Spacecraft Doppler tracking as a narrow-band detector of gravitational radiation

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Abstract
We discuss a filtering technique for reducing the frequency fluctuations due to the troposphere, ionosphere, and mechanical vibrations of the ground antenna in spacecraft Doppler tracking searches for gravitational radiation. This method takes advantage of the sinusoidal behavior of the transfer function to the Doppler observable of these noise sources, which displays sharp nulls at selected Fourier components. The non-zero gravitational wave signal remaining at these frequencies makes this Doppler tracking technique the equivalent of a series of narrow-band detectors of gravitational radiation distributed across the low-frequency band. Estimates for the sensitivities achievable with the future Cassini Doppler tracking experiments are presented in the context of gravitational wave bursts, monochromatic signals, and a stochastic background of gravitational radiation.