Satellite belt around the Earth

THE LATEST REPORT about the recently launched INSAT 3-B having reached its orbiting point in space should put at rest whatever doubts might have still lingered about launching of the satellite having been a hundred per cent success.

The ``collocation'' - which means placing together or side by side - of the INSAT 3-B with INSAT 2-E launched much earlier puts India among the nations which have thrown a ring of satellites around Planet Earth.

The slots along different east longitudes in the geo-stationary orbit are INSAT 2-B AND INSAT 2-C, INSAT 1-D and INSAT 2A and INSAT 2DT. They should together ensure a comprehensive availability of satellite services for broadcasting, tele- communication, television and weather forecasting.

The satellites at different orbits are separated from each other by quite a few thousand kilometres. If at all there has been any cluttering in space, it should be due to the ``space junk'' estimated at around three to four thousand pieces of varying sizes of debris floating above the Earth.

INSAT-3B weighing 2000 kg when it goes into orbit would have shed about 700 kg of its lifting material which would have been jettisoned among the space junk.

They gradually descend to be burnt when they slip into the Earth's atmosphere and if parts of the much larger ones escape being wholly destroyed, they drop into the sea or the ground as space souvenirs.

A space vehicle, shorn of all the equipment which become junk when they are left in the corrosion-free space either to orbit round the Earth or race towards distant planets (the U.S. spacecraft Galileo has been studying the planet Jupiter and its moon, the Ganymede, for some time now) which would take several years is not exactly quite ``lonely'' like the cosmonauts aboard a manned flight.

It is under unremitting surveillance from the ground-space tracking stations. The space environment is not wholly friendly to space vehicles because of its being corrosion-free.

The findings of an exploratory pilot project launched way back in 1991 by the European Space Agency (ESA) were that the space environment is not as hospitable as might have been believed.

The exposure of space vehicles to the unfiltered ultraviolet rays leaves them in an irradiated environment of the kind known to the Earth-based nuclear industry.

Arthur Clarke in his Greetings, Carbon-based Bipeds, writes about the hazards of electromagnetic pollution from communication and meteorological satellites and this is said to have wrecked radio astronomy. While the new technologies which have supported the space programme covered the development of solid lubricants carried out by the Sriharikota Rocket Centre, the longevity of the satellites lasting for not less than seven years will depend upon the durability of the systems built into it and their giving the required response to the commands given from the Earth-based tracking stations.

The life of a geo-stationary communications satellite is determined by the amount of fuel which can be carried for orbital manoeuvres.

Keeping the satellite exactly in the right orbit requires periodical manoeuvres to compensate for changes in it resulting from perturbations from the gravity of the sun and the moon and from the other unsettling space presence, the solar wind.

The satellite's orbit begins to change when its fuel runs out. Even if it has enough fuel to keep it in orbit, the efficiency of its solar arrays which meet its power requirements begins to weaken.

The instructions to the satellite given by the ground-based electronics are in the binary coded digital form which the computer can understand and execute and they will have to be appropriately coded and the coded signal is telemetred to the satellite.

The computer system aboard the satellite receives and processes the instructions which are then decoded. The instructions could range from the supply of specified quantity of fuel to the boosters, the firing of the engines for a certain period of time and the reorientation of the antenna.

This requires a total grasp at the earth- based monitoring stations of the performance of the satellite-borne systems. The codes used are generally the American Standard Code for Information Interchange (ASCII).

Advancing space technology has shielded the exposure of satellites to radiation while deriving the maximum advantages from the space environment being corrosion free arising from the absence of oxidation hazards.

The INSAT system operates over 5,000 two-way circuits covering 141 routes amounting to as many as 1,20,000 route kms of communication facilities as against the not more than 70,000 route kms of terrestrial communication facilities.

The range of services provided by the satellites in the 2000 kg range extends from land use\cover mapping for agroclimate zonal planning, flood-mapping and damage assessment, acreage and yield estimation of major crops, national level forest cover mapping and ground water potential zone mapping, estimates of snow-melt run off for the Sutlej and Beas basins, to mention only a few.

With the satellites extending such a wide range of services from as far away as 35,000 kilometres, the very large number of two- way circuits just mentioned provide for the monitoring of them by the ground tracking stations.

One should remember that so much has become possible when we are only just stepping into the space age. Though multi-purpose satellites like INSAT 3-B weighing 2000 kgs, throwing images of heavy tonnage, they are a far more miniaturised outfit when seen against comparable terrestrial gadgetry weighing several times more with with the availability of services being far more limited.

Such a mega-performance in space would not have become possible but for the technology of a genre far different from what terrestrial engineering had depended upon before the launching of satellites was even thought of.

The cryogenic engine which hurls satellites into orbit has emerged from a space age chrysalis making its demands for propellants which have to be maintained at minus 253 degrees C.

Satellite technology is advancing at a pace which had not been matched at any time earlier and it represents a response to the demands from space age science a good part of which has to ensure operations and performances in freezing wildernesses under the sun and a canopy of stars.

INSAT 3B will be generating 3,700 watts from a single-sided solar array configuration while the generation from the double-sided configuration would be as much as 1650 megawatts.

When the ISRO puts satellites into geo-synchronous orbit with its Geo-Stationary Launch Vehicle (GSLV) in the coming years, it will be a major milestone in space.

The orbit of the geo-stationary satellite will be synchronous with the orbit of the Earth and will, therefore, appear to be stationary at a distance of 35,900 kilometres.

It was again Arthur Clarke who is credited with the concept of the geo-synchronous satellite. He visualised how communications around the world would become possible with a network of three geo-stationary satellites spaced at equal intervals around the Equator.

His concept of the geo-synchronous satellite was successfully tested with the launching of Syncom 2 on July 1963 while Syncom 3 launched in August 1964 was the first geo-stationary stellite.

C. V. Gopalakrishnan

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