Sensitive Broadband Receivers for Microwave Limb Sounding

Microwave limb sounding is a proven remote-sensing technique that resolves the spectra of microwave thermal emission along a limb view of the earth's atmosphere with a cold space background. The temperature and composition of the atmosphere as a function of altitude is retrieved by analyzing the spectra returned from a vertical scan of the limb. The Microwave Limb Sounder (MLS) instrument on the NASA Upper Atmosphere Research Satellite (UARS) was the first experiment to study the microwave limb from space, and was followed by the current EOS MLS instrument on the Aura spacecraft. We are developing a new class of highly-sensitive broadband receivers for a next-generation microwave limb sounder that scans the limb in elevation and azimuth to generate a 3-D map of atmospheric composition. Sensitive receivers are needed to reduce integration times to allow the addition of rapid horizontal scanning while maintaining high measurement precision. The Scanning Microwave Limb Sounder (SMLS) will sample a 6000 km cross track swath with 50 km resolution while doubling the vertical resolution of its predecessor, Aura MLS. The wide swath allows for six or more daily samples for most of the mid latitudes. These frequent measurements, combined with the good horizontal and high vertical resolution of SMLS are key to enabling the study of fast processes in the upper troposphere affecting chemistry, climate, and air quality. Two receivers are being developed for SMLS: a 230 GHz channel will be used to study the upper troposphere and a 640 GHz channel will focus on measurements of the stratosphere. Both receivers feature broad tunable bandwidth (100 GHz each) to enable measurements of many important species including water, ozone, CO, HCN, NO, SO2, and acetone. Each receiver will downconvert the signal using a superconductor-insulator- superconductor (SIS) heterodyne mixer to achieve sensitivities of 100 K and 200 K for the 230 GHz and 640 GHz channels, respectively. The high spectral resolution achieved with heterodyne detection enables precision measurement of the profiles of pressure-broadened lines. Broad instantaneous bandwidth (up to 24 GHz for each channel) will enable multiple species to be measured simultaneously. The receiver for the lower-frequency channel features a 180-280 GHz sideband-separating SIS mixer. The input signal is split into two paths with a 90° relative phase shift using a waveguide branch-line quadrature hybrid coupler. Each path is separately downconverted using niobium / aluminum nitride / niobium SIS mixers. The downconverted outputs of the two mixers are recombined with a 6-18 GHz stripline 90° hybrid coupler to separate the two sidebands, with the final output in two 12 GHz wide sub-bands. Separating the sidebands increases the effective instantaneous bandwidth of the receiver, rejects unwanted flux from the image sideband, and improves calibration accuracy by eliminating the sideband imbalance uncertainty inherent to double-sideband mixers. The research described herein was carried out at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA, under contract with the National Aeronautics and Space Administration.

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