Huygens 4th Probe Checkout
One of the planned activities of the 4th checkout was a critical one for the Aerosol Collector and Pyrolyser (ACP) instrument, which involved opening the exhaust valve. For background information about the ACP instrument and detailed technical information relevant to this test, see: http://www.aero.jussieu.fr/experience/ACP/ . The valve in question is 'P2' on Figure 1.
The ACP inner chamber was pressurised with dry nitrogen a few months before launch prior to integration in the Huygens Probe. This made it possible to maintain a chemically clean environment inside the instrument during the launch campaign and during the early cruise phase. The ACP team had been monitoring the evolution of the gas pressure during the first three in-flight checkouts as it slowly decreased, as expected, due to the low leak rate of the sealed chamber. The ACP team had planned to open the valve two years after launch as it was expected that there would still be sufficient gas left in the chamber to see the pressure change following the opening of the valve. Everything went just as predicted.
The 4th checkout was the last to be conducted with the High Gain Antenna (HGA) pointed towards the Sun. In fact, at the time of this checkout in mid-September 1999, the Earth was in opposition, so during that period, since the Sun and the Earth were aligned, the HGA was also pointed to Earth. During the checkout itself, the Sun was about 2° away from the HGA boresight; but some radio noise from the Sun was picked up, and it showed up in the receiver AGC data as in the previous checkouts. The variation of the AGC reflected the attitude variation of the spacecraft during the checkout. This is illustrated in Figure 2.
In summary the 4th checkout went very well. All instruments performed nominally. The variation of the DWE Doppler signal was again observed as in previous checkouts when DWE was activated. The behaviour has essentially not changed from one checkout to the next. We will continue monitoring the DWE behaviour during future checkouts. It will be interesting to study the behaviour of DWE during the next checkout which will be conducted in a noise-free radio environment, as the HGA is turned away from the Sun.