Parent Topic: DETAILS

THERMAL CHANNEL CALIBRATION

AVHRR thermal channels 3, 4, and 5 are calibrated using the procedure described in the NOAA Polar Orbiter Data User's Guide. Each output (or calibrated) pixel value is computed as follows:

 PO = PlanckEqn(E)

 E  = S(c) * PI + I(c)

 where:  
       PI   is the input pixel value
       PO   is the output (calibrated) pixel value
       E    is the radiance of the input pixel
       S(c) is the slope coefficient for channel c
       I(c) is the intercept coefficient for channel c

 units:  
       PI   is a pixel count value (usually 0-1023 for AVHRR)
       PO   is surface brightness temperature in degrees K
       E    is radiance in milliWatts/(m^2 sterad cm^-1).
       S(c) is radiance/count
       I(c) is radiance

The PlanckEqn looks like this:

 T = C2 * v / ln(1 + C1 * v^3 / E)

 where:  
       T    is the brightness temperature in degrees K.
       C1   is a constant in milliWatts/(m^2 sterad cm^-1).
       C2   is a constant in cm K.
       v    is central wave number in cm^-1.
       E    is radiance in milliWatts/(m^2 sterad cm^-1).

The constants C1 and C2 are defined as follows:

 C1 = 1.1910659e-5  milliWatts/(m^2 sterad cm^-4)
 C2 = 1.438833 cm K

Note that after applying Planck's Equation, AVHRRAD does not perform any non-linearity corrections of the computed surface brightness temperatures. The non-linearity corrections can be applied manually afterwards.

AVHRRAD tries to obtain the channel slope and intercept coefficients from the AVHRRSEG text segment. If these values are not present, AVHRRAD will extract further information from the text segment in order to compute the slope and intercept coefficients. This extra information includes PRT counts, BLACKBODY counts, SPACE counts, and optional "A" coefficients for converting PRT counts to temperatures. The procedure for computing the slope and intercept coefficients is described in NESS 107, section 5.1.1.

Parent Topic: DETAILS
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