Microwave Band Doppler Tracking of Spacecraft

Observations in the low-frequency (LF, ~0.0001-0.1 Hz) spectral band require space-based detectors. Laser interferometers in space, such as LISA, will fly by 2010 and promise very high sensitivity. However, the only current broadband technique in the LF band uses the Earth and a distant spacecraft as free test masses in a "one-armed" interferometer, coherence being maintained through a high-precision frequency standard on the ground. In this technique, the Doppler tracking system of the NASA/JPL Deep Space Network (DSN), referenced to an ultra-high-quality frequency standard, transmits a microwave tone to a distant spacecraft. The signal is received at the spacecraft and phase-coherently retransmitted back to the earth. The DSN antenna receives this transponded signal and compares its frequency with that of the transmitted signal. The tracking system thus continuously measures the relative dimensionless velocity (2 Dv/c ~ Df/f_o) between the Earth and spacecraft. Unlike other GW detectors, the ~1-10 AU Earth-spacecraft separation makes the Doppler detector large compared with the wavelength for most candidate signals. In this regime, a GW incident on the Earth-spacecraft system is resolved into three events in the Doppler time series: buffeting of the Earth, buffeting of the spacecraft, and the initial buffeting of the Earth transponded back to the tracking station. These perturbations are of order h, the GW strain amplitude, in Df/f_o (see line drawing below).

The sum of the Doppler perturbations of the three pulses is zero. Pulses with duration longer than about the one-way light time produce overlapping responses in the tracking record and the net GW response begins to cancel. The tracking system has a passband to gravitational excitation: the low-frequency band edge is set by pulse cancellation to be roughly (1/one-way light time), while thermal (white phase noise in the receiver system) and other noises in the radio system limit the high frequency band edge to about (1/10 seconds). For a more detailed discussion of the Doppler tracking technique, including principal noise sources, their transfer functions, and the expected sensitivity of the Cassini Ka-band experiments in 2001-2004 click here.