McIDAS Programmer's Manual
Version 2003

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Appendix D

POES AVHRR
Calibration Information

This appendix contains the following additional POES AVHRR calibration information resulting from the launch of NOAA-15 satellite in May 1998.

AVHRR Calibration Background

Since NOAA-12 and -14 AVHRR use the older TIRO calibration while the NOAA-15 AVHRR uses the newer AVHR calibration, changes have been made in McIDAS-X area structure between the NOAA-12 and -14 areas and the NOAA-15 areas. All bands of the NOAA-15 AVHRR/3 has calibration that differs from NOAA-14 AVHRR, but most of the changes are internal to the area structure and calibration module and are transparent to McIDAS-X users.

All POES images are now produced and delivered only through SDI (SSEC Desktop Ingestor). Areas created from NOAA-15 data will have the AVHR calibration type; areas created from NOAA-12 and -14 data will have the TIRO calibration type. The AVHR calibration exactly duplicates the output of the TIRO calibration in all respects. For NOAA-12 and NOAA-14, the TIRO and AVHR calibration appears interchangeable.

The advantage of the new calibration type is that only relatively minor changes need to be made once to SDI software (assuming the raw data stream format does not change). Any further changes required by NOAA-15 orbit performance is made only in the AVHR calibration module, rather than the SDI ingestor. The TIRO and QTIR calibration modules will gradually become obsolete; they will be used only for archived data.

SDI can now deliver NOAA-12 and -14 ADDE areas in either the TIRO or AVHR format; a logical switch has been installed to produce either calibration type. Only the AVHR format can be delivered by SDI for NOAA-15. For those installations that do not use SDI, an updated SATBAND file, an updated kbprep.f, and kbxavhr.dlm still will not be able to manipulate NOAA-15 imagery,because kbxtiro.dlm will not be upgraded.

TIRO and AVHR Differences

The changes from TIRO formats for AVHR are summarized below:

  1. The TIRCAL line-by-line calibration code is now moved to the SDI. This allows retension of the use of the platinum resistance thermometers (PRT) and samples from target and space looks, but it identifies channel three as either a near IR or thermal IR sensor (using bit 10 of word 7 in the HRPT minor frame). It renames channel three to channel six if it is identified as near IR by bit 10 of word 7, which can switch while going from one line to another, but normally switches only twice per orbit.

    Following a switch of bit 10 in word 7, there may be one or two bad lines in the raw data, and up to 10 bad lines of poorly calibrated data (worst case), due to the five-point subcommutation of the PRT temperatures between reference values. These bad lines appear totally black or totally white. After a reference line is encountered, it takes four more lines to acquire all of the PRT temperatures and to calculate their weighted average. If this is done on-the-fly, there is no way to recover any bad or poorly calibrated lines already sent. However, if you use post-processing of a completed area, it is possible to backfill the erroneous line prefix data.

    Each image still contains five bands or less, but they are numbered 1,2,3,4,5 or 1,2,6,4,5 with corresponding changes in the line prefix LEV section. All calibration constants that are generated by SDI and passed in the line prefix CAL section are converted to scaled integers with scaling of 100,000 for slopes and offsets (except for the band 3 slope, which is scaled by 10,000,000) and a scaling of 100 for temperatures.

    The DOC section content is unchanged, as compared to XSD; it is copied just as received, but left-shifted five bits. The length of the DOC section does not change with NOAA-15. For each line, the DOC section captures the raw data in an HRPT minor frame that does not constitute imagery that is converted to interleaved pixels.

  2. Prior to NOAA-15, every AVHRR channel had a linear calibration that required a single gain and a single offset constant in the line prefix for each channel. With NOAA-15, this changes to a bi-linear calibration for the visible (1,2) and near IR (6) channels, which requiries two slopes and two offsets (four constants and two conversion equations) for each visible and near IR channel identified in the line prefix.

  3. The three thermal IR channels (channels 3, 4 and 5) from the AVHRR/3 instrument have a new non-linear correction method applied, which requires a constant from the line prefix CAL section that contains the average space count. TIRCAL used this constant to calculate the slope-offset constants for previous AVHRR data in bands 3,4, and 5. Since the NOAA-15 raw data stream continues to look exactly like NOAA-12 and NOAA-14, the slope-offset calculations currently done in TIRCAL are preserved in SDI for all AVHRR data, including AVHRR/3. This permits the AVHR calibration module to be used for NOAA-12 and NOAA-14 data from SDI as well as for NOAA-15 data. At SSEC, all POES data has been switched from TIRO to the new AVHR calibration type within SDI after NOAA-15 began operation; TIRO is calibration used only as a fall-back position.

    An inverse Planck function developed by SSEC is used for converting to emission temperature in both SDI and kbxavhr.dlm.

  4. The line prefixes for AVHR calibration data have been reformatted to allow eight constants per line for each of the channels (160 bytes total) instead of two constants (40 bytes) for TIRO. For the visible channels, the first four constants parameterize the bilinear calibration, while the last four constants are unused. For the IR channels, we use only the first two constants for linear calibration. The fifth and sixth constants are five scan averages of the space look and internal target. The internal target temperature is now redundantly written into the CAL section of the line prefix for bands 3,4, and 5 (at constant position 7) instead of overwriting the channel 3 patch temperature and spare word. The eighth constant is an unused spare in all channels. Note that the scaling factors for the constants are also different from TIRO. These line prefix changes will make the AVHR calibration type images incompatible with TIRO calibration.

  5. The CAL=QUICK option is no longer available for AVHR.

 

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