Introduction
The story of GNSS begins not with GPS, but with a small beeping satellite launched by the Soviet Union in 1957. That satellite, Sputnik, inadvertently revealed the fundamental principle that would become satellite navigation.
The Discovery: The Doppler Effect
When Sputnik launched, scientists at Johns Hopkins University noticed something curious: as the satellite approached, its radio signal frequency increased; as it passed and moved away, the frequency decreased. This was the Doppler effect in action.
More importantly, they realized that by measuring this frequency shift, they could determine exactly where the satellite was in its orbit. Within years, the thinking flipped: if you know the satellite's orbit precisely, you can determine your position on Earth by measuring the same Doppler shift.
Transit: The First System
The US Navy developed Transit (also called NAVSAT) in the 1960s:
- First operational satellite navigation system
- Used by submarines for positioning
- Required 10–15 minutes of tracking for a fix
- Accuracy: ~200 meters
- Operational until 1996
The Birth of GPS
In 1973, the US Department of Defense combined ideas from multiple programs to create the NAVSTAR GPS (Navigation System with Timing and Ranging):
- 1978: First GPS satellite launched
- 1983: After Korean Air Lines Flight 007 was shot down, President Reagan announced GPS would be available for civilian use once complete
- 1989: First Block II satellite launched
- 1993: Initial operational capability declared
- 1995: Fully operational with 24 satellites
Selective Availability: The Intentional Error
For years, civilian GPS had intentional errors added (Selective Availability) to limit accuracy for non-military users:
- Civilian accuracy: ~100 meters
- Military accuracy: ~20 meters
- 2000: President Clinton turned off Selective Availability, instantly improving civilian GPS to ~10 meters
Other Systems Emerge
- GLONASS (Russia): Developed in parallel with GPS, fell into disrepair after USSR collapse, fully restored by 2011
- Galileo (European Union): Civilian-controlled system, first test satellite 2005, fully operational 2019
- BeiDou (China): Regional system by 2012, global by 2020
- Regional systems: QZSS (Japan), NavIC (India)
Modern Era
Today, we have over 130 operational navigation satellites. Modern receivers use all constellations simultaneously, achieving accuracies that would have seemed impossible just decades ago.
Vital Points
- GNSS began with Sputnik and the discovery of the Doppler effect
- Transit was the first operational system
- GPS became fully operational in 1995
- Selective Availability intentionally degraded civilian signals until 2000
- Today, multiple systems provide redundancy and improved accuracy