The Quantitative Estimate Of Path Radiance On The Remote Sensing Images

The radiance reaching the satellite sensor includes the photons coming from the sun,which do not reach the surface and are backsacttered towards space, namely path radiance.This is an inerference term that does not carry any information about the terrestrial objects. Asecond contribution is due to photons scattered by the atmosphere into the path from thesurface to the satellite. A third contribution represents the radiance reflected by the target thatis not absorbed or scattered on its way to the sensor, this signal carries information about theterrestrial surface.The interaction of solar radiation with the constituents of terrestrial atmosphere is animportant problem in atmospheric correction of the remote sensing images. The intensity ofscattered radiation and its angular distribution depend on the nature of the scatterer and itssize in relation to the wavelength of the interacting radiation. With the increasing use ofair-borne and space-borne techniques for remote sensing of the earth's surface, quantitativestudy of the atmospheric effects on the upwellling solar radiation, reflected by terrestrialobjects, has become more important since these effects cause the radiance of a ground objectas measured by the remote sensor to differ from its actual value. Meanwhile, quantitativestudy of the effects of path radiance on remote sensing data aslo has become very important.Solar radiation, as it passes through the terrestrial atmosphere, undergoues extinction due toabsorption and scattering by both air molecules and aerosols. Though both these processescause attenuation of the direct radiation, scattering causes diffuseness due to the angularredistribution of the scattered radiation. Consequently, a scene on ground is illuminated notonly by the directly transmitted solar radiation but also by the scattered radiation from thesurrouding hemispher of the sky. The gound scene reflects a part of the incident radiationtowards the sensor, which forms the directly transmitted solar radiance. To this direct radiance,is added the path radiance(Lp) comprising of the radiances reflected by the target surroundingand subsequently scattered diffusively by the atmosphere. The total radiance reaching thesensor is the sum of solar radiance and path radiance.In this paper, using a set of simplified radiative transfer equations, the effects of thechanges in the physical properties(such as columnar size distribution, optical depth andrefractive index)of atmospheric aerosols on the upwelling atmospheric path radiance areinvestigated for a nadir viewing remote sensor at various solar illumination angles in thecontext of developing operational methods for correcting path radiances received by remotesensing satellites for atmospheric effects. It is seen that at low solar illumination angles andhigh haze conditions (aerosol optical depths>0.5) the path radiance depends strongly on thenature of the aerosol size distribution, whereas for low to moderate values of aerosol opticaldepths (<0.5), the path radiance increases nearly linearly with optical depths. Spectralvariations indicate a large increase in path radiance with decrease in wavelength.Simultaneous measurements of spectral optical thickness and path radiance are used totest the relationship between path radiance and aerosol optical thickness for a given geometryof illumination and observation. The selected geometry is representative of satellite remotesensing conditions. Finally, the quantitative study of the effects of path radiance on remotesensing images has been performed. This method has been tested to be feasible and havingmore precision than methods used before.