Automated spectro-goniometer: A robitic arm for the measurement of snow bi-directional reflectance distribution function

We describe the UCSB Automated Spectro-Goniometer (ASG), a two-axis robotic arm paired with a field spectroradiometer for the rapid and accurate measurement of the optical bi-directional reflectance distribution function (BRDF) of snow. Improvements in radiative transfer models and remote sensing algorithms for snow properties require a more accurate characterization of the angular distribution of reflectance of snow. Previous measurements of snow BRDF have been incomplete with respect to one or more of spectral range, spectral resolution, angular resolution and sampling, solar zenith sampling, and snow characterization. The kinematics of the ASG may be described by Rodrigues ' fonnula for a 2 degree of freedom arm. We describe the forward kinematics for the ASG and then solve the inverse problem from given view angle to necessary rotation about each axis. Its two-dimensional hemispheric sampling space facilitates the measurement of spectral reflectance from snow into any direction. The ASG will usually sample a 10deg hemispheric grid at a 0.5 Hz sampling rate. We will also sample the solar principal plane at 1deg angular resolution at 0.5 Hz sampling rate. With a total weight of 41 kg, the ASG is easily portable by an individual via sled.