Data Set Overview
=================
 
THe Huygens Atmospheric Instrument is one of the six experiment onboard
the Huygens probe. HASI is a muultisensor package designed to measure
the physical quantities characterizing Titan's atmosphere during entry
and descent phase and at the surface.
 
Science Objectives and Observational Strategy:
HASI science objectives for the Huygens missione are to:
- determine the atmospheric density, pressure and temperature profiles
- Determine the atmospheric electric conductivity and charge carrier
  profiles
- Investigate ionisation processes
- Survey of wave elelctric fields, lightining and analyse the quasi-static
  electric fields leading to storm formation
- Detect acoustic noise due to turbulence or storm
- Characterise the roughness, mechanical and electric properties of the
  surface material, whatever its phase, solid or liquid
 
These objectives are addressed at one location and time, but a very
high resolution ground thruth for the calibration of remote sensing
observations.
The observational strategy for the Huygens probe is fixed by a
programmed sequence, HASI observations, mode switching are organized
in a timeline triggered by events (e.g. T0 detection), time and
altitude information provided by Huygens CDMS via the DDBL (Data
Distribution  Broadcast Line).
 
Parameters
==========
 
HASI S/W provides two independent telemetry queues: one dedicated to
each Huygens Command and Data Management Unit (CDMU). HASI handles TM
packets queues by the internal operating system, that allocate
dynamically the memory for the packets. The memory remains allocated
until the tM packet is discarded from the queue (TM queue capability
of about 130 packets).
Few seconds (about 132 s) after HASI power ON HASI telemetry starts
(both on CDMU-A and CDMU-B). Acquired data are transmitted redundant
on both CDMUs (with the exception for PWA science data, same packet
content is transmitted on both CDMU TM line, but only one chain is
considered valid). Till TdataH (T0+2.5 min) HASI data are trasmitted,
but remains stored in the memory. After TdataH (when the link between
Huygens probe and Cassini should be safelly established, stored
packets are retransmitted as original).
During entry phase, when HASI is the only operative instrument, in
order to cope with the  higher CDMS polling rate also copies of the
packets stored in the TM queues are re-transmitted.
 
HASI presents to the Huygens Probe Command and Data Management System
(CDMS) about 8-9 TM packets for CDMS cycle (16 s).
Each HASI telemetry packets is composed of a header containing packet
identification, sequence count and lenght. The first data contained
in the packets data field corresponds to the time stamp relevant to
the HASI mission time when the first value has been measured. The time
relevant to the single values contained within the data field is
computed according to their position (sensor sampling and packet
filling rate).Time relevant to each data value contained in the
telemetry packet is computed starting from the timestamp, according to
their position within the packets and from the sampling rate (constant).
 
 
HASI marks each telemetry packets with a timestamp relevant to the DDB time
when the first source data contained in the packet data field is created.
Timestamp resolution is a BCP count (125 ms); the resolution on this time
value is 1 LSB = 1ms.
 
DATA
====
 
The HASI telemetry packets are filtered into data format relevant to the
different subsystem and operational mode. The raw binary data are
reformatted into ASCII tables reporting as first column the relevant
mission time.
 
Structure of DATA Directory
===========================
HASI data are organized in different folders divited by subsystem.
They are:
ACC: Triaxial ACCelerometer subsystem
TEM: TEMperature sensors
PPI: Pressure Profile Instrument
PWA: Permittivity, Wave and Altimetry package
HK:  DPU (Data Processing Unit) housekeeping
PROFILES: higher level products
 
Parameter within data filename are indicative for:
- sensor, data processing level, datatype, mission phase and/or mode.
 
File naming convention
======================
 
The data product file naming convention is as follows:
HASI_L{data processing level}_{subsystem}{missionphase}_{datatype}.{ext}
data processing level {2, 3, 4}  according to CODMAC processing level
subsystem {ACC, TEM, PPI, PWA, DPU}
mission phase {E = entry , D = descent, S = surface} when applicable
sensor data type {SERVO, SERVOSTD, SERVOT, PIEZO, PIEZOSTD, PIEZOT,
PIEZOIMP}; {FINE1, COARSE1,...}; {NSA,HCA, HK, NSA11P, NSA13T, ...};
{RP, AC_ELF, AC_VLF, ACU, LGH, MIP, RAU}
extension {LBL = label, TAB = table}
 
 
Ancillary Data
==============
 
The time reference (mission time) is given by the Huygens timer
provided to the instrument in the Descent Data Broadcast (DDB) by the
Huygens Command Data and Management System (CDMS).
HASI receives a DDB information every 2 s and is synchronized every
125 ms by the BCP (BroadCast Pulse) signal.
In preT0 (entry phase), time is referenced to Huygens probe power on (S0);
after entry time is referenced to T0 (arming time of the Descent Device
Deployment System pyros).
HASI marks each telemetry packets with a timestamp relevant to the DDB time
when the first source data contained in the packet data field is created.
Timestamp resolution is a BCP count (125 ms); the resolution on this time
value is 1 LSB = 1ms.
Conversion from DDB time (Mission Time) to UTC time will be provided by
HPOC (Huygens Probe Operation center).
Native time (enlapsed from T0) in s a sign '-' for preT0.
 
Huygens Probe auxiliary data such as reference dimensions, mass, CoG,
products of Inertia evolution, parachute characteristics and the Huygens
aerodynamical database have been provided by the Project and are archived
within the Huygens HouseKeeping (HK) data set.
 
 
Coordinate System
=================
 
The reference system for HASI is the Huygens probe system axes.
The official trajectory reconstruction is the result of a common effort,
based a subset of Huygens experiments and housekeeping data, produced by
the Huygens Descent Trajectory Working Group (DTWG) and archived within
the Huygens DTWG data set.
Huygens reference system: Titan centric coordinates (as defined by DTWG).
 
 
Software
========
 
HASI data files are tables of ASCII format fields for each
housekeeping and science data elements.  This simple conversion from
the raw, binary EDR records involves no irreversible change or data
reduction.  As such, the level 2 data set represents an appropriate
primary archive of all of the raw HASI data delivered to the ground and
there is no expectation of any need for reprocessing to perform this
conversion in the future.  Accordingly, the software used to read the
HASI raw, binary files and create the higher level data is not archived
with this data set.
All the processing, transfer function and calibration applied to the raw
data are described in the subsystem Calibration reports contained in the
CALIBRATION directory. Within this documents are also reported the error
budget and measurement uncertainties.

Confidence Level Overview
=========================
 
The HASI data set represents all HASI data from the Huygens probe mission
at Titan as recovered from the Cassini Solid State Memory and Cassini-
Huygens Project ground system.
The multiple redundant data streams, downlinked have been checked and
merged to provide the most complete and correct sequence.
 
 
Data Coverage and Quality
=========================
 
The HASI data set is organized into a separate data file for each sensor
package as listed in the Data section under DATA_SET_DESC above.
The overal data quality is very good. The loss of channel A resulted into
the loss of some PWA data that were not redundant (e.g. Relaxation probe
and Radar). No data gaps caused by telemetry dropouts have been recorded.
 
For higher level products some concerns are still open:
- calibration for PWA data: only level 2 is provided in this dataset.
- The Huygens DTWG trajectory is incoherent to the altitude profile
retrieved from Huygens radar altimeter.
Different approaches and methods applied to HASI data resulted in altitude
and velocity profiles showing discrepancy not consistent with uncertainty
range relevant to models and measurements.
The reason for this discrepancy is still under investigation.
Future analysis could imply the need for reconsolidate the trajectory.
 
 
DOCUMENTS
=========
 
The DOCUMENT directory contains the files that provide documentation and
supplementary information to assist in understanding and using the data
products.
 
 
DATASET VERSION
===============
The data set id is HP-SSA-HASI-2-3-4-MISSION-V1.1. The file ERRATA.TXT
explains the differences with the previous archived data set
HP-SSA-HASI-2-3-4-MISSION-V1.0.
 
CALIBRATION Directory
=====================
 
This directory contains files providing the calibration
information of the HASI subsystems.
 
HASI ACKNOWLEDGEMENT/CREDITS
============================
 
The Cassini-Huygens mission is a cooperative project of NASA,
the European Space Agency (ESA) and the Italian Space Agency
(ASI). The Huygens Atmospheric Structure Instrument (HASI)
onboard the Huygens probe is a multisensor package designed
to measure the physical quantities characterizing Titan's
atmosphere.
Prof Marcello Fulchignoni is the HASI Principal Investigator.
The HASI Project Office is at CISAS 'G. Colombo' of the
University of Padova (UPD), Italy and is coordinated by ASI's
Observation of the Universe Directorate.
 
The Accelerometer (ACC) subsystem has been designed, developed,
assembled and provided by the United Kingdom. The ACC team is
based at Planetary and Space Sciences Research Institute (PSSRI)
of the Open University, UK.
 
The Pressure Profile Instrument (PPI) subsystem has been
designed, developed, assembled and tested by the Finnish
Meteorological Institute (FMI), Helsinki, Finland.
 
The Temperature sensors (TEM) have been designed, developed,
assembled and tested by CISAS 'G. Colombo' of the University
of Padova (UPD), Italy.
 
The Permittivity, Wave and Altimetry (PWA) subsystem has been
designed, developed and tested by a collaboration of institutes:
the Research and Science Support Department (RSSD), ESTEC, ESA,
the Institut fuer Weltraumforschung (IWF), Graz, Austria;
the Laboratoire de Physique et Chimie de l'Environnement, (LPCE),
CNRS - CNES, Orsay, France; the Instituto de Astrofisica de
Andalucia (IAA-CSIC), Granada, Spain.
 
The acoustic sensor is part of the Permittivity, Wave and
Altimetry (PWA) subsystem and has been designed, developed and
tested by the Space Research Institute (Institut fuer
Weltraumforschung, IWF) of the Austrian Academy of Sciences, Graz,
Austria.
 
 
For more information on the HASI instrument, visit
http://cisas.unipd.it/hasi/
 
 
CREDITS
============================
HASI_ACC                                  ESA/ASI/UPD/OU
HASI_TEM                                  ESA/ASI/UPD
HASI_PPI                                  ESA/ASI/UPD/FMI
TRAJECTORY altitude and vertical profile  ESA/ASI/UPD/OU/FMI
HASI_PWA                                  ESA/ASI/UPD/RSSD/CNR/IWF/IAA-CSIC
 
Review
======
 
      The Huygens data sets were reviewed in two steps:
      (1) a review of the Experimenter-to-archive-ICD, in 2004.
      (2) a review of the data sets, in 2006.
      Procedures and details of this process can be obtained from
      the ESA archive team:
 
      Olivier Witasse or Joe Zender
      ESA ESTEC
      Keplerlaan 1
      2200 AG Noordwijk
      The Netherlands
      Olivier.Witasse@esa.int or Joe.Zender@esa.int
 
      This data set was reviewed by Ralph Lorenz and an anonymous reviewer.