Limitations of triple-angle radiometric geosurface measurementsfrom high altitude to mitigate low-absorption atmospheric effects

摘要

The remote measurement of geosurface radiances using microwave andnthermal infrared radiometers from satellite platforms has been annattractive and challenging goal. A significant impediment is datancorruption by the intervening atmosphere. Attempts to correct for thenatmosphere apparently have not been reported at microwave frequencies,nbut have been addressed at thermal infrared wavelengths. For thermalninfrared, papers have been written reporting on multi-spectral andnmulti-angle measurements to infer the absolute value of geosurfacentemperature. A dual-angle radiometric technique to measure sea-surfacentemperature from an aircraft has been reported by Saunders. Hisntechnique made measurements at zenith angles near 0° and 60° ,nand the reported absolute accuracies are 0.2 degree C. Since the pathnlength was short with small path loss and insignificant path radiance,ntwo unknowns are involved and measurements at two angles were sufficientnfor solution. As the platform altitude is increased, the number ofnsignificant factors increases, such as upwelling radiance from a lossynatmosphere and downwelling radiance scattered from the surface. With annincrease in the number of significant factors, the number of independentnmeasurements must also increase for solution. In this paper, an attemptnis made to examine the utility of a triple-angle measurement method onnthe same area or pixel to extend the earlier dual-angle measurementntechnique. It is assumed that the next significant factor for somenexamples maybe the radiance from the intervening path. The purposes ofnthe present approximate analysis are to quantify the surface radiancenand its uncertainty due to measurement limitations. It is believed thatnthere are scenarios within limited latitudinal zones, local times, andnseasons where the assumed path characteristics are reasonably valid, andnthe error would be a small bias term to the calculated values