Physics Event Datagrams

Event data collected by the LAT Physics Acquisition (LPA) application during planned physics observations and unplanned GRB observations is transmitted to the science data partition on the SSR as CCSDS packets that wrap datagrams in a a custom format known as LAT Science Format (LSF).

An event is included in the data stream if any of the event data handlers active in the current data collection mode "accept" that event. For instance, if both a gamma filter and a heavy ion/CNO filter are active in PHYSICS mode, if the event passes the CNO filter but is rejected by the gamma filter, the event is still put into the data stream. For each event put into the data stream, FSW adds summary bits summarizing how each active handler evaluated that event. In our example, the event passed by the CNO filter would contain a summary explaining why the event was rejected by the gamma filter.

In addition to the event data itself, physics event data packets also contain "auxiliary data", which, at a minimum, consists of mission elapsed time and GEM clock counter data, and configuration keys (which can be used to identify the pedestal/gain, filter geometry, software configuration, and other configuration files that were utilized to support the collection and packaging of the event data). Finally, when data is requested with the highest level of statistical compression, the physics event data stream also contains the compression tables required to uncompress the data.

The FSW can be commanded to store physics data at various levels of compression. LPA may compress data, but it never summarizes or discards any data delivered by the hardware. Regardless of the format and compression level of the data they encapsulate, datagrams contain the complete set of event data assembled by the LAT Event Builder Module for events which have passed the filtering process.