Cassini as a Narrowband Detector of Gravitational Radiation

Abstract

A filtering technique allowing the reduction of two-way Doppler frequency fluctuations from spatially localized noise sources (like the frequency fluctuations introduced by the troposphere, the ionosphere, or the mechanical vibrations of the ground antenna) is presented in the context of the Cassini 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 gravitational wave signal remaining at these frequencies makes this Doppler tracking technique the equivalent of a series of narrowband detectors of gravitational radiation, distributed across the low-frequency band. Estimates for the sensitivities achievable with the future Cassini Doppler tracking experiments are presented for different classes of gravitational wave signals.