Abstract: Over the past ten years ground-based mm-wave atmospheric spectroscopy has emerged as a powerful tool for measurements of the vertical profile of water-vapor abundances in the middle atmosphere. The use of masers to provide front-end amplification in previous experiments has, because of their inherent limited rf bandwidth, limited the measurement technique to the mesosphere and upper stratosphere. However, solid-state amplifiers have now emerged with sensitivities nearly as good as those obtained with masers, but with much larger bandwidths. These amplifiers have made it possible to extend the measurement range of this technique into the lower stratosphere. In response to this development, and the need for long- term continuous stratospheric water-vapor measurements, NRL has designed and built a new mm-wave radiometer system specifically designed for making measurements of water vapor in the stratosphere. The instrument, which is based on a high electron mobility transistor (HEMT) front-end amplifier, is described in detail. The data retrieval scheme and the results of an extensive instrument data simulation study are also presented. It is found that knowledge of the tropospheric water-vapor content and profile are important in order to obtain good retrieval accuracy below 30 km.!
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