Retrieval of atmospheric optical parameters from airborne flux measurements: application to the atmospheric correction of imagery.

Methodologies that employ auxilliary flux data collected by upward- and downward-looking optical sensors to improve atmospheric corrections of airborne multispectral images are presented and evaluated. Such flux data often are collected in current airborne sensors to produce bidirectional reflectance factor (BRF) images and estimates of hemispherical-hemispherical reflectance. The fact that these images must then be corrected for atmospheric interference raises the question as to whether the auxilliary flux information can be employed to estimate some of the input parameters required by atmospheric correction models. Radiative transfer simulations are employed to demonstrate that the utilization of the downwelling and upwelling fluxes as a means of inferring intrinsic atmospheric optical information can be used to better characterize the local atmosphere and accordingly to improve the atmospheric corrections applied to the apparent BRF images.

Sensitivity of surface reflectance retrieval to uncertainties in aerosol optical properties.

We formulate a procedure to investigate the sensitivity of surface reflectances retrieved from satellite sensor data to uncertainties in aerosol optical properties. Aerosol optical characteristics encompassed in the study include the aerosol optical depth, the Junge parameter (i.e., spectral dependence), and the imaginary part of the refractive index (i.e., aerosol absorption). The study includes both clear and hazy atmospheric conditions, wavelengths of 0.550 and 0.870 µm, three solar zenith angles, and five viewing geometries. Key results are presented graphically in terms of accuracy requirements on the aerosol property under consideration for a 5% uncertainty in predicted surface reflectance.

Investigation of continental aerosols with high-spectral-resolution solar-extinction measurements.

A modified implementation of the Langley method has been used to measure the atmospheric optical-depth spectrum at 5-nm intervals from 0.36 to 1.10 microm. Extensive measurements of the aerosol optical depth at 550 nm and the Junge exponent showed that there was a distinct separation of atmospheric conditions into clear and hazy conditions. A study of the sensitivity of the retrieval of the 550-nm surface reflectance factor from spaceborne observations was carried out, using the above characterization of typical atmospheric conditions in terms of mean and standard-deviation values for the aerosol optical depth and Junge exponent.

Rayleigh optical depth comparisons from various sources.

Rayleigh optical depth values obtained from various computations, tabulations, and parameterizations are not always in good agreement. Important differences as large as 3 or 4% can arise depending on the choice of depolarization factor, the formula used for the refractive index of air, and the choice of standard values for columnar and molecular number densities. The fitting equations generally give rise to the largest differences. The use of different standard altitude profiles for atmospheric pressure and temperature causes a variation of 1% or less in Rayleigh optical depth.