Imagine a sweltering August day. A refrigerator truck carrying tons of frozen chicken to a major distributor develops engine trouble and the refrigeration system fails.

A sure-fire disaster? No, thanks to the on-board alarm system that automatically beams a signal to a special black box on a satellite orbiting 23,000 miles overhead.

The signal is instantaneously relayed back to Earth, to the trucker's dispatcher, who can pinpoint -- within a few meters -- the truck's exact location 400 miles away.

The dispatcher responds promptly with a message saying help is on the way. Then, working back through the same ground-to-satellite system, the dispatcher locates another company truck deadheading home.

The empty truck is diverted to the aid of the disabled carrier. The chicken is transferred and delivered with only a few hours lost.

This scenario is now possible as a result of the successful launch March 28 of the McLean-based GTE Spacenet Corp.'s communications satellite GSTAR II on the Ariane rocket.

Riding piggyback on the GSTAR II was a small black box, named the Geostar Satellite System by its founder and promoter, Dr. Gerard O'Neill.

"Geostar solves one of the most fundamental problems in our highly mobile society: how to find and establish two-way communications with someone, wherever he is," said O'Neill, who is president and chief executive officer of Geostar.

The motorist, the boater, the cop on the beat and even the man on the street will be able to have access to Geostar through a small, hand-held lightweight transceiver -- not much larger than a checkbook -- powered by two AA batteries.

Each transceiver will have its unique digital identification code -- or fingerprint -- assuring privacy and confidentiality for the user while, at the same time, providing Geostar a simple way to bill the user.

The transceiver will send signals to the black box on the GSTAR satellite, which in turn will relay the messages back to Earth through a supercomputer in Princeton, N.J.

The computer will pinpoint the location of the transceiver, which it keeps track of through the identification codes.

Then, through telephone lines, the computer will forward the signal to its destination.

O'Neill envisions many commerical uses for Geostar, including:

Use by police departments to pinpoint positions of cruisers or to communicate with crime victims.

Alarm systems for cars, homes and businesses.

Use by state highway officials to monitor the transportation of nuclear and toxic wastes.

Communications by rescue officials in air and sea rescue operations.

Prevention of midair collisions.

And, if the family breadwinner is a salesman on the road and you have an urgent need to reach him but don't know where he is, one could just press the right button on the transceiver and, presto, be able to locate him through Geostar's alphanumeric (telegram) capability.

Westinghouse Electric Corp., which has a contract to monitor the movement of fissionable materials throughout the country, is one of the first companies to contract for Geostar's service.

Geostar also has signed Railstar Control Technology Inc., a subsidiary of Massachusetts Guilford Transportation Industries, to develop computer hardware and software systems for use in railroad and other fixed-route transportation systems.

The Geostar system is expected to be accurate enough for railroad control centers to manage train movement precisely.

Geostar claims to have orders for some 12,000 transceiver units.

Eventually, the complete Geostar system will comprise black boxes aboard three communications satellites.

By piggybacking them on satellites, the costs to launch the boxes is only $13 million (compared with the minimum $85 million that it cost GTE Spacenet to launch the satellite itself).

Initially, the company figured it would have to build and operate its own satellites -- at a cost of about $400 million.

But by attaching the system to other satellites, Geostar pared its costs substantially -- to about $100 million. Most of that money has come from private investors and potential corporate partners.

For consumers using Geostar, the transceivers cost about $450 apiece, but as they become widespread, the price is expected to drop to $150. The monthly service charge will be an additional $50.

RCA's Astro Electronics Division, builder of the receiver that will fly on GTE's next communications satellite, calculates that Geostar can carry 50 million messages an hour, eliminating any fear that the system can become saturated.

While early plans contemplate only coverage of the continental United States from three satellites, the Geostar engineers are aiming to provide global coverage, which would require receive/relay packages on six satellites.

How did this all begin? O'Neill told a group of navigation experts in London that a midair collision in San Diego in 1978 convinced him that a system must be developed that would make such collisions unlikely.

"I'm going to invent that system," he vowed.