# Is GPS time measuring the proper time on the mean sea level or the GPS station itself?

LeapSecond.com states:

Global Positioning System time is the atomic time scale implemented by the atomic clocks in the GPS ground control stations and the GPS satellites themselves.

Does GPS time measure the proper time on the mean sea level (rotating geoid) or the proper time of the GPS station itself?

In other words, is GPS time exactly 19 SI seconds behind TAI time, where an SI second is defined as the proper time on the rotating geoid equal to the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom at rest and at a temperature of 0 K?

Put another way, is GPS time and TAI time simultaneous?

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## 1 Answer

GPS time is locked to TAI time, with a constant difference of 19 seconds as you say. In fact GPS is used as part of the TAI process. See for example http://tycho.usno.navy.mil/ptti/1993/Vol%2025_13.pdf and http://tycho.usno.navy.mil/ptti/ptti2000/paper12.pdf for details.

If you're interested in this area there's a lot of useful stuff on the US Navy web site. Have a Google for something like "gps time site:navy.mil" for lots of useful articles.

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Is the 19 seconds merely an estimate to "link" TAI time and GPS time or is the difference exactly 19 seconds? Also, do you mean that GPS time is based on the proper time of the atomic clocks at sea level or the atomic clocks at the GPS station? – Pacerier Jul 8 '12 at 12:34
GPS time is the time at the GPS control stations i.e. at geoid level. GPS time was fixed to be the same as UTC time at 1980, and this also fixes it relative to TAI time. The difference between TAI time and GPS time is exactly 19 seconds and this difference will remain constant in the future. – John Rennie Jul 8 '12 at 13:13
But the GPS control stations are either at the mountain tops or in the space right? In other words, they aren't at the mean sea level and thus aren't simultaneous with TAI time? en.wikipedia.org/wiki/Relativity_of_simultaneity – Pacerier Jul 9 '12 at 9:02
The control stations are at or around sea level. See gps.gov/systems/gps/control. – John Rennie Jul 9 '12 at 9:27
Hmm then perhaps off topic, but are you aware why are there both GPS time and TAI time if they are in fact the same thing? – Pacerier Jul 9 '12 at 11:58