[GreenKeys] Accurate clocks...

[Craig Sawyers]

> AHH yes now I recall...  Cesium.
> That is the element that is accurate to one second over 400 years.

The constellation of orbital satellites that provide the signals for GPS
navigation use Caesium clocks.  However, the precise frequency has to be
corrected for the effects of both special relativitity (because of relative
motion between the satellites and the receiver) and general relativity
(because the clocks and the receiver are in different locations in the
earth's gravity field).

Although this amounts to only a few tens of microseconds a day, the
registered position of the receiver would drift by around 6 miles a day if
relativistic effects were not corrected for.

Craig