Narrow-band Searches for Gravitational Radiation with Spacecraft Doppler Tracking

We discuss a filtering technique for Teducing the two-way Doppler frequency jluctuations of noise sources localized in space (like the troposphere, or the master clock) that aflect the sensitivity of spacecraft Doppler tmcking searches for gravitational mdiation. This method takes advantage of the sinusoidal behavior of the tmnsfer function to the Doppler observable of these noise sources, which displays s h a v nulls at selected Fourier components. The non-zero gravitational wave signal remaining a t these frequencies makes this Doppler tmcking technique the equivalent of a series of narrow-band detectors of gmvitational radiation[l], distributed across the Iow-frequency band. Estimates for the sensitivities achievable with the future Cassini Doppler tracking experiments a% presented in the context of broad-band gravitational wave bursts, monochromatic signals, and a stochastic background of gravitational radiatiod2I.