An optical radar for airborne use over natural waters

An optical radar for detecting targets in natural waters was built and tested in the Gulf of Mexico. The transmitter consists of a Q-switched neodymium-glass laser, with output amplified and doubled in KDP to 0.53 micrometer wavelength. The receiver incorporates a novel optical spatial filter to reduce the dynamic range required of the photodetector to a reasonable value. Detection of targets to a depth of 84 feet was achieved with a considerable sensitivity margin. The sensitivity of the radar is highly dependent on the optical attenuation coefficient. In general, measured returns fell between the values predicted on the basis of monopath and multipath attenuation. By means of simple physical arguments a radar equation for the system was derived. To validate this theoretical model, measurements of optical attenuation and of water surface behavior were also instrumented, and some of these results are given. Volumetric backscatter was measurable over the entire depth range; such radars should therefore be useful for monitoring natural water quality. Airborne bottom-profiling is another application.