Evaluation of a Prevailing Visibility Sensor Based on a Scanning Solid State Video Camera
PHILLIPS LAB HANSCOM AFB MA
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As part of its effort to automate surface weather observations, the U.S.Air Force funded the development of a visibility sensor that duplicates the function of a trained weather observer it scans the horizon and estimates the prevailing visibility by the reduction in visibility target contrast in many different directions. Just as for the human observer, the accuracy of this method depends upon the number and quality of visibility targets. Good targets have a known range and a high intrinsic contrast i.e., close to black that is stable with time of day and season of year. Methods were developed for assessing time variations in inherent target contrast and for detecting significant simultaneous changes in the inherent contrast of many targets, such as caused by sticking snow. The system was operated at two sites with significantly different sets of visibility targets. One site included distant mountains as targets. The other was limited to nearby targets. Plywood targets. The other was limited to nearby targets plywood targets painted black were constructed at this site to assess sensor performance with ideal targets. In contrast to the point visibility sensors used in most automated weather observing systems, the prevailing visibility sensor can, in principle, detect sector variations in visibility. In order for this capability to be effective in practice, many targets must be located at different ranges and in different directions.... Visibility, Prevailing visibility, Weather observations, Automated weather observations, Aviation weather.