Atmospheric attenuation studies in the 183-325 GHz region

Abstract

The absorption characteristics of the earth's atmosphere in the 183-325 GHz frequency region of the electromagnetic spectrum are investigated. Instrumentation problems associated with coherent radiometric detection dictated the use of a wide-band Germanium bolometer detector. Upon helium cooling the Germanium element to 4.2\deg K the bolometer was observed to have a noise equivalent power of 10^{-9} watts for a 1-Hz bandwidth. Using the sun at two zenith angles as the signal source, measurements were made of the solar radiation as seen at the earth's surface through a set of wire mesh bandpass filters. The filters' transmission responses were determined by scaling the results of 109 GHz measurements. The total zenith atmospheric attenuation was then obtained as a function of frequency through the use of a spectral zonvolution technique. The results, especially in the window between the 183 and 325 GHz water vapor lines, seem to agree favorably with the calculated values made according to the Van Vleck-Weisskopf equations modified by the Schulze-Tolbert line-shape factor [1]. The minimum attenuation in the region was measured to be 0.6 dB/g.m3of surface water vapor density at 240 GHz.