  Recent advances in space-qualified atomic clocks (low-mass, low
power-consumption, frequency stability comparable to that of ground-based
clocks) can enable interplanetary spacecraft radio science experiments at
unprecedented Doppler sensitivities. The addition of an on-board digital
receiver would allow the up- and down-link Doppler frequencies to be measured
separately. Such separate, high-quality measurements allow optimal data
combinations that suppress the currently-leading noise sources: phase
scintillation noise from the Earth's atmosphere and Doppler noise caused by
mechanical vibrations of the ground antenna. Here we provide a general
expression for the optimal combination of ground and on-board Doppler data and
compute the sensitivity such a system would have to low-frequency gravitational
waves (GWs). Assuming a plasma scintillation noise calibration comparable to
that already demonstrated with the multi-link CASSINI radio system, the
space-clock/digital-receiver instrumentation enhancements would give GW strain
sensitivity of $2.0 \times 10^{-17}$ for randomly polarized, monochromatic GW
signals over a two-decade ($\sim0.0001-0.01$ Hz) region of the low-frequency
band. This is about an order of magnitude better than currently achieved with
traditional two-way coherent Doppler experiments. The utility of optimally
combining simultaneous up- and down-link observations is not limited to GW
searches. The Doppler tracking technique discussed here could be performed at
minimal incremental cost to also improve other radio science experiments (i.e.
tests of relativistic gravity, planetary and satellite gravity field
measurements, atmospheric and ring occultations) on future interplanetary
missions.
