How Does GPS Work?
GPS receivers calculate position by measuring the time signals take to arrive from multiple satellites. Each satellite broadcasts its position and the exact time; your device measures how long the signal took to arrive, calculating distance. With signals from at least 4 satellites, the receiver triangulates your 3D position within a few meters.
Key Takeaways
- The GPS system has 31 satellites orbiting about 12,500 miles up, arranged so at least 4 are visible from anywhere on Earth at any time.
- A-GPS (Assisted GPS) in phones uses cell towers to speed up initial location fix.
- GPS doesn't work well indoors or in dense urban areas where signals are blocked.
Explanation
The GPS system has 31 satellites orbiting about 12,500 miles up, arranged so at least 4 are visible from anywhere on Earth at any time. Each satellite contains precise atomic clocks and continuously broadcasts its position and exact time. Radio signals travel at light speed (similar to how WiFi transmits data), so the receiver can calculate distance based on signal delay.
Here's the math: if a signal took 0.067 seconds to arrive, and radio waves travel at 186,000 miles/second, the satellite is about 12,500 miles away. But knowing distance from one satellite only tells you you're somewhere on a sphere around it. Two satellites narrow it to a circle; three give two possible points; four (or more) pinpoint your exact 3D location.
Accuracy depends on several factors: number of satellites visible (more is better), atmospheric conditions (ionosphere can slow signals), signal reflections off buildings (multipath error), and satellite geometry (spread-out satellites give better accuracy than clustered ones). Consumer GPS is typically accurate to 10-15 feet; high-end systems achieve centimeter precision.
GPS satellites orbit at approximately 12,550 miles altitude, completing two full orbits every 24 hours (roughly 8,700 mph). Each satellite carries multiple atomic clocks accurate to within 1 nanosecond per day. This extreme precision is essential because a clock error of just 1 microsecond translates to a position error of about 1,000 feet. The clocks must also be corrected for Einstein's general relativity - time runs 38 microseconds per day faster in orbit than on Earth's surface due to weaker gravity. These satellites were placed in orbit by rockets and must maintain precise positions.
Modern smartphones combine GPS with GLONASS (Russia's 24-satellite system), Galileo (Europe's 30-satellite system), and BeiDou (China's 35-satellite system), giving access to over 100 navigation satellites simultaneously. This multi-constellation approach reduces the time to get an initial fix from 30-45 seconds to under 5 seconds and improves accuracy in challenging environments like urban canyons. Dual-frequency GPS receivers, now common in flagship phones, further improve accuracy to 1-3 feet by compensating for ionospheric delays.
Things to Know
- A-GPS (Assisted GPS) in phones uses cell towers to speed up initial location fix.
- GPS doesn't work well indoors or in dense urban areas where signals are blocked.
- GLONASS (Russian) and Galileo (European) are alternative satellite systems; most phones use multiple systems.
- Your phone doesn't send signals to satellites—GPS is receive-only, so it doesn't reveal your location. Using public WiFi safely is a separate privacy concern.