four inclination changing manoeuvres of GSAT-12 were carried out during March 20-22 to spend the remaining propellant.
To deal with the growing uncertainty in the exact availability of fuel near depletion, this inclination-changing strategy helped avoid any disruption in the circularity of the already achieved graveyard orbit.
On March 23, the final passivation manoeuvre was carried out to vent out the remaining fuel by firing the oppositely mounted thrusters, cancelling the net thrust without affecting the orbit. As part of electrical passivation, all rotating mechanisms such as the momentum wheels, reaction wheels and gyros were turned off, batteries were disconnected from the solar panels and discharged.
Finally, the transmitters were switched off to avoid any potential RF interference. The passivation activities were completed on March 23, 2023.
GSAT-12 is the 23rd GEO satellite to undergo PMD before decommissioning. The finally achieved circular orbit was nearly 400 km above the GEO altitude. The PMD operation is fully compliant with the IADC and UN space debris mitigation guidelines as well as the UN guidelines for the long-term sustainability of outer space activities.
