The SOHO Mission Halo Orbit Recovery from the Attitude Control Anomalies of 1998 Craig E' Roberts Co

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The SOHO Mission Halo Orbit Recovery from the
Attitude Control Anomalies of 1998Craig E.
RobertsComputer Sciences CorporationFlight
Dynamics FacilityGoddard Space Flight Center

• Libration Point Orbits and Applications
• 10 - 14 June 2002
• Parador dAiguablava
• Girona, Spain

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Contents

• Introduction to SOHO and its Halo Orbit
• SOHO Stationkeeping Technique
• June 1998 Anomaly and Recovery
• December 1998 Anomaly and Recovery
• Conclusion

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Solar Heliospheric Observatory (SOHO) Spacecraft
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SOHO Mission Overview

• Launched 3 December 1995 Sun-Earth/Moon L1 Halo
  Orbit Insertion (HOI) March 1996
• Joint ESA/NASA mission second NASA mission
  dedicated to Sun-Earth L1 halo orbit
• Purpose Sun and solar wind science
• 3-axis stabilized closed loop attitude system
  with momentum wheels, gyros (formerly), Sun
  sensors, FHSTs
• Steering laws keep S/C X-axis and instrument
  boresights Sun-pointing Z-axis nods to stay
  aligned with Suns spin axis over course of year
• Hydrazine blowdown propulsion for translational
  control, momentum management and backup attitude
  control
• Two fully redundant strings of 8 4.5 N thrusters
  per string

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SOHOs Halo View from NEP
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SOHOs Class 2 Halo Side View
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SOHOs Halo Looking Sunward from Earth
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SOHO Halo Orbit Stationkeeping Technique

• Single-axis control
• Thrust vectors aligned with S/C body X-axis
  (X-axis always Sun-pointed)
• Thrust/?V then parallel or anti-parallel to
  S/C-Sun line
• Upshot ?V basically along Earth-Sun line (GSE
  X-axis)
• Trajectory propagated to a candidate SK epoch ?V
  applied and differentially corrected as
  trajectory propagated toward a target goal (RLP
  VX 0) in subsequent RLP XZ-plane crossing
  repeated until two halo revolutions are achieved
• ?V toward the Sun increases orbital energy,
  preventing halo decay back toward Earth
• ?V away from Sun decreases orbital energy,
  preventing escape into free heliocentric orbit

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Stationkeeping Realities

• LPO correction costs increase exponentially from
  epoch of last impulse doubling constant ? 16
  days
• Burn performance deviations (errors) dominate
  orbit knowledge errors so, are biggest
  contribution to the magnitude of the next SK burn
• For SOHO, we prefer to burn well before
  correction cost has grown to 1.0 m/sec
• SOHO SK technique nominal performance 3 to 4
  burns per year 90 to 120 days apart 2 to 3
  m/sec/year
• However, SOHO attitude control realities intrude
• - Momentum dumps needed at intervals (3 to 4
  per year) dump residual ?Vs can be 2 to 8
  cm/sec along Sun line
• - SK schedule tied to momentum dump schedule
  so that the dumps residual ?Vs can be offset

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Stationkeeping Realities, Part 2 ESRs

• Worse than momentum dumps, frequent ( 3 to 4 per
  year) onboard attitude anomalies called Emergency
  Sun Re-acquisitions (ESRs) occur randomly and
  require thrusters
• ?V impact can be of order 1.0 m/sec before
  attitude stabilization restored (thrusters off/
  wheels back on)
• ?Vs from ESR events must be countered by special
  orbit recovery burns as soon as possibleusually
  within a day or twoafter the ESR
• ?Vs to recover from typical ESRs comparable in
  magnitude to normal SKs, i.e., lt 1.0 m/sec
• Recovery burn ?Vs opposite in direction to ESR
  ?Vs
• Although identical in maneuver technique, we
  distinguish between normal SK and post-ESR orbit
  recovery burns

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Post-ESR Recovery

• Meaningful post-ESR trajectory re-targeting
  requires that the ESR ?V first be modeled into
  the trajectory (cant wait for OD)
• Unfortunately, in SOHO ESR mode, the telemetry
  lacks usual burn-related parameters like thruster
  counts so ESRs cannot be reconstructed from
  propulsion data alone
• Fortunately, both the ESR timeframe and net ?V
  can be measured directly from DSN Doppler
  tracking data (actual ESR burn Doppler compared
  to no-burn predicted Doppler)
• ESR ?V then modeled into the trajectory as a
  finite burn event using trajectory design
  software (Swingby) model is adjusted until
  output radial delta-V matches Doppler
• Trajectory then propagated up to a candidate
  recovery burn epoch, and halo is re-targeted from
  that point

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The June 25, 1998 Disaster

• Problematic response to an ESR event by
  spacecraft controllers led to the spacecraft
  rolling off into a tumble, with a resulting loss
  of communications and power
• Out of contact for several weeks, many were
  fearing the mission lost.though there were
  reasons for hope
• Attitude simulations suggested that SOHO likely
  settled into a slow spin about its major axis of
  momentum, but with the solar panels roughly edge
  on to the Sun (depriving SOHO of power)
• Predicted that attitude geometry would be such
  that by mid-summer the panels would begin getting
  some Sun
• Radar skin contact was made via Arecibo/Goldstone
  test on July 23, verifying predictions of
  position in halo
• DSN successfully made brief radio contact on
  August 3

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Disasters Impact on Trajectory

• Doppler data covering the ESR prior to tumble was
  available but once tumble began station lock was
  lost, depriving us of tracking and telemetry
• Though there was intermittent dropout prior to
  complete loss of contact, Doppler indicated that
  a total of only several cm/sec was imparted prior
  to the tumble, but resulting in a net delta-V of
  just 1.4 cm/sec Sunward
• Doppler and some later analysis suggested a
  tail-off of the thrusting and a possible thruster
  shutdown not long after tumble began (but this
  was highly uncertain)
• So, the best guesstimate was that a mere 1.4
  cm/sec (net Sunward) was imparted to the orbit
  this was modeled into the reconstructed best
  estimated trajectory, which was used to support
  subsequent search operations

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Early Assessment

• Next 8 slides are a sampling of historical
  documents from the 1998 crisis they are
  hand-annotated screen snaps from a brief study of
  possible post-accident trajectories
• They show the Telemetry Dropout Case, or TDC
  (the trajectory as known at the point of loss of
  communication), trajectory as well as a narrow
  range of dispersions from that case
• Wildly different outcomes from such a narrow
  range of dispersions underscore the extreme
  sensitivities of LPOs to small perturbations
• The plots were used to impress on the SOHO
  Project that there was a very wide range of
  possible outcomes, and that hopes of recovering
  SOHO depended on detecting it, and restoring it
  to health, sooner rather than later

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Six Possible Escape Trajectories to Solar Orbit
for 1 cm/sec Dispersions (1 to 5 cm/sec)
from Telemetry Dropout Case, or TDC
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Post-Escape 11.6 Year Heliocentric Earth-Return
Trajectoryfor the TDC Case (Solar Rotating Frame)
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Escape Trajectories for other possible small
dispersionsfrom TDC ?1 cm/sec to TDC ?6 cm/sec
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First Dispersion Case (TDC 7 cm/sec) with
Fall-back to Earthwith Temporary Capture and
Eventual Escape
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TDC 8 cm/sec Dispersion Case
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TDC 9 cm/sec Dispersion case
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TDC 10 cm/sec Dispersion Case First Capture
into High-Energy, Long-Duration Chaotic Orbit
with Lunar Encounters
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TDC 11 cm/sec Dispersion Case
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SOHO Recovery 1

• Luckily and amazingly, the predicted trajectory
  turned out to be very close to truth, which led
  to the successful Arecibo contact
• Carefully, over 5 weeks, batteries were
  re-charged, the propulsion system was thawed, and
  communications were ramped up in a pre-planned
  manner
• OD operations resumed, though with spotty,
  poor-quality tracking data from intermittent
  contacts
• Though OD solutions were uncertain, they appeared
  to confirm that the June 25 anomaly could not
  have imparted more than 5 to 10 cm/sec, at most
• A planned 5-day, Sun-pointing attitude
  reacquisition procedure commenced on Sept. 16 and
  finished Sept. 22 however this involved roughly
  5 days of thrusting, imparting 3 m/sec more in
  the Sunward direction

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SOHO Recovery 1, Continued

• Thrusting effects from the September attitude
  reacquisition (done in ESR mode) were modeled
  into the trajectory, again relying on Doppler
  observations
• The first halo recovery ?V maneuver (6.2 m/sec)
  was performed on Sept. 25, correcting for both
  the original halo degradation and the excess
  energy imparted by the September ESR
• More or less normal operations resumed, and two
  more orbit recovery maneuvers in October and
  November fully restored the halo orbit
• SOHO systems came through it all in flying colors
  (science worked as well as, or better than,
  before), with the exception of two of the three
  gyros that were now dead
• Optimism was high going into December, yet the
  specter of just a single gyro remaining hung over
  the mission

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December 21, 1998 SOHO Disaster 2

• As we were preparing for MM and SK maneuvers on
  Dec. 21, the last gyro suddenly died
• SOHO plunged into another ESR, with no way of
  returning to a normal mode of Sun-pointing
  stabilization
• In ESR mode, the two Sunward jets that control
  yaw, pulse as much as 5 times more frequently
  than the two canted, anti-Sunward jets
  responsible for pitch
• This behavior imparts a net ?V in the Sunward
  direction, increasing energy of the orbit,
  inducing it toward escape
• Hence, we had a virtual continuous, low-level
  thrust situation on our hands, with no known end
  in sight
• Initially, Sunward ?V amounted to as much as 0.65
  m/sec imparted per day, using as much as 0.7 kg
  of fuel

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SOHO Disaster 2 (continued)

• Given the sensitivity of halo orbits and
  exponential behavior of correction, SOHO was in a
  very grave situation
• While the SOHO team sought ways to re-configure
  SOHO to operate without attitude thrusting or
  gyros, my job was to come up with a scheme to
  keep SOHO home at L1
• We faced the prospect of never getting out of ESR
  mode in that case, attitude thrusting would have
  exhausted the 186 kg of fuel then remaining in
  some 6 to 9 months (depending on assumed
  consumption rate)
• This estimate was made difficult by lack of
  thruster telemetry data
• Duty cycling had to be estimated via indirect
  means, and we could see from Doppler observations
  that thrusting varied
• Needed a way to model what was happening to the
  orbit

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Orbit/Thrusting Modeling Method

• Monitor/record R/T Doppler and measure net radial
  (station line-of-sight) delta-V over a given time
  period
• Model ESR events in Swingby as reconstructed
  finite burn events using B-branch yaw and pitch
  thrusters
• The yawpitch pulsing ratio was 51, or adjusted
  as needed to track observed radial delta-V
• Thrust duty cycles were inferred that would yield
  the radial delta-V actually observed
• ESR duty cycles were low compared to planned
  burns, but over 24 hours cumulative ?V could
  approach 0.65 m/sec
• This approach was applied in the early
  post-anomaly period as a way to update the
  pre-anomaly orbit the technique was then
  continued/extended over the course of the 40-day
  ESR to compute orbit updates

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ESR Delta-V Day by Day (from Doppler) Dec. 21
to Jan. 6
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ESR Cumulative Delta-V (from Doppler) Dec.21 to
Jan. 6
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Low Thrust Trajectory Evaluation

• Halo orbit was rapidly degrading escape into
  solar orbit was inevitable if intervention was
  not forthcoming
• Computed single-shot correction maneuvers for
  various dates (assuming constant-level ESR
  thrusting), but they would prove too large for
  crippled S/C to perform
• Point of no return reached by February, if
  nothing done at all
• Constant ESR duty-cycle modeling showed fuel
  exhaustion by May at earliest, August at latest,
  depending on assumed duty-cycle level
• Continuous fuel drain was definitely a concern,
  but of lesser priority than prompt intervention
  and recovery on behalf of halo orbit
• S/C engineers estimated that they might devise a
  way to escape ESR mode by approximately February
  1st, 1999

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Nominal Halo Shown with an Indefinite ESR Escape
Trajectory, and an Un-recovered One-day ESR Event
Trajectory for Comparison
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Indefinite ESR Escape Trajectory, and Guided
Continuous Thrust Trajectory through fuel
exhaustion and fallback with Earth Capture
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SOHO Recovery 2 Maneuvers

• Could correct with one or two big burns but it
  was deemed not advisable by S/C experts to do a
  ?V over 10 m/sec
• This meant corrections would need to begin in
  early January waiting much longer would make it
  difficult to recover, and before too long,
  impossible to recover
• Conceived of a series of approximately four
  burns, all under 10 m/sec, spread over January to
  counter the ESR and reduce orbital energy
• But new, open-loop style commands needed to be
  devised and implemented ultimately, regular
  orbit burn duty cycles were reduced from normal
  75 to just 5, making burns of given delta-V 15
  times longer to execute than formerly
• Final cleanup burn(s) could wait until S/C back
  in normal mode (attitude thrusters off, control
  returned to wheels)

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SOHO Recovery 2 Complicating Factors

• Halo recovery maneuvers faced numerous
  complicating
• factors, including but not limited to
• No thruster counts available, making thrust
  reconstruction possible only via indirect
  methods/estimations
• Usable OD was not attainable during the entire
  ESR period due to presence of effective
  continuous thrust
• Loss of gyros meant onboard software no longer
  viable for closed-loop maneuver control
  workaround methods improvised to implement
  delta-V maneuvers
• Perturbations from the ESR were unpredictably
  variable
• Continuous monitoring of the Doppler necessary to
  gauge ?accumulating ESR ?V and the spacecraft
  roll rate
• Correction maneuver schedules shifted frequently
  for a variety of reasons, making
  prediction/planning difficult

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SOHO Recovery Maneuver History
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Conclusion How Was It Done?

• Modeled the ESR event as a sequence of finite
  burns, updating the thrust rate piece-wise as the
  Doppler data accumulated model extended daily
  over the entire 40-day eventproviding us with
  the needed orbit updates
• Predicted trajectories were propagated in
  continuous low-thrust finite burn mode, assuming
  an average duty cycle
• A series of small (lt10 m/sec) correction
  maneuvers were placed at intervals over a month,
  with the final burn of the series used as the
  independent halo re-targeting variable
• The series of correction burns countered the
  cumulative ESR damage and offset the halo energy
  just enough such that continued ESR thrusting
  added the energy back at just the right rate to
  maintain a halo-like orbit
• The correction series was tuned as we went,
  such that at the end of the ESR there was only 15
  cm/sec error left