# That 10km/day error predicted if GPS satellite clocks not corrected for relativity

Some authorities have stated publicly and without explanation that if the theories of Special and General Relativity were not taken into account in the design of the GPS (by building the satellite clocks to run 38us/day slower than GPS time before launch aka 'the factory offset), the position indicated by an earthbound GPS user device would drift by about 11km/day. I've considered this for various GPS models but can only predict much smaller effects. That multiplying the 38us/day uncorrected difference from GPS time by the speed of light yields 11.6km/day, does not for me seem to relate to GPS receiver function. I'd be very glad for any pointers.

• I was desperately trying to work out how to do LaTeX when I found this en.wikipedia.org/wiki/… , it goes through the calculations in reasonable detail. This also gives a value of around 38 us. Whether converting this time into path length gives a reasonable estimate of the error I don't know. It seems like it might be a estimate of sorts although the exact error would depend on the distance of the unit from each of the satellites I suppose. I hope this is of some help. Commented Dec 4, 2011 at 19:46

The satellites' clocks are corrected for GR and SR but this is (mostly) irrelevant for how the GPS system works. Your receiver is comparing the time difference between the time sent by a number of different satellites.

If this time is in 'earth' seconds or 1 part in $$10^{10}$$ speeded up 'space seconds' is to first order irrelevant - so long as all the satellites experience the same effect. So the choice is to broadcast at $$10.23$$ MHz and let the signal be a slightly different frequency when it reaches the ground, or adjust the frequency to $$10.22999999543$$ MHz onboard so it's $$10.23$$ MHz on the ground. [As described in the GPS spec itself.][1]

I think this is where the urban legend of the 'USAF didn't believe in relativity and weren't going to correct the clocks' comes from.

Of course although your position relative to the satellites is unaffected by the time dilation - the satellites' own knowledge of time and so its position in its orbit would accumulate an error. To allow you to find your absolute position the satellite also broadcasts its own orbit data and the time, allowing your receiver to calculate the satellites position in space.

The satellites are in orbit at about $$20,000$$ km altitude, $$26500$$ km from the centre of the Earth so have an orbit of $$165,000$$ km which they cover every $$12$$ hours. An error of $$38.6$$ μs/day in a path of $$333,000$$ km/day still gives a position error (of the satellite) of only a fraction of a meter - although this accumulates with time.

This could be corrected by giving the satellites an adjusted figure for their orbital speed or by updating their empheris as they pass over the ground station.

[1]: Section $$3.3.1.1$$ Frequency Plan in the GPS spec at https://www.gps.gov/technical/icwg/IS-GPS-200D.pdf

• Re To allow you to find your absolute position the satelite also broadcasts its own position in orbit -- This is incorrect. Each GPS satellite broadcasts its ephemeris, which is updated every four hours or so. The ephemeris allows one to accurately compute the satellite position as a function of time. Commented Apr 29, 2018 at 2:27
• @DavidHammen: the satellite also broadcasts a set of data which allows you to determine its own position in orbit - better ? Commented Apr 29, 2018 at 3:11
• I'm a little confused by this answer - does relativity matter or not, besides the blueshifting of the frequency? First you say that the correction is irrelevant, but then you say that there would be an error if the clocks weren't corrected. Commented May 15, 2018 at 16:17
• The relativity effect is real, it is much smaller than the niave 10km/day. It could be corrected in other ways Commented May 15, 2018 at 16:40

If you look at the wikipedia page about GPS and relativistic corrections, they make it clear that this 10km/day drift applies to the 'pseudoranges' - the initial distance calculated between the receiver and each satellite. This error would cancel out in solving the triangulation problem to obtain the receiver position, since it is a an equal error in all the satellite clocks.

Here is my guess as to why they chose to correct this effect: the individual clocks all have some drift as well, and are periodically synced to a master timebase on earth. If they were allowed to drift so drastically from the master, it would be necessary to adjust every clock simultaneously, or navigation would be completely out of whack. Somewhat simpler to just adjust individual units as their drift becomes noticeable.

Found the answer after drawing a blank with several experts. Two US professors of high GPS pedigree, independently explained that the '10km/day' claim presupposes that between 1 and 3 of the satellites used for a 4 satellite fix do not incorporate the 38us/day clock rate ('factory') offset. They also remarked that the GPS is often used as a time source where observed time shifts are clearly important.

I and others have been vexed by several scientific authorities publicly repeating the 10km/day position error claim without any mention of that presupposition. The question is resolved but the presupposition seems strange because relativity shifts all the observed satellite clock rates approximately equally. That presupposition seems only to allow GPS position finding to be shown to be about as susceptible to transmitter clock differences as radio-location systems such as Loran, where relativity is not a consideration.

Sincere thanks to those who replied to my question.

• > They also remarked that the GPS is often used as a time source where observed time shifts are clearly important. Definitely. I'm wondering, though, how GPS time signals achieve an accuracy of <= 40ns relative to UTC if (approximate!) relativistic effects are already about 38µs.
– balu
Commented Jun 18, 2020 at 14:09