The Huygens Atmospheric Structure Instrument (HASI) is a multi sensor package
which has been designed to measure the physical quantities characterizing Titan\u2019s
atmosphere during the Huygens entry and descent phases and at the surface.
HASI\u2019s suite of sensors measured the physical and electrical properties of Titan\u2019s
atmosphere. Profiles of temperature, pressure, density, atmospheric conductivity have
been collected. HASI investigated the electric properties and the nature of the surface.
Accelerometers measured deceleration in all three axes as the probe was in the entry
phase. With the aerodynamic properties of the probe already known, it has been
possible to determine the density of Titan\u2019s high atmosphere and to deduce temperature
and pressure profiles. During the descent (from 162 km down to the surface),
temperature and pressure of the atmosphere were measured directly and the Permittivity
and Wave Analyzer sensors measured the electron and ion conductivities as well as the
complex permittivity of the atmosphere and searched for electric wave activity
including lightning. Acoustic signals collected by the HASI microphone were
processed by the on board FFT and their amplitude and main frequency, averaged
over two minutes, were transmitted to the ground. The HASI instrument package had a
total mass of 5.7 kg. The accelerometers package is mounted at the center of mass of
the probe and the temperature sensors and the pressure probe inlet are mounted on a
stem outside of the probe. Two deployable booms carrying Permittivity ad Wave
electrodes stowed under the thermal shield of Huygens, has been released at the
beginning of the descent into Titan\u2019s atmosphere. HASI provided, in addition to its own
scientific data, calibration information of use by other instruments on Huygens and by
remote sensing observations from the Cassini orbiter
