Low-Frequency Gravitational Wave Searches Using Spacecraft Doppler Tracking.
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, whereΔνis the Doppler shift andν0is the radio link carrier frequency. A gravitational wave having strain amplitudehincident on the earth-spacecraft system causes perturbations of order h in the time series ofΔν/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.
Jet Propulsion Laboratory, California Institute of Technology, Mail Stop 238-725, 4800 Oak Grove Dr., Pasadena, CA 91109-8001 USA.
J W
JW
Armstrong
Cassini; Doppler stability; Doppler tracking; frequency stability; gravitational wave detectors; radio