SOHO Enters – and Emerges From – ESR (emergency sun reacquisition)

From SOHO’s Whats New page

January 16, 2001

SOHO is in Normal Mode as of 20:49 UT. The last thruster activity was
completed at about 19:50 UT. Some instrument checks and recovery
activities were also performed today, but most of the instruments that
were in safe mode because of the ESR will be returned to observation mode
tomorrow or Thursday.

January 15, 2001

The FOT and spacecraft engineers brought SOHO back into coarse roll
pointing (CRP) mode at 17:49 UT on Monday, 2001
January 15, and obtained star maps with the star sensor unit (SSU) that
allowed them to determine the spacecraft roll orientation
(-55.89 deg). With an accurate roll, a high gain antenna (HGA) pointing table
was assembled, and medium-rate telemetry
re-established through the HGA.

After a brief period of instrument commanding, the FOT will:

• Transition the spacecraft to reaction motor wheels (RMW) mode
• Perform a momentum management maneuver
• Transition to CRP
• Roll back to nominal.

The spacecraft should remain in CRP overnight.

January 14, 2001

SOHO has been in ESR (emergency sun reacquisition) mode since
Sunday, 2001 January 14 at approximately 21:02:02 UT. The proximate
cause of the ESR was an attitude control unit (ACU) reset. Flight
Operations Team personnel are in communication with the
spacecraft, and are gathering information on the status of the ACU at
the time of the reset before a decision can be made on when to return
to Coarse Roll Pointing (CRP) and normal mode.

The cause of the attitude control unit (ACU) reset remains unknown after
inspecting the telemetry values prior to the reset, and there are no
indications of energetic particle events causing the upset. The present
roll rate is +0.2 deg/sec. A status meeting at 00:15 UT decided to leave
the spacecraft in ESR mode for the night and start the recovery tomorrow
morning. Instrument safing activities were completed at about 00:45 UT.