Characterization and field use of a CCD camera system for retrieval of bidirectional reflectance distribution function

Vicarious calibration and field validation is a critical aspect of NASA's E arth Observing System program. As part of calibration and validation resear ch related to this project, the Remote Sensing Group (RSG) of the Optical S cience Center at the University of Arizona has developed an imaging radiome ter for ground-based measurements of directional reflectance. The system re lies on a commercially available 1024 x 1024 pixel, silicon CCD array. Angu lar measurements are accomplished using a fish-eye lens that has a full 180 degrees field of view with each pixel on the CCD array having a nominal 0. 2 degrees field of view. Spectral selection is through four interference fi lters centered at 470, 575, 660, and 835 nm. The system is designed such th at the entire 180 degrees field is collected at one time with a complete mu ltispectral data set collected in under 2 min. The results of laboratory ex periments have been used to determine the gain and offset of each detector element as well as the effects of the lens on the system response. Measurem ents of a stable source using multiple integration times and at multiple di stances for a set integration time indicate the sq stem is linear to better than 0.5% over the upper 88% of the dynamic range of the system. The point spread function (PSF) of the lens system was measured for several field an gles, and the signal level was found to fall to less than 1% of the peak si gnal within 1.5 degrees for the on-axis case. The effect of this PSF on the retrieval of modeled BRDFs is shown to be less than 0.2% out to view angle s of 70 degrees. The degree of polarization of the system is shown to be ne gligible for on-axis imaging but to have up to a 20% effect at a field angl e of 70 degrees. The effect of the system polarization on the retrieval of modeled BRDFs is shown to be up to 3% for field angles of 70 degrees off na dir and with a solar zenith angle of 70 degrees. Field measurements are mad e by mounting the camera to a boom mounted to a large tripod that is aligne d toward south. This tripod obstructs sampling of the surface reflectance p ast 25 degrees off nadir northward. The system is typically operated at a h eight of 1.5 m to view over a large sampling of surface features, such as c racks. To evaluate the surface BRDF, measurements are collected throughout the morning as a function of Sun angle. A single measurement consists of al l four bands and a dark-current measurement. Data sets have been collected over several vicarious calibration sites and calibration tarpaulins. Compar isons with measurements made by a simple goniometer-based system indicate t hat the camera system is as accurate as the goniometer. Scattering phase fu nction values derived from the camera system are fit to a modified Pinty-Ve rstraete equation. This function is shown to fit the data to better than 0. 3% for data collected during an example RSG vicarious calibration experimen t. Bidirectional reflectance data derived from the camera system also compa re well to those predicted from the Walthall model. These BRDF models are c ritical for determining the applicability of measurements taken over small areas to represent the BRDF properties of an entire site, which in some cas es is of the order of several kilometers in size.