6. Data Processing Level: Derive_ERD

The Telemetry Distribution Files (TDFs) from an entire revolution have been scanned in order to obtain a number of histories which are used to control the succeeding processing phases. These histories are: (i) the Continuous Compact Status History (CCSH) and (ii) the Executed Observation History (EOH). The CCSH and EOH contain all necessary information to extract the telemetry data covering a Target Dedicated Time (TDT). This first global step is called TDF_FIRST_SCAN.

The telemetry is accessed again to generate the first level of data products, called Edited Raw Data (ERD), being the format in which raw ISO telemetry is distributed to observers. ERD is basically reformatted TDF data from which invalid sections are removed. No scientific nor instrument-specific processing is performed. ERD products are in the form of binary FITS tables.

ERD generation is performed in units of an observation. The start and end times which are recorded in the EOH determine the interval of telemetry for a given observation^6.1.

The ERD processing also cuts the CCSH into Compact Status History (CSH) products. A CSH product contains all compact status information belonging to a given TDT for a given instrument.

As the telemetry data are in `raw' engineering units, conversion algorithms should be applied to convert these engineering units to `physical' units. In Appendix B descriptions of the main conversions are provided.

In order to facilitate the processing steps in the succeeding Derive_SPD level, the orbital phase of the spacecraft for 3 reference times (start, middle, and end) of a TDT are included in the ERD data. The corresponding ERD header keywords are TREFCORn and TREFPHAn (with n=1, 2, 3) for the UTC of reference time and corresponding orbital phase, respectively. The information is necessary for the application of calibration data which depend on the position of the spacecraft in the orbit. The orbital phase data are derived from the orbit file. Since this is processed with the APH (aperture pointing history) data, the orbital phase information is already available on ERD processing level.

The ISOPHOT ERD products have been defined on the basis of the data format of the different subsystems. There are 5 types of ERD:

1. with prefix PPER for single detector subsystems PHT-P1, P2, and P3 (see Section 13.2.2),
2. with prefix P1ER for PHT-C100 (see Section 13.2.3),
3. with prefix P2ER for PHT-C200 (see Section 13.2.4),
4. with prefix PSER for PHT-S (both PHT-SS and SL together in one file, see Section 13.2.6), and
5. with prefix P2ES for PHT-C200 Serendipity Mode scans (see Section 13.2.5).

The CSH data format is the same for all detector subsystems.