Development of system for measuring visibility along the free space optical link using digital camera

Fog and low cloudiness are the worst phenomena for Free Space Optical (FSO) links. High density of small water particles of the size comparable to the wavelength of radiation attenuates the signal and cause communication outages. We present the results from the constructed visibility measurement using camera with black and white target in real conditions at the mountain meteorological observatory Milešovka. It is proved that the camera measurement system is able to measure inhomogeneous fog along the FSO link and detects quick changes in visibility in contrast to the professional visibility meters. The correlation coefficient between the measured attenuation and theoretic attenuation of optical link computed from measured visibility reaches value 0.97 in the selected time interval. The camera measurement differs from the calibrated visibility meters by a maximum of a few meters in homogeneous fog.