An infrared heterodyne radiometer for high-resolution measurements of solar radiation and atmospheric transmission

Abstract

A tunable, Dicke-switched, infrared heterodyne radiometer (IHR) has been designed, fabricated, tested, and used to observe solar radiation and determine the atmospheric transmissivity in the <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9- 11-\mu</tex> m spectral band. The IHR provides a spectral resolution of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6.7 \times 10^{-3}</tex> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , a minimum detectable power level of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.9 \times 10^{-23}</tex> W/Hz, and a temperature resolution of less than 1 K for a source temperature of 1000 K, an IF predetection bandwidth of 100 MHz, and an integration time of 30 s. Detailed design equations and measured IHR performance are presented. The IHR was used to make solar and atmospheric transmission measurements and a vertical path atmospheric attenuation of 2.3 dB has been established under favorable weather conditions. The attenuation of solar radiation due to cloud cover and haze have also been investigated.