Hyperspectral Radiometer

Passive microwave radiometers are an important remote sensing tool for observing and understanding physical processes in Earth’s oceans, atmosphere, land and cryosphere. They measure temperature, composition and water in all its forms, albeit at low spatial (20-60 of km) and spectral resolution. Models of the Earth system are continually evolving to higher spatial resolutions (sub-km scale) with less reliance on parameterizations in favor of physical models. Passive microwave sensor technology must evolve to sample the Earth over increasing broad portions of the microwave spectrum at km-scale spatial resolutions (< 10 km). To achieve this goal, JPL will mature broadband microwave imaging spectrometer array technologies to enable km-scale imaging from space. This task will harness and maintain JPL’s unique expertise in array systems and self-calibrating ultra-low noise MMIC RF receiver technology. We focus on two array technologies: thinned arrays for UHF to Ka-band and dense focal plane arrays for mm-wave systems (>45GHz). Both array topologies will use advanced digital spectrometer backends with integrated correlation and spectrometer capability. Applications for km-scale resolution imaging arrays include atmospheric tomography for observing 3D thermodynamic processes (e.g. boundary layer temperature and moisture, cloud and precipitation structure) and high-resolution surface imaging (e.g. temperature, soil moisture, surface wind vector, ocean salinity, vegetation water content, snow depth). Achieving spatial resolution parity with visible, infrared and active microwave sensors will offer the ability to observe small-scale processes with unprecedented detail. Advances in AI driven sensing will need high-resolution passive microwave systems for autonomously targeting equally high-resolution active instruments. Task A: Novel hybrid beam-forming, synthetic thinned-aperture radiometer technology for large in-space imaging arrays. This task builds on novel concepts developed through the ESTO IIP-ICD (2021) Separated Thinned Array for Sensing Ice Sheets (STASIS) project (PI: A. Akins/JPL). The key innovation is combining beam-forming within individual sub-array antenna elements which are then combined via aperture synthesis to effectively embed multiple-arrays into the same structure achieving the unprecedented sensitivity needed to sample signals at small spatial scales (0.5-10km). The array technology we will mature is applicable to any frequency, but game-changing for low-frequencies where large aperture sizes are needed and mechanically scanned approaches are limited to many 10’s of km resolution. For this task, we will focus on L-band arrays due to the dense information content in this portion of the microwave spectrum that is relevant to answering science questions from the 2017 Decadal Survey. We will develop a scalable, low-cost and mass-producible sub-array topology which can be arbitrarily arranged to form a larger array in a deployed structure. Our efforts will include (1) detailed trade studies to maximize design scalability and minimize mass, power, size and cost and (2) implementation of a component demonstrator to verify the results of these trades, thereby elevating the system TRL to at least 4. Task B: Dense focal-plane self-calibrating mm-wave arrays At mm-wave frequencies, dense focal plane arrays feeding mechanically scanned reflectors (solid or deployable) become practical to achieve km-scale resolution. JPL will develop the first focal-plane array (FPA) of compact, low-power continuous correlating imaging spectrometers around 118 GHz oxygen and 183 GHz water vapor lines to enable novel observational approaches such as “cloud-slicing,” where frequency-dependent atmospheric opacity is used to “scan” storms vertically. The noise precision of total-power radiometers (those currently flying and those in development for PBL missions) is limited by short term fluctuations referred to as “flicker” or “1/f” noise. This noise manifests as image striping and is the limiting factor that presently dominates overall sensor precision. The high stability correlating radiometer technique is self-calibrating and eliminates legacy front-end (Dicke) switches that have high losses at millimeter waves and eliminates the need for bulky free-space calibration targets which become impractical for large arrays. JPL will design custom RF MMIC front-ends and pair them a digital correlation spectrometer ASIC developed under a JPL supervised NASA SBIR. This resulting system enables (a) high spectral resolution for sensing of the narrow high-altitude oxygen and water-vapor absorption lines using 64 spectral channels, (b) greater-than 10 GHz total instantaneous bandwidths for simultaneous near surface sensing, (c) improved 1/f receiver stability and sensitivity without sacrificing observation time, and (d) one watt power consumption for scaling into FPAs of such receivers – all necessary for attaining both high sensitivity and sub-km spatial resolution from low Earth orbit.

Increase scientific understanding of natural phenomena using remote sensing.

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