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Emissive Bands

MODIS has 16 Emissive Bands (Bands 20-25 and bands 27-36), each with a 1 km ground resolution and wavelengths ranging from 3.75 to 14.24 micrometers (microns).

Key Uses of the MODIS Emissive Bands

 BAND
Cloud Fraction24, 25, 33, 34, 35, 36
Cloud Temperature32, 33, 34, 35, 36
Cloud Temperature22, 23, 31
Forest Fires & Volcanoes21, 31, 32
Mid Troposphere Humidity27
Sea Surface Temperature20
Surface Temperature22, 23, 29, 31, 32
Total Ozone30
Troposphere Temperature24, 25
Upper Troposphere Humidity28

L1B Emissive Calibration

The MODIS emissive calibration algorithm is designed to determine the at-aperture spectral radiance of the Earth scene with its associated uncertainties. Level 1A data is Earth-located raw sensor digital numbers and Level 1B data is Earth-located, calibrated data in physical units.

MODIS Emissive Calibration Data Flow

The RED print (upper half of figure) denotes parameters which change every scan. The BLACK print (lower half of figure) denotes periodic or pre-launch determined parameters.

The On-board Blackbody and Space View are used every scan to calibrate the emissive bands. The on-orbit emissive band MODIS calibration is a two point method which fits a nonlinear response by using pre-launch measurements.


Thermal Emissive Band (TEB) Calibration

The following figure is a simplified flow diagram for the TEB calibration.

TEB Flowchart

The major steps in this process are:

  1. Preprocess the L1A telemetry and onboard calibrator (OBC) data and retrieve the required LUT files
  2. Retrieve L1A Earth-view (EV) DN (digital number) and subtract the average (50 samples per scan) space-view (SV) background TEB Equation 1
  3. For Bands 32-36, apply optical crosstalk correction (Photons from Band 31 leak into Bands 32-36 via filter substrates)TEB equation 2
  4. Use BB and SV DNs to calculate scan by scan linear calibration coefficients
    TEB Equation 3
    where eS = source(s) emissivity
    dnS = detector response to source (s) with background subtracted
    LS = source (s) band averaged radiance over relative spectral response (RSR)
    a0 and a2 = temperature dependent offset and nonlinear (quadratic) coefficient
    RVS = response versus scan angle
  5. Calculate the Earth-view radiance for each (BDF) (F=134 cross-track frames: there are no subframes for TEBs)TEB Equation 4
  6. Convert radiance to a scaled integer (SI). See below in Scaled Integer Conversion.
  7. Calculate RSS uncertainty and convert to an uncertainty index (UI). See below in Uncertainty Index

Uncertainty Index

For each pixel, the Uncertainty Index (UI), with a range of 0-15, is computed online using the following:

UI = scaling_factor[B]•ln(sEV/specified_uncertainty[B])

The band dependent scaling factors and specified uncertainty values come from LUT inputs. The RSS uncertainty, sEV (in percent), is computed dynamically.

UI utilizes the first four bits of an 8-bit unsigned integer, which yields a range of 0-15.

Scaled Integer Conversion

The TEB radiance values, LEV (W/m2/µm/sr), are scaled to an integer range of 0-32767 over the LUT-supplied dynamic range of Lmin to Lmax using:

SI(BDF)=32767*(LEV(BDF)- Lmin(BDF))/(Lmax(B)- Lmin(B))

LEV (a 32-bit floating-point number) is scaled to a valid integer of 0 to 32767, using 15 bits of a 16-bit integer number. SIs greater than 32767 are reserved for invalid data.

The LUT-fuished values of Lmin and Lmax are indexed by band only.

The RSB reflectance factors are scaled in a similar manner.