  Gravitational waves imprint apparent Doppler shifts on the frequency of
photons propagating between an emitter and detector of light. This forms the
basis of a method to detect gravitational waves using Doppler velocimetry
between pairs of satellites. Such detectors, operating in the milli-hertz
gravitational frequency band, could lead to the direct detection of
gravitational waves. The crucial component in such a detector is the frequency
standard on board the emitting and receiving satellites. We point out that
recent developments in atomic frequency standards have led to devices that are
approaching the sensitivity required to detect gravitational waves from
astrophysically interesting sources. The sensitivity of satellites equipped
with optical frequency standards for Doppler velocimetry is examined, and a
design for a robust, space-capable optical frequency standard is presented.
