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

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, has performed global brightness temperature observations at 87, 164, 174, 178 and 181 GHz, throughout its nearly 3-year mission. Successful on-orbit demonstration of the TEMPEST-D mission has clearly demonstrated that microwave imagers and sounders on CubeSats are capable of 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 GMI instrument on the GPM NASA/JAXA mission.

TEMPEST-D, RainCube and other CubeSat missions, including the recent TROPICS 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 cost, 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 microwave radiometer calibration for heterogeneous small satellite sensors, based on a trustable, SI-traceable calibration source.

Our 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 commonly used for atmospheric sounding and imaging. Individual metamaterial unit cells have been combined to form supercells, to realize broadband high emissivity at a number of desired millimeter-wave frequencies, 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 and 90 GHz, and are extending their performance to the 118 GHz temperature sounding and 183 GHz humidity sounding bands. To improve the repeatability and predictability of dielectric properties above 100 GHz, our metamaterial prototype fabrication is currently being expanded from commercial PCB production to nanofabrication in university laboratories. These developments are expected to lead to the design, production and validation of a broadband metamaterial emitter operating at both millimeter-wave sounding and imaging channels from 50 GHz to 230 GHz, to enable a thin, cost-effective calibration target for millimeter-wave sounding and imaging from small satellites.

At the same time, the team is developing an SI-traceable calibration target for broadband operation from 50 GHz to 230 GHz. The thermal homogeneity and emissivity of the NIST blackbody calibration target have been modeled and experimentally validated using a thermal vacuum chamber at Northrop Grumman Space Systems in Azusa, CA, with target temperatures near 80 K. This blackbody target will provide an SI-traceable standard for pre-launch calibration for a wide variety of heterogeneous millimeter-wave radiometers for atmospheric remote sensing from small satellite constellations.