TITLE: GCN CIRCULAR NUMBER: 6946 SUBJECT: Swift autonomous slewing re-enabled DATE: 07/10/19 21:31:28 GMT FROM: Neil Gehrels at GSFC Neil Gehrels (NASA-GSFC) reports on behalf of the Swift team: This is an update on Swift status following earlier Circulars (Gehrels 6760; Barthelmy et al. 6781; Burrows et al. 6791; Gehrels 6825). As of today, Swift is re-enabled for full-time autonomous GRB slewing. The gyro problems of the past 2 months are resolved and the observatory is back to slewing accurately within its 3 arcmin requirement. BAT and XRT continue to be fully operational and performing science, as they have been for several weeks. UVOT is in engineering mode, and will be activated for science observations in the middle of next week. The TOO web site is back on line. More details on the problem and solution are as follows: On August 10 gyro #3 (known as Two Axis Rate Assembly - TARA 3) exhibited jumps in its bias offset level (voltage output when gyro is not rotating) that caused the spacecraft to go into safe hold. Redundant TARA 2 was switched in for recovery. It was then found that TARA 2 is slightly misaligned from TARA 1, with the consequence that slew accuracy was degraded. Due to memory limitations of the onboard spacecraft computer, assumptions were made in the matrix manipulation mathematics that did not permit a simple correction for the misalignment. However, a clever technique was found by Craig Markwardt to use available parameters to combine signals from all 3 gyros for accurate slewing. This mode of operation, which is now the baseline, is called TARA 123 and uses TARA 3 to correct the TARA 1 and 2 alignments. The TARA 3 term comes in at low weight and, as such, is not sensitive to its small bias shifts. A more elaborate software patch has been developed that could be used in the event of a failure of TARA 3.