Emerging Microwave Radiometer Calibration Technologies using Metamaterials to Enhance Atmospheric Sounding and Remote Sensing of Clouds and Precipitation from Small Satellites

Passive microwave sounders and imagers provide critical observations for initialization and validation of regional and global weather forecasting models. Although much progress has been made in global climate modeling, extreme weather events are still misrepresented in parameterizations, largely due to a lack of sufficient observations. However, passive microwave sensors perform Earth observation exclusively from low Earth orbit (LEO), and their limited revisit times on the order of twice daily lead to substantial temporal sampling gaps from the tropics and sub-tropics to the mid-latitudes, where cyclones and other precipitating storms cause the greatest damage to life and property.

In this context, the Temporal Experiment for Storms and Tropical Systems Technology Demonstration (TEMPEST-D) NASA Earth Venture Technology demonstration mission, originally intended for 3 months of operation, performed the first global multi-frequency microwave brightness temperature observations throughout its nearly 3-year mission. Successful on-orbit demonstration of the TEMPEST-D mission has demonstrated the capability of microwave imagers and sounders on CubeSats to provide well-calibrated, highly stable, low-noise microwave radiometer observations. TEMPEST-D observations have led to improved temporal resolution in satellite observations of quantitative precipitation and advances in characterization of convection and humidity profiles in the vicinity of storms. TEMPEST-D passive microwave observations of precipitation systems were shown to be highly quantitatively correlated with observations from the active microwave instrument RainCube, the first-ever weather radar on an orbiting CubeSat. Finally, cross-validation between TEMPEST-D and the Global Precipitation Measurement (GPM) microwave imager (GMI) over precipitation systems showed that the TEMPEST-D CubeSat sensor performed similarly to the GPM/GMI instrument.

TEMPEST-D, RainCube and other CubeSat missions, including the TROPICS Pathfinder and constellation, have demonstrated the capacity for constellations of smaller, more affordable satellites to improve thermodynamic sounding of water vapor and temperature as well as microwave imaging of convective systems for operational weather forecasting. One of the principal benefits of deployment and operation of small satellite constellations of microwave sounders and imagers is to substantially improve temporal revisit times from low-Earth orbit (LEO). Other advantages include reduced sensor and launch costs, rapid infusion of new technology, and risk reduction based on deployment of larger quantities of satellite sensors, depending on available resources for deployment and operation, as well as satellite lifetimes. Incorporation of small satellite constellations into NASA’s scientific and NOAA’s operational LEO satellite systems will involve accurate and precise calibration of heterogeneous microwave sensors produced by a larger variety of instrument providers than previously thought possible. NASA and NOAA have critical unmet needs in terms of repeatable microwave radiometer calibration for heterogeneous small satellite sensors.

The Miniaturized Microwave Absolute Calibration (MiniMAC) team has demonstrated the design, development, and verification of metamaterial-based microwave absorbers, fabricated on organic-based printed circuit boards, as promising alternatives to traditional, bulky microwave absorbers for the calibration of microwave radiometers for atmospheric remote sensing. Their use is particularly attractive for on-board calibration of sensors on CubeSats and SmallSats. Thin metamaterial target prototypes fabricated from relatively inexpensive PCB technologies have been demonstrated to achieve near-unity emissivity at millimeter-wave frequency bands from nearly 50 GHz to 200 GHz that are commonly used for atmospheric sounding and imaging. Individual metamaterial unit cells have been combined to form supercells, to realize broadband high emissivity at multiple millimeter-wave frequency bands of interest, enabling a lightweight, cost-effective and thermally homogeneous alternative to more bulky blackbody sources. Our team has fabricated and experimentally verified near-unity emissivity supercell metamaterial calibration targets at 54 GHz temperature sounding, 90 GHz imaging, 118 GHz temperature sounding and 183 GHz humidity sounding bands. The emissivity performance of these metamaterial calibration targets has been validated in a millimeter-wave radiometer over a range of operating temperatures. These developments are expected to lead to the design, production and validation of a broadband metamaterial emitter operating at four millimeter-wave sounding and imaging channels, to enable a thin, cost-effective calibration target for millimeter-wave sounding and imaging from small satellites.