Payload Scheduling with Dynamic (Re)Tasking

Tasking Information for Spring 2006 UCD Math Clinic

Dynamic Scheduling of Prioritized Tasks Tasking and Allocation information using commercial remote sensing satellite tasking as a real-world example

Task Data A task represents a user’s request for the satellite to perform some activity. In the context of the 2006 Math Clinic, a task represents a request to perform remote sensing of the Earth (e.g. to take a picture). A more accurate term for task is Collection Requirement (CR). In some systems, the CR is the external request and the Task is the translation of the collection requirement into a software object that the mission scheduling system can interpret. For our purposes the two terms can be used interchangeably.

The format for a task will be an ASCII text file in XML format. There are freeware libraries that support XML parsing, such as Xerces (for both Java and C++, at http://xml.apache.org) or xmltools.m (for Matlab Release R13, at http://mathworks.com/matlabcentral/ under the File Exchange link do a search for “xml parsing”). XML parsing can also be done natively within Matlab if you know JAXP.

XML is a text format that holds elements and attributes. An element is defined with angle brackets () to indicate the start of the definition and angle brackets with a preceding slash () to indicate the end of the definition. Elements can have attributes and contain child elements. For example, This is text for this child element. All XML documents start with a special element, with at least one special version attribute. You don’t have to worry about this, just know that it needs to be there to be considered a valid XML document. It looks like this,

For our XML tasking document, it will have this basic structure, Obviously there will be one or more CollectionRequirement elements as child elements to the CollectionRequirements root element. All attributes are quoted strings. You can ignore the timeCritical, quality and requester attributes. The duration attribute will always be a fixed value of 5 for all CRs and its

1 Tasking Information for Spring 2006 UCD Math Clinic units are in seconds. The priority is an integer (0 – 999), and latitude and longitude are units of degrees (expressed as a float or double). The strategy is either simple or monitor. If the strategy is monitor, then the sensor must periodically revisit the same target. All the monitor collections must be within the preferred window (hard constraint) and the min/max revisit values will be greater than zero. The minRevisit attribute will specify the minimum number of collections that must take place on the target, and the maxRevisit specifies the maximum number of collections that could take place on the target. Scheduling more than the minRevisit and less or equal to the maxRevisit is a bonus (objective function incentive). Scheduling more monitor collections than the maxRevisit is a soft constraint violation (a waste of the resource). The rule of thumb for monitor tasks is that they should be evenly distributed across the preferred (surveillance) window.

Soft Constraint Violation

Preferred Interval Effective Interval Simple Task

Hard Constraint Violation

Preferred Interval Effective Interval Monitor Task

2 Tasking Information for Spring 2006 UCD Math Clinic

Allocation Data To schedule a task means that you have “allocated” that task to a resource, or that you have created an “allocation”. The allocations you produce, that represent your schedule, should have the following format: ID a string that identifies this allocation TaskID a string that references the associated task ID State a single digit (0-2) that indicates NEW/MODIFY/DELETE StartTime a date + time, down to the second EndTime a date + time, down to the second Your schedule could be held in memory, a flat file or a database. If it is held in memory for some number of runs, then there should be a facility to eventually persist the schedule to a flat file or database for post analysis.