HASI engineering operations at Titan

On the 14th of January 2005 the Huygens probe entered the mysterious previously quite unknown atmosphere of Titan. One of the six instruments on board the Huygens probe was the Huygens Atmospheric Structure Instrument (HASI) devoted to the analysis of the atmosphere of Titan and to the study of its surface. A big effort was put to implement the automatic sequence to avoid any loss of scientific data. The HASI instruments, within the Huygens probe, was powered-ON before all the other instruments and started its measurements at an altitude of around 2800km, when the telemetry link with the Cassini probe was not yet set or established. In this phase of the mission 44kbts of housekeeping and acceleration data had to be stored inside the experiment memory and a specific saving sequence was implemented with several copies of each packet of data. In the successive descent phase, triggered by the 10m/s^2 deceleration threshold, temperature and pressure measurements sampling sequence was added to the accelerometer sequence; the starting of the sampling was anticipated as much as possible compatibly with the storage capability of the system and before the front shield separation (which occurred 32s after the threshold passing). The critical sequence of the deployable booms with the electric package instrument was implemented in the software in order to have execution in a 40 seconds window and a backup deployment sequence 80 seconds later. Due to the distance of the probe from the Earth and the configuration of the mission no real-time sequence operation could be executed during the 2.5 hours of the mission. Furthermore the link with the Cassini spacecraft did not allow a downlink to the probe during the mission phase, for this reasons all the operations have been programmed before the probe release which was performed the 25th of December 2004. The flexibility of the onboard software allowed the instrument to perform exactly as planned. The implementation of the Preheating phase on board the Huygens probe required a significant upload in the onboard software of the probe itself and on the software of most of the instruments. The HASI on board software had to be reprogrammed to run within this new scenario. Tests where conducted during the last part © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.