GDA calibration methodology relies on physical models that employ (i) sensor specific orbital and imaging information, (ii) sensor and band specific atmospheric radiative transfer calculations, (iii) sensor, location, and time dependent estimations of surface radiative budget, and (iv) modeling of surface topographic and land cover anisotropy. Reliance on physics based models ensures the predictability, transparency, and accuracy of the results, allows for the simulation of data acquisition, and enhances the comparison and analysis of data from various sensors and/or acquisitions.
The earth’s atmosphere has a significant effect on remote sensing imagery. Atmospheric gases, aerosols, and clouds scatter and absorb the incoming solar radiation and the reflected and/or emitted radiation from the surface. As a result, atmosphere greatly modulates the spectral dependence and spatial distribution of the surface reflectance. Understanding the atmospheric radiative transfer and correcting atmospheric effects from remotely sensed imagery are therefore very critical for land surface characterization. GDA has developed a set of processing algorithms that rely on physical models for modeling the atmospheric radiative transfer and image correction for atmospheric effects. The procedures explicitly model the atmospheric radiative transfer while relying on scientifically accepted and widely used 6S (second simulation of satellite signals in the solar spectrum) software (Vermote et al, 1997). The resulting product represents true surface reflectance.
The SR (true Surface Reflectance) product is a further improvement of GDA’s SASR product.
GDA's true Surface Reflectance product is a per-pixel, per-band estimate of the surface spectral reflectance (in W m-2 µm-1) as it would have been measured at ground level as if there were no atmospheric contribution. It first calibrates imagery to the at-sensor (or top-of-atmosphere or planetary) reflectances and then, using daily water vapor, ozone, surface pressure and air temperature, corrects it for the atmospheric transmittance terms, atmospheric depth deviation from the sea level, sensor off-nadir view angles, solar off-zenith angles, and the curvature of the earth.
The product is a 16-bit raster image, linearly scaled between 0 (0% reflectance) to 10,000 (100% reflectance). The calibration improves radiometric properties of the imagery and allows for superior land cover mapping, change analysis, as well as image mosaicing and comparison.