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gaia data release 3 documentation

4.4 Processing steps

4.4.1 AGIS pre-processing

Author(s): Jose Hernandez

The AGIS pre-processing takes care of preparing the data produced by other systems which is needed by AGIS to arrange and convert the data in a way which will be more suited for the AGIS execution.

The types of data used by AGIS are:

  • The Gaia satellite and main solar-system bodies ephemeris.

  • The time ephemeris needed to convert between the different time scales used in DPAC processing (OBMT, TCB, etc.).

  • The current Gaia DPAC source catalogue originating from the IGSL.

  • The IDT or IDU AstroElementaries (FOV Observations, each one typically containing 1 SM CCD transit and 8 or 9 AF CCD transits), in the first release only IDT AstroElementaries were used.

  • The IDU match information, linking AstroElementaries with Gaia catalogue sources.

  • The IDU new sources which were created during the AstroElementary to Gaia catalogue match process.

  • The commanded attitude files, used to generate the initial attitude needed to bootstrap AGIS.

  • The BAM data was analysed offline in order to find out the discontinuities where calibration boundaries should be placed.

AGIS works internally in the TCB time scale while typically most of the input data coming from the MDB are tagged in OBMT. During the pre-processing the OBMT times get converted into TCB times.

The pre-processing also takes care of filtering those CCD transits where the IPD was not successful as reported in the corresponding IDT/IDU flags provided in the AstroElementary, during the pre-processing the sources and their observations are sorted so that during the AGIS processing all the observations of each source are available to the core algorithm computing the source parameters.

At the end of the pre-processing we have the source catalogue and the observations arranged in a convenient way for the AGIS processing. The next step consists of the selection of the primary sources and their associated Gaia observations. These are the sources which will be used in the iterative process to determine the nuisance parameters (Attitude and Calibration).