Here is the straight dope on gps clocks and how relativity effects them...
I'm posting the introduction, for those who don't want to download a 4+ MB pdf file:
quote:
A GPS receiver must make two corrections that are related to relativity in order to
provide time or position to a user. We discuss these corrections and focus mostly on
estimating the geometric range delay t D , the time for GPS signals to propagate from the
transmitter to the receiver in vacuum. Proper estimation of t D is essential for solving for
position or time. This is an application of the relativistic principle of the constancy of the
speed of light, which states that electromagnetic signals travel in Euclidean straight lines
with speed c relative to an inertial reference frame. We present a few cases which apply
to many common uses of GPS and illustrate them numerically.
We present the theory behind corrections with references given for any derivations not
done here. Through the derivations, we show that the Interface Control Document (ICDGPS-
200) specifications, as issued by the Joint Program Office of the Global Positioning
System [l], consistently cover the requirements of relativity down to within 1 ns or less.
We show that the ICD specifications include relativity corrections with enough accuracy
for most applications [2,3]. In particular, we discuss the relativistic Doppler effect, the
formula for its instantaneous magnitude, and its relationship with typical GPS receiver
operation. We also address the use of carrier-phase measurements, which is not discussed
in the ICD.
Relativistic effects which are not usually modelled in GPS are: (a) a small effect on
satellite clock rates due to the earth’s oblateness and (b) a small time delay due to the
slowing of electromagnetic signal propagation in the earth’s gravitational field. The earth’s
oblateness contributes a very small constant rate correction for satellite clocks, which is so
small (less than 50 ps per day) that it can be neglected. It also causes a periodic variation in
the GPS satellite clock time having twice the orbital period and a peak-to-peak amplitude
less than 200 ps. For a GPS satellite-to-user link the gravitational time delay is less than
200 ps [2]
Our goal is to give simple recipes, so that GPS users may understand how to implement
receiver designs consistently with the requirements of relativity. We then back
up these prescriptions with full explanations and derivations, so that those interested can
understand where the prescriptions come from and what approximations are involved. The
result is that this document provides a brief tutorial on how relativity is applied for users
of the GPS broadcast ephemerides.
Hopefully that came out correct - The copy/paste from the pdf was a bit flaky.
Moose
™ Version 4.2
