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Annex

HUYGENSMISSION AND PROJECT

OVERVIEW

Note: This overview is an introductory support to the HUYGENS Enquiry Board activities as defined by ESA Director General in his Board mandate letter.

It is intentionaly limited to the elements that are necessary to recall the mission and the Huygens Probe design and development programme where relevant for the enquiry investigations.

December 2000

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The HUYGENS Probe is carried to the vicinity of Titan, the main Saturn moon, by the CASSINI Orbiter after a complex interplanetary journey of nearly 7 years including four gravity assist fly-bys (twice Venus, Earth and Jupiter). The mission started with a successful launch October 15, 1997 and the HUYGENS science mission is expected to take place in November 2004.The probe is in a dormant mode during the Cruise phase; some short electrical check-out sessions are however performed every six-month period by powering the Probe from the Orbiter and by using a coupling interface for communicating.When CASSINI is about to cross Titan orbit, then the Probe is released, nominally 22 days before Titan encounter. The Probe is now energised with its batteries and the whole sequence is ruled by the autonomous on-board software and hardware functions including 3 timers in hot redundancy and 2 redundant “g-switches”. The PSE, Probe Support Equipment, stays with the CASSINI Orbiter in order to allow communications from the Probe to the Orbiter during the discovery of Titan atmosphere and “landing site”.

CASSINI Orbiter

HUYGENS Probe

High Gain Antenna

MISSION PROFILE

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

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

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

Entry and descent phases, all under full power supply and on-board data management control. These phases will last 2 to 3 hours, thanks to aero -thermal shell protections and chutes’ deployment, until “landing”.Orbiter/Probe motion, with a maximum relative velocity of 6 km/s, is adjusted with an Orbiter Delay Time of 4 hours for the trailing trajectory of CASSINI Orbiter. It enables an identical time slot of communications from the Probe transmitters and antennas to the CASSINI Orbiter via its High Gain Antenna and the PSA receiving channels(Probe Support Avionics). Appropriate CASSINI steering towards Titan is performed and 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 distance between the Probe and the CASSINI Orbiter.The six scientific experiments are activated in a predefined sequence and the corresponding data are streamlined by the Probe CDMS (Command and Data Management System). Then data are transmitted at fixed digital rate together with other data 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 to Earth via the NASA Deep Space Network and then distributed to the scientific Investigators.

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 the various Titan atmospheric constituents. If the Probe makes a safe impact at landing it may also measure the surface composition.

ACP Aerosol Collector and Pyrolyser

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

DISR Descent Imager and Spectral Radiometer

It takes images and makes spectral measurements using several sensors from the ultraviolet to the infrared spectrum. Clouds as well as Titan surface will be observed 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 the atmosphere such as temperature, pressure, electrical conductivity and permittivity.

DWE Doppler Wind Experiment

Backed by the presence of Ultra stable Oscillators on the Probe and the Probe Avionics staying on the Orbiter, it will permit to measure winds in Titan atmosphere and may also help reconstitute the Probe flight dynamics during descent phase.

SSP Surface Science Package

The package contains several sensors capable of measuring several physical properties of the surface like thermal,acoustic, dielectric and optical ones. It will also indicate impact deceleration and Probe attitude if stable enough on solid surface bed.

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HUYGENS SCIENTIFIC EXPERIMENTS and PROBE Equipment ACCOMODATION

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PROBE to ORBITER COMMUNICATION FEATURES

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

Probe data transmission

Probe data reception on-board Orbiter (PSA)

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After the usual ESA managed competition the industrial Phase B activities began in January 1991 under the leadership of Aerospatiale Cannes, now part of Alcatel Space. MBB, now part of ASTRIUM GmbH, had the responsibility of all Probe system integration 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 Data Management Subsystem) in the course of phase B . LOGICA UK was selected for both the POSW (Probe On-board Software) and the SAWS ( Support Avionics Software). CAPTEC performed an independent validation of these software.SCHRACK,now Austrian Aerospace, developed the PDRS Electrical Ground Support Equipment as well as a Link Simulator under specifications of Alenia Spazio.

HUYGENS INDUSTRIAL ORGANISATION

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Four Probe system models were build:

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

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

Special Model, called SM2, used for a balloon drop test checking the whole sequence 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 SM2 which was done by FOKKER Space. Aerospatiale residents were permanently present in all sites where probe models’ integration and testing took place including final mating with CASSINI at JPL facilities.

The main milestones and major project reviews were conducted by ESA with 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 intervened under a direct mandate from the NASA Administrator during the 1995 -1997 period.

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