ABSTRACT

TITLE:	Reasoning About and Scheduling Linked HST Observations with SPIKE

AUTHORS: Laurence Kramer (STScI), Mark Giuliano (STScI)

TEXT:	Scientific observations executed by the Hubble Space Telescope 
	(HST) are subject to a number of complex and interacting timing 
	and orientation constraints. We classify these constraints as 
	"absolute" and "relative." The former apply to only one observation, 
	whereas the latter apply to two or more linked observations or 
	"visits." Examples of absolute constraints are "Schedule Visit A 
	BETWEEN Day 10 	and Day 20" or "Schedule Visit A at an 
	ORIENT in the range 30 to 40 degrees." Relative constraints 
	express either how the visits should be linked in time (for example 
	"Schedule Visit2 AFTER Visit1 by 10 days" or linked in orientation
	(for example "Schedule Visit2 ORIENT FROM Visit1 by 20 degrees").

	This paper will focus on the implementation of relative constraints,
 	how they are combined with absolute constraints, and the problems
 	encountered and solved in combining relative constraints
	along the orthogonal dimensions of time and orientation.

	We also briefly present the SPIKE Plan Window Scheduler [1],
	which creates a Long Range Plan by assigning "plan windows" where the
	visits might feasibly be scheduled, and which optimizes the
 	placement of windows to satisfy a number of criteria. 

	Since the assigned plan windows are much larger (nominally eight weeks) 
	than the actual times needed to schedule the visits, we confront
	the problem of "link set flexibility." 	This implementation allows
	 a great deal of latitude for a short term scheduler to later 
	assign actual spacecraft execution times to visits. We also describe 
	an enhancement of the Plan Window Scheduler to schedule orientation
	angles along with plan windows for orient linked visits, while 
	confronting computational constraints.

[1] "Achieving Stable Observing Schedules in an Unstable World", Mark Giuliano,
    Proceedings of ADASS '97, Sonthofen, Germany, in press.