The SOHO Mission Halo Orbit Recovery from the Attitude Control Anomalies of 1998 Craig E. Roberts Computer Sciences Corporation Flight Dynamics Facility Goddard Space Flight Center - PowerPoint PPT Presentation

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The SOHO Mission Halo Orbit Recovery from the Attitude Control Anomalies of 1998 Craig E. Roberts Computer Sciences Corporation Flight Dynamics Facility Goddard Space Flight Center


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Title: The SOHO Mission Halo Orbit Recovery from the Attitude Control Anomalies of 1998 Craig E. Roberts Computer Sciences Corporation Flight Dynamics Facility Goddard Space Flight Center

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

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

Solar Heliospheric Observatory (SOHO) Spacecraft
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
  • Two fully redundant strings of 8 4.5 N thrusters
    per string

SOHOs Halo View from NEP
SOHOs Class 2 Halo Side View
SOHOs Halo Looking Sunward from Earth
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
  • 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

Stationkeeping Realities
  • LPO correction costs increase exponentially from
    epoch of last impulse doubling constant ? 16
  • 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
  • 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

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
  • Although identical in maneuver technique, we
    distinguish between normal SK and post-ESR orbit
    recovery burns

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

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

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

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

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

SOHO Recovery 1, Continued
  • Thrusting effects from the September attitude
    reacquisition (done in ESR mode) were modeled
    into the trajectory, again relying on Doppler
  • 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

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
  • 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

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/Thrusting Modeling Method
  • Monitor/record R/T Doppler and measure net radial
    (station line-of-sight) delta-V over a given time
  • Model ESR events in Swingby as reconstructed
    finite burn events using B-branch yaw and pitch
  • 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

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

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

SOHO Recovery 2 Complicating Factors
  • Halo recovery maneuvers faced numerous
  • factors, including but not limited to
  • No thruster counts available, making thrust
    reconstruction possible only via indirect
  • 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
  • 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

SOHO Recovery Maneuver History
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