The Hindu : METSAT launch: a technological feat

Online edition of India's National Newspaper
Saturday, Sep 14, 2002

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METSAT launch: a technological feat

By R. Prasad

CHENNAI SEPT. 13. The first firing of the propellant on board the METSAT satellite happened this morning at 7.30 a.m.. With this, the height of the satellite's orbit has been modified to 11,000 km perigee (nearest to the earth) and 35,000 km apogee (farthest from the earth). At the time of launch, the perigee was 218 km and apogee 35,000 km. Two more firings are pending and are scheduled for tomorrow and Monday.

Tomorrow's firing would be able to place the satellite in a more or less 36,000 km circular orbit and fine tuning, if required, would be done on Monday.

With this ISRO has met a tall order of using a Polar Satellite Launch Vehicle (PSLV) to launch a satellite into Geosynchronous Transfer Orbit (GTO). As a norm, Geosynchronous Satellite Launch Vehicle (GSLV) is used to launch a satellite into GTO.

The PSLV launches a polar orbiting satellite in a north-south direction thus scanning the entire earth while GSLV orbits in east-west direction.

But in the case of GSLV, the satellite stays `fixed' above a particular area of the earth as the satellite orbits at the same speed as the earth.

Why then did ISRO use a PSLV for this mission? "The answer is very simple," said G. Madhavan Nair, Director of the Vikram Sarabhai Space Centre, Thiruvananthapuram. "We have effected a net saving of nearly Rs. 70-80 crores by doing so."

This works out to nearly 50 per cent savings in the cost of the launch vehicle. The cost of a METSAT launch vehicle is Rs. 70 crore.

The trade off has been a reduction in the weight of the satellite. Typically, a GSLV carries a payload of 2 to 2.5 tonnes though the METSAT carries only 1055 kg.

Has it been easy to use a PSLV to launch satellites into GTO? "No," claimed Dr. Nair. "On the other hand, managing this mission has been particularly difficult." There are several reasons for this. Adding nearly 50 per cent more injection velocity while maintaining a proper orientation, altitude, flight path and sequence of path have proved very difficult.

A typical PSLV has an injection velocity of 7.5 km/sec while GSLV has 10 km/sec velocity. The velocity has to be increased to 10km/sec to overcome gravity and centrifugal force and take the satellite to 36,000 km orbit. At the same time care need to be taken not to exceed 11.2 km/sec velocity as the satellite would be out of earth's gravity and be lost forever. In short the height of the orbit determines the velocity to be imparted to the launch vehicle.

But to achieve this the scientists had to optimise on several parameters. For one the weight of the payload was limited to 1055 kg. A typical GSLV payload is in the order of two tonnes. Again, the third stage of the vehicle was made lighter by reducing the structural mass by nearly 70 kg. The efficiency of the motor was also improved by increasing the nozzle area ratio. The fourth stage fuel capacity was increased from 2 to 2.5 kg and the engine was made to burn the fuel for 25 per cent more duration. ISRO scientists are optimistic about increasing the payload of the PSLV launched into the GTO from 1055 kg to 1200 kg.

METSAT is unique in several other aspects too. First and foremost, it is a satellite dedicated for meteorological purpose. It is for the first time that water vapour band will be used (in addition to visible and thermal infrared bands). Water vapour band is particularly useful for countries with tropical climate. "The water vapour band would allow the meteorological department to measure the water content in the atmosphere and arrive at meaningful and more accurate prediction of rainfall," Dr. Nair said.

The resolution in the visible band has been improved from 8 x 8 km to 2 x 2 km. This has become possible by the use of better optics and sensors. The use of lightweight planar array antenna has helped the satellite. "We can claim to possess a world class meteorological satellite. And this has been made possible by combining water vapour band with other two bands," Dr. Nair said. The dedicated satellite gives the freedom to scan a particular zone of interest. Though this was technologically feasible even earlier, it was not put to use as earlier satellites were also meant for communication and broadcasting.

The next big target is to develop a Mark 3 GSLV capable of carrying a four-tonne payload to the GTO. Today's Mark 2 GSLV can carry only a two-tonne payload. "We are confident of launching Mark 3 by 2007," Dr. Nair said.

"And we intend using the indigenously developed cryogenic stage in lieu of the Russian one in the Mark 2 GSLV in two year's time. We may launch such a vehicle by 2004-2005."

The ability to maintain the METSAT flight to a predetermined path in conjunction with altitude and orientation has given a great boast to ISRO's lunar mission.

In the case of lunar mission, the precision of the launch vehicle path is very demanding. A difference of more than 0.01 degree can prove fatal. In the case of METSAT launch, the precision required was 0.2 degree. "There are many challenges to be met but this launch has made us more confident of a lunar mission," he said.

The lunar mission's aim is to explore the moon as there are several regions left unexplored.

Surveying the moon for rare metals and studying the origin of planets are also on the agenda.

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