SOHO in CRP mode until 8 December
06 Dec 1999SOHO is in Coarse Roll Pointing (CRP) mode at a roll angle of -76 degrees. The current plan is to leave the spacecraft in that configuration until Wednesday 8 December. Some instruments (MDI, EIT, LASCO and SWAN) are performing observations with some precautionary procedures (so-called 'safing flags') in place.
The initial understanding of the events experienced on Wednesday 1 December have been confirmed by reproducing the events on the spacecraft simulator at ESTEC.
The loss of roll control in CRP was intimately linked to the timing of this particular CRP mode trigger. A telemetry buffer pointer was not reset, causing memory outside the buffer space to be zeroed as the pointer moved out of bounds.
Whenever CRP mode is invoked by normal commanding and verified to be functioning correctly after initialization, it should keep on functioning as expected.
The decision to leave the spacecraft in CRP mode until Wednesday 8 December is based on the fact that staying in CRP mode gives the lowest probability of a repeat of the repeat of the events of last week.
The current plan is to have a fully tested patch available on Wednesday, for uplink prior to the recovery maneuvres. This long-term solution will prevent the memory from being zeroed in case of a similar timing problem.