Operations
The Ground System
The data telemetered down from Fermi enters the Fermi ground system through the Mission Operations Center (MOC), staffed by the Flight Operations Team (FOT) and also hosted at GSFC. The MOC 'cleans up' the telemetry, monitors the spacecraft through the housekeeping portion of the telemetry, and transmits the appropriate telemetry to the other ground system elements. The MOC also sends commands to the spacecraft, particularly a weekly load of observing and operational commands based on timelines created by the instrument teams and the FSSC.
Each instrument team maintains an Instrument Operations Center. The LAT Instrument Science and Operations Center (LISOC) is located at SLAC in Palo Alto, CA, while the Fermi GBM operates the GBM Instrument Operations Center (GIOC) in Huntsville, AL. The Instrument Operations Centers receive the 'cleaned up' telemetry, monitor their detectors through the housekeeping portion of the telemetry, process the science data, and transmit the resulting science data products to the FSSC. The LAT science data processing is quite extensive, starting with event reconstruction from the 'hits' in different parts of the LAT and ending with a characterization of these events. The Instrument Operations Centers also serve data to members of their teams.
This model of the ground system is more decentralized than usual, with the instrument teams taking a more active role in the routine processing of the science data. Data processing occurs at each ground system element, leading to a straightforward set of data 'levels.' The MOC removes corrupted or duplicate telemetry packets, and ensures that the packets are in time order; the resulting 'cleaned up' telemetry is called Level 0 data. The Instrument Operations Centers process the instrument-dependent data, creating event lists; the data that leaves is called Level 1 data. Finally, processing by scientists using the Fermitools results in Level 2 data. Catalogs and compendia of Level 2 data are considered Level 3 products.
Observations
The FOVs of Fermi's two detectors are very large and therefore the concept of an 'observation' of a given source is not very meaningful. Useful LAT data can be taken over approximately 30% of the sky at any time (although the effective area will vary over the FOV), and therefore while data are accumulated for a given source, data are also accumulated for a large number of other sources. Survey mode is the predominate mode. The LAT alternates pointing 50 degrees (default) above or below the orbital plane; this observing mode provides uniform sky coverage every 2 orbits (~3 hours) with the equivalent of 30 minutes of on axis exposure. Pointed and modified survey observations must be justified. Even during these deviations from the standard survey mode, counts accumulated from one source will be interspersed with counts from many other sources.
Thus instead of thinking about the LAT data as consisting of a number of independent observations of discrete sources, the LAT data should be conceptualized as a continuous event list from the beginning of the mission to the end with counts from your source appearing with greater or lesser frequency depending on the LAT's effective area towards the source at a given time.
During the first 10 years of the mission, deviations from the standard sky-survey mode, pointed observations, or alternative survey strategies to enhance coverage of selected regions of sky, could be submitted by users. As a consequence of a transient phenomenon in the sky, the Fermi Project Scientist (or his/her designee) could declare a 'target of opportunity' (ToO) observation resulting in a pointed observation. Such ToO observations could result from a Guest Investigator (GI) notifying the Project Scientist that the criteria for a ToO observation in a successful GI proposal had been met or a request from the community. These requests for a ToO observation, whether or not previously proposed, were submitted through a website. The data accumulated during a ToO observation was not proprietary to the requester.
Due to an issue with the
-Y SADA array, ToO observations are now selected by the Project Scientist directly.
Data Latency
In practice, LAT data latency is about 12 hours. Data from the Fermi GBM and the LAT are stored in the spacecraft's Solid State Recorder (SSR) as the data are taken; the instruments do not store these data internally. While the system specifications permit 72 hours from the taking of source data until data are available from the FSSC, the actual data latency is typically much less. Although the SSR can hold up to 30 hours of data, the data is downlinked from the spacecraft through a TDRSS satellite 6-7 times per day on the Ku band. Thus in normal operations the data should reside on the spacecraft for only ~3 hours before being downlinked. While the LAT instrument team is permitted up to 24 hours to process the data, the data pipeline is designed to process the data from a given downlink before the data from the next downlink arrives. Similarly, while the FSSC is permitted 24 hours to ingest data and make it available, the FSSC plans to complete processing the data from one data transfer before the next such transfer.
The GBM team processes their data after every downlink. Burst data is transferred to the FSSC as soon as processing is completed. Daily data products are packaged into one hour units, allowing for a shorter latency than routine daily data. Prompt alerts are issued for each recorded gamma-ray burst via NASA's GCN network .