Researchers have demonstrated a sub-millimeter-wave spectrometer that combines extremely broad bandwidth with extremely high sensitivity and spectral resolution to enable future spacecraft to measure the composition of the Earth’s troposphere in three dimensions many times per day at spatial resolutions as high as a few kilometers. Microwave limb sounding is a proven remote-sensing technique that measures thermal emission spectra from molecular gases along limb views of the Earth’s atmosphere against a cold space background.
The new receiver will down-convert thermal emission spectra in the 180–300 GHz band using superconductor-insulator-superconductor (SIS) heterodyne mixers. A technique called sideband separation is used to provide 24 GHz of instantaneous bandwidth from a single receiver, enabling many chemical species to be measured simultaneously by a single receiver with accurate calibration. The high sensitivity provided by SIS mixers will enable accurate measurements of chemicals at low concentrations with very short integration times. A novel scanning telescope, also under development at the Jet Propulsion Laboratory, will take advantage of these short integration times to measure three-dimensional maps of the concentration of a large number of key chemical species in the troposphere over nearly the entire planet five to nine times per day. These frequent measurements will enable researchers to both monitor air quality and to understand how pollution is transported by the atmosphere.
This work was done by John S. Ward, Bruce Bumble, Karen A. Lee, Jonathan H. Kawamura, Goutam Chattopadhyay, Paul Stek, and Frank Rice of Caltech for NASA’s Jet Propulsion Laboratory. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com /tsp under the Electronics/Computers category. NPO-46205
This Brief includes a Technical Support Package (TSP).
Sideband-Separating, Millimeter-Wave Heterodyne Receiver
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Overview
The document discusses the development of sensitive broadband receivers for a next-generation Microwave Limb Sounder (SMLS) designed to enhance atmospheric remote sensing capabilities. Microwave limb sounding is a technique that analyzes the thermal emission spectra from the Earth's atmosphere, allowing for the retrieval of temperature and composition data as a function of altitude. The SMLS aims to improve upon the previous Microwave Limb Sounder (MLS) instruments used on NASA's Upper Atmosphere Research Satellite (UARS) and the Aura spacecraft.
The SMLS will feature two receivers operating at different frequencies: a 230 GHz channel for studying the upper troposphere and a 640 GHz channel for stratospheric measurements. Each receiver is designed with a broad tunable bandwidth of 100 GHz, enabling the simultaneous measurement of various atmospheric species, including water vapor, ozone, carbon monoxide (CO), hydrogen cyanide (HCN), nitrogen oxides (NO), sulfur dioxide (SO₂), and acetone. The use of superconductor-insulator-superconductor (SIS) heterodyne mixers allows the receivers to achieve high sensitivities of 100 K and 200 K for the respective channels.
The document highlights the technical specifications of the lower-frequency receiver, which employs a sideband-separating SIS mixer. This design enhances the effective instantaneous bandwidth and improves calibration accuracy by eliminating sideband imbalance uncertainties. The receiver splits the input signal into two paths with a 90° phase shift, downconverts the signals, and recombines them to separate the sidebands, thus optimizing performance.
The SMLS is expected to provide a 3-D map of atmospheric composition with improved horizontal and vertical resolution, sampling a 6000 km cross-track swath at 50 km resolution. This capability will enable frequent measurements—up to six times daily in mid-latitudes—facilitating the study of rapid atmospheric processes that influence climate, chemistry, and air quality.
The research and development of these advanced receivers are conducted at the Jet Propulsion Laboratory (JPL) under NASA's auspices, with the goal of enhancing our understanding of the Earth's atmosphere and its dynamics. For further inquiries, contact information for JPL is provided in the document.
