SOHO Status Report - October 2003
Since showing the first signs of degradation in early May 2003, the HGA Z-axis mechanism has become unusable in nominal mode. The root cause is still unknown, but the Z-axis mechanism can still be moved if necessary, using double motor windings (prime and redundant). The fixed Z-axis position introduces inkeyholel. periods, alternating between 18 and 25 days every three months, for downlink through regular 26-meter DSN stations. Larger 34 or 70-meter stations are required to fill these periods. The current Z-axis position minimises the duration of the keyhole periods when coupled with a 180 roll every 3 months.
DSN support of the SOHO mission can now be classified into three types based on the achievable telemetry downlink rates using the HGA or LGA (Low Gain Antenna) on 26, 34, or 70-meter stations:
- High rate telemetry on 26-meter stations through the HGA (9-10 weeks every three months).
- High rate telemetry on 34 or 70-meter stations through the HGA (first & last 4-6 days each keyhole)
- Medium rate telemetry on 34-meter stations, high rate telemetry on 70-meter stations, through the LGA (central 9-15 days of each keyhole period; recorder dumps require high rate telemetry).
The fall keyhole is from 22 September to 16 October. DSN coverage with 34/70 m stations for this keyhole is very good, with insignificant data loss expected. The outlook for the December/January and March/April keyholes is much worse, however, with severe competition for 70 m stations even between the different Mars missions. Science data loss seems inevitable. This is most serious for the long-term helioseismology experiments (GOLF, VIRGO, and MDI's medium-rate program). Industry (SES) has been contacted for a feasibility study and cost estimate for a Central OnBoard Sofware (COBS) patch involving a modified recording mode of only these critical low-bandwidth data during longer periods without possibilities for recorder dumps. In normal recording mode, the recorder fills in approximately 12 hours. In addition, ESOC has been contacted for a feasibility study and ROM cost estimate for using the new 35 m ESA station in New Norcia during future keyholes.
A planned 60-day MDI continuous contact period was decimated by the fall keyhole - reduced to about 30 days. Future keyholes through 2006 are being worked into the long-term DSN schedule, to move 60-day continuous contacts between keyholes.
On 8 July 2003, SOHO entered Emergency Sun Reacquisition mode (ESR 19) due to a spurious Fine Sun Pointing Attitude Anomaly Detector reading. The cause appears to be a Single Event Upset in the circuitry involved, similar to ESR 14 in January 2000. Later the same day, a planned station keeping, momentum management and 18º roll manoeuvre was performed. The next manoeuvre was scheduled to take place on 7 October 2003.
Operations and Archiving
All instruments are nominal and SOHO science operations are progressing smoothly.
SOHO was recently awarded the Laurels for Team Achievement Award of the International Academy of Astronautics (IAA). On 7 May 2003, MDI, EIT and CDS observed the Mercury transit. Although the observations were geared towards improving instrument characterisation, the event was widely observed by the media and the online community.