How GPS Works: Satellites, Signals & Triangulation
Your position, from 20,000 km above
Your position, from 20,000 km above
Each satellite tells you its distance but not direction. That distance defines a circle of possible positions. Click to place your phone, and watch three satellite circles narrow down the location to a single point.
Click anywhere to place your position. Satellites measure their distance to you.
31 satellites orbit Earth at 20,200 km altitude. They are arranged so at least 4 are visible from any point on Earth at any time. Watch them orbit.
GPS started as a military project. Now it is embedded in nearly every device and industry.
Car GPS, walking directions, airplane routing. 4 billion GPS devices are in use worldwide. The original military-only system was opened to civilians in 1983.
Cell towers, financial markets, and power grids use GPS for nanosecond-accurate time synchronization. GPS is as much a timing system as a location system.
Geologists track tectonic plate movement (cm/year). Wildlife biologists track animal migration. Farmers use GPS-guided tractors for precision planting.
GPS satellites must account for Einstein's relativity. Their atomic clocks tick 38 microseconds faster per day due to lower gravity (general relativity) and time dilation from orbital speed (special relativity). Without this correction, GPS would drift 10 km per day.
You've learned how satellites orbiting 20,000 km above Earth pin down your exact location using nothing but precise timing. Three circles of distance, overlapping at one point. Simple geometry, extraordinary precision.
Each of 31 GPS satellites broadcasts its position and the exact time from its atomic clock. Your phone receives these signals.
Light travels at 300,000 km/s. If a signal takes 0.067 seconds to arrive, the satellite is 20,000 km away. Distance = speed * time.
One satellite gives a sphere of possible positions. Two satellites narrow it to a circle. Three satellites give a single point. Four corrects for clock error.
GPS atomic clocks are accurate to 1 nanosecond. A 1-nanosecond error = 30 cm position error. Civilian GPS is accurate to about 3 meters.
Put your new knowledge into practice!