The Cassini gravitational wave experiment

Doppler tracking experiments using the earth and a distant spacecraft as separated test masses have been used for gravitational wave (GW) searches in the low-frequency (similar to0.0001-0.1 Hz) band. The precision microwave tracking link continuously measures the relative dimensionless velocity, Deltav/c, between the earth and spacecraft. A GW incident on the system produces a characteristic signature in the data, different from the signatures of the principal noises. For 40 days centered about its solar opposition in December 2001, the Cassini spacecraft was tracked in a search for low-frequency GWs. Because of instrumentation upgrades on the spacecraft and on the ground, this joint NASA/ASI collaboration is the most sensitive Doppler tracking experiment to date. The improved sensitivity was mainly due to two technical upgrades. First, the use of a higher radio frequency tracking link (Ka-band, approximately 32 GHz) strongly suppressed noise from radiowave phase scintillation due to plasma irregularities along the line of sight. Second, the use of an advanced tropospheric calibration system allowed calibration/removal of most of the tropospheric phase scintillation. The new instrumentation should also allow broadening of the experiment's band to lower Fourier frequencies, where the GW sources radiate more strongly and, may appear with larger SNR. These upgrades support not only the GW experiment, but also a Cassini relativity experiment at solar conjunction and the Cassini radio science experiments at Saturn. Here we describe the GW experiment, including the transfer functions of the signals and noises to the Doppler observable, and present the noise statistics and compare them with the pre-experiment noise budget.