Measuring the 557 GHz Water Vapor Absorption Line with Radar Speckle Averaging

A 557 GHz short-range radar has been built to demonstrate how absolute humidity can be measured remotely in a low pressure atmosphere, such as on Mars. This concept relies on accurately measuring terrain reflectivity over bandwidths of up to a few GHz, and comparing the result to well understood models of the water vapor absorption line centered at 557 GHz. For realistic terrain profiles, accurate reflectivity measurements can only be obtained by averaging over many independent speckle realizations. In this paper, we measure the water vapor absorption resonance in a laboratory gas cell using speckle-averaging of reflectivity signals from a rough, moving target.