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Meeting	ACGS Committee Meeting 101 - Salt Lake City - March 2008
Agenda Location	9 SUBCOMMITTEE B - MISSILES AND SPACE 9.2 An Overview of the PredGuid Skip Entry Algorithm
Title	An Overview of the PredGuid Skip Entry Algorithm
Presenter	Gregg Barton
Affiliation	Draper Lab
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	The lunar missions to be performed by the Crew Exploration Vehicle (CEV) must be able to return from the Moon any time and still be able to land accurately either at designated continental US sites or coastal US water locations. However, the reentry range needed to enable this capability cannot be realized with direct entry, but requires instead a skip entry. While the Apollo vehicle was capable of skip entry, this capability was never applied in flight both because of availability of Naval recovery resources in the Pacific for Apollo, and because landings with Apollo skip guidance were expected to be very inaccurate (with uncertainty of hundreds of miles). This is because the Apollo guidance algorithm was unable to adaptively correct its skip trajectory during its entry sequence to statistical variations in vehicle dynamics and naturally varying atmospheric conditions. Draper Laboratory has since developed a Predictor-Corrector guidance algorithm known as PredGuid which overcomes the Apollo skip trajectory limitations. Using PredGuid, accurate skip entry trajectory landings in or near the US are enabled for any CEV lunar return scenario. This presentation reviews the lunar return mission, the guidance phases utilized by PredGuid, and the differences in those phases from those applied by Apollo.