Abstract
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 2Δv/c = Δν/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 Δν/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.
Highlights
Radio communications systems on deep space probes are used for both command and control of the spacecraft and for returning telemetry to the ground
The ∼ 1 – 10 AU earth-spacecraft separation makes the detector large compared with millihertz-band gravitational wavelengths, and times-of-flight of signals and radio waves through the apparatus are important
Arrival and for a detector large compared with the gravitational waves (GWs) wavelength and derived the spectral distribution of Doppler fluctuations due to an isotropic GW background
Summary
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 25 and 26 about the antenna mechanical test, and five references; changed title of Section 8 to include “LISA”, updated throughout estimate of launch date. Page 17: Corrected typographical error X-(749/3344) Ka1 to X-(880/3344) Ka1. Page 28: Corrected entry for stochastic spacecraft motion from ≃ 3 × 10−16 to ≃ 2 × 10−16. Page 44: Added short discussion of flyable clocks and discussion of E(t) below.
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