Low-Frequency Gravitational Wave Searches
Using Spacecraft Doppler Tracking

J. W. Armstrong 
Jet Propulsion Laboratory
California Institute of Technology
Mail Stop 238-725, 4800 Oak Grove Dr.
Pasadena, CA 91109-8001, U.S.A.

This article has been revised on 15 Jan 2008 and 11 Jan 2016 (see changes in detail).


This paper discusses spacecraft Doppler tracking, the current-generation detector technology used in the low-frequency (∼millihertz) gravitational wave band. In the Doppler method the earth and a distant spacecraft act as free test masses with a ground-based precision Doppler tracking system continuously monitoring the earth-spacecraft relative dimensionless velocity 2Δvāˆ•c = Δ νāˆ•ν0, where Δν is the Doppler shift and ν0 is the radio link carrier frequency. A gravitational wave having strain amplitude h incident on the earth-spacecraft system causes perturbations of order h in the time series of Δ νāˆ•ν0. Unlike other detectors, the ∼ 1 – 10 AU earth-spacecraft separation makes the detector large compared with millihertz-band gravitational wavelengths, and thus times-of-flight of signals and radio waves through the apparatus are important. A burst signal, for example, is time-resolved into a characteristic signature: three discrete events in the Doppler time series. I discuss here the principles of operation of this detector (emphasizing transfer functions of gravitational wave signals and the principal noises to the Doppler time series), some data analysis techniques, experiments to date, and illustrations of sensitivity and current detector performance. I conclude with a discussion of how gravitational wave sensitivity can be improved in the low-frequency band.

Keywords: Cassini, Doppler stability, Doppler tracking, frequency stability, gravitational wave detectors, radio

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