Space

November 21, 2003
Special purpose satellites and lunar probes: ISRO's new dimension

Rajendra Prabhu

NEW DELHI -- The recent launch of a purely communication satellite and the forthcoming launches of ResourceSat and preparations for the moon probe, takes the Indian Space Research Organization (ISRO) to a new dimension in getting deeply involved into the socio-economic development of the country.

The communication satellite INSAT-3E marks a departure from the earlier series of geosynchronous satellites (those which appear to be at a fixed point in relation to the earth) which were multipurpose. Though more of these "one-in-three" satellites are still on the drawing board and launch pad, ISRO is concentrating at each future satellite doing specialised work such as checking on record, education upgradation for millions of people, etc.

All of these satellites help our development objectives. The 1,360kg RESOURCESAT-1 is in the series of earlier remote sensing satellites like IRS, six of which have already been used to probe the Indian agriculture, ground water resources, forest cover, minerals and other natural resources. All of these are sun synchronous satellites, that is, they are in a circular orbit always facing the sun. When they orbit over the Indian territory, their cameras open up and take images of the country that are collated with other observations to provide macro images of Indian vegetation, water and minerals.

RESOURCESAT-1 is the heaviest and most modern of them all with high resolution cameras, linear imaging self-scanner (LISS-4), LISS-3 and an advanced wide field sensor (AWiFS). It has a solid-state recorder with a capacity of 120Gigabits to store images taken by its cameras, which are then read out to the ground stations during specific points in its orbits. LISS-4 operates in three spectral bands with 5.8-meter spatial resolution and steerable upto +/-26 degrees to obtain stereoscopic imagery and achieve five-day revisit capability. The LSS-3 has, in addition to three spectral-band cameras, a short-wave infrared band with 23.5-meter spatial resolution. The AWiFS similarly has an additional band with 56-meter spatial resolution. With all of these cameras, ground stations can receive enormously improved imagery. It would continue the imaging being done by the earlier satellites, IRS-1C and IRS-1D, launched in 1995 and 1997, respectively, but whose life is at its sunset point.

While these remote sensing satellites help India to map its natural resources and then manage them more efficiently, the geo-synchronous INSAT series of satellites are helping build communication bridges across the country, beam TV signals and take meteorological images every half an hour. Here too, the focus has shifted to single-purpose satellites, instead of a multi-functional spacecraft INSAT was initially designed for. The INSAT series are playing a major role in social development projects, such as spreading social awakening, awareness of developmental issues, bring literacy to people in the deep interiors, connect rural doctors to specialists in metros, provide tele-medicine facility to villagers, etc. A significant step in spreading literacy and improving educational facilities would be taken with the orbiting of EDUSAT in the next few months.

ISRO, which at one time had to depend upon foreign launch vehicles for placing its satellites into different orbits, is now moving toward self-sufficiency with tremendous improvements in its PSLV launch vehicle and the tests of GSLV launchers. PSLV was originally conceived to place into polar orbit (about 900km circular orbit from the earth) a spacecraft weighing upto 1,000kg. However, with RESOURCESAT-1, this launch vehicle has demonstrated its capacity to go beyond that earlier design limit.

Incorporating changes to PSLV's original four-stage design has provided the much higher capacity for payload beyond one tone. The earlier concept of the four stages with alternately solid and liquid propulsion remains. However, in the first stage, the six boosters or strap-on motors attached to the main motor fire at different times. Using the solid fuel hydroxyl-terminated poly butadiene, the fist stage develops a thrust of 4,672kiloNewton (kN). The second stage uses liquid propellant UH25 with Nitrogen Tetroxide as the oxidizer. Together, the 41.5-ton liquid fuel produces a thrust of 800kN. The Vikas engine for the rocket was designed indigenously, just as all the other materials, including the maraging steel for the casing, the various carbon composites, etc.

The final two stages have undergone extensive modification from the original design for substantially improving the vehicle's payload capacity from the original 1,000kg to over 1.5 tons. (The payload capacity for the GSLV is over two tons.) The third stage uses 7.6 tons of HTPB-based solid propellant and produced a maximum thrust of 246 kN. With its polyramide motor casing, there is a substantial saving in weight. The last stage has a twin engine configuration using liquid propellant. The 2.5-ton mono-methyl hydrazine and mixed oxides of nitrogen together make up the fuel. Each one of these twin engines generates a maximum thrust of 7.3kN. It is the fourth stage that pushes the payload into the desired orbit, and is therefore, a very critical item for the success of the launcher. The fourth stage houses the inertial navigation system that guides the vehicle from the lift-off to spacecraft injection into the orbit.

The tracking system provides real-time information for flight safety and for preliminary orbit determination after the satellite attains its designed height. The spacecraft itself has corrective mechanism in the form of thrust control motors using hydrazine fuel to correct the orbit exactly according to design. The whole operation of tracking and spacecraft control is monitored and maneuvered from the master control facility at Hasan near Bangalore.

PSLV, which was first tried out in 1993 as a launch vehicle, has now become a major launch vehicle for ISRO and could be used for launching satellites of space organisations from other countries. This is the sixth launch using PSLV. It has only failed once to meet the design objectives. It is to be the launch vehicle for ISRO's projected unmanned flight to the Moon named Chandrayan-1. The indigenously designed and built GSLV is under construction. It develops far greater thrust than the PSLV by using, among other things, liquid hydrogen and oxygen as fuel. Initially, GSLV had to use Russian-built cryogenic engines for GSLV. However, indigenous cryogenic engines will finally power India's largest launcher soon.