ACES

Schematic of ACES containing PHARAO and Space H-Maser.

An atomic clock in space will allow a new kind of Earth research: relativistic geodesy. A radio signal from the surface of a planet climbs 'uphill' through the gravitational field (out of a gravity well), so when it is received by an observer in a weaker gravitational field its wavelength is
seen to be longer, as predicted by Einstein's theory of relativity. Measuring this change in wavelength can be used to map the gravity of our planet. These measurements require a timekeeper of almost unimaginable accuracy – such as
ACES.