Joel,
Leigh is correct GPS units have corrections for both Special and
General Relativistic effects. You have very accurate clocks not only moving
around the earth, but they are also in a gravitional field. The Sagnac
effect is the name of the source of the error. There was a nice general
interest article in Mercury (May, 1996, page 23). Mercury is a publication
of the Pacific Astronomical Society.
In the article mentions that the satellites have a synthesizer that
generates a correction. In the first one they had turned the correction off
for 20 days and the clock ran 442.5 parts in ten to the twelfth faster than
clocks on the ground. GR predicted 446.5 parts!
If you are at all interest in GPS, the Mercury article is a good
general discussion of the topic.
Gary
Gary Karshner
St. Mary's University
San Antonio, Texas
KARSHNER@STMARYTX.EDU