SBN NEAR: MSI Calibration Proessing Levels  

MSI Calibration Processing Levels

MSI images are archived data in up to 7 forms which have undergone differing calibration pathways and levels of subsequent processing, depending on the target body, time during the mission, and intent of the observation:

Level 1 RAW The image is provided in units of raw data number as received from the spacecraft after data were uncompressed. These raw are also known as NEAR Level 1 data.
Level 2 RAD The image is calibrated to units of radiance (watts per meter squared per micron per steradian) using radiometric calibration equation (1) below. These data are the most scientifically meaningful.
  RADDBL Same as RAD. In addition, the image is deblurred by application of the Fourier filtering restoration of a degraded image PSF.
  IOF The image is calibrated to units of I/F using radiometric calibration equation (1) below. The radiance image is divided by pi times the solar irradiance expected at the target body's distance from the Sun.
  IOFDBL Same as IOF. In addition, the image is deblurred by application of the Fourier filtering restoration of a degraded image PSF.
  CIF The image is cleaned and calibrated to units of I/F using radiometric calibration equation (2) below. Calibration is performed with removal of background levels by subtraction of a 0-ms exposure image. Then the image is divided by the pi times the solar irradiance expected at the target body's distance from the Sun.
  CIFDBL Same as CIF. In addition, the image is deblurred by application of the Fourier filtering restoration of a a degraded image PSF.

For more information about the MSI calibration processes , read the MSI calibration document (TEXT).

Radiometric Calibration Equations

For radiometric calibration, the main objective of inflight calibration is solving for variables in the calibration equation for converting image data numbers (DNs) to physical units of radiance, W m-2 micro m-1 sr-1. The calibration equation has two forms. The first form is applied when the field-of-view is underfilled, or when the field is filled by accompanying 0-ms exposures taken close in time and in the same filter are not available: 

  Radiance(x,y,f,T,t,c) =

     {[DN(x,y,f,T,t,c) - Dark(x,y,t,T,MET)] - Smear(x,y,t)} * 100
     ------------------------------------------------------------   (1)
       Flat(x,y,f,c) * Coef(f) * Resp(f,T) * Atten(f,c) * Exp(t)
where DN(x,y,f,T,t,c) is raw DN measured by the pixel in column x, row y through filter f at exposure time t and temperature T with the cover status c open or closed. Dark(x,y,t,T,MET) is the dark closed. Dark(x,y,t,T,MET) is the dark level modeled for this pixel at exposure time t, temperature T, and mission-elapsed time MET, time t, temperature T, and mission-elapsed time MET, derived from a model based on dedicated dark frames taken throughout the the mission. Smear(x,y,t) is the scene-dependent readout smear for the pixel at exposure time t. Flat(x,y,f,c) is the flat field for filter f with the cover status c open or closed. Coef(f) is the coefficient for converting dark-removed, flat field and smear-corrected DN from filter f to radiance, for a baseline exposure time of 100 ms. Resp(f,T) is the responsivity for this filter at temperature T relative to the baseline, inflight operating temperature (-29.6 deg C). Atten(f,c), if appropriate, is the attenuation of incoming signal by the lens cover in filter f when the cover status c is closed. Exp(t) is exposure time in milliseconds between 1 and 999 ms. The second version of the calibration equation is used for monochrome sequences having as an objective photometric accuracy, or for color sequences, in either case when the asteroid overfills the FOV: 
  Radiance(x,y,f,T,t,c) =

     {[DN(x,y,f,T,t,c) - Dark(x,y,MET,T,t)] -
                  [DN(x,y,f,T,0,c) - Dark(x,y,MET,T,0)]} * 100
     ---------------------------------------------------------     (2)
     Flat(x,y,f,c) * Coef(f) * Resp(f,T) * Atten(f,c) * Exp(t)
where DN(x,y,f,T,t,c) is raw DN of an intended image scene. DN(x,y,f,T,0,c) is an image acquired a few seconds later at an exposure time of 0 ms. The 0-ms image contains no real scene information, but has the same transfer smear and leaked light as the primary image. It differs only in (a) the exact position of the scene at the sub-pixel level and (b) a slightly lesser accumulation of dark current at the shorter exposure time. For a typical Eros image exposed to a DN of approximately 2000, this approach removes approximately 20 DN of leaked light ignored in equation 1. Application of this version of the equation requires two raw images to produce one calibrated, reduced image, and this is much more resource-intensive.

This section is an excert from the MSI calibration document (TEXT).