L1A to L1B Conversion

1
L1A to L1B Conversion
Dark Restore Correction

• SeaWiFS sensor produces counts even in total
  darkness
• Sensor takes a reading of an area of known
  darkness every scan
• The median of all scans in that image is
  subtracted from L1A_data to produce
  
  L1A_data
• This is done separately for each band

2

• Calculated from the radiance-to-counts slopes and
  zero-offset counts in the sensor calibration
  table
• Function of band, detector, and band gain
  settings
• Calculation algorithm taken from the SeaWiFS Data
  Analysis System (SeaDAS) l1bgen subroutine

3

• There is a slight variability of the sensor
  sensitivity depending on which of the two mirror
  sides is used in each scan
• Different for each band
• Side 0 produces smaller counts than side 1
• This factor is taken directly from the sensor
  calibration table

4

• Difference image mirror side corrected image
  uncalibrated image (subset of band 1, 4x zoom)
• Black - 4 White 4
• Mirror side generally alternates for each scan
  line producing a striping effect
• This correction factor is small (lt 0.5)

5

• Count shows a dependence on detector temperature
• Stronger at the lower wavelength bands
• Digitized scan temperatures are provided with the
  L1A image data
• Temperature correction factors are included in
  the sensor calibration table

6

• Scan modulation factor accounts for the
  sensitivity of the detector to the position of a
  pixel in a particular scan
• For all odd-numbered bands, the gain factor
  decreases from the first to last pixel
• For even-numbered bands, the gain factor
  decreases towards the center of the image
• The center pixel gain factor is 1
• These factors are available directly from the
  sensor calibration table

7

• Difference images scan mod corrected image
  uncalibrated image (left band 1, right band 2)
• Odd band correction factor goes to 1.5 for the
  leftmost pixels
• Even band correction factor is lt 1 for all pixels

8

• Detector sensitivity is deteriorating with time
• Of all the correction factors, the time-dependant
  one is the most significant
• The time-dependant correction factor is
  calculated by subtracting the reference time
  (time of launch) from the time of scan to get Dt
• Dt is then multiplied by the correction factors
  from the sensor calibration table according the
  formula below
• Bands 7 and 8 have the largest gain factor
• Band 6 is deteriorating the fastest
• Bands 3 and 4 have no time-dependant gain
  correction