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E. Marouf (San Jose State University), R. French (Wellesley College), N. Rappaport (Jet Propulsion Laboratory), F. Thomson (Stanford University), C. McGhee (Wellesley College), S. Asmar, D. Johnston (Jet Propulsion Laboratory)
On July 1, 2005 at 01:12 SCET-UTC, Cassini started the engine burn required to insert the spacecraft into orbit around Saturn (SOI). Almost 30 minutes later, Cassini was occulted by Saturn's rings as seen from the Earth. The geometric ring occultation covered all main ring features, starting at the outer edge of Ring A at 01:42 and ending at the inner edge of Ring C at 02:40. From 01:12 to 03:07, Cassini X-band radio signal (3.6 cm-wavelength) was turned on, primarily to monitor the burn. The sinusoidal transmitted signal was referenced to the on board ultrastable oscillator, allowing measurement of the signal amplitude and phase at the 70-m ground receiving station of the Deep Space Network at Canberra, Australia. As a useful by-product, a complete ring occultation observation, including free-space baseline, was achieved. Because of the special orientation of the spacecraft during the burn, the Cassini low-gain antenna was used to transmit the signal. Nominal radio occultations are conducted using the high-gain antenna, hence have intrinsic free-space signal-to-noise ratio (SNR) higher by a factor of ~10,000 than the SOI occultation. Nonetheless, clearly detectable signal was observed during occultation by features in Rings A, Cassini Division, and Ring C, but not Ring B. The measurements, after reconstruction to remove diffraction effects, may be used to obtain an optical-depth and phase-shift profiles of resolved ring features. Achievable radial resolution primarily depends on the ring-opening-angle B, available free-space SNR, and occultation geometry. We compare radial resolution achievable for the Cassini SOI occultation (B = 24.7 deg, SNR = 10 dB-Hz) with those of the Voyager ring occultation (B = 5.9 deg, SNR = 50 dB-Hz), and contrast the results with those expected from nominal radio occultations during the Cassini tour. Example optical depth profiles from the Cassini SOI occultation are presented.
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Bulletin of the American Astronomical Society, 36 #4
© 2004. The American Astronomical Soceity.