1、Lessons Learned Entry: 0405Lesson Info:a71 Lesson Number: 0405a71 Lesson Date: 1996-04-26a71 Submitting Organization: JPLa71 Submitted by: J.A. RobertsSubject: Stepping Commands Cause Positioning Problems (1970/76) Abstract: Unlike explicit commands, stepping or incremental commands, however, simply
2、 cause the device to move by the commanded increment from its current position to the new position. If the current position is incorrect, the next position and all subsequent positions will be incorrect. The use of explicit commands is recommended whenever no significant operational advantage exists
3、 using incremental commands. Should an incremental command capability be desired for other reasons, the system design should consider also implementing the explicit command capability to ensure fault tolerance.Description of Driving Event: (Relevant Historical Lessons Learned)Mariner (M)69 and Vikin
4、g Orbiter (VO)75 had problems because the designs used stepping commands rather than explicit commands. Explicit commands cause a device (such as the scan platform) to move directly to the desired state or position, regardless of its current position. Stepping or incremental commands, however, simpl
5、y cause the device to move by the commanded increment from its current position to the new position. If the current position is incorrect, the next position and all subsequent positions will be incorrect.When a power problem occurred as M69 was approaching its Mars fly-by, the scan platform moved in
6、advertently and the engineering telemetry needed to determine the platforms position was destroyed. The platform then had to be commanded by trial-and-error until Mars appeared in the returning pictures. Had this search not been successful in the short time remaining until Mars encounter, the entire
7、 science sequence would have been irretrievably lost.Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-A less serious problem occurred on VO-1 when the position of the camera filter wheel got out of step with the picture-taking sequence. Once out of st
8、ep, the several frames subsequently exposed were lost due to over/under-exposure. Correct exposure had to be restored by ground command.Additional Keyword(s): SoftwareReference(s):a71 M69 P/FR Nos. 204634, 204660a71 VO75 VISA # 1450 (IOM 3623-BB-167)Lesson(s) Learned: All commands, whether stepping
9、or explicit, have within them a potential for error. However, since explicit commands do not depend on the success of previous commands to be successful, they tend to be “fault-tolerant,“ in the sense that they do not perpetuate errors in the command sequence.Recommendation(s): The use of explicit c
10、ommands is recommended whenever no significant operational advantage exists using incremental commands. Should an incremental command capability be desired for other reasons, the system design should consider also implementing the explicit command capability to ensure fault tolerance.Evidence of Rec
11、urrence Control Effectiveness: N/ADocuments Related to Lesson: N/AMission Directorate(s): N/AAdditional Key Phrase(s): a71 Flight Equipmenta71 SpacecraftProvided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Additional Info: Approval Info: a71 Approval Date: 1995-10-16a71 Approval Name: Carol Dumaina71 Approval Organization: JPLa71 Approval Phone Number: 818-354-8242Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
