Similarly, an in-flight engineering demonstration of the Probe Relay Sequence, the 2 nd orbiter critical sequence, is planned on 29 th February-4 March 2004. All orbiter functions that are needed to support the Probe Relay Sequence will be exercised. In addition, the JPL and the ESOC ground segments will also be exercised.

Revised Huygens mission implementation: status report
Following the timely delivery of all instrument flight software patches in June, an extensive system validation of the pre-heating configuration was carried out on the Engineering Model at ESOC in July. All instrument teams provided excellent support during the validation campaign that required a large amount of test results to be evaluated. The probe and instrument patched software codes are fully compatible with both the pre-heating and the no pre-heating configuration.

The fifth Huygens ESA/JPL Quarterly Meting took place on 24-26 September. A number of technical issues regarding the Huygens mission were discussed. The most important outcome of the meeting was the decision to baseline the pre-heating option. The purpose of pre-heating is to warm the data system circuitry to further reduce the Doppler effect which has troubled the link performance.

Pre-heating is accomplished by anticipating the Probe power ON by 4 hours prior to entry at Titan. All analyses show that there are sufficient power reserves and that the mission is completely robust to this option. No mission single point failure was introduced by the pre-heating configuration. The Delta Flight Acceptance Review is being planned to start in early December.

The in-flight implementation and validation of the pre-heating software patches will be carried out on 7-17 December. A careful approach is being implemented to allow full verification of the proper loading of the instrument patches before activating the probe payload. For this reason, the test is carried out over several DSN passes with ground interactivity from JPL, ESOC/HPOC and the PI-institutes.

Science results
A test of general relativity was carried out using the Cassini orbiter multi-frequency radio links during the Solar Conjunction Experiment between 6 June and 7 July 2002 by an Italian team (Bertotti et al., Nature 425, 374-376, 25 Sept. 2003). According to general relativity, photons are deflected and phase-delayed by the curvature of space-time by any mass. The bending and delay are proportional to γ+1, where the parameter γ is unity in general gravity but zero in the Newtonian model of gravity. The results obtained by Cassini, which measured the bending and delay of radio photons by the Sun improved the value of � by a factor of 50.

They agree with the prediction of standard general relativity with a sensitivity that approaches the level at which, theoretically, deviations are expected in some cosmological models.

Of great relevance to what Huygens might encounter at landing, recent Earth-based measurements (using upgraded Arecibo and Green-Bank radio telescopes), found echoes consistent with liquid surfaces in 12 out of 16 radar looks.

A highly successful balloon flight was conducted from the ASI balloon base in Trapani, Sicily on 6 June. A full-scale Huygens Probe mock-up was dropped from 33 km altitude and descended under parachute for a safe landing about 40 min later. The mock-up was instrumented with the full HASI instrument and additional sensors including the Huygens radar altimeter and probe system accelerometers.

The data set is being used to characterise the HASI instrument in the Probe aerodynamic environment and prepare for the data analysis of the real mission data set at Titan. It also provided an excellent data set for testing the tools being prepared by several Huygens science teams to reconstruct the Huygens descent trajectory and probe attitude.

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