US seeks to pack more comsats into orbit

Some 22,000 miles above the equator, the first United States domestic communications satellite, Westar 1, is going out of service. Its retirement symbolizes the end of the growing-up period for the communications satellite (comsat) business.

Comsats have gone from novelty to necessity in less than a decade. From now on, the managers of this business, the design engineers, and the regulators will be hard pressed to meet the rising demand for service. According to a study sponsored by the NASA Lewis Research Center, the radio frequency bands allotted for the service and the favored satellite orbit will be saturated in little more than a decade.

This preferred, so-called geosynchronous orbit, girdling Earth 22,300 miles above the equator, is where Westar 1 has perched. Traveling along it, a satellite moves at the same rate at which Earth spins. Thus the satellite hovers over a given spot on the planet's surface below. There are more than 60 comsats, belonging to various nations and the Intelsat international system, in this orbit.

Right now, US satellites such as Westar 1 are spaced at least 4 degrees (about 1,800 miles) apart along the geosynchronous orbit to avoid interference. However, in late April, the Federal Communications Commission (FCC) authorized 2 degree spacing. The agency anticipates the number of comsats designed for US domestic coverage will grow from 15 to 38 by 1987. Communications technology can now control radio interference enough to allow tighter spacing.

The FCC's action is only a slight reprieve for the satellite business, however. International Telephone & Telegraph and Western Union Telegraph (Westar's owner), assumed a 2 degree spacing when they made the Lewis-sponsored study.

As service expands to include such new features as direct satellite-to-home TV links, and as other nations make heavier demands on the geosynchronous orbit, saturation will be inevitable, says Steven Stephenson, market assessment technical manager for NASA Lewis. He predicts 2 degree spacing will postpone saturation by only a few years.

What is needed, he says, is to bring in a new generation of comsat technology such as is now being developed at the Lewis center. This would enable satellite channels to use higher frequencies and multiple transmission beams.

Mr. Stevenson explains this would involve moving to frequencies in the 20 to 30 gigahertz (billion-hertz) range from the present bands at 4 and 6 gigahertz and at 12 and 14 gigahertz. The higher frequencies would accommodate something like four times as many communications channels as do the present bands. Also, interference could be controlled well enough to allow 1 degree spacing of satellites. Thus twice as many satellites could be accommodated in a given orbital space.

In addition, Stevenson says, each satellite could use a number of narrow beams as opposed to a single beam today. This, too, would allow more individual channels.

Hughes Aircraft Company has suggested another possibility - clusters of satellites around a given orbital position. Some experts have proposed using a large space platform with many communications units and a central controller to guide their operation in an efficient and noninterfering manner. Hughes, instead , has patented a system of several individual satellites deployed in a close formation. In orbit, one satellite would act as the controller. This plan would help relieve orbital crowding.

Taken together, improvements such as these would expand satellite capacities enormously. Saturation would be avoided, Stevenson says. However, he adds that this requires continued support of work such as that at Lewis. Given continuing pressures to cut the NASA budget, he says such support is not now ensured.

Stevenson notes that, were the Lewis program to be cancelled, other countries would develop similar technology. ''We would end up buying from them and getting inferior systems,'' he observes.

Meanwhile, controllers have used the last of Westar 1's rocket propellant to move the satellite up about 40 miles, to a higher orbit. In a few months, it will be officially shut down. Then it will drift out of the way of active satellites, continuing to orbit Earth as a monument to the comsat pioneers.