Realization of PATH Goals using a Small Satellite Constellation

In their most recent decadal assessment (Earth Application from Space, 2007) of Earth science space missions the U.S. National Research Council identified the Precipitation and All-weather Temperature and Humidity (PATH) mission as one of ten recommended medium cost ($300M-$600M) missions. Based on the NRC's outlined goals, PATH would have the unique capability of providing all-weather temperature and moisture soundings and cloud and raincell imagery at spatial scales comparable to AMSU-A/B or ATMS, but at sub-hourly temporal resolution. The essential capability of PATH is to provide the atmospheric penetrability and spatial resolution of operational microwave sensors but with temporal resolution commensurate with the natural rate of evolution of convectively driven weather. Although not specifically required for PATH, microwave spectral imagery at 50, 118, and 183 GHz with spatial resolution of ~10-30 km and temporal resolution of ~15-60 minutes could be expected to significantly enhance forecasting of mesoscale convective weather and hurricane rain band evolution, along with provide valuable temporal gap-filling data for synoptic weather forecasting.

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 ~20-30 Cubesats hosting 50-57, 118, and 183 GHz cross-track scanning spectrometers. Several Cubesat missions using these bands are under study and development at CU, MIT/LL, NASA/JPL, and NASA/GSFC. In order to achieve the necessary spatial resolution at these bands within the CubeSat form factor orbital altitudes between ~425-500 km altitude are desirable. Also desirable is the capability to assimilate radiances from such sensors into either 4D-Var or interpolative numerical weather models in order to permit low-cost access to space using “as-available” launch slots.

We discuss in this presentation the merits of such a random constellation of passive microwave sounding/imaging Cubesats from the multiple viewpoints of data assimilation and global sampling, downlink capability and latency, orbital lifetime, launch availability, deorbiting capability, system reliability, and operational risk. Attention is given to constellation technology infusion as impacted by the anticipated economy of scale necessary for refreshment of the constellation. We argue from a joint technology, science, and operational standpoint that a cost-effective realization of the PATH goals, but with the additional feature of global coverage, can be achieved by a low-cost random-orbit constellation of 3U 118- and 183-GHz Cubesats supplemented by a comparably orbiting set of 50-57 GHz Cubesats. The CU PolarCube mission, designed to demonstrate the essential features of a 3U Cubesat member of this constellation, will be discussed.