How is data acquired by the spacecraft stored in the central computer and prioritized for return to Earth?
All Pathfinder downlink data are packetized and assigned to APID (“Application Process IDentifier”) queues, from which data is downlinked in FIFO (first in first out) order. The packetization and assignment takes place immediately as a result of execution of commands which acquire data — separate commands to packetize and enqueue the data are not used. APIDs can be configured as rings, where old data is overlayed, or as queues, where new data is rejected if the size limit is exceeded. APIDs are identified by both name and number (0-42). Specific data formats are permanently assigned to specific classes of queues. For instance, one queue is for rover health data, and only data of that format can be assigned to it. There are multiple queues for IMP image data. Any IMP image can be assigned to any of these IMP queues, and within limits, the assignment is negotiable. Within a single downlink session, APIDs are prioritized according to a two-dimensional priority matrix called a DPT (“Downl
