At least three geostationary sensor concepts have been identified that respond to PATH goals, which call for a “microwave array spectrometer” to meet these needs. A 2-m filled-aperture GEostationary Microwave (GEM) instrument was defined by Staelin et al. in 1997 as part of a joint NOAA-NASA study. This instrument used a set of spectrometers operating at 50-57, 118, 183, 380, and 425 GHz and scanned by motion of the main reflector and subreflector. A Geostationary synthetic Thinned Aperture Radiometer (GeoSTAR) based on aperture synthesis and requiring no moving components has been under development by Lambrigsten et al. at NASA Jet Propulsion Laboratory. The GeoSTAR baseline design provides sounding at 50-57 and 183 GHz, although with some compromises on area of coverage, sensitivity, and spectral channel set. A hybrid geostationary system using a filled aperture 183 GHz scanning radiometer and a slowly rotating thinned aperture clock radiometer is under development at the National Space Science Center in Beijing.
However, none of these systems will alone provide global coverage, but instead will be stationed so as to observe critical regional events (hurricanes at landfall, typhoons, nor'easter storms, etc…). Due to recent advances in microwave receiver and filter technology a more cost effective means of achieving PATH goals is proposed to be based on a fleet of nanosatellites, and specifically a constellation of ~30-40 CubeSats hosting cross-track scanning spectrometers observing using the ATMS channel set along with several sounding channels at the 118.750 GHz O2 line. Several CubeSat concepts using these bands or a subset of them are being demonstrated at the University of Colorado, MIT/LL, NASA/JPL, NASA/GSFC. A commercial mission to realize PATH goals using CubeSats and these bands is under commercial development at Orbital Micro Systems (OMS, Inc.).
Critical to achieving the goals of PATH and to using CubeSat passive microwave fleet data for numerical weather prediction (NWP) and climate change studies is precise knowledge of the radiometer's spatial and spectral responses and noise and stability characteristics. This sensor metadata must be accurately predictable from pre-launch studies and verifiable while on orbit, but by moderate means given the anticipated low required cost of fleet deployment (~$2M per spacecraft). Accuracies of ~0.05-0.1 K will facilitate the assimilation of CubeSat fleet radiances into NWP models over all-weather conditions and with effective “fleet-averaged” sensitivities that are a factor of ~5-7 times lower than those of existing microwave satellite sensors. The use of NWP assimilation will in turn promote lower-cost access to space using “as-available” launch slots that result in quasi-randomized orbital sets.
We discuss in this presentation the merits of a random constellation of passive microwave sounding and imaging CubeSats from the multiple viewpoints of calibration accuracy, data assimilation and global sampling, downlink capability and latency, orbital lifetime, launch availability, deorbiting capability, system reliability, operational risk and economy of scale. We argue from a joint technology, science, and operational standpoint that a cost-effective realization of the PATH goals, but with the additional features of global coverage and improved NWP sensitivity, can be achieved by a low-cost random-orbit constellation of CubeSats supporting the ATMS and 118 GHz bands. The CU PolarCube and PATHSat missions are designed to demonstrate the essential features of this constellation, for which a commercial operational fleet to satisfy NOAA's EON/MW requirements is being developed by OMS.