Advances in Designing, Building, and Testing Configurable Reflectarrays for High-Resolution Microwave Sounding and Imaging from Small Satellite Platforms

There are current critical needs of NASA, NOAA, and other agencies carrying out Earth environmental monitoring for higher-performance observing systems (lower noise, finer resolution, broader coverage, etc.) but also smaller, lower-cost sensing platforms that offer flexibility in how they are deployed and used. To achieve these ambitions, it is necessary to consider the observing system, comprising not only the sensor, but also the concept of operations, processing, and potential for collaborative and synergistic observations. Here we present a new approach that enables dynamic, data-driven sensing and provides a way to test and evaluate the overall end-to-end system performance in the laboratory prior to launch with realistic Earth scenes. Recent technology advances now enable the utilization of new sensing concepts that reconfigure the sensor in real time to adjust where they are looking, their dwell time, their spatial resolution, and depending on the platform, their geometrical vantage point. For example, at frequencies spanning approximately 10-100 GHz, phased array and reflectarray observations of wind (speed and direction), wide-swath polarimetric imagery, soil moisture and sea surface temperature, and atmospheric thermodynamic state that are deemed critical by NASA’s strategic planning are now possible. These measurements would all be improved by this work, since the sensor would be configured for maximum resolution, coverage, and dwell time for regions in the scene that exhibit the highest variability and would thus benefit the most from high-fidelity sensing. This approach also efficiently optimizes use of a fixed set of resources.

Here we describe two new systems recently funded by the NASA Earth Science Technology Office (ESTO) to improve present capabilities for high-resolution atmospheric sensing from small satellite platforms. First, the Configurable Reflectarray Wideband Scanning Radiometer (CREWSR) is a high-resolution, lightweight, low-power multiband (23, 31, and 50-58 GHz) radiometer with a deployable scanning reflectarray. It is envisioned to be fielded on an ESPA-class small satellite platform, with a stowed volume that fits within a 0.61 m x 0.71 m x 0.97 m envelope. Once in orbit, the platform will deploy a large Reconfigurable Reflective Surface (RRS), as well as a multi-feed antenna connected to a multiband radiometer. These components allow for an electronically-scanned beam for radiometric Earth observation. CREWSR would operate with a single, linear polarization, but fully polarimetric operation is also possible in principle. The reflectarray is also compatible with radar use, thus enabling wide-swath radar from a small satellite.

Second, there is a critical need to enable development, test, and evaluation of Versatile, Intelligent, and Dynamic Earth Observation (VIDEO), and one key enabler is the recent emergence of metamaterials for use in high-performance blackbody radiometric targets. These materials are very thin (~200 microns) and lightweight (tens of grams), allowing them to be easily scaled up to realize very large targets (> 1 m^2) to subtend an entire sensor field of regard during laboratory measurements. Furthermore, the thin planar structure of the metamaterials provides a relatively small thermal mass, thereby permitting the projection of thermal features with very high spatial frequency content into the sensor field of view at the subpixel level. We will produce a 50 cm x 75 cm (20” x 30”) Radiometric Scene Generator (RSG) operating near 54 GHz with very large thermal contrast at the subpixel level for a typical spaceborne microwave radiometer full-width-at-half-maximum (FWHM) beam width in the range of 1-3 degrees. The RSG will be used for two purposes: (1) to project spatial features into the radiometer field of regard that can be detected and acted upon by the intelligent processing algorithms, and (2) to spatially encode spectral information in the atmospheric sounding band used for temperature profiling. The intelligent processing algorithm will utilize feature detection and machine learning techniques to recognize regions of interest in the atmospheric scene and cause the sensor to react to the scene characteristics by changing the sensor response function. In this presentation, we will provide an overview of these new technologies and discuss how they address current unmet needs for high-resolution sensing from small satellites in the critical frequency bands spanning 20-60 GHz.