Common: Radiometric coefficients are generated in two steps. The first step is an off-line process from Level-1 data product generation to prepare radiometric coefficients at predefined reference temperature. These coefficients will be effective for a long time, depending on the instrument stability, and available in radiometric correction data base files along with the temperature coefficients. One set of offset and sensitivity data are necessary for the VNIR and SWIR bands and one set of offset, linear sensitivity and nonlinear sensitivity data are necessary for the TIR bands. Destriping parameters will be generated from the image data if necessary, analyzing the image data obtained during the initial checkout operation period.

The second step is an on-line process executed during Level-1 product generation to correct the radiometric coefficients for instrument conditions such as detector temperature and dewar temperature, which may change for every observation.

Generation of Radiometric Correction Data Base: The radiometric coefficients for the reference temperatures were evaluated during the preflight test period using integration spheres, followed by a regular evaluation during the inflight period with on-board calibration and vicarious calibration data, and then updated if necessary. On-board calibration is scheduled every 17 days to check the stability.

These radiometric coefficients are available in the radiometric correction data base with temperature coefficients as on-line parameter files applied in the Level-1 processing. All temperature coefficients were prepared during the preflight test period and will be used throughout the mission period.

The required absolute accuracies are 4% (s) for VNIR and SWIR, and 1 to 3K for TIR depending on target temperatures.

Generation of VNIR Observation-Unit-Specific Radiometric Coefficients: Figure 2-4(a) shows the VNIR radiometric coefficients generation algorithm flow. Detector temperature is the only reference parameter by which the radiometric coefficients (offset and sensitivity) have to be corrected for a specific observation unit data set. Under normal operating conditions within the designed temperature range, this correction process will not be necessary, since the correction value is expected to be very small.

Generation of SWIR Observation-Unit-Specific Radiometric Coefficients: Figure 2-4(b) shows the SWIR radiometric coefficients generation algorithm flow. Detector temperature and dewar temperature are the reference parameters by which the radiometric coefficients (offset and sensitivity) have to be corrected for a specific observation unit data. Under normal operating conditions the detector temperature is controlled to within ±0.2 K at around 77 K. Therefore the correction process for detector temperature will not be necessary as long as the SWIR is operating normally .

The dewar temperature correction, which is necessary for compensating only for thermal radiation from the dewar, is applied only to the offset, since the SWIR detector is sensitive to room temperature thermal radiation up to 5 µm and the band pass filter can not completely remove this out-of-band radiation. The dewar temperature is used as a representative value for the internal thermal radiation.

Generation of TIR Observation-Unit-Specific Radiometric Coefficients: Figure 2-4(c) shows the TIR radiometric coefficients generation algorithm flow. The linear and the non-linear sensitivities coefficients in the data base are corrected only by the detector temperature. Under normal operating conditions the detector temperature is within ±0.2 K at around 80 K. Therefore, correction for the detector temperature will not be necessary as long as TIR is operating normally.

The offset data, common throughout the observation unit, are generated from the short term calibration data acquired at the beginning of each observation by using the blackbody temperature in the TIR supplementary data.

The correction for the offset data due to chopper temperature is calculated using the chopper temperature changes from the short term calibration period. The chopper temperature correction is carried out on the TIR Level-0A image data with the DC clamp correction. This correction will be possible for each scan data (each ten lines of data in the along-track direction), since the chopper temperature data is included in the supplementary data.

This image data correction will result in slightly different TIR Level-0B image data DN values from Level-0 data. The Level-0 data is digitized to 12 bits. The LSB (Least Significant Bit) value of Level-0B image data is very small compared to NEDT (less than one third for the 300 K target). Therefore, this difference will not give rise to any significant round-off error.

Figure 2-4 Radiometric correction coefficients generation flow

TIR DC Clamp Correction: TIR output voltage is clamped at -1.4 V ± DV_n for bands 10-12 and -0.9 V± D V_n for bands 13 and 14 when the chopper plate is observed by the detectors at every scan. The small voltage D V_n is the noise voltage at the moment of the clamp which changes randomly in every scan and must be corrected to exactly set the clamp voltage.

Figure 2-5 shows the TIR DC clamp correction flow. The exact clamped output (DN value) is available in the TIR supplementary data as the chopper data. The chopper data in the one previous scan are used for the correction. The 100 chopper data acquired for one scan are averaged to reduce noise component, followed by the DC clamp error calculation which corresponds to ± D V_n . This clamp error is transferred to the TIR radiometric correction module to subtract it from the TIR Level-0A image data to generate Level-0B image data.

Figure 2-5 TIR DC clamp correction