Although conceptually important – and used with excellent success as part of the hydrogen-maser-based suborbital
GP-A experiment [126] – one-way Doppler presents a practical problem for precision tracking of deep space
probes: Flight-qualified frequency standards for deep space are currently (2015) substantially less stable than
ground-based standards. The quality of one-way spacecraft Doppler GW measurements is severely limited by
noise in the flight frequency generator. Deep space tracking systems circumvent this by measuring two-way
Doppler. In the two-way mode the ground station transmits a radio signal referenced to a high-quality frequency
standard. The spacecraft receives this signal and phase-coherently retransmits it to the earth. The transponding
process adds noise, but at negligible levels in current observations (see Table 2), and does not require a good
oscillator on the spacecraft. The ground station then measures the two-way Doppler shift by comparing the
frequency of the received signal against the frequency of a local reference derived from the ground frequency
standard.