Scientific objectives and implementation of the Pressure Profile Instrument (PPI⧹HASI) for the Huygens spacecraft

Abstract

The Huygens entry probe will be deployed into the Titan atmosphere by the Cassini spacecraft. During the 3 h descent the Huygens Atmospheric Structure Instrument (HASI) will observe a comprehensive set of variables and phenomena, encompassing pressure, temperature, density and atmospheric electricity. The Titan atmospheric vertical pressure profile will be recorded by the Pressure Profile Instrument (PPI:HASI) provided by the Finnish Meteorological Institute (FMI). The principal sections of the PPI are:• sensor boom extending out of the Huygens main body, • Kiel probe with pitot tube in the end of the sensor boom,• pressure sensors (Barocap ®) inside the Huygens body, and• pressure hose conveying the pressure signal from the Kiel probe to the pressure sensors.The decision to measure total pressure instead of static pressure and the design of the Kiel probe was based on aerodynamic simulations. Simulations were performed for the airflow around the Huygens probe and in the vicinity of the tip of the sensor boom. During the descent the Huygens probe is constantly changing its attitude. Hence a pitot tube alone would not give a reliable pressure reading. By using the Kiel probe the total pressure reading is insensitive to the angle between the streamlines and the Kiel probe up to 45°. The PPI uses pressure sensors with three different sensitivities to cover the pressure range of 0-180 kPa. The sensor technology is a heritage from a concept that has been applyed in earlier space and terrestrial applications. The PPI starts measurements at an altitude of 160 km, producing 28 bits of data per second. Measurements are designed to continue beyond the time of impact on the surface of Titan until Huygens stops operating. The flight unit has been integrated to the Huygens entry probe and tests have been successful. A special balloon test session of PPI and other HASI instruments simulating the actual mission of Huygens was carried out successfully in 1995.