CHAMP LONGTERM ANALYSIS OF THE POWER AND THERMAL SUBSYSTEMS

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CHAMP LONG-TERM ANALYSIS OF THE POWER AND THERMAL
SUBSYSTEMS
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CHAMP

• Geo-scientific satellite operated by DLR/GSOC
• Obtains data for improved models of Earths
  gravitational, magnetic and electric fields.
• Mass 530 kg
• Polar orbit, 450 km altitude
• Launch 15 July 2000
• Mission duration 5 years

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CHAMP POWER SUBSYSTEM

• Solar array, 5.3 m2, 570 W
• 3 Panels, Starboard, Port and Zenith.
• Battery
• 10 NiH2 cells in series
• Power Control Distribution Unit (PCDU)
• Receives electrical power from
• solar array and/or battery
• Distributes power according to
• commands received from the OBDH
• Delivers housekeeping data
• to the OBDH
• Performs battery charge regulation
• via a BCR unit.

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CHAMP POWER SUBSYSTEM

• Battery Charge Regulation (BCR)
• Constantly monitors the batterys state of charge
• Charges the battery with maximum available solar
  current during the first part of the sun phase
  (FULL Charge)
• Reduces the charge current to aprrox. 0.25 A once
  the battery reaches a programmable End Of Charge
  (EOC) level (TRICKLE Charge)
• Subsequently shuts down pairs of solar strings
  from the two side panels, then from the top
  panel.
• Automatically returns to FULL Charge mode once
  the battery starts discharging.

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CHAMP POWER SUBSYSTEM

• Example of a charge/discharge cycle over 1 orbit

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CHAMP THERMAL SUBSYSTEM

• S/C interior isolated by foam-core solar panels
  and MLI
• Large screen radiator facing Earth for internal
  heat rejection
• 60 Thermistors, measuring temperatures of the
  main instruments and equipment
• 22 Thermistor-controlled electrical heaters, of
  5W, 7W or 10W.
• Active thermal control via a simple ON/OFF logic
  based on different thermal control heater set
  point tables
• Requirements
• Maintain all instruments and equipment within
  operating temperature limits.
• Stabilize the accelerometer (ACC) sensor to 1C/orbit
• Stabilize the Flux Gate Magnetometer (FGM) sensor
  to
• Stabilize star sensors to
• Stabilize star sensors DPU (data processing unit)
  to

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CHAMP OPERATIONS OVERVIEW

• Contact over Weilheim ground station on two
  consecutive orbits, every 12 hours.
• Uplink of time-tagged commands for future
  contacts
• Reception of real-time telemetry
• Reception of dumped telemtery recorded on-board
• Separation of housekeeping data from science data
• Forwarding of science data to scientific centre
• Offline processing of dumped housekeeping data
• Generation of various offline products.

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CHAMP OPERATIONS Offline processing

• Rapid extraction of a set of housekeeping
  parameters from memory dump files.
• Creation of a set of ASCII parameter files, one
  per telemtery packet
• DATE TIME THPC0011 THPC0012 THPC0013
  
• 2001/03/19 141500 0.12 0.13 0.6975
  
• 2001/03/19 141510 0.28 0.29 0.7005
  
• 2001/03/19 141520 0.43 0.45 0.6900
  
• 
  
• ASCII files imported by Excel tool, plots
  automatically created.
• 7-day ASCII files created each week by meging
  together ASCII files obtained from single memory
  dumps.

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LONG TERM ANALYSIS

• Sun-to-orbit (Beta) angle
• 0 (Noon orbit)
• Sun in the S/C X-Z plane
• Maximum eclipse duration
• Maximum solar current intensity
• Positive Beta angle
• Sun on the port side of S/C
• Negative Beta angle
• Sun on the starboard side of S/C
• Full sun orbits
• When Beta 70
• No eclipses
• Battery Charge Regulator in constant TRICKLE
  Charge mode.

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LONG TERM ANALYSIS

• Example of Beta angle effect
• Solar panel temperatures
• Sun moving from port
• side to starboard side
• Side panels have equal
• temperatures at Beta0
• Battery Capacity
• Full-sun orbits
• Eclipse duration increasing
• Commanded changes
• in battery EOC level.

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LONG TERM ANALYSIS - BATTERY

• Battery efficiency degradation
• Recharge Factor (RF) defined as
• Where t2 - t1 is the charge time, t3 t2 is the
  discharge time, U is the voltage and I is the
  current.
• RF calculated for all orbital cycles from the
  available telemetry parameters (battery
  charge/discharge currents, battery voltage).
• Full-sun orbits excluded from the calculation
  battery always in TRICKLE Charge mode and no
  eclipses (no discharge).
• Minimum and maximum RFs calculated according to
  the uncertainty in the measurements (current and
  voltage sensor accuracies).

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LONG TERM ANALYSIS - BATTERY

• Linear prediction by 2005 (end of mission), the
  recharge factor shall lie between1.20 and 1.25
  (efficiency between 80 and 83).

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LONG TERM ANALYSIS SOLAR ARRAY

• Complex solar array model
• Based on solar cell specification and on-ground
  test results
• Calculates the theoretical output power at any
  (UTC) time
• Based on
• Precise orbital position (from on-board GPS data)
• Exact solar intensity (solar constant, W/m2)
• Bus voltage
• Solar cell thermal model
• Comparison of measured power (from telemetry)
  with theoretical power (from the model).
• Only relevant when operating in FULL Charge mode
• TRICKLE Charge phases excluded from calculation
• Full-sun orbits excluded from calculation
• Generated maximum and minimum curves based on
  uncertainty in the measurements (current and
  voltage sensor accuracies).

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LONG TERM ANALYSIS SOLAR ARRAY

• Results Minimum and maximum difference between
  the measured and predicted power of the solar
  array
• Linear prediction A definite degradation shall
  only be measurable in 2010. The sensor accuracies
  are not sufficient to currently measure
  degradation.

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LONG TERM ANALYSIS THERMAL SUBSYSTEM

• Collected all temperature measurements since
  beginning of mission.
• Comparison of in-orbit behaviour with on-ground
  test results (e.g. from TBTV tests) and
  predictions.
• All requirements were met
• So far, CHAMP has not experienced two identical
  thermal environments separated in time for a
  possible degradation analysis (identical
  sun-to-orbit angle, identical solar intensity,
  same flight direction).
• Earliest possible time for a comparison March
  2003, with data from October 2001

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CONCLUSIONS

• CHAMPs power and thermal subsystems should meet
  the required 5-year mission lifetime.
• Long-term analyses are extended with the new data
  on a regular basis.
• First power problems (negative power margin)
  could occur during the first noon orbits after
  November 2005
• Generated power not sufficient to completely
  charge the battery
• Can reduce the required heater power to increase
  power margin if necessary
• The results are significant for the GRACE
  mission, which has an almost identical power
  subsystem to CHAMP.