Cassini Bistatic Radar Titan
El pasado sábado 20 de mayo la Estación de Robledo ha recogido los datos de este interesante experimento con sus antenas DSS-63 (70 m) y DSS-55 (34 m).
OVERVIEW:
Second Cassini tour opportunity for Radio Science (RSS) to observe Titan's ionosphere and neutral atmosphere using radio occultation, and Titan's surface using bistatic scattering. The radio occultation is the third ever of Titan, the first being a sole Voyager occultation in 1980. The three Cassini radio signals (Ka/X/S) are planned to probe the ionosphere and atmosphere of Titan on both the ingress and egress sides. A critical spacecraft maneuver is implemented to steer the Cassini high-gain antenna (HGA) boresight so that the radio signals refracted in the atmosphere are still received on the Earth down to Titan's surface. The measurements provide important high-spatial-resolution information about the large-scale structure of the ionosphere and atmosphere of Titan, including the electron number density profile, the temperature-pressure profile, and profiles of any microwave absorbing species. They also provide information about atmospheric small-scale structure such as gravity waves, turbulence, and sharp layers. During approach to Titan, the Cassini HGA boresight will be pointed to illuminate regions of Titan's surface for which mirror-like (quasi-specular) reflections of the incident radio signals can be observed at the the DSN ground receiving stations (bistatic scattering). The strength and polarization properties of the reflected signals, if detectable, provide important information about the dielectric constant of the surface region probed (physical nature) as well as the surface roughness.
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Cassini experiment report. CONGRATULATIONS.
Report written by Essam Marouf and Dick French (Cassini RSS science team).
RSS completed its second Titan radio occultation and bistatic observations early Saturday morning (May 20, 2006). All indications from real-time monitoring of the events are that the experiments executed very well.
The Madrid DSN complex covered both the ingress and egress observations.
The 70-m DSS-63 covered S- and X-bands. The 34-m DSS-55 covered Ka-band and provided backup X-band. RCP and LCP polarizations were recorded at the earth receiving stations.
A "quick-look" analysis of the signal intensity profiles computed from the 1 KHz bandwidth recorded data indicate high quality S-, X-, and Ka-band ionospheric and atmospheric occultation data were acquired on both the ingress and egress sides. Signal frequency data was also collected and appears to be of equally good quality.
As for T12 on March 18, 2006, the S-and and X-band signals were tracked down to Titan's surface. The Ka-band signal was extinguished about 20 seconds before reaching the surface. Remarkable similarity between the
T12 and T14 signal extinction profiles leaves little doubt that true microwave extinction by atmospheric species in Titan's atmosphere is responsible for the earlier extinction of the Ka-band signal.
The similarities between T12 and T14 also indicate little pointing errors, although quantitative assessment is yet to be completed. The limb-track maneuver appears to have executed as planned.
Update of the DSN Ka-band 34-m antenna pointing model the day before the occultation (based on ORT data collected a week earlier) proved critical for the good T14 Ka-band pointing, thanks to the dedicated work of David Rochblatt and his DSN colleagues, particularly Jesus Calvo and Pablo Perez at Madrid. The ORTs completed before the occultation events proved yet one more time how critically needed the ORTs are for timely troubleshooting and addressing potential problems ahead of an actual event.
The RSS Titan occultations are among the most demanding observations of the entire tour, requiring precise delivery of the spacecraft past Titan to sub-km accuracy, as well as rapid steering of the spacecraft while on thrusters to follow the virtual image of the Earth along Titan's limb.
We thank our NAV, AACS, SP, SCO and ULO colleagues in helping to make these challenging observations a great success.
The exceptional support of the Madrid station personnel were critical factors for the success of the T14 RSS observations. We are also very grateful to Jeff Boyer, Chau Buu, Dave Doody and the Cassini ACEs, and the S20 leads for their contribution and great work.
The T12 and T14 occultations are the only two RSS Titan occultations during 2006 (T27 and T31 are to follow in March and May of 2007). The excellent data from T12 and T14 will keep the RSS team busy till T27!