December 2004

Space Tracker

The earliest satellite watchers' ideas led to GPS.

By Dan Cho

Over the past decade or so, the Global Positioning System has grown from a military navigation tool to a near ubiquitous tracking system. Biologists use it to follow migrating animals, while motorists depend on it to avoid getting lost on highways. But while the system's wide availability has sparked an explosion in innovative applications, satellite navigation itself can be traced back to the first satellite in orbit: Sputnik.

On October 4, 1957, the Soviet Union launched its famous orbiter, beating the United States into space and shocking the world with its technological prowess. For 23 days, Sputnik circled the globe emitting a radio signal, a constant beep that could be heard by anyone tuned to its frequency.

A few days after Sputnik's launch, physicists William Guier and George Weiffenbach at the Johns Hopkins Applied Physics Laboratory set up a receiver to listen for its signal. Because of the Doppler effect, the tone they heard varied slightly in frequency as the satellite passed overhead. The men realized they could use this shift to calculate Sputnik's position relative to their receiver and soon were able to accurately predict Sputnik's path.

A few months later, Guier and Weiffenbach were called before their boss, Frank McClure, who asked them to reverse the Sputnik tracking problem: use a known orbit and the same Doppler shift to calculate the position of a ground receiver. McClure envisioned a handful of satellites that continually broadcast their coordinates to submarines at sea, whose onboard computers would determine the vessels' locations every few hours, whenever a satellite passed above. After Guier and Weiffenbach confirmed that the idea was feasible, McClure and another Hopkins scientist, Richard Kershner, outlined what would be known as the Transit Navigational System.

In 1960, the first Transit satellite entered orbit. In 1967, the government opened the system to civilian ships and surveyors, who soon constituted the majority of users. The Transit program was gradually replaced by GPS -- a different tracking method that employs many more satellites and does not require any waiting time to pinpoint position. The Transit system was retired in 1996, after more than 30 years of successful service.