Effects of Resonances in Corrugated Horn Antennas for a 22 GHz Balancing Radiometer

Abstract-The Stratospheric WAter vapor RAdiometer (SWARA) is a microwave radiometer designed for ground-based measurements of water vapor (H 2 O) in the middle atmosphere (20 to 80 km), including the stratosphere and mesosphere. The instrument is operating in a noncryogenic balancing calibration mode. Since its deployment, features have been observed in the spectrum which can be attributed to resonant variations of the antenna pattern of the corrugated horn. This paper presents copolar and crosspolar antenna pattern measurements of two sister antennas of the SWARA horn, as well as water vapor measurements from both antennas on the ground-based microwave radiometer MIddle Atmospheric WAter vapor RAdiometer. We show that small irregularities in the frequency spectrum at the −20-dB level are visible in the copolar pattern, which, due to the balancing operation scheme used for the radiometer, lead to features in the spectrum that have the same or even higher brightness temperature as the line of interest. Index Terms-Antenna pattern measurements, microwave remote sensing, middle atmosphere, water vapor

Stratospheric Water Vapour in the Tropics: Observations by Ground-Based Microwave Radiometry

This thesis reports on observations of tropical stratospheric water vapour by the ground-based microwave radiometer/spectrometer WaRAM2 in 2007. The 22GHz receiver is set up at Mérida Atmospheric Research Station on top of Pico Espejo, Venezuela (8°32'N, 71°03'W, 4765m above sea level). It is the only such sensor that continuously operates at tropical latitudes. The high altitude site is ideally suitable for microwave observations, because most tropospheric water vapour is located below the sensor. Water vapour plays a key role in middle atmospheric processes. Because of its large infrared resonance, it strongly participates in the radiative budget, both in terms of a greenhouse effect at lower altitudes and radiative cooling at higher altitudes. It is a source gas for the highly reactive hydroxyl radical, and exerts indirect effects on ozone destruction in the formation of polar stratospheric clouds. Due to its long lifetime, water vapour also serves as a dynamical tracer