Press Release

SOHO is in Coarse Roll Pointing mode

By SpaceRef Editor
January 10, 2000
Filed under

January 10, 2000

SOHO is progressing smoothly with operations geared towards a return to Normal Mode late this evening or early tomorrow.

The reason for the entry into Emergency Sun Reacquisition (ESR) mode last Friday remains under investigation. The status of the spacecraft appeared to be
nominal right up until the time of the ESR trigger, and all operational systems appear to function flawlessly as the recovery proceeds.

The spacecraft has now been returned to its nominal roll position, aligned with the solar rotational axis. This translates into about -6 degrees in ecliptic
coordinates. Momentum managements and a station keeping (orbit trim) manoeuvre will be performed before returning to Normal Mode.

January 7, 2000, 21:20 UT

SOHO transitioned into Coarse Roll Pointing (CRP) mode at 21:20 UT, ending a period just short of 21 hours in Emergency Sun Reacquisition mode.

The current roll position is about -24 degrees, and the switch to the High Gain Antenna for medium rate telemetry occurred at 01:03 UT. The nominal FPSS
offset has been applied. The offpointing flag has been enabled for MDI.

High rate telemetry was achieved at 01:28 UT. NRT was enabled at 02:02 UT. NRT was disabled at 03:29 UT after allowing MDI, LASCO, EIT and UVCS
to finish their commanding.

Plans for the weekend is to leave the spacecraft in CRP until Monday morning, when further actions will be decided upon. Tentative plans for Monday
includes momentum management, roll maneuver, momentum management, station keeping, momentum management. The first momentum management may
not be required due to the small roll offset, and plans may of course change.

January 7, 2000

As of Friday 7 January 2000 at 00:28 UT, SOHO is in Emergency Sun Reacquisition (ESR) mode. Since the entry into ESR happened during a scheduled
gap in the contact with the spacecraft, the event was not detected until a while later. A spacecraft emergency was declared at 02:16 UT, to ensure adequate Deep
Space Network (DSN) coverage with 34-meter antennas.

Initial operations were successfully completed during the night, including some instrument safing activities that are not performed automatically.

Operations toward a recovery from ESR mode are progressing today, as is the investigation into the cause of the ESR trigger.

In ESR mode, attitude control is accomplished onboard by the Attitude and Orbit Control Subsystem (AOCS) firing thrusters automatically. Roll control is
achieved by sending thruster commands from the ground.

When in ESR, the spacecraft uses the Low Gain Antenna (LGA) to broadcast low-rate engineering data only. This antenna is not sensitive to the roll and
offpointing excursions that normally occur under ESR conditions, because it has a wider beam than the High Gain Antenna. However, it provides a weaker
signal to the ground stations, requiring 34-meter dishes to reliably pick up the signal.

The High Gain Antenna, which provides a higher data rate, cannot be used until the spacecraft enters the more stable Coarse Roll Pointing mode.

SpaceRef staff editor.