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Meeting	ACGS Committee Meeting 94 - Reno - November 2004
Agenda Location	9 SUBCOMMITTEE D – Dynamics, Computations and Analysis 9.2 Station Keeping Control of the High Altitude Airship
Title	Station Keeping Control of the High Altitude Airship
Presenter	Tom Myers
Affiliation	Systems Technology, Inc
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	The Missile Defense Agency (MDA) High Altitude Airship (HAA) will be a large superpressure airship that will maintain station at about 70,000 ft. over a fixed point on earth. Limited thruster power presents problems for station-keeping in high winds. STI has developed an approach to station keeping in the “Post Thrust Limit” (PTL) regime where the winds exceed the headway that can be made under the limited thrust available. This builds on shear direction control (SDC) concepts developed for balloons. PTL SDC for the HAA involves exploiting the superpressure hull to control buoyancy and thus altitude. The basic control concept is to adjust the altitude such that the wind tends to convect the HAA toward its nominal station. The basic control law developed causes the HAA to perform a helical orbit. The basic closed loop system involves the action of a nonlinear “helical attractor” which is similar to a limit cycle. A refinement to the basic design enforces altitude limits that produce more complex trajectories and suggest the possibility of chaotic dynamics. The initial assessment of PTL SDC indicates that even at low values of wind shear it can at least reduce drift of the HAA in high wind events.