SPACE

New heights for the PSLV

The versatile Polar Satellite Launch Vehicle is all set to put a meteorological satellite in the geo-synchronous transfer orbit.

T.S. SUBRAMANIAN
in Bangalore

METSAT, a 1,050 kg meteorological satellite, will be launched by a Polar Satellite Launch Vehicle, PSLV-C4, from Sriharikota, Andhra Pradesh, in September. This is the first time that the Indian Space Research Organisation (ISRO) is using a PSLV to launch a satellite at a height of 36,000 km, into a geo-synchronous transfer orbit (GTO). The previous five successive PSLV flights launched remote-sensing satellites in polar sun-synchronous orbits at a height of 800 km to 900 km.

The mission has thrown up various challenges and ISRO has a busy schedule ahead.

PICTURES COURTESY ISRO
METSAT, the meteorological satellite that is to be launched by the PSLV into a geosynchronous transfer orbit.

"There are two unique features about this mission," Dr. K. Kasturirangan, ISRO Chairman, told Frontline. One, the PSLV has been configured to launch METSAT in the GTO. Secondly, the spacecraft is meant exclusively for meteorological purposes as distinct from the three-in-one multi-purpose INSATs that have been used for meteorology, telecommunications and broadcasting, he said.

Dr. P.S. Goel, Director, ISRO Satellite Centre, Bangalore, called it a new mission in itself, "the PSLV in GTO". In his assessment, the most important thing is that "if we can prove this methodology of using the PSLV to launch the METSAT in the GTO, we can modify the METSAT into a small communications satellite". The cost will be lower too because it is launched by the PSLV. (Communications satellites have to be launched at 36,000 km altitude).

V. Sundararamaiah, Scientific Secre-tary, ISRO, said the third stage of PSLV-C4 had been upgraded to enable it to launch the METSAT in the GTO. "This is a high-performance motor, which uses solid propellants. It has more propellants now because its structural weight has been brought down, he said. The motor has undergone tests on the ground successfully. Another innovation is that the fourth stage will have more fuel - 2.5 tonnes of liquid propellants instead of two tonnes.

Extensive exercises have been done to "characterise" the vehicle for the GTO mission. "All aspects of the mission have been characterised and simulations done to realise this mission," said D. Narayana Moorthi, Director, Launch Vehicles Programme, ISRO, Bangalore. Mission design and implementation have been done for many disciplines.

S.S. Balakrishnan, Deputy Director, Mission and Launch Services, Launch Vehicle Programme Office, characterised the PSLV's third-stage motor as one of the most efficient upper solid-stage-motors in the world. "It is comparable to the best anywhere in the world," he said. This high-performance motor underwent a test on March 20, 1992 at SHAR, when about 270 parameters such as pressure, strain, temperature and vibration, were monitored. The new motor would increase the payload capability of PSLV-C4 by about 70 kg, he said.

METSAT's lifespan is seven years. ISRO Satellite Centre took two years to develop it. METSAT will beam pictures of approaching cyclones and moving clouds and data on water vapour content in the atmosphere. Vikram Sarabhai Space Centre, Thiruvananthapuram, has built PSLV-C4. The Mission Director is S. Ramakrishnan. The Vehicle Director is S.P. Sharma.

With a record of five successful flights, the PSLV has become a baseline vehicle for the GSLV, which had its maiden flight from SHAR in April 2001. Its easy adaptability to various configurations has earned it the reputation of being a cost-effective vehicle. ISRO plans to reconfigure it for its lunar mission.

Between now and 2005, modified versions of the PSLV will launch a series of satellites: METSAT, Resourcesat, Cartosat, Meghatropique and Astrosat. Resourcesat will be a world-class remote-sensing satellite and Astrosat will undertake science missions.

INSAT-3A will be launched by Arianespace from Kourou in French Guyana in October. INSAT-3A, being built by the ISRO Satellite Centre, will weigh about 3,000 kg at the launch pad. The second flight of the GSLV will rise from SHAR in the first quarter of 2003, bearing a GSAT-2.

According to V.R. Katti, Programme Director, GEOSAT, METSAT has several novel features. ISRO's satellite builders have used carbon fibre reinforced plastic (CFRP) in the structure of the spacecraft instead of aluminium monocoque. He said: "We want a structure that is light and meets various requirements. In addition, we prefer to have a surface that is a fairly good conductor of electricity. All this can be met by the CFRP." The satellite builders developed a new propellant tank for METSAT. INSAT-2A, 2E and others had a solar array on the one side and a boom and a sail on the other. This was a common feature in multi-purpose satellites like INSATs, which had a meteorological payload called Very High Resolution Radiometer (VHRR). METSAT does not have a boom and a sail. "It uses a specially designed magnetic torquer to take care of the imbalances resulting from radiation pressure exerted on the solar panel," Katti said.

There was a rationale behind ISRO's decision to use the PSLV to launch METSAT in the GTO, Balakrishnan said. Any launch vehicle can impart a certain velocity to the satellite and put it in a certain orbit. There were six PSLV missions, including three developmental and three operational ones, and all barring the first were successful. Every mission had something new. With each mission the PSLV's capability to throw a heavier satellite into a polar sun-synchronus orbit grew. The second PSLV mission was used to put an IRS weighing about 804 kg in an 825-km polar sun-synchronous orbit. The third orbited a 930-kg satellite. The fourth flight deployed IRS-1D, weighing 1,200 kg, in a 700-km orbit. The fifth PSLV mission accomplished a very critical operation, of launching three satellites from a single vehicle. They were ISRO's IRS-P4 weighing 1,050 kg, South Korea's Kitsat weighing 105 kg, and Germany's 45-kg Tubsat. The sixth flight again performed a multiple launch. It deployed both ISRO's Technology Experiment Satellite (TES), which weighed 1,100 kg, and Germany's BIRD in a 569-km circular orbit. Then the vehicle put Belgium's PROBA in a higher elliptical orbit by means of a tricky flight manoeuvre. BIRD and PROBA weighed about 94 kg each.

"This way, when you want to go to the GTO, you can put a 1,050 kg satellite there," Balakrishnan explained. "So the opportunity has now come to launch a satellite for meteorological purposes. It suits the PSLV's capability to do so. We are able to do this (just) with the cost of the PSLV," he added. The PSLV can also be used to a put a satellite weighing three tonnes in a low-earth orbit.

In 2005, the PSLV will launch Meghatropique, an Indo-French project. According to Sundararamaiah, the unique mission will launch two satellites each weighing 600 kg, one on top and the other below, by a dual launch adapter. CARTOSAT will also be launched in this configuration with another passenger. The co-passenger mode is different from the piggyback mode of the last two PSLV flights. Sundararamaiah added: "So there is scope not only for piggyback satellites but for co-passengers of 600 kg class to fly by the PSLV." In order to enable this dual mode launch, the PSLV's fourth stage would be reconfigured to create more space to accommodate two satellites. The quantum of propellants in the fourth stage would be reduced from 2.5 tonnes to one tonne.

Katti said that the building of METSAT had an evolutionary history. The first-generation multi-purpose INSATs with meteorological and communications components were built abroad and launched from foreign launch pads in the 1980s. It was prudent to continue to build these multi-purpose satellites even in the 1990s because they were cost-effective then. ISRO pioneered these satellites. But the demand for communications satellites with more transponders and higher signal strength has gone up now. So it was logical to break up these multi-purpose satellites into separate missions, one for meteorology and another for communications, Goel argued. Besides, multi-purpose satellites were very complex. Since they generated more power, the temperature-related issues arising in them were difficult to handle, he said.

For instance, INSAT-3A is a massive satellite that weighs almost 3,000 kg in the launch pad. It has 24 transponders, the meteorological payload called VHRR and a charge-coupled device camera. INSAT-3A will generate three kilowatts of power. Since the demand for communications services was going up, ISRO decided to separate the meteorological segment from the communications segment so that the latter could have more transponders with higher power. The communication services would pay for themselves from the user agencies. Thus was born METSAT.

Besides, the multi-purpose concept was not cost-effective for very large satellites. It was so for satellites in the 2.5-tonne class.

With the PSLV proving its capability to deploy satellites weighing from 800 kg to 1,400 kg in a polar sun-synchronous orbit, studies revealed that it could also put in orbit a 1,050 kg class satellite in the GTO. Besides, the India Meteorological Department (IMD) preferred an independent meteorological spacecraft. "A decoupled meteorological satellite met several objectives and so it made sense, from many angles, to develop METSAT," Goel said.

Meteorological satellites have a social purpose. This means that meteorological services should be cost-effective and this can be accomplished by a flight on board the PSLV. "So this is a novel use of launch vehicle theory... The challenge in this mission was to evolve a platform (satellite) that can be flown within the lift-off mass constraint of the PSLV," Katti said. A lot of effort was made to achieve this. It was found that a one-tonne-class satellite would suit the PSLV's lift-off weight for a GTO mission.

According to V.K. Kaila, Project Director, METSAT, the spacecraft has a VHRR for imaging in visible, infra-red and water vapour bands. It will give pictures of 2-km resolution of the globe. Infra-red pictures have a resolution of 8 km. In addition, mapping of water vapour content in the atmosphere can be done. METSAT has a data relay transponder too. It will collect weather data from various unattended platforms in the country and relay them to a central station in New Delhi so that all the weather data and pictures of clouds and approaching cyclones can be provided, said Kaila. To relay VHRR weather data from the spacecraft to the ground station, ISRO has used for the first time a planar array instead of a C-band parabolic antenna.

Another new feature of METSAT is that there will only be 12 thrusters aboard to control its altitude compared to 16 used in INSATs. The solar panel of METSAT will be deployed immediately after its injection into the GTO to take advantage of full power generation in the GTO itself.

Kaila explained the advantages of launching a meteorological satellite in the GTO. In the low earth orbit, a satellite took pictures of the earth periodically and it visited the same region only once in a few days. It took finer pictures, and at a higher resolution of, say, five metres or 2.5 metres, depending on its capability. But a VHRR in METSAT in the GTO could give a synoptic view of one-third of the global region at a closer resolution of 2 km. The advantage in this, Kaila said, was that METSAT in the GTO could give pictures of the same place and the same region once every half an hour. Thus, changes in the movement of clouds and cyclones could be observed better.