Abstract
During the second half of the '70's Ben-Shalom et al.1 developed a system for research of the spectral transmittance of the atmosphere in the infrared range on nearly horizontal atmospheric paths between 2.5 and 14 microns. The system configuration is bi-static: a source (of known emission) and a measuring sensor station linked by line of sight. Comparison of the measured radiation with the known emission gives the fraction of transmitted radiation. The longest path published there was a respectable 44 Km. and provided significant data. However, the system required a very high temperature (2400K) short lived (~3 hours) emitter element, home-built projection optics, and a cumbersome closed cycle water cooling system for the 6 KWatt source. The sensor end of the system was a Circular Variable Filter (CVF) based spectroradiometer using liquid nitrogen (LN2) cooled detectors. The signal processing electronics was based on a synchronous detection method using a chopper at the source and a radio-transmitted reference signal in phase with the chopper. In this paper we describe the Atmospheric Transmittance Measurement System (ATMS) recently built by CI and presently undergoing reliability and accuracy tests. Its main advantages over the old system are: it is built of only commercial off-the-shelf items (COTS), it can measure in both the IR and visible ranges simultaneously, it is cost effective and easy to use and maintain. The calibration method and transmittance measurement algorithm are also described. A transmittance measurement of a nearly horizontal sea level path of 6.5 Km. is shown here as an example for the whole spectral range of 0.4 to 14 microns in less than very clear and dry weather conditions. The result shows agreement with the MODTRAN model on the spectral behavior but at the time of publication we are still investigating the accuracy of the actual transmittance values. We believe that the ATMS can be used for longer paths, based on the signal to noise ratio encountered at 6.5 Km. In addition and in contrast to the old '70's system, which could be used only for measurements above 1 Km. paths, the ATMS is built and calibrated so that it can measure transmittance through short paths of few tens of meters.
Published Version
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