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Annex

HUYGENSMISSION AND PROJECT

OVERVIEW

Note: This overview is an introductory support to theHUYGENS Enquiry Board activities as defined by ESA DirectorGeneral in his Board mandate letter.

It is intentionaly limited to the elements that are necessary torecall the mission and the Huygens Probe design anddevelopment programme where relevant for the enquiryinvestigations.

December 2000

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The HUYGENS Probe is carried to the vicinity of Titan, the main Saturn moon, bythe CASSINI Orbiter after a complex interplanetary journey of nearly 7 yearsincluding four gravity assist fly-bys (twice Venus, Earth and Jupiter).The mission started with a successful launch October 15, 1997 and the HUYGENSscience mission is expected to take place in November 2004.The probe is in a dormant mode during the Cruise phase; some short electricalcheck-out sessions are however performed every six-month period by powering theProbe from the Orbiter and by using a coupling interface for communicating.When CASSINI is about to cross Titan orbit, then the Probe is released, nominally22 days before Titan encounter. The Probe is now energised with its batteries andthe whole sequence is ruled by the autonomous on-board software and hardwarefunctions including 3 timers in hot redundancy and 2 redundant “g-switches”. ThePSE, Probe Support Equipment, stays with the CASSINI Orbiter in order to allowcommunications from the Probe to the Orbiter during the discovery of Titanatmosphere and “landing site”.

CASSINIOrbiter

HUYGENSProbe

High Gain Antenna

MISSION PROFILE

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The HUYGENS autonomous sequence is split up in several phases of events, eachone corresponding to pre-defined operational modes and specific probe configurations.

Coast phase from CASSINI separation (after proper Timer loading) till atmosphericentry;

Wake-up phase, nominally executed through the Probe redundant Timer;

Entry and descent phases, all under full power supply and on-board datamanagement control. These phases will last 2 to 3 hours, thanks to aero -thermalshell protections and chutes’ deployment, until “landing”.Orbiter/Probe motion, with a maximum relative velocity of 6 km/s, is adjusted with anOrbiter Delay Time of 4 hours for the trailing trajectory of CASSINI Orbiter. It enablesan identical time slot of communications from the Probe transmitters and antennas tothe CASSINI Orbiter via its High Gain Antenna and the PSA receiving channels(Probe Support Avionics). Appropriate CASSINI steering towards Titan is performedand the digital Probe data streams are recorded by CASSINI on-board data system.

HUYGENS AUTONOMOUS SEQUENCE

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The HUYGENS discovery mission of Titan is executed during these phases.The winds blowing in Titan atmosphere may move the relative angle and distancebetween the Probe and the CASSINI Orbiter.The six scientific experiments are activated in a predefined sequence and thecorresponding data are streamlined by the Probe CDMS (Command and DataManagement System). Then data are transmitted at fixed digital rate together with otherdata from Probe instrumentation among which two Radar Altimeters in hot redundancy.Once received and recorded on-board the Orbiter, all the data will be retransmitted toEarth via the NASA Deep Space Network and then distributed to the scientificInvestigators.

HUYGENS ENTRY AND DESCENT PHASES

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HUYGENS SCIENTIFIC EXPERIMENTS

GCMS Gas Chromatograph and Mass Spectrometer

It is a very versatile gas chemical analyser able to identify and quantify thevarious Titan atmospheric constituents. If the Probe makes a safe impact atlanding it may also measure the surface composition.

ACP Aerosol Collector and Pyrolyser

It collects aerosols during descent phase at various atmospheric layers, and thenheats up the collected samples in three different steps in order to producepyrolysis products that will be flushed to the GCMS.

DISR Descent Imager and Spectral Radiometer

It takes images and makes spectral measurements using several sensors fromthe ultraviolet to the infrared spectrum. Clouds as well as Titan surface will beobserved with the capacity of performing mosaic panoramas.

HASI Huygens Atmosphere Structure Instrument

It consists of a various set of sensors to measure the physical properties of theatmosphere such as temperature, pressure, electrical conductivity andpermittivity.

DWE Doppler Wind Experiment

Backed by the presence of Ultra stable Oscillators on the Probe and the ProbeAvionics staying on the Orbiter, it will permit to measure winds in Titanatmosphere and may also help reconstitute the Probe flight dynamics duringdescent phase.

SSP Surface Science Package

The package contains several sensors capable of measuring several physicalproperties of the surface like thermal,acoustic, dielectric and optical ones. It willalso indicate impact deceleration and Probe attitude if stable enough on solidsurface bed.

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HUYGENS SCIENTIFIC EXPERIMENTS and PROBE EquipmentACCOMODATION

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PROBE to ORBITER COMMUNICATION FEATURES

After separation the Probe is able to do one-way transmissions to the Orbiter thanksto two 10-Watt transmitters connected with two Probe antennas without cross-coupling. The respective carrier frequencies are 2040 MHz and 2098 MHz and thelink margin on carrier recovery is very good at all Probe/Orbiter aspect anglesbetween 20° and 60°.Carrier modulation is of BPSK type and carrier frequencies are respectively producedby an Ultra stable Oscillator (TUSO) and two classical Crystal Oscillators. Two digitaltelemetry streams of NRZ type are delivered by the redundant CDMUs (Command &Data Management Units) at a basic 8192 bps rate. Modulating sub-carriersfrequencies are multiples of the basic digital rate. A time shift of some seconds isintroduced between each individual stream in order to cover some potential linkfading and ease, later on, a full data stream reconstruction.On-board the Orbiter a Probe Support Avionics front End is connected to the Highgain Antenna. Two parallel chains process the received signals from the Probe inseveral steps: low noise amplification followed by a sophisticated, softwarecontrolled, receiver digital section including digital frame restitution. Bit error rate isexpected to be less than 10 E-5.

Probe datatransmission

Probe data reception on-board Orbiter (PSA)

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After the usual ESA managed competition the industrial Phase B activities began inJanuary 1991 under the leadership of Aerospatiale Cannes, now part of AlcatelSpace.MBB, now part of ASTRIUM GmbH, had the responsibility of all Probe systemintegration and testing activities.Alenia Spazio had the responsibility of the PDRS (Probe Data Relay Subsystem),including the Probe Support Avionics on-board CASSINI Orbiter). LABEN replaced MARCONI Space Systems for the CDMS (Command and DataManagement Subsystem) in the course of phase B . LOGICA UK was selected for both the POSW (Probe On-board Software) and theSAWS ( Support Avionics Software).CAPTEC performed an independent validation of these software.SCHRACK,now Austrian Aerospace, developed the PDRS Electrical GroundSupport Equipment as well as a Link Simulator under specifications of AleniaSpazio.

HUYGENS INDUSTRIAL ORGANISATION

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Four Probe system models were build:

STPM Structural, Thermal and Pyro Model to qualify the sophisticated “shelland kernel” probe configuration;

EM Electrical Model with its usual purpose of pre-qualifying electricalinterfaces and functional performances;

Special Model, called SM2, used for a balloon drop test checking the wholesequence of pyro separations and chutes’ deployments;

FM Flight Model, finally mated with the CASSINI Orbiter at JPL facilities.

MBB Ottobrunn performed the AIT operations of all system models but SM2which was done by FOKKER Space. Aerospatiale residents were permanentlypresent in all sites where probe models’ integration and testing took placeincluding final mating with CASSINI at JPL facilities.

The main milestones and major project reviews were conducted by ESAwith attendance of NASA-JPL according to the following schedule:

SRR, System Requirements Review, April 1991PDR, Preliminary Design Review, October 1991SDR, System Design Review, October 1992MHDR, Mechanical Hardware Review, May 1994EHDR, Electrical Hardware Design Review, July 1994CDR,System Critical Design Review, September 1995FAR, Flight Acceptance Review, April 1997LRR, Launch Readiness Review, October 3, 1997

The CASSINI-HUYGENS launch occurred October 15, 1997

A special ‘External Independent Readiness Review’ EIRR also intervenedunder a direct mandate from the NASA Administrator during the 1995 -1997period.

PROBE MODELS AND PROJECT REVIEWS

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CASSINI - HUYGENS PREPARATION FOR LAUNCH

Huygens Probe is mounted and shown up-front on its heat shield face