Any frequency will do... What is important is to account for the
difference (due to time dilation) of satellite clocks and ground
based clocks, as they must be synchronized in global navigation
satellite systems. General relativity beautifully predicts those
differences and is incorporated in to the GPS Specs. The later URL
below is a graphic showing the sign and degree of time dilation as
a function of altitude.
See: Relativistic Effects on Satellite Clocks
http://relativity.livingreviews.org/open?pubNo=lrr-2003-1&page=node5.html
http://relativity.livingreviews.org/Articles/lrr-2003-1/frctfrq.png
Temptation gets the better of me now:
If we beamed you up, Scotty, to one of the GPS satellites, the
relativistic corrections would be applied to the ground based clocks
from your perspective. But the more traditional perspective is for
the ground (or near ground) down on the planet.
