Remote Sensing - a tool for environmental observation

However real detectors have some degree of non

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However, real detectors have some degree of non-linearity and will give a small signal out even when no radiation is detected: dark current, or the offset in figure 5.4. The slope of the characteristic is called transfer gain or gain. Consequently, two steps are generally necessary for radiometric correction 1) correction for gain and offset of the detector and 2) correction for the atmospheric conditions at the time of data acquisition. Two approaches are possible for radiometric corrections:
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65 1) if sufficient measurements of the conditions of the atmosphere such as optical thickness, scattering and sun elevation are available, and if the surface reflectance of several dark and bright targets in the study area are determined, then the image can be radiometrically corrected using models such as the 5S code, LOWTRAN or ATCOR. A detailed description of the atmospheric effect on remote sensing imagery is provided by Kaufman (1989), Kaufman and Sendra (1988), Richards (1986) and Lillesand and Kiefer (1994, p.531-536). Model descriptions of LOWTRAN and 5S-code are available from Kneizys et al. (1988) and Tanré et al. (1985). 2) Most often the previous mentioned data are not available. In these cases, other approaches can be followed. For example, for the Landsat MSS and TM conversion tables can be applied to translate Digital Numbers into reflectance or into absolute spectral radiance (mW/(cm*ster*μm)². These conversion formulae are based on gain and offset values measured in the laboratory before launch and on the sun elevation at the time of data acquisition (sun elevation is included in the header of each digital TM and MSS image). The conversion formulae for Landsat MSS and TM are described in Markham and Baker (1986). A bulk correction method for atmospheric effects is called the histogram method or darkest pixel method or haze removal. This technique is based on the fact that the near infrared spectral band (Landsat MSS band 7 or Landsat TM band 4) is essentially free of atmospheric effects (figure 5.5). This can be verified by examining DN-values of clear water and shadows. Their DN values should be close to zero. The assumption of this technique is that atmospheric scattering has added a constant value to each pixel in a band. Hence, the lowest value in the histogram of each band is subtracted from all pixels in that specific band (figure 5.6). Figure 5.4 A) Transfer characteristic of a radiation detector: gain and offset and B) Hypothetical mismatches in detector characteristics (Richards, 1986).
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66 Figure 5.3 Interactions of incoming solar radiance with the atmosphere and the contamination of the satellite-measured radiance (Hill, 1993). Figure 5.5 Atmospheric scattering as a function of wavelength (Sabins, 1987).
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67 Figure 5.6 Illustration of the effect of path radiance on Landsat MSS histograms (Richards, 1986).
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