Validation of a laser-based system for ground measurement of backscatter surface reflectance

The recent deployment of on-orbit active sensors operating at optical wavelengths requires new calibration methods to be investigated. In response to this, a ground-based active radiometer for measuring backscattered surface reflectance has been developed by the Remote Sensing Group at the University of Arizona. This instrument, known as the reflectometer, was designed to match the illumination and detection geometry of spaceborne lidar systems. The reflectometer uses a Nd:YAG laser operating at 1064 nm (with the capability of 532 nm), illuminates the target sample at normal incidence by use of a beam expander and fold mirror, then collects the reflected light at nadir through an aperture in the fold mirror. In order to reduce stray light, a 3 nm bandwidth filter centered on the laser wavelength is mounted in front of the silicon detector and a half cylinder shell encloses the optical system. Previous measurements at White Sands Missile Range, NM have produced results that are within 3% of coincident measurements using a field spectrometer. The results of these measurements are presented, including further laboratory testing using tarpaulin witness samples and future improvements of the original system design. In addition, comparison of reflectometer measurements to MODIS derived reflectance as it relates to on-orbit lidar retroreflection will be discussed. The benefits of validating MODIS derived reflectance will become essential with the launch of CALIPSO and its incorporation into the A-train.