The Next Frontier: Nanosatellite Constellations for High-Resolution Microwave Earth Remote Sensing

The need for low-cost, mission-flexible, and rapidly deployable spaceborne sensors that meet stringent performance requirements pervades the NASA Earth Science measurement programs, including especially the recommended NRC Decadal Survey missions. The challenge of data continuity further complicates mission planning and development and has historically been exacerbated by uncertain and sometimes substantial shifts in national priorities and budget availability that have degraded and delayed critical Earth Science measurement capabilities. Indeed, the recently published Midterm Assessment of NASA's Implementation of the Decadal Survey finds that, "The nation's Earth observing system is beginning a rapid decline in capability as long-running missions end and key new missions are delayed, lost, or canceled. The projected loss of observing capability could have significant adverse consequences for science and society." To address these challenges, we present nanosatellite constellation architectures that would profoundly improve both the performance and cost/risk/schedule profiles of NASA Earth and Space Science missions by leveraging recent technology advancements and present a path forward to bring these emerging capabilities into operational use. As a key enabling element, we describe the MicroMAS (Microsized Microwave Atmospheric Satellite) 3U CubeSat mission to be launched by NASA in 2014. Furthermore, we assess a possible evolution to a scalable and mission-flexible 6U CubeSat-based self-organizing constellation architecture (the Distributed Observatory for Monitoring of Earth, henceforth "DOME") that would achieve state-of-the-art performance (and beyond) relative to current systems with respect to spatial, spectral, and radiometric resolution.

MicroMAS is a 3U CubeSat (30x10x10 cm, ~4kg) hosting a passive microwave spectrometer operating near the 118.75-GHz oxygen absorption line. The focus of the first MicroMAS mission (henceforth referred to as MicroMAS-1) is to observe convective thunderstorms, tropical cyclones, and hurricanes from a near-equatorial orbit at approximately 500-km altitude. The MicroMAS-1 mission is internally funded and is based on systems analysis funding provided by NOAA. A parabolic reflector is mechanically rotated as the spacecraft orbits the earth, thus directing a cross-track scanned beam with FWHM beamwidth of 2.4-degrees, yielding an approximately 25-km diameter footprint from a nominal altitude of 500 km. Radiometric calibration is carried out using observations of cold space, the earth's limb, and an internal noise diode that is weakly coupled through the RF front-end electronics. A key technology feature is the development of an ultra-compact intermediate frequency processor module for channelization, detection, and A-to-D conversion. The antenna system and RF front-end electronics are highly integrated and miniaturized. In this talk, the mission concept of operations will be discussed, the radiometer payload will be described, and the spacecraft subsystems (avionics, power, communications, attitude determination and control, and mechanical structures) will be summarized. Test data from the recently completed MicroMAS Engineering Development Model (EDM) will also be presented.

A second focus of this presentation is the evolution of MicroMAS-1 to a cross-linked 6U CubeSat constellation with onboard propulsion systems for high-fidelity Earth and Space Science research. Such architecture could provide game-changing advances by reducing costs by at least an order of magnitude while increasing robustness to launch and sensor failures, allowing fast-track insertion of new technologies, and improving science performance. High-resolution passive microwave atmospheric sounding is an ideal sensing modality for nanosatellite implementation due to rapidly advancing microwave and millimeterwave receiver technology. The DOME constellation would nominally comprise 6U CubeSat Microwave Atmospheric Sounder (CMAS) satellites. Each CMAS satellite would host a complete 6U CubeSat atmospheric sounder, including a radiometer payload module with passive microwave receivers operating near atmospheric absorption lines near 60 and 183.31 GHz, and a spacecraft bus with attitude determination and control, avionics, power, cross-linked communications (spacecraft-to-spacecraft and spacecraft-to-ground), and propulsion systems. A spacecraft spinning mechanism provides a 60 RPM cross-track scan as the satellite orbits the earth. Spatial, spectral, and radiometric performance is comparable to present state-of-the-art systems with costs exceeding $100M. The propulsion systems would be used to achieve formation flight (the satellites would be separated by approximately 500 ± 5 km) and to facilitate de-orbit. The cross-linked communication would provide: 1) reduced communications latency to ground, a key performance attribute that is currently lacking in present systems leading to suboptimal utilization of observations of dynamic meteorological events such as tropical cyclones and hurricanes, and 2) data-driven sensing whereby the lead sensor observes dynamic meteorological phenomena and sends a message to the following sensor to temporarily enable a very high resolution sensing mode (a higher sample rate, for example) to better capture the interesting event and preserve spacecraft resources for when they are most needed. The DOME constellation would allow global, high-resolution, persistent observations of the Earth's surface and atmosphere for studies of the hydrologic cycle and climate feedback processes.