Mars Express Power Subsystem Inflight experience

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Mars Express Power SubsystemIn-flight experience

• Roberto Ferretti
• VEGA Group

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Power Mission Scenario

• Lifetime One Martian year plus one year of
  extension (already obtained)
• Five eclipse periods occurring during the 2
  Martian years (three of them are over). The most
  critical will be next August 2006
• The need for a high autonomy, due to the absence
  of real-time practical monitoring and control at
  Mars distance and in observation attitudes and
  because of 2 Solar conjunction phases
• Ability to cope with a highly variable
  environment Sun-to-Mars distance (impact on
  Solar flux), Earth-to-Mars distance, changing
  attitude, etc.

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Power Design
PCU from ROSETTA 28V regulated BUS Maximum Power
Point Tracker (MPPT) available
Two Solar Array Wings from GLOBALSTAR 11.18 m2,
cells Si-BSR 2PR/200 Worst Case EOL Power 684W at
1.65AU
PCU
PDU
Three Li-ION Batteries 72Ah Nominal Capacity
BOL Sony Hard Carbon type 18650
PDU from ROSETTA 78V Latch Current Limiters (LCL)
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Power AnomalyArray Power Regulator routing
Envisaged Wiring Design
684W EOL worst case
Real Wiring discovered during the first battery
discharge test
Only 60 of power was available
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Power Anomaly Management and Solution
Dissymmetric APR setting Strategy has been
adopted in order to minimise the loss of power

• Different APR Clamp Voltage on the APR 1,2,3
• APR 1 always maximum clamp 66V
• APR 2,3 Clamp changed to values close to Maximum
  Power Point according to season (i.e Manual
  regulation)

Power Extraction is approximately 72 ofDesigned
Estimation
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Power Anomaly Consequence

• Thermal conditioning only when the payload is
  needed
• No x-TX in eclipse
• S/C recharge relaxation after NADIR
• Safe Mode power management and dedicated FDIR
  software applications
• Mission Planning driven by the seasonable power
  available

All this allows NO reduction of Science
Operations with respect to the intended Baseline
(actually 50 more), except for a few power
limited periods
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SA Power Status

• A correlated SA model is used to
• supports the power operation
• optimised the APR setting changes

The APR setting will change within each orbit
during the next Aphelion in August 2006
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Power Operations Activities

• Two main activities characterize the Mars Express
  Power Operations

Monitoring
from Short to Long Time Scale Control
Engineering
Statistics Modelling Tools
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Power Operations - Monitoring

• Very Short Time Monitoring Performed
  by Controller on duty Tools Real time TM, S/C
  visible The aim Recognize power anomalies,
  maximise the level of attention during the
  eclipse seasons
• Short Time Monitoring Performed by Power
  Engineer Tools Retrieval of TM data. S/C not
  visible The aim Recognize power anomalies
  occurred with the S/C not visible
• Long Time Monitoring Performed by Power
  Engineer Tools Retrieval from the Long Term
  Archive The aim Statistic, Seasonal Behaviour
  Analyses, Operation Optimization, S/C status
  assessments, etc

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Power Operations - Control

• Solar Array Power Assessment
• No sensors have been envisaged in order to asses
  the SA status (temperature sensors, Solar Panels
  power independent measurement, etc). Outside the
  eclipse seasons, the assessments are performed
  via indirect tests.

Adopted Process In Battery Charge Mode (BCR), the
SA power demand is maximum. The SA provides the
power to manage the S/C and charge the batteries
at the same time. Outside the eclipse season this
scenario is deliberately induced by means of a SA
rotation (i.e. eclipse of the SA). The batteries
discharge for a certain time. When the SA rotate
back to nominal, the power generation is maximum.
The comparison of this power with the model
prediction (seasonable estimated theoretical
power) provides the assessment of the SA status.
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Power Operations Engineering Overview

• Statistics and AnalysesBattery assessment,
  Operation Margin elaborations
• ModellingSA Power Model, SA Voltage Model in
  Albedo, Thermal Power Consumption Model
• Tools and conceptsSoftware tools, Operation
  Margin Concept

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Power Engineering - Statistics

• Battery Discharge Statistic
• The aim is to asses the Battery ageing/usage
  comparing the number of mission discharges with
  the manufacturer predictions

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Power Engineering - Analyses

• Battery Ageing Assessment
• The aim is to asses the Batteries ageing
  measuring the nominal capacity fade. Life
  Ageing and Cycling Ageing are the main reasons of
  capacity fade in the Li-ION batteries.

Adopted Process Calibration of the Li-ION Battery
Model by means of a deep discharge (occurred or
deliberately generated). The deep discharge is
necessary to see the effects of negative
electrode ageing. (Negative electrode EMF is
nearly constant at high states of charge so only
positive electrode ageing is visible from shallow
discharges). The calibrated model is used to
generate Battery Discharge Voltage vs Time curves
at constant current. The Battery capacity is
given by the Discharge Current multiplied by the
time necessary to discharge completely the battery
MEX Battery Degradation 18.5 _at_ DOY2005.306
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Power Engineering - Modelling
Solar Array Voltage Model in AlbedoReproduces
the SA voltage reduction observed when the S/C
flies close to the Planet. The Mars Albedo
increases the SA temperature hence decreases the
SA efficiency.
Model description A simplified SA thermal model
estimates the SA Voltage(T) curve at apocentre. A
thermal model, based on the radiative SA View
Factor with respect to the planet, reproduces the
SA temperature at pericentre (albedo) Knowing the
temperature at pericentre, the apocentre
Voltage(T) function provides the SA voltage at
pericentre.
This model makes more robust the APR setting
strategy(also within an orbit)
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Power Engineering - Tools and Concepts
Operation Margin ConceptHow much more Operations
can be performed with the available not used
extra power? The Operation Margin replies to
this question. It measures the energy used to
manage the S/C and fully recharge the battery in
each orbit, then compares this with the Solar
available energy.
The Operation Margin measures the S/C Operation
Power Efficiency and support the FCT in the
science Planning It provides a measurable
operation performance parameter for the I/F with
the management and the science community
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Mission Achievement

• Average 1.5 science pointings per orbit
• NADIR pointings of up to 68 minutes in duration
• Return 2Gbits per day comapared with 0.5Gbits
  mission baseline
• Such pointing profiles have been possible even
  during the maximum eclipses, albeit with the
  occasional need trim singular orbits to prevent
  power problems
• The power approaches and techniques have allowed
  tremendous science
• Direct detection of water ice at the poles
• Methane, aurora, fantastic vistas from the stereo
  camera
• Evidence of recent geological formations.
• This has been possible mainly due to the
  performance and reliable predictability of the
  power subsystem, allowing to safely fly with very
  low absolute power margins, for example in the
  last eclipse season full science was safely
  conducted with a maximum DoD approaching 60 and
  average operational margin of only 10 Watts.

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Future

• The preparation of the strategy is on going in
  order to cope with the next aphelion low power
  season (August-October 2006). The S/C will be
  managed in Survival Mode minimizing the power
  consumption. The predicted margin is a few tens
  of Watts (for about 400W consumption average).
• The present S/C power status and the further
  preservation strategies give today the confidence
  that Mars Express can survive further mission
  extensions, if approved.
• The reduction of the State of Charge and Battery
  temperature outside the eclipse seasons will be
  applied in order to reduce the Batteries ageing
  rate (if compatible with the operation scenarios).

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Lessons Learnt

• Separated Solar Panel sensors (Voltage, Current)
  are almost mandatory to periodically asses the
  Solar Array status and promptly recognize
  failures.
• Temperature sensors on the solar array should be
  available to analyze power reduction phenomena
  (i.e. albedo effect, temperature transient
  effects coming out from eclipse, etc).
• The possibility of disconnecting the Batteries
  from the power subsystem should be available
  in-flight to allow accurate and controlled
  battery degradation assessment.
• The previous point returns information (in-life
  EMF curve) useful for the periodical State of
  Charge synoptic calibration. This test scenario
  removes all the affect of the internal
  resistance.

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Acknowledgements

• E. Ecale and his team, EADS Atrium, Toulouse
• G. Dudley, European Space Agency, AG Noordwijk,
  The Netherlands
• M. Denis and A. Moorhouse, European Space Agency,
  Darmstadt, Germany
• E. Rabenau, NOVA Space, Bath, UK
• O. Page, SciSys, Bristol, UK, BS4 5SS and J.
  Schulster, SciSys, Chippenham, UK
• T. Ormston, ESA-YGT Darmstadt, Germany

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