GPS notes

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This was part of a GPS tracker I built around 1999/2000. It consisted of a Metricom Ricochet radio modem, a Garmin eTrex GPS, and a generic Linux PC. The version I installed in my 1969 Beetle used a full sized PC and inverter instead of a Palm Pilot. This photo was taken with a Polaroid iZone camera. The real picture was a little bigger than my thumb.

GPS Tracker circa 1999


GPS clock precision and accuracy

General Relativity causes satellite clocks to run fast 38 microseconds per day, which corresponds to a drift of 10 km per day.

Due to General Relativity the clocks in a GPS satellites experience a different speed of time than we do on the ground. Two effects come into play. First the gravity of the Earth makes time slow down for us, so the satellites appear to speed up. But to maintain orbit the satellites have to move very fast, so time slows down for them, too, which from our perspective makes them appear to speed up. The gravitational effect is about 6 times bigger than the speed effect. The speed of the satellites makes their clocks appear to slow down 7 microseconds per day. But the distortion of space due to gravity is higher at sea level so our time slows more which makes the satellites' clocks appear to speed up by 45 microseconds per day. So the satellite clocks appear to tick fast by 38 microseconds per day overall. That corresponds to a drift of about 10 km per day