Improved spacecraft radio science using an on-board atomic clock: application to gravitational wave searches

TL;DR

This paper proposes using advanced on-board atomic clocks and digital receivers on spacecraft to enhance Doppler measurements, significantly improving sensitivity to low-frequency gravitational waves and benefiting various radio science experiments.

Contribution

It introduces a new method for combining ground and on-board Doppler data to suppress noise and improve gravitational wave detection sensitivity in interplanetary radio science.

Findings

Achieves GW strain sensitivity of 2.0 x 10^-17 in 0.0001-0.01 Hz band

Enhances Doppler sensitivity by about an order of magnitude over current methods

Offers a versatile technique applicable to multiple radio science experiments

Abstract

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.