The Effect of a Stale Earth Pole Vector on Navigation Errors for the Space Shuttle

Russell L. Strachan

Abstract:	This paper presents the analysis and results of an investigation into navigation errors of the Space Shuttle ascent state vector that exceeded the 1-sigma criteria. It describes the interim changes, permanent solution, and verification that resolved this condition. This problem was first noted in the Shuttle Avionics Integration Laboratory (SAIL) in January 1991. These observed errors are a comparison between the navigation/guidance system math model and the Orbiter FSW. Primary Avionics Shuttle Software (PASS) for STS-48 was in use during a Return-to-Launch-Site (RTLS) flight scenario. The down range position (V) error exceeded the Main Engine Cutoff (MECO) l-sigma limit (800 feet). Repeating the RTLS with Backup Flight Software (BFS), the downrange error was exceeded. The Navigation Initialization UVW velocities at T-l 1 seconds were noted to be higher than expected (~0.1 FPS). The earth-pole vector was found to be a constant in the ascent mass memory defined for an epoch in January 1980 (pre STS-1). It had not been updated even though the location of the earths rotation axis is not inertially fixed. Earth pole and earths rotation rate are used to account for the effect of the earth’s angular velocity. Since earth pole was stale, meaning that it was pointing in the wrong direction, an error was introduced. This error compromised the ability to discriminate potential real navigaton software errors. A FSW patch was developed that set the earth pole and earth-pole-rate in the PASS Mass Memory since they correspond to the RNP matrix third row elements. PASS and BFS results were notedly improved for all flight senarios. This Earth Pole Initialization (EPI) method was originally intended as an interim solution until onboard RNP computation was available. However, after a cost/thousand-line-of-code (KLOC) assessment, the EPI method was deemed sufficient as a permanent fix.
Published in:	Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994) June 6 - 8, 1994 Antlers Doubletree Hotel Colorado Springs, CO
Pages:	501 - 509
Cite this article:	Strachan, Russell L., "The Effect of a Stale Earth Pole Vector on Navigation Errors for the Space Shuttle," Proceedings of the 50th Annual Meeting of The Institute of Navigation (1994), Colorado Springs, CO, June 1994, pp. 501-509.
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