Envisat Flight Experience: FDIR and Lifetime Optimisation

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Envisat Flight Experience FDIR and Lifetime
Optimisation D. Milligan ESA - ESOC 9th
International Conference on Space Operations June
2006, Rome
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• Introduction
• Envisat satellite launch 1st March 2002
• Satellite Design lifetime 5 years (i.e. until
  March 2007)
• Specificity of Envisat Largest Earth
  Observation civilian satellite with 9 instruments
  on-board

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• Envisat Satellite
• Sun Synchronous LEO. Alt 800km 35 repeat
  cycle incl. 98.55deg.
• Satellite Mass 8.2 tonnes, Power 7kW, Size
  10.5x5m with 14x5m SA.
• PEB with 9 instruments SM contains support
  systems (AOCS, Power, Thermal, DHS, COMMs etc)

http//envisat.esa.int/instruments/tour-index/
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• Envisat Ground Segment
• Flight Operations Control Centre in Darmstadt,
  DE
• Main GS in Kiruna, SE (S-band HKTM, X-band
  Science data)
• Artemis data relay satellite (Ka-band) Science
  and HKTM via ESRIN, Frascati, IT

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• Envisat Orbit
• Sun Synchronous 10 AM Local Time Descending Node
• 100mn period, Alt 800km 35 repeat cycle
  incl. 98.55deg.

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• Envisat Orbit Air Drag
• Orbit degrades due to perturbations. Ground
  track maintained to /- 1km.
• Air drag lowers altitude. Below optimum, causes
  easterly drift at ANX (above optimum, westerly
  drift)

Air drag lowered altitude causes easterly drift
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• Envisat Orbit Solar Gravity Perturbation
• Orbit degrades due to perturbations. Ground
  track maintained to /- 1km.
• Solar Lunar Gravity, causes a secular decrease
  in inclination

Solar Gravity causes inclination decrease
Ground track perturbation at high latitudes
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• Envisat Orbit Correction Manoeuvres
• Corrections required from propulsion system to
  maintain /- 1km ground track
• Air Drag compensated with periodic in-plane
  SFCMs (every 30-50 days)
• Solar gravity compensated with out-of-plane OCMs
  (3 per year)

Ground track OCM correction
Ground track SFCM correction
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• Envisat Fuel consumption
• The 13 Out-of-plane OCMs have consumed 95 of
  the total hydrazine used
• SFCMs have used remainder (for air drag and
  twice for collision avoidance manoeuvres)
• Assuming nominal performance of S/C hydrazine
  depletion is the ultimate limit on life extension

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• Envisat OCMs
• Out-of plane OCMs require a 90deg slew (thruster
  controlled)
• They must be performed inside eclipse (including
  slews) ? ANX only
• The eclipse latitude changes with the seasons

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• Reducing Hydrazine Consumption
• Quality of Science data dictates /- 1km
  deadband
• Goal is to reduce hydrazine consumption whilst
  fulfilling this requirement
• A loss is associated to each manoeuvre (slew
  offset spread)

offset
spread
ANX
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• Reducing Hydrazine Consumption
• Slew Loss 0.7kg per manoeuvre (constant)
• Spread Loss increases with thrust duration (fn
  of upstream pressure)
• Offset Loss increases with offset of thrust
  centre wrt ANX

TA
Alpha Beta
ANX
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• Reducing Hydrazine Consumption
• Slew Loss dominates at beginning of life - leads
  to minimising the total no. manoeuvres / year
• /- 1km deadband dictates a minimum no.
  manoeuvres / year
• Dropping tank P ? dropping thrust ? manoeuvre
  size increase

Eclipse length limited
Deadband limited
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• Reducing Hydrazine Consumption
• Below a threshold hydrazine pressure ? incr.
  manoeuvre freq
• There is not enough time in the eclipse (for
  thrusting slews)
• Manoeuvre must be split increased slew losses
• But what if the slews were outside the eclipse?

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• Reducing Hydrazine Consumption
• Slews take 300s each ? possible extra 600s for
  the thrust phase to spread into
• Cumulative effect of all losses must be
  carefully assessed
• Graphs show losses (one and two burn, 2000m
  deadband solutions)
• Best / worst cases seasons (eclipse centre moves
  /- 23.5deg wrt ANX)

Thrust only in eclipse
Thrust and slews in eclipse
One burn no longer possible
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• Reducing Hydrazine Consumption
• One burn solutions generally better
• If slews are allowed outside eclipse ? reduced
  hydrazine consumption
• e.g. when eclipse is filled, at ANX one ? two
  burn (2km dV) needs 0.42kg
• Changing constraint saves fuel but can it be
  safely done?

Thrust only in eclipse
Thrust and slews in eclipse
One burn no longer possible
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• Slewing outside eclipse
• Solar illumination different during /- 90deg
  slew about Z
• Temperature change minimal (lt5 mins per slew)
• Solar illumination into sensitive optics
  assessed (FDIR surveillances)

Slew
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• Slewing outside eclipse
• e.g. Star tracker optics
• At eclipse exit x face exposed to sun
• minimum angle still gt47deg ? OK
• Similar analysis across spacecraft (e.g. Earth
  sensor, instruments) ? OK

Star Trackers
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• New strategy implemented January 2006 (2 so far)
• Projecting Forward ? extension to 2010 feasible

Hydrazine available onboard
today
optimal
Low freq failures
High freq failures
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Envisat Power Subsystem

• 7kw Solar Array
• 8 NiCd batteries
• RSJ Solar Array Shunt regulator, with voltage
  and current limited battery charging

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Envisat Battery Charge Control

• Batteries critical items (in LEO, fast charging
  and high no. cycles)
• Hardware charge control (temperature compensated
  voltage and current limit)
• battery charge software algorithm fine tunes
  k-factor (charge/discharge ratio)

Software commanded trickle charge
Hardware voltage limit
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Envisat Battery Charge Control

• Certain failures would stop software charge
  control
• Leads to increased k-factor ? overcharge
• battery overcharge dangerous ? build up of
  gasses ? severe cases battery loss
• Ground can only recover in pass (lt1 pass per
  orbit 10mins per 100min orbit)
• Review of Power FDIR.

Software commanded trickle charge
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CFS update of Power FDIR

• Goal ? reduce no. failure cases leading to
  battery charge algorithm disabling
• Concept of backup charge reference battery
  introduced
• An algorithm to automatically re-enable charge
  management
• No. failure cases leading to any overcharge
  reduced
• CFS update prepared, validated (at industry
  ESOC) ? operationally active 7/2005

Software commanded trickle charge
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Safe Mode Fast-track

• If satellite SFM occurs (so far in flight not),
  CFS not running
• Substantial ground intervention required ? long
  duration ? cumulative overcharge ? danger of
  battery damage
• SFM CRPs reviewed ? SFM Fast-track recovery
  created
• hardware configuration pre-selected based on
  highest probability of success
• Analysis of ground station coverage
• Sequences optimised to increase speed and
  robustness (using G V flags)
• Training of engineers and SPACONs
• Result ? recovery to battery charge algorithm
  activation reduced to 3-4 passes

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• Conclusions
• Review of Spacecraft carried out in view of
  mission extension
• 5 year design life expires March 2007
• Assuming good performance hydrazine limiting
  factor
• Orbit maintenance strategy reviewed losses
  evaluated
• Pre-flight constraint re-evaluated and changed
  efficiency boost
• Extension to 2010 foreseen
• Robustness of battery system to overcharge
  increased
• Failure cases leading to overcharge reduced by
  software upgrade
• SFM fast-track recovery created

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• Questions?

http//envisat.esa.int/