A setback and some lessons

Published : May 12, 2001 00:00 IST

The failure of GSAT to reach the geo-synchronous orbit does not detract from the success of the Geo-synchronous Satellite Launch Vehicle mission.

A SHORTAGE of 10 kg of liquid propellants on board the GSAT, which was launched by the Geo-synchronous Satellite Launch Vehicle (GSLV) from Sriharikota, Andhra Pradesh, on April 18, made the spacecraft go into a "drift orbit" instead of the geosynchronous orbit. The satellite now circles the earth once every 23 hours instead of 24 hours and thus fails to match the earth's rotation. As a result, the Indian Space Research Organisation (ISRO) will not be able to carry out many of the novel communication experiments that it had planned to do with the GSAT's payloads. The experiments can now be done only for a limited period, when GSAT is visible over ISRO's Master Control Facility (MCF) at Hassan, Karnataka. It went off the MCF's visibility range on April 30 and is expected to come back by May 15, when ISRO can conduct experiments related to digital audio broadcasts.

An ISRO engineer said: "The GSAT in the drift orbit is like a fantastic car without petrol." The satellite was in "good health" although it could not reach its space home at 48 East in the geosynchronous orbit.

The GSAT's failure to reach its geosynchronous orbit does not detract from the success of the GSLV mission because it was essentially a developmental flight aimed at testing the capability of the vehicle to put a communication satellite weighing around 1,500 kg into a geosynchronous transfer orbit (GTO). The vehicle, carrying the 1,540-kg GSAT, did this successfully. A top ISRO scientist said: The mission was to prove the capability of the GSLV and it has been done. This is a fantastic achievement for a developmental vehicle."

According to Dr. K. Kasturirangan, Chairman, ISRO, besides proving the vehicle's capability ISRO was able to perform all the critical manoeuvres with regard to the satellite. The massive solar panels and the long solar boom were deployed, the antennae spread out, and the satellite itself was put in a three-axis stabilised mode. Thus the spacecraft was in its final configuration.

G. Madhavan Nair, Director, Vikram Sarabhai Space Centre, Thiruvanantha-puram, said that the drifting of the satellite came as a disappointment after a perfect launch. He called it "a narrow miss". He praised R.V. Perumal, GSLV Mission Director, for the mission's success. "Perumal sweated blood for the GSLV's success and brought maturity to its complex technology," Madhavan Nair said.

There was much jubilation at Sriharikota when the three-stage GSLV lifted off from its launch pedestal at 3-43 p.m. and deployed the satellite into the GTO. The first stage of the GSLV comprised two strap-on liquid engines attached to the core solid stage; the second was a liquid engine stage; and the third was a cryogenic stage, which was used for the first time in an ISRO vehicle. Although the upper cryogenic stage was imported from Russia, the electronics, control and guidance systems for the entire vehicle, including the cryogenic stage, were done by ISRO engineers (Frontline, May 11, 2001).

It was a perfect flight with the three stages, including the liquid strap-on engines, igniting accurately and jettisoning with clock-work precision. The upper cryogenic stage should have injected the satellite into orbit at a velocity of 10,200 metres a second but the velocity fell short by 0.6 per cent, that is, 70 m a second. As a result, the satellite reached a highly elliptical orbit with a perigee of 181 km and an apogee of 32,051 km against the target of 180 km by 35,975 km, plus or minus 675 km. That is, the apogee was shorter by about 3,900 km.

Dr. Kasturirangan attributed this to "the dispersions in the orbital injection parameters expected in the very first test flight." The shortfall was made up by firing the propulsion motors on board the satellite and GSAT reached the intended GTO on April 19. "The shortfall of 0.6 per cent in the final velocity of injection (of the satellite) was correctable and was corrected using the satellite's propulsion," Dr. Kasturirangan said. Through a series of six orbital manoeuvres conducted between April 19 and 23, the satellite's orbit was raised close to near-geosynchronous height with an apogee of 35,665 km, a perigee of 33,806 km and an inclination of 0.997.

But an unexpected problem arose. When ISRO engineers at the MCF were giving commands to raise GSAT's orbit, they fired its liquid apogee motor (LAM). For this, they used fuel from two tanks: one German-made tank and the other Indian-made. An ISRO engineer said that the flow of fuel from one tank was higher than that from the other. As a result, one tank got emptied faster than the other and the satellite's centre of gravity shifted.

The MCF personnel worked to keep the satellite in the right attitude, that is, direction. The attitude control thruster held the satellite in the proper direction despite the shift in its centre of gravity. The engineer said: "The attitude control thruster kept firing and holding the satellite in the proper direction. That is how we lost the fuel."

ISRO engineers resorted to a bold strategy to raise the GSAT's orbit. Instead of using the LAM alone for this purpose, they used four (out of 22) Newton thrusters. The smaller Newton thrusters are normally used to correct the satellite's orbit and not to raise it. An ISRO engineer said: "For the first time in the world, the Newton thrusters were fired for 85 minutes continuously. It is a record." Another achievement was that the experience gained by the engineers in this operation would help them in future missions of INSAT (Indian National Satellite). "We have learnt how to use the smaller Newton thrusters in the back-up mode in case there is a problem with the LAM while raising the apogee," the engineer said.

Dr. Kasturirangan said: "Several new communication technologies have been evaluated using GSAT." They included the fast recovery star sensor, a sophisticated instrument on board the satellite that is used to measure its orientation and to recover the earth-lock quickly in case of its loss. (The loss of earth-lock in an INSAT mission had left the spacecraft doomed). A new technology for thermal control of satellites using heat pipes was proved. A new technique of power management through charger arrays for improving the overall efficiency of power systems was validated.

An ISRO scientist pointed out: "We basically wanted to prove the hardware on the satellite. This has been done." Since GSAT was drifting 13 towards west, it kept going out of the MCF's visibility range and coming back.

Madhavan Nair said: "We will not be able to do all the experiments with GSAT. We will do the maximum number of experiments when it reaches our slot. It will come over the MCF in the middle of May. Then we will test the various sub-systems of the satellite." He added: "Whenever GSAT is visible over the MCF, we can still use it for digital audio broadcasts. It is only an experimental satellite. We did not want to put a dummy payload on the GSLV. So we used GSAT. If it had reached the precise geosynchronous orbit, we could have done these communication experiments for three years."

International regulations demanded that ISRO switch on GSAT's payloads only in its slot, he said. They could be switched on for a short duration.

According to Madhavan Nair, it is not correct to attribute the 0.6 per cent shortfall in velocity to the under-performance of the Russian cryogenic stage. He said: "It is wrong to say that. The shortfall was only 0.6 per cent, that is, 70 metres less than a velocity of 10,200 m a second. It is a very short gap. The satellite can do this kind of correction. It (the shortfall) was made up... But one of the fuel tanks got depleted. The centre of gravity shifted. We could not fire the main thruster, which gives 310 metres a second of specific impulse. The secondary thruster gave only 280. So we consumed more fuel in the process. We ran short of our goal. We reached 35,000 km by 36,000 km. It is continuously circling the globe." (The satellite should reach a geosynchronous circular orbit, that is, 36,000 km by 36,000 km).

THE sixth Polar Satellite Launch Vehicle (PSLV), said Madhavan Nair, would be launched from Sriharikota towards the end of August. The launch campaign was progressing according to plan, the modification of the launch-pad used for the GSLV liftoff had started and the PSLV stage motors had arrived in Sriharikota, he said.

S. Ramakrishnan, Mission Director for the PSLV flight, said that the vehicle would put in orbit three satellites: Phoba from Belgium, BIRD from Germany, and the Technology Experiment Satellite (TES) of ISRO. Phoba had payloads that would try autonomous management of the satellite, Ramakrishnan said. BIRD was for remote-sensing. Both were micro-satellites weighing 100 kg each. The TES was a remote-sensing spacecraft to survey the earth's resources.

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