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 $2 \Delta v/c = \Delta \nu/\nu_0$, where $\Delta \nu$ is the Doppler shift and $\nu_0$ 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 $\Delta \nu/\nu_0$. 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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Since a Living Reviews in Relativity article may evolve over time, please cite the access <date>, which uniquely identifies the version of the article you are referring to:
John W. Armstrong,
"Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking",
Living Rev. Relativity 9, (2006), 1. URL (cited on <date>):
|Title||Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking|
|Author||John W. Armstrong|
|Date||accepted 2 November 2005, published 24 January 2006|
|Date||accepted 10 January 2008, published 15 January 2008|
|Changes||Included three way in Table 1; modified Figure 9 (parallel structure with Figure 12) and corrected typographical error regarding correct factor to isolate dispersive plasma in Section 4.2; updated the propagation noise Figure 10; corrected entry in Table 2 for stochastic spacecraft motion; added short discussion of flyable clocks and discussion of E(t), two Figures 24 and 25 about the antenna mechanical test, and five references; changed title of Section 8 to include “LISA”, updated throughout estimate of launch date.
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