ISRO hopes to overcome orbit shortfall

By N. Gopal Raj

BANGALORE, APRIL 19 There appears to have been some under- performance in the Geosynchronous Satellite Launch Vehicle (GSLV) launched on Wednesday. Nevertheless, the Indian Space Research Organisation (ISRO) believes that it can ensure that the GSAT-1 satellite meets its design life of three years.

The orbit figures released by ISRO show that the GSAT- 1's farthest distance from the earth (apogee) after the launch was 32,051 km, compared to the 35,975 km which should have been achieved. The shortfall of 3,900 km in apogee is well beyond the launch vehicle's permissible orbital dispersion. In addition, the orbital inclination - the angle at which the satellite's orbit is inclined to the equator - is 19.2 degrees, compared to the 19 degrees planned for.

Additional propellant would therefore be needed to correct these shortfalls. The amount of propellant left when the satellite reaches its final position in orbit is a major determinant of its life since fine orbit trimming operations have to be regularly carried out throughout its lifetime.

Calculations show that the satellite would have had about 100-120 kg available to it for such corrections at the beginning of life, if it had been injected into the planned orbit. Calculations suggest that the additional propellant required by the GSAT-1 to compensate for the shortfalls of its orbit could be as much as 50 kg. One would, therefore, have expected the life of the satellite to be considerably reduced.

But ISRO appears confident that the satellite can be made to serve its design life of three years. According to one source, to allow for contingency situations, the satellite had been loaded with more propellant than was strictly necessary for three years of life. But this additional propellant alone will not suffice and ISRO will be looking to two other measures as well.

One is to minimise the propellant consumed in moving the satellite from its initial orbit to its destination in the geostationary orbit, some 36,000 km above the equator. Today morning, the first firing of the GSAT-1's onboard liquid-fuelled rocket engine went on for just 20 minutes, half that originally planned. The idea is to carefully check the satellite's orbit accurately before making further corrections. The onboard motor will be fired four or five times altogether, instead of the three firings originally planned. The other path for eking out the propellant would be to reduce the periodic corrections carried out after the satellite reaches its destination. This was done both in the case of Insat-2A and Insat-2B satellites when a problem in their tank design reduced the propellant available for corrections.

But there is no word yet on what caused the under-performance of the GSLV. Two systems could be the prime suspects. One is the imported Russian cryogenic engine which has only been ground- tested and has never before flown on any launch vehicle. The other is the rocket's guidance and navigation system. This system has been successfully flown four times on the Polar Satellite Launch Vehicle (PSLV). But that was for polar launch missions. This is its first debut for an equatorial launch.

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