Next Generation Hyperspectral Millimeter-wave Radiometers for Sub-orbital Atmospheric Sounding

The 2017 National Academies’ Earth Science decadal survey has identified the planetary boundary layer (PBL) as a new frontier for remote sensing that requires investment in new technologies, retrieval approaches and observational techniques. Current passive microwave systems and those planned to be launched in the next decade are not optimized for the frequency-resolving flexibility and resilience necessary for near-surface remote sensing. These conventional microwave sounders typically rely on difficult-to-tune analog back-ends with total power detectors and are therefore limited to only a few tens of spectral channels. This limitation results in sub-sampling of the vertical atmospheric temperature and moisture profile and increases retrieval errors. Additional drawbacks include potential pass-band variability between detectors, susceptibility to a changing radio frequency interference environment and limited use of the available spectral bandwidth per sounding level. Next-generation software-defined spectrometers based on high-speed digitizer circuits, or so-called ‘hyperspectral’ radiometers, overcome these limitations. Such systems are well poised to meet the needs of the scientific community while taking advantage of the maturation of analog and digital components for high frequency system design.

Two NOAA-sponsored radiometer technology development and demonstration activities ongoing at JPL, one of these in collaboration with Colorado State University (CSU) and World View Enterprises, that leverage software-defined spectrometer capabilities. The first is a wide-bandwidth three-channel airborne atmospheric microwave sounder (AMS) sensitive to the oxygen (O₂) absorption complex near 60 GHz and the O₂ and water vapor absorption lines at 118 GHz and 183 GHz respectively. This radiometer will be installed onto the NOAA G550 aircraft and be operated by their OMAO division to provide operational temperature and water vapor profile information for the study of convective storm systems, along with other weather phenomena. The second is a high-altitude (~20 km) balloon-mounted instrument based on an upgraded version of the CSU High-frequency Airborne Microwave and Millimeter-wave Radiometer High-Definition (HAMMR-HD) for acquisition of up to 30 days of continuous data. The HAMMR-HD instrument will provide a rich data set by sampling the full microwave spectrum from 18-200 GHz over a variety of atmospheric scenes.

At the heart of both these instruments are high-speed application specific integrated circuits (ASIC) for signal detection. These ASICs allow for development of low-power instruments while still providing significant improvements in information content and flexibility. Such software-defined systems are therefore very attractive for space-borne applications. The technology development presented here is also an opportunity to mature integration, test, and qualification, techniques for deploying such ASIC-based radiometers in space in the future. Furthermore, datasets acquired with these two instruments will be valuable for testing the ability to extract PBL structure from observed microwave spectra and support mission/instrument concept development for future PBL-focused missions for NASA and NOAA.

In this presentation, we will cover design principles of implementing very wide-band millimeter-wave radio frequency (RF) front-ends (> 40-GHz frequency span) as well as high speed ASIC-based digital back-ends (18-GHz frequency span) and the associated calibration challenges. We will also present extensive sub-system characterization that includes spectral response, white noise characteristics, and flicker (or 1/f) noise performance and their respective effects on instrument sensitivity as well as operational concepts. Lastly, outdoor open-sky performance of these instruments will be discussed, along with radiometric calibration requirements and techniques.