Huygens communications link enquiry board

Annex
HUYGENS
MISSION 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
1
MISSION PROFILE
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”.
High Gain Antenna
CASSINI
Orbiter
HUYGENS
Probe
2
HUYGENS AUTONOMOUS SEQUENCE
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.
3
HUYGENS ENTRY AND DESCENT PHASES
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.
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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
6
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 crosscoupling. 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 onboard Orbiter (PSA)
7
HUYGENS INDUSTRIAL ORGANISATION
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.
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PROBE MODELS AND PROJECT REVIEWS
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,
PDR, Preliminary Design Review,
SDR, System Design Review,
MHDR, Mechanical Hardware Review,
EHDR, Electrical Hardware Design Review,
CDR,System Critical Design Review,
FAR, Flight Acceptance Review,
LRR, Launch Readiness Review,
April
October
October
May
July
September
April
October 3,
1991
1991
1992
1994
1994
1995
1997
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.
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CASSINI - HUYGENS PREPARATION FOR LAUNCH
Huygens Probe is mounted and shown up-front on its heat shield face
10