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On 2011-04-23 the National Geographic aired a TV special, in part about GPS. They said a number of things that struck me as highly unlikely. I wonder if you could confirm:

1. they said that the GPS hand held unit sends its time to 3 satellites. I would have imagined it would be reverse.

2. They said the satellites do the calculation then send the position back to to hand held unit. That made no sense. That would be a huge computational burden on the satellites which are strapped for power and weight.

3. They said the satellites are adjusted for relativistic effects prior to launch and suggested there is no way of adjusting their clocks ever after. That struck me as absurd. Obviously you need ways to control the satellite in may ways from ground.

4. They said the hand held GPS unit has an extremely accurate clock. This seems plausible, but I would have imagined there would be some mechanism to keep it accurate so that that clock itself would not need atomic accuracy.

5. They said if they did not adjust for relativistic effects, GPS would be out by 7 miles each day. That sounds cool, but higher than I would have expected.

6. They said something about how accurate it was. I don't remember the number, but it struck me as too high. It obviously must be accurate enough to nail down which street you are on.

I don't own a GPS unit myself, though my brother, and my sister each let me play with theirs for a few hours. A satellite company uses my BigDate software for helping keep all the satellites in the world co-ordinated. They never told me the detail of just how they use it.

You are correct in that it is the satellites that transmit the signals. The hand held is a receiver only and must be receiving data from a minimum of three satellites to calculate a position.

You are correct again.

Each satellite has very accurate atomic clocks but I'm not sure how much adjustment can be done from the ground or would ever be required

The handhelds do not have atomic clocks but their clocks are updated from the satellite signals and so are very accurate.

I'm not sure of the exact numbers but there are several sources of error of which clock errors are one.

The best accuracy you can expect from recreational grade GPS receivers is +/- 3 metres (10 ft) under ideal conditions. Under tree cover or in difficult topography accuracy will be much worse.


GPS signals are used for timing in a number of applications including cell phone networks.

Affect of relativity requires GPS satellite clocks to tick faster than if on the ground hence they are set to tick slower before launch so as to tick correct once in orbit. Each satellite has multiple clocks as they do not have an infinitive life.



No handhelds don't have atomic clocks and neither do they require very accurate time, Time and Timing are two very distinct clock requirements. Time isn't that important but Timing or time difference is and this time difference is something that is a computed value as part of each and every position solution.



They might be getting confused with 7 microseconds per day if relativity was ignored?

All in all National Geographic don;t pass on this one, give them a fail.

From http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html

"The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds. If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time. This kind of accumulated error is akin to measuring my location while standing on my front porch in Columbus, Ohio one day, and then making the same measurement a week later and having my GPS receiver tell me that my porch and I are currently about 5000 meters in the air somewhere over Detroit."