Monday 12 June 2000 8.30am EDT
|Last week was an eventful week for the Chandra Team.
1. Command Timing Overlap
During the 8am EDT pass on 6/5 the spacecraft was not in the configuration expected from the daily load. The reason was traced to a timing overlap between the completion of one command and the start of the next. The first command (which issued a set of ACIS commands) was completed after the absolute start time of the next command (an HRC door open command), and resulted in the OBC waiting in a loop for the correct time to occur - effectively waiting for many days. The event occurred at 3:02am EDT on 6/5. After assessing nominal spacecraft health, SCS 107 was run in real-time to stop the loads and safe the Science Instruments. Real-time procedures were run to re-enable momentum monitors, to enable momentum auto-unloading, and to re-open the HRC door to the partially closed position. The daily loads were rebuilt to continue science operations beginning at 11:59pm EDT on 6/5. A series of calibration observations of PSR B0540-69 were impacted by the event, and will be rescheduled in an upcoming load. Checks have been implemented to detect and avoid similar timing overlaps in future loads, and modifications are being assessed to the ground software.
2. High Radiation Trigger
At 5:41 EDT on 6/8 the ACIS high radiation flag in the Flight Software Radiation Monitor was set to TRUE, resulting in activation of SCS 107 to safe the science instruments. This was due to high counts for protons in the EPHIN P4GM channel, exceeding the threshold of 300 counts/cm^2/s/sr (the channel reached 460). The high proton counts occurred following passage of an interplanetary shock wave due to a halo coronal mass ejection associated with two X-class solar flares on 6 June. SCS 107 ran nominally and all safing actions were completed with the exception of the HETG retraction. The HETG was not retracted because the MCE-B patch has not yet been uplinked - this is planned for next week. This was the first occurrence of an SI safing action due to high radiation environment for the mission.
The observations impacted by the trigger were MCG-6-30-15 and X0918-54 which will be rescheduled in an upcoming load. The daily load was rebuilt to restart at 8.30am EDT on 7/9 to continue with the observation of RXTE J1550-564 (TOO). Radiation levels had fallen to acceptable levels and science operations were resumed.
3. Real time procedures.
In addition to the procedures above, a routine update of ephemeris coefficients was completed on 6/6, and the fifth Squeegee mode test was run on 6/9. This Squeege test, conducted at 7pm EDT on 6/9, tested an approach that combined on-chip summing, slower that normal parallel transfer time, and a 24 row exclude window.
Note that analysis indicates that the likely cause of the auto-unloading of momentum reported last week was a difference between the predicted and actual solar torque pressure at low Sun pitch angles, rather than higher than predicted gravity gradients at perigee.
The planned observations for the coming week are as follows.
PSS 1443+2724 ACIS-S Jun 11 NGC 4472 ACIS-S M82 ACIS-I Jun 12 Radiation Belts Serpens X-1 ACIS-S/HETG Jun 13 1812-12 ACIS-S 3C 273 ACIS-S/LETG Jun 14 3C 273 ACIS-S/HETG PSS 1317+3531 ACIS-S PSS 1057+4555 ACIS-S NGC 1399 ACIS-S/HETG Radiation Belts Jun 15 Circinus Galaxy ACIS-S/HETG NGC 1399 ACIS-S/HETG Jun 16 ARK 564 ACIS-S/HETG Jun 17 Radiation Belts TZ Crb ACIS-S/HETG Jun 18
All spacecraft subsystems continued to operate nominally.
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