GP-B Post-Flight Analysis—Final Report - Gravity Probe B - Stanford ...

Table 8-4. SCRs Addressed in On-orbit S/W Change3243 6/4/2004 Modified thrusterdistribution algorithm3244 6/11/2004 Modification to nulldumping scheme3245 6/11/2004 Mode 1B bias estimateupdate3246 6/11/2004 Update hard-codedstar sensor db3247 6/11/2004 Modifications to dragfreesafemode test3248 6/11/2004 Modify loss of dragfreesafe mode macroDue to 2nd thruster failure, and the possibility of losingadditional thrusters, modify the MSS to include GeometricLogic algorithm to mitigate this problem. Provide commandsand data base readout capability to determine theconfiguration. This capability will co-exist with the currentpseudo-inverse approach, and a control flag which can beverified by DBRO will be used to select which approach to use.The default will be the Geometric Logic approach.See attachmentThe null dumping scheme needs to be modified due to thethruster distribution change. Ref. SC3243During Mode 1B operations it was discovered that the gyro biasestimate was only correct for one vehicle axis. Modify the codeto make it correct for x and y axes.The star sensor data base has the following hard-codedparameters (Jon-TBD)Star_Mag_Limit_Comp(1)=3.0Star_Mag_Limit_Cat(1)=3.0It was determined from flight experience that system efficiencyis increased if they are updated. A CSTOL patch was used toupdate them via release message, but with MSS3.4.3 the CSTOLwill no longer have valid addresses.Stanford has requested two new capabilities1) In primary drag-free mode, provide a return to drag-freeoption for 3 attempts in the event drag-free is dropped.2) In backup drag-free, stop translation control in the event ofreturn to analogIn a requirements review meeting held on Thursday, 6/10/04, itwas decided that modifying this safemode test was the favoredapproach. The test will check the following, and each check canbe independently enabled or disabled by command, withDBRO verification:1) RSS of error above a limit2) Arbiter state - analog3) DF_state (not=) DF_cmdThere will also be macro changes as specified in SCR3248 and3249As part of SCR3247, the loss of drag-free safe mode macroneeds to be modified to accommodate the options specified inSCR3247.[6/14/04 O. Chan] The safing responses for drag-free Test asbelow:SMACRO07 to mask for GSS arbiter state checkSMACRO29 to mask for drag-free data limit and status checks[7/22/04 O.Chan] No further implementation on Automaticallyreentry to primary drag-free.244 March 2007 Chapter 8 — Other Spacecraft Subsystems Analyses