Prompt processing of Level 0 data through to Level 1 event quantities
Providing near real time monitoring information to the IOC
Monitoring and updating instrument calibrations
Creating high level science products from Level 1 for the PI team
Reprocessing of instrument data
Providing access to event and photon data for higher level data analysis
Performing bulk production of Monte Carlo simulations
Interfacing with mirror PI team site(s) (sharing data and algorithms)
Interfacing with the SSC (sharing data and algorithms)
Supporting Engineering Model and Calibration tests
Supporting the collaboration for the use of these tools
The IOC and Data Processing Facility are co-located at SLAC. The SSC will be located at Goddard Space Flight Center.
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Click on the left-hand image to see the flow of data from the spacecraft to the scientist. The right-hand image shows the data being processed from raw through to photons in the Reconstruction and Monte Carlo paths.
There are three major components: instrument simulation and event reconstruction; an operations facility to do automated event processing; and tools to perform higher level analyses on the processed data. In addition, there are utility elements for access to data, low level analysis tools and support of the user community.
Event reconstruction takes raw data, uses calibration constants to convert to physical units and suppress bad readouts, and performs pattern recognition and fitting procedures to interpret the data in terms of interacting particles in the instrument and to determine their identity, direction and energy as well as possible. There is a tradeoff between photon efficiency and purity in this process as selection criteria are applied to the events to suppress background.
Simulation is an alternate source of raw data for the reconstruction. It takes models of the fluxes of particles incident on the instrument, coupled with a physical model of the device and performs Monte Carlo simulation of the passage of the particles through the materials. The output of this simulation is then formatted as raw data, in the same form as seen from the real instrument. This tool provides the ability to assess the expected performance of GLAST, develop algorithms in a controlled environment, and to estimate efficiencies and purities of the final products.
During routine flight mode, there will be regular downloads of data from the instrument to the ground station and thence to the IOC. The ops component of SAS will take Level 0 data (raw data corrected for transmission effects) from the IOC and automatically perform the event reconstruction on it.
The facility will also tag and make available, appropriate events for performing calibrations. It will keep track of high level diagnostics (eg correlated performance of instrument subsystems that are not available from the raw data alone) and make them available to the IOC Operations staff for monitoring purposes.
This facility will also be able to produce bulk simulations.
All data coming and out of the facility will be catalogued and made available to the SSC.
The collaboration is responsible for a number of high level data products. The table below summarizes the science data products higher than level 0 that the SAS will produce. Several of the data products are databases of one sort or another, primarily because they are large datasets that will need to be accessed in ways other than time order. The exact list of products is still under negotiation. These products will be generated in a coherent way from the Level 1 data output from reconstruction.
| Data Product | Description |
| Instrument response functions | Effective area, energy resolution, energy redistribution, and point-spread function for all gamma-ray event types |
| Timeline (as observed) | Observing mode and spacecraft position & orientation as a function of time. Command and performance states. |
| Exposure history | Detailed timeline that includes live-time and coverage information for a grid on the sky, for rapid generation of exposure maps |
| Source catalog | Positions, fluxes, and uncertainties for all detected sources in the sky survey. Includes flux histories, spectral indices, and identifications |
| GRB/transient alerts | Most initial GRB and bright AGN flare alerts will be generated on the spacecraft; these SAS alerts will provide refined information, or for many AGN flares, the initial notification. |
| Interstellar emission model | The interstellar emission model is only loosely speaking a data product; it will be refined as necessary using flight data. It is essential for the production of the source catalog, and for likelihood analysis of GLAST gamma-ray data in general, so in any case it is a deliverable. |
Low level analysis tools will be supported (eg Root and IDL) for accessing and examining the Level 0 and 1 data, as well as event display tools for visualization.
Code architecture, development and release management will all be provided.
This organization is also responsible for providing support to the collaboration user community and to the SSC members in their connection to the general community.
R.Dubois Last Modified: 07/23/2001 11:11