The Automated Unmanned Surface Vehicle LimnoVIS as a Tool for Measuring BRDF in Inland Waters

Kurzfassung

As the observed scattering of light at suspended particles in the water column depends on the directions of illumination and observation, measuring the bi-directional distribution function (BRDF) of water bodies can help to validate both physical models and multi- or hyperspectral satellite data. BRDF measurements with spectrometers mounted on a ship can be tedious, inaccurate and time-consuming. LimnoVIS is an unmanned surface vehicle (USV) developed at the German Aerospace Center (DLR) for the collection of hyperspectral in-situ data at inland waters while avoiding the influence of a larger ship’s hull or superstructures on the measurements. With its capability to stay at a certain position and changing the orientation at the same time, it represents a tool perfectly suited for systematic BRDF measurements. It allows for automated measurement series of upwelling radiance (both with blocked and non-blocked skylight) and sky radiance with an azimuth angle relative to the sun (ΔΦ) between 0 and +/-180 degrees and an off-nadir angle (Δ ϑ) between 0 and 50 degrees with only little vehicle-induced distortions on the measurements. Downwelling irradiance is measured both with a cosine corrector and a reference panel. The control software of LimnoVIS provides an intuitive user interface for setting up any sequence of measurements with varying angles even while the USV is already deployed. Cameras are installed to give both a live view and a documentation of the water surface at the time of the measurements.