Sputnik and the Doppler insight (1957)
Within weeks of Sputnik 1's launch, two physicists at Johns Hopkins APL — William Guier and George Weiffenbach — noticed they could track the satellite by listening to its radio beacon and measuring the Doppler shift as it passed overhead. A colleague then asked the inverse question: if you know where the satellite is, could you locate the receiver? Within months they had the math for a global satellite navigation system.
Transit, Timation, and 621B (1959–1973)
The US Navy built Transit (1959) for submarine navigation — five satellites, fix every 90 minutes, accurate to a few hundred metres. The Air Force ran a parallel programme called 621B. The Naval Research Lab built Timation, which carried the first atomic clocks into orbit. In 1973 the Pentagon merged all three into a single programme: NAVSTAR-GPS.
Launch and KAL 007 (1978–1983)
The first GPS satellite, Navstar 1, launched in February 1978. The system was meant to be military only. In September 1983 a Korean Air Lines 747 strayed into Soviet airspace and was shot down by a Su-15. The crew had no way to know they had drifted off course. President Reagan announced two weeks later that GPS would be made free for civilian use once it was operational. That promise turned out to be the foundation of the modern geolocation industry.
Selective Availability and its end (1990–2000)
Until May 2000, the Pentagon deliberately degraded the civilian signal — 'Selective Availability' — so that consumer GPS was accurate only to about 100 metres. President Clinton turned it off at midnight on 1 May 2000. Civilian accuracy improved 10× overnight. Every navigation app, ride-share company and food delivery service that exists today depends on that single decision.
What came next
Russia's GLONASS reached full coverage in 2011, the EU's Galileo in 2016, China's BeiDou in 2020. Modern receivers use all four constellations simultaneously — 100+ satellites visible from anywhere on Earth — and combine them with inertial sensors for sub-second fixes in tunnels and city canyons.