SOHO ESR ends
30 November 1999As of Monday 29 November, 18:35 UT, SOHO is no longer in Emergency SunReacquisition (ESR) mode. The SOHO spacecraft emergency that was declaredto ensure adequate Deep Space Network (DSN) coverage has officiallyended. Instruments are in safe configuration and appear to beunscathed from the episode.
Spacecraft recovery is proceeding with maneuvres to compensate for a stay of about 21 hours in ESR mode, which has perturbed the angular momentum, orbit and roll of the spacecraft.
Planned activities for 30 November and 1 December include several momentum management segments, station keeping (orbit trims), and roll maneuvres to return SOHO to its nominal configuration. Instrument recovery operations will take place after SOHO returns to Normal Mode.
The Emergency Sun Reacquisition (ESR) mode
ESR mode is a 'safe hold mode' or a 'safety net' configuration entered autonomously by the spacecraft in the event of anomalies. It is a hard-wired analogue control mode that is part of the Failure Detection Electronics (FDE). Unlike the other control modes, it is not under the control of the Attitude Control Unit (ACU) computer. Thrusters are used in ESR to control the spacecraft attitude. Once the spacecraft has entered the ESR mode, a recovery sequence must be commanded and executed under ground operator control to proceed to the 'normal mode' from which science observations are made. The first step in this recovery sequence involves the use of the ISA mode, in which the ACU computer takes over the commanding of the thrusters to point the spacecraft towards the Sun using the onboard Sun acquisition sensor SAS-1.