The on-orbit verification of spaceborne lidar systems relies on several approaches including those based on measuring the returned signal from the surface. This returned signal depends on both the atmospheric transmittance and the surface reflectance in the backscattered direction. Thus, knowledge of the surface reflectance is critical to such approaches. The Remote Sensing Group at the University of Arizona has developed a laser-based system to measure the backscatter surface reflectance at the ground for use in vicarious calibration of lidar systems. The device relies on a beam expander and fold mirror system to illuminate the ground. A detector is placed behind an aperture cut into the fold mirror to allow the backscattered signal to be measured. This work describes the results from the use of this system for the ground-based validation of GLAS using data from White Sands Missile Range in New Mexico. Measurements from a field-portable, passive spectrometer are also shown for comparison