Re: Christmas Presents (GPS Receivers)

[Leigh Palmer <palmer@sfu.ca>]

> I assume you meant to say that its applied Special Relativity,
> rather than GR.

Nope. GR is what I said & meant.

The atomic clocks aboard the GPS satellites do not run at the same
speed as the clocks on the Earth below. They must be corrected, just as
were the famous "flying clocks" of the seventies that circumnavigated
the Earth via commercial airliners. In both cases SR and GR corrections
had to be made.

The clocks in the satellites are moving quite fast relative to those on
earth, and therefore they appear to run slow by an amount predicted by
SR time dilation. However the satellite clocks are at a higher
gravitational potential than those on the surface, and by GR that means
they will run fast compared to the surface clocks. The GR effect is
larger than the SR effect when both are taken into account. The moving
clocks run faster, not slower. What is observed very nearly (but not
exactly) matches the prediction.

There is a good article on GR and GPS in a 1995 or 1996 issue of
"Mercury". I'd be more specific, but my collection is a mess just now.
Surely your book "Relativity and Engineering" must also go into this.

Leigh