Atmospheric Propagation Effects on a Laser Ship Mast Detector

Abstract

The performance of a laser gate ship mast detector is evaluated. The laser gate consists of transmitting optics and laser on one side of a shipping channel and receiving optics and detector on the other side of the channel. Expressions relating the probability of detection and the false alarm rate to laser power, noise, visibility or atmospheric attenuation of the beam are developed. Finally, at power levels meeting safety standards and for useful probabilities of detection and false alarm rates, the visibilities which limit system performance are determined as a function of laser gate path length. The gate, at visible wave lengths, will function at visibilities only slightly less than the path length. In the next step, the expected performance of the laser gate using a CO₂ laser radiating at 10.6μ in the infrared is analyzed. The extinction coefficient, which is only a fraction of that in the visible case, is estimated for poor visibility conditions (RVR = 1,000 ft.) from a combination of attenuation measurements relating attenuation to total water content and meterological results relating visibility to total water content. Then, for the infrared system, the same arguments as above are invoked and, in this case, it is shown that the infrared system will provide the required performance at much longer path lengths.