GPS in Space

The Global Positioning System has helped many people find their way on earth. Now, this network of satellites may help fellow satellites find their way in space, and keep orbiting communications devices from colliding.

The conventional way to keep satellites on course involves controllers on the ground who monitor any changes in the satellites’ orbits using radar, for example, or radio transmissions. In recent years, however, several low-flying satellites (just a few hundred kilometers above the earth) have carried GPS receivers that allow them to determine their positions instantly and adjust their orbits faster and less expensively than if ground controllers had to intervene. But until now nobody knew if the GPS system would work above its own network of satellites, which orbit at 20,000 kilometers. This is an important question, since some 300 communications satellites already reside at 36,000 kilometers, the altitude at which a device can achieve “geosynchronous orbit,” hovering over the same part of the earth at all times.

In September, the amateur satellite AO-40, in cooperation with NASA, successfully detected a GPS signal at an altitude of 52,000 kilometers. Because GPS satellites only beam their transmissions toward the earth, AO-40 had to rely on small portions of beams that made it past the earth from satellites on the other side of the planet, up to 70,000 kilometers away.

“This capability can revolutionize the way we fly satellites in the future,” says Frank Bauer of the Guidance, Navigation and Control Center at NASA’s Goddard Space Flight Center in Greenbelt, MD, who arranged for the experiment. “We will be able to autonomously control spacecraft and keep them in the right orbit.” That ability may open up geosynchronous orbit to more satellites. The number of new devices the orbit can accommodate is rapidly dwindling, because the satellites must be widely spaced to avoid collisions. Bauer believes that satellites using GPS navigation could be safely placed closer together. This could help meet the growing demand for satellite communications services in areas like North America, Europe and East Asia, where space for satellites is shrinking the fastest.

Eventually, GPS could prove itself at even higher heights. San Diego-based TransOrbital plans to include a GPS experiment on TrailBlazer, an unmanned commercial lunar mission scheduled for launch in mid-2002. That experiment will test the ability to use GPS signals for navigation as far away as the moon, about 385,000 kilometers away.