Spacecraft Doppler tracking: Noise budget and accuracy achievable in precision radio science observations

We discuss noise in Doppler tracking of deep space probes and provide a detailed noise model for Doppler radio science experiments. The most sensitive current experiments achieve fractional frequency fluctuation noise of about 3 × 10 −15 at 1000‐s integration time, corresponding to better than 1 micron per second velocity noise. Our noise model focuses primarily on the Fourier range ≈10 −4 –1 Hz, but we briefly discuss noise in lower‐frequency observations. We indicate applications of the noise model to experiment planning, identify phenomena limiting current Doppler sensitivity, and discuss the prospects for significant sensitivity improvements.