System for imaging through inclement weather at submillimeter wavelengths

Abstract

A concept for an active imaging system which includes a laser to scan the scene in raster fashion boresighted with a scanning receiver is under investigation. This is in effect a laser-aided forward looking infrared system which will be employed to recognize vehicular targets in inclement weather at ranges of several kilometers. The scattering properties of fog and clouds require operation at wavelengths greater than 100 µm in order that the range may not be severely limited by attenuation. The scattering properties of rain, and the need to have sufficient angular resolution consistent with a receiver diameter no greater than 1-2 m, place an upper limit to the wavelength of about 2 mm. The optical transmittance of the clear atmosphere permits operation only in a few windows between 350 µm and 1.5 mm. The range equation based upon an optical heterodyne detection system has been derived for operation in the selected windows in clear weather, fog, and rain. The properties of submillimeter lasers and detectors have been reviewed. A program is now underway at U.S. Army Missile Command to develop atmospheric attenuation and target signature information.