The system by the numbers
31working satellites, about
12,550miles up
2trips around Earth each day
4satellites needed for a fix
The system is run by the U.S. Space Force and is free for anyone in the world to use.
How your phone finds itself
- Each satellite broadcasts the timeFrom an onboard atomic clock
- Your receiver notes when each signal arrivesThe delay tells how far away the satellite is
- Three distances pin your positionLike circles that cross at one point
- A fourth fixes your own clockYour phone's clock isn't accurate enough alone
Einstein keeps GPS honest
Two effects on the clocks
- Fast-moving clocks run slow (special relativity)
- Clocks in weaker gravity run fast (general relativity)
- Together, satellite clocks gain about 38 microseconds a day
Why it matters
- Light travels about 7 miles in 38 microseconds
- Uncorrected, positions would drift about 6 miles a day
- Engineers set the satellite clocks slow before launch
Remember
Every time you use a map app, you're using relativity.
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Answer key
- How many satellites does a receiver need for a full position fix?4Three give position; the fourth corrects the receiver's own clock.
- About how high do GPS satellites orbit?12,550 milesThey circle about 12,550 miles up, twice a day.
- What does each satellite broadcast?A precise time from an atomic clockYour receiver turns the signal delay into distance.
- Without relativity corrections, GPS would drift about:6 miles a daySatellite clocks gain about 38 microseconds a day, enough to drift miles.
- Who runs the GPS system?The U.S. Space ForceIt's operated by the U.S. Space Force and free for everyone.