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,Libration Point Orbits and Applications 10 - 14 June 2002 Parador dAiguablava Girona, Spain,Contents,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 control 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 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,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,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., 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,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,