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.