Sunglint Detection for Unmanned and Automated Platforms

Shungudzemwoyo Pascal Garaba, Jan Schulz, Marcel Robert Wernand, Oliver Zielinski
2012 Sensors  
We present an empirical quality control protocol for above-water radiometric sampling focussing on identifying sunglint situations. Using hyperspectral radiometers, measurements were taken on an automated and unmanned seaborne platform in northwest European shelf seas. In parallel, a camera system was used to capture sea surface and sky images of the investigated points. The quality control consists of meteorological flags, to mask dusk, dawn, precipitation and low light conditions, utilizing
more » ... coming solar irradiance (Es) spectra. Using 629 from a total of 3,121 spectral measurements that passed the test conditions of the meteorological flagging, a new sunglint flag was developed. To predict sunglint conspicuous in the simultaneously available sea surface images a sunglint image detection algorithm was developed and implemented. Applying this algorithm, two sets of data, one with (having too much or detectable white pixels or sunglint) and one without sunglint (having least visible/detectable white pixel or sunglint), were derived. To identify the most effective sunglint flagging criteria we evaluated the spectral characteristics of these two data sets using water leaving radiance (Lw) and remote sensing reflectance ( R r s ). Spectral conditions satisfying 'mean L w (700-950 nm) < 2 mW-m 2-nm 1 ■ Sr or alternatively 'minimum Rrs (700-950 nm) < 0.010 Sr-1', mask most measurements affected by sunglint, providing an efficient empirical flagging of sunglint in automated quality control. Sensors 2 0 1 2 ,12 12546 Keywords: sunglint; empirical quality control; ocean colour; coastal and shelf seas; hyperspectral sensing 60 58 56 54 52 50 48 Instrumentation A RAMSES-ACC hyperspectral cosine irradiance meter (TriOS, Germany) was used to measure incoming solar radiation, E s (k). Two RAMSES-ARC hyperspectral radiance meters (7° field-of-view in air), were used to detect the sea surface radiance Lsfc (9sfc, , X) and sky radiance L±y (9sky, ®, X). A frame (see Figure 2 ) designed to hold the irradiance sensor facing upwards, with the sky and sea surface radiance sensors at zenith angles 9sfc = 45° and 9sky = 135°, was fixed to the mast of the ship facing starboard, 12 m above sea surface. These spectral measurements were automatically collected at 15 min intervals over a spectral range X = 320-950 nm in steps of 5 nm. A DualDome D12 (Mobotix AG, Langmeil, Germany) camera system with field-of-view set to 45°, was used to capture sky and sea surface images simultaneous to hyperspectral measurements, as illustrated in Figure 2 . Positioning height (-12 m above sea surface) of camera and optical sensors proved to be unaffected by sea spray. The camera's field-of-view was set congruent with the area observed by the radiometers, L±y and LS fC. Ship's position and heading were recorded by a Differential Global Position System (DGPS) and sampling times were logged in Coordinated Universal Time (UTC). North Sea "■sv Celtic S e a
doi:10.3390/s120912545 fatcat:h7playzpc5apldithlrboj6kua