Comparison of two multisheet transmission windows for millimeter-wave radiometers

The design and performance of a triple-sheet Mylar window and a Teflon-and-foam window are studied theoretically and experimentally to determine which window produces the smaller brightness temperature variations and, therefore, leads to better accuracy of radiometers operating at frequencies of 20-90 GHz. Theoretical brightness temperatures due to emission and reflection by the windows are calculated and compared; experimental measurements obtained by using the clear atmosphere as a radiation source are also compared. Both theory and experiment indicate that the Teflon-and-foam window produces smaller brightness temperature variations at these operating frequencies. Effects of the windows on integrated water vapor and liquid measurements are also examined; errors are significant for the triple-sheet Mylar window but are negligible for the Teflon-and-foam window. It is observed that use of the Teflon-and-foam design results in acceptably small errors in radiometric measurements of water vapor and cloud liquid