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TEB PC Bands 33, 35, 36 Saturation Problem

Description

The Aqua MODIS instrument on-board Blackbody (BB) routinely operates at a nominal temperature of 285K. On a quarterly basis, the BB performs a warmup/cooldown (WUCD) cycle over the entire BB operational temperature range of 270K to 315K, as part of the MODIS thermal emissive band (TEB) on-orbit calibration and characterization. During pre-launch testing it was determined that three PC bands (B33, 35, and 36) saturate at BB temperatures above 295K.

Impact to L1B Data Set

Operational scan-by-scan b1 calculations can be affected by the saturation to return a potentially incorrect coefficient value. Science Team examination of the data sees no obvious impact on the L1B products due to this saturation characteristic.

Estimated Impact to L2 Science Products

Negligible.

Cause of Effect

Aqua MODIS pre-launch testing demonstrated that these three PC bands (B33, 35, and 36) saturate at BB temperatures of approximately 295K (B33), 300K (B35), and 305K (B36). TEB detectors are calibrated on a scan-by-scan basis using observations of the BB. During the WUCD activity the BB temperature will exceed the saturation limits for these three bands and the calibration algorithm as designed for Terra MODIS can return an incorrect b1 calibration coefficient for saturated detectors.

Mitigation Approach/Added Testing to Characterize Effect

An algorithm has been designed and implemented in the Aqua MODIS L1B code for data processing. The Aqua-specific algorithm defaults to using a fixed b1 coefficient instead of the scan-by-scan derived b1 coefficient for these three bands once the BB reaches and exceeds a certain temperature. The BB temperature value, defining the switch from scan-by-scan to default b1, is stored in a LUT (BB_T_sat_aqua). For Collection 5 (and all previous collections) the T_sat switch values were derived at pre-launch. In Collection 6, the trend of on-orbit T_sat values was re-analyzed and the LUT values were adjusted. The following table shows the BB T_sat values above which the default b1 coefficient is used in the L1B processing.

 Band 33Band 35Band 36
Collection 5290 K295 K300 K
Collection 6293 K296 K301

The default b1 coefficients are stored in a LUT (BB_T_sat_default_b1_aqua) and are derived from observations of the BB at the temperature values in the table above. The default b1 coefficients are monitored every WUCD cycle and are updated as needed.

Estimated Schedule to Complete

Finished and implemented in L1B code.

History of Characteristic

The default b1 coefficient strategy has been applied over the lifetime of the Aqua mission. Several on-orbit updates to the LUT have been applied:

Collection 5 LUT timestamps (coefficient updates)

  1. 2002001.0000
  2. 2002158.1815
  3. 2002183.1520
  4. 2002218.1455

Collection 6 LUT timestamps (coefficient updates)

  1. 2002001.0000
  2. 2007349.0300
  3. 2008236.0300
  4. 2009038.0300
  5. 2009206.030