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Subsections



5.4 TDF_First_Scan and Derive_ERD Products

ISOCAM collected data on a continuous basis and passed it to the OBDH for insertion into the 8192-byte telemetry buffers that were downloaded every 2 s and assembled into one TDF per revolution. When ISOCAM was designated prime instrument, its data were downloaded through 24 of the 32 frames; otherwise, when parallel, through only 1 frame. Pointing data occupied 4 frames and housekeeping data the rest.

TDF_First_Scan scans an entire TDF to divide the revolution into individual observations and deliver their scheduling information via the Executed Observation History (EOH) files. Of relevance to ISOCAM are the EOHA and EOHC files from which data are extracted later in the pipeline and inserted into raw-data product header records so that the files themselves do not always need to be used. The EOHC file was a relatively late addition to the pipeline with the main purpose to ensure continuous data coverage. The EOHA observation schedule contains gaps between the nominal end of one observation and the start of the next. ISOCAM's continuous operation meant that potentially useful data sometimes fell in those gaps. Such data did not form part of an observer's TDT but were of help in reconstructing the history of the illumination of individual detector pixels when modelling instrument transients. The EOHC file, where the `C' stands for continuous, was devised to plug those gaps and make all ISOCAM data available and was used in later stages of the ISOCAM pipeline to define observation boundaries.

Derive_ERD is run once per observation using, in ISOCAM's case, the EOHC file to define the relevant parts of the TDF from which to extract, expand and record prime CIER or parallel CPER edited raw data and CDER diagnostic raw data. The on-board system interleaves two types of ISOCAM data into telemetry, namely housekeeping F1 blocks and imaging F2 blocks. Whenever the OBDH recognised that an F2 block was ready at the end or beginning of an integration time, it was inserted into telemetry which otherwise contains a continuous series of F1 blocks. Other instrument-independent parts of telemetry are processed simultaneously to transfer raw pointing data from the 4 relevant frames into the AOCS file and general housekeeping data into the GEHK file. At the end of the TDF, the AOCS file is used during derivation of pointing files.


5.4.1 Compact Status products: CCSH & CSTA

For ISOCAM, it is the function of TDF_First_Scan to enumerate the instrument changes that took place over the course of a revolution in the CCSH file. As mentioned above, the CCSH primary header contains ISOCAM clock calibration keywords. The 20 or so parameters that define the operational status of ISOCAM are known collectively as the compact status and are given in chronological order in the CCSH file along with the start and end UTC, UTK and UTC time keys of the periods in which they apply. Individual observation's compact status values appear later in the CSTA file. For the CVF scans which are used for the spectrophotometric CAM04 AOT, only a single, nominal filter wheel position is stored in CCSH[1].CCSHFLTW(*) and CSTA[1].CSTAFLTW(*), instead of the full set employed.


5.4.2 Raw data primary header keywords

The PRIMARY headers of CIER, CPER, CDER, CISP and CPSP files contain a common set of keywords derived from various sources to give important descriptive data, as in the following example:


   OBJECT  = 'CB195/L701'         / Target ID as given by proposer
   OBSERVER= 'DCLEMENS'           / Proposer ID in ISO Mission DB
   EQUINOX =               2000.0 / Equinox
   TMRATE  =                   32 / Telemetry rate in Kbps (Kbits/sec)
   EOHAUTCS= '96085090612'        / Approx. UTC of start of observation
   EOHAUTCE= '96085092918'        / Approx. UTC of end of observation
   EOHAAOTN= 'C01     '           / AOT name
   EOHAPLID= 'STARLESS'           / Proposal ID
   EOHAOSN = '07      '           / Observation sequence number
   EOHAPSN = '00      '           / Pointing sequence number
   EOHAPCAT= '4       '           / Proposal category
   EOHACIND= '        '           / Calibration indicator
   EOHATTYP= '002     '           / Target type
   AOTVERS = '03.01   '           / AOT-to-OCT logic version
   ATTUTCSL= '96085090534'        / UTC of start time of slew to intended target
   ATTUTCS = '96085090550'        / UTC of time of first arrival at intended target
   ATTOTFTH=                 10.0 / On-target flag threshold (arcsecs)
   ATTRA   =            293.69733 / Intended Right Ascension of instrument viewing
   ATTDEC  =             12.34669 / Intended DEClination (with ATTRA)
   ATTTYPE = 'R       '           / Type of attitude operation (P/R/T)
   ATTRNPTS=                    5 / Number of points per line of the raster
   ATTRNLNS=                    5 / Number of lines in whole raster pointing
   ATTRDPTS=                   32 / Distance (arcsecs) between adjacent points
   ATTRDLNS=                   32 / Distance (arcsecs) between adjacent lines
   ATTRORIE=                    1 / ORIENTation flag (0 or 1)
   ATTRROTA=                  0.0 / ROTATE of raster pattern (degrees)
   ATTGUIDE=               109046 / Guide star reference number
   ATTSAANG=                 71.0 / Solar aspect angle (degrees)
   ATTERROR=                    2 / CONTINGEncy flag(0=success; 1=target not acq'd)
   TREFCOR1=            228128772 / UTC of 1st reference time
   TREFPHA1=            129.47567 / Orbital phase at TREFCOR1
   TREFCOR2=            228129465 / UTC of 2nd reference time
   TREFPHA2=            129.48371 / Orbital phase at TREFCOR2
   TREFCOR3=            228130158 / UTC of 3rd reference time
   TREFPHA3=            129.49175 / Orbital phase at TREFCOR3
   TREFUTC1=            228128772 / UTC (whole seconds since 01-01-1989)
   TREFUTC2=              5112273 / UTC (remaining fraction of second)
   TREFUTK =            301458521 / ISO Uniform Time Key (UTK)
   TREFITK =             89720116 / ISO INSTRUMENT Time Key (ITK)
   TREFITKU=     0.13999950000000 / ITK unit length in seconds
Of those keywords that are not obviously self-explanatory, the EOHA data are from the EOHA record that applies to the observation; the ATT data are pointing-related quantities from the first relevant APPH record; and the TREF data give the clock calibration or are calculated at times near the beginning, middle and end of the observation to show progress through the spacecraft's orbit.


5.4.3 Edited Raw Data products: CIER or CPER & CDER

Derive_ERD makes one CIER or CPER row for every F2 block found in telemetry, expanding the compacted format into bytes, words or longwords and adding supplementary housekeeping data from an adjacent F1 block. Similarly, each of the larger number of F1 blocks is used to make one CDER row, without imaging data, thus providing a complete history of the instrument's housekeeping. The F1 contents of the CDER record are identical to the F1 part of CIER or CPER records and comprise data for 244 different housekeeping parameters, including those in vector columns related to a wide variety of quantities such as settings of the internal calibrator source or the cryo motor; temperature measurements at different detector locations; on-board processor status and error codes; and voltage settings. Most of these data are of limited general interest. The rest of the CIER or CPER records are occupied by raw imaging data in CIER[1].F2IMAG(1122,*), for example, and 33 other associated F2 block items. The dimensions of F2IMAG are greater than $32\times32=1024$ in order to accommodate extra data required by the SW readout scheme. Unless on-board integration was in operation, records alternate between RESET and EOI data with identical CAM on-board times CIER[1].F2TIME(*) and CIER[1].F2BOOTTI(*) that are used to calculate the CAM ITK in CIER[1].GPSCTKEY(*).


5.4.4 Pointing History products: IRPH or CRPH & IIPH or CIPH

Pointing data are obviously vital for data analysis and these are calculated at the end of Derive_ERD using the AOCS file derived earlier at the same time as other raw data. Two files are produced, namely a Reference Pointing History file and an Instantaneous Pointing History file. When ISOCAM was prime, the IRPH and IIPH files contained CAM pointing data. Otherwise, CAM parallel pointing data are found in the CRPH and CIPH files. The pointing system operated independently of any other instrument, so these files are of a general design described in the ISO Handbook Volume I, [40]. Because parallel pointing files were only made for CAM parallel's benefit, it is worth pointing out here, though, the extra CRPH[1].PRIME(*) and CIPH[1].PRIME(*) columns that show the aperture of the prime instrument in use. A change in the PRIME aperture implies a change in the spacecraft's and ISOCAM's pointing direction requiring a new data boundary. The other significant pointing event for both prime and parallel data is a change of RPID. The expected time of changes of IRPH[1].RPID(2,*) are given in IRPH[1].UTC(2,*) and IRPH[1].UTK(*), for example, which may be compared with the actual times by inspection of IIPH.RPID(2,*), IIPH.UTC(2,*) and IIPH.UTK(*).


next up previous contents index
Next: 5.5 Derive_SPD Products Up: 5. The Data Products Previous: 5.3 ISOCAM Data Products
ISO Handbook Volume II (CAM), Version 2.0, SAI/1999-057/Dc