HASI ACC XSERVO higher level product derivation
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Vertical profiles of atmospheric *density*, *pressure* and *temperature* 
have been derived during entry using accelerometric data considering 
the drag force exerted by the atmosphere on the Probe and using the 
assumption of hydrostatic equilibrium and the perfect gas law (refer 
to ZARNECKIETAL2001 [RD1] and FULCHIGNONIETAL2002).

The probe *trajectory* and *attitude* can be reconstructed by analysing 
vehicle acceleration. The profile of axial and normal accelerations 
during the Huygens entry and descent in Titan's atmosphere will be 
used in order to retrieve the probe entry track, velocity and altitude 
profiles, and attitude, during the high-speed entry phase (ref. 
FULCHIGNONIETAL2002).

The *Huygens auxiliary data* (e.g.reference dimentions, mass, CoG, 
products of Inertia evolution, parachute characteristics and the 
Huygens aerodynamical database) necessary to for post flight analysis 
have been provided by the Project and are archived in the Huygens HK 
data set.

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   ENTRY
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Starting from the *acceleration* profile of the Huygens probe, 
*Vertical velocity* and *altitude* profile has been derived from 
trajectory reconstruction by double integration.

Vertical profiles of atmospheric *density*, pressure and temperature 
have been derived during entry using accelerometric data considering 
the drag force exerted by the atmosphere on the Probe and using the 
assumption of hydrostatic equilibrium and the perfect gas law (refer 
to ZARNECKIETAL2001 [RD1] and FULCHIGNONIETAL2002).

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  DESCENT
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Altitude and vertical Velocity profiles as derived from trajectory 
reconstruction during descent

Altitude and vertical Velocity profiles have been derived by Kalman 
filtering relying mainly on reverse HASI Servo Accelerometer 
integration from impact up to start of the descent phase. 
In order to contain altitude uncertainties related to direct double 
integration, probe dynamics has been modeled by propagating probe 
motion through state equations, and is corrected with both 
accelerometer data and PPI total pressure measurements 
(which contain the possible drift in descent velocity) [Bettanini]