3 THE ORIGINS OF SATELLITE NAVIGATION

All gyroscopes and accelerometers tend to drift over time. Drift was not the most serious issue for designers of guidance systems for ballistic missiles, which accelerate for only a few minutes. Navigators on commercial aircraft could correct for drift by contacting radio beacons as the craft approached land. The Apollo astronauts corrected for drift by taking periodic celestial sightings as they journeyed to the Moon and back. However, submarines faced a different challenge: they operate for long periods underwater and do not wish to broadcast their position. To correct for drift, they had to come close to the surface, get an accurate fix from the stars, and update their onboard inertial systems. The time required to take celestial sightings exposed the sub to an enemy wishing to locate it. The US Navy's response to that problem was to orbit a set of satellites, collectively called Transit, to work with the submarines. Beginning in 1959, Transit satellites performed this service, and the system remained in constant use until it was replaced by GPS in 1996 (see figure 7). Transit's positioning technology had little to do with the way that GPS and other modern satellite positioning systems operate. However, it occupies an important place in history. Its success showed that satellite-based navigation and positioning were practical and effective alternatives to classical techniques. Its widespread use beyond ballistic missile—carrying submarines, for which it was designed, showed that satellite-based systems would find a large civil and military market.