Satellite Navigation (GPS)
The Global Positioning System (GPS) began as a military project but now guides everything from smartphones to aircraft. 31 satellites in medium Earth orbit provide precise location and time to billions of users worldwide.
How GPS Works
GPS satellites orbit at 20,200 km, each continuously broadcasting its precise location and a time signal from atomic clocks. A GPS receiver triangulates its position by measuring the time delay (and thus distance) to at least 4 satellites. With 4 measurements, the receiver can determine latitude, longitude, altitude, and time.
GPS Signal Structure
- L1 (1575.42 MHz): C/A code (civilian) + P(Y) code (military)
- L2 (1227.60 MHz): P(Y) code + L2C (modernized civilian)
- L5 (1176.45 MHz): Safety-of-life signal, aviation
- C/A Code: 1.023 MHz chip rate, 1 ms code period
- P Code: 10.23 MHz, encrypted as Y code for military
GPS Constellation
The nominal constellation is 24 satellites in 6 orbital planes (4 per plane), inclined 55 degrees. This ensures at least 4 satellites are visible from any point on Earth at any time. The actual operational constellation typically has 31+ satellites for redundancy.
Positioning Accuracy
- Standard GPS: 3-5 meters with C/A code
- Differential GPS (DGPS): 1-3 meters
- RTK (Real-Time Kinematic): 1-2 cm with base station
- Military P(Y) code: ~1 meter accuracy
Global Navigation Satellite Systems (GNSS)
GPS is not alone—multiple global constellations now operate:
- GLONASS (Russia): 24 satellites, 64.3° inclination
- Galileo (EU): 30 satellites, 56° inclination
- BeiDou (China): 45 satellites (global as of 2020)
- NavIC (India): 8 satellites, regional coverage