A Geostationary Microwave Sounder: What, Why, How, When

A geostationary microwave sounder, capable of providing continuous monitoring of temperature, water vapor, clouds, precipitation, and wind in the presence of clouds and precipitation, will add tremendously to our ability to observe dynamic atmospheric phenomena, such as hurricanes and severe storms, monsoonal moisture flow, atmospheric rivers, etc. Such a sensor is now feasible, enabled by technology that has been developed under NASA’s Instrument Incubator Program. The Jet Propulsion Laboratory has led that development in partnership with the University of Michigan. A prototype, the Geostationary Synthetic Thinned Aperture Radiometer (GeoSTAR), is essentially an “AMSU in GEO”, i.e. it will provide similar measurements as are now obtained with AMSU and ATMS but every 15 minutes instead of every 12 hours and covering an entire hemisphere instead of a narrow swath. GEO orbits are almost 50 times higher than the LEO orbits that current microwave sounders operate from, and the corresponding scaling of aperture size required to maintain spatial resolution had stymied the development of such a sensor for many decades in spite of an expectation in the atmospheric science community that a geostationary microwave sounder would revolutionize the field. The aperture synthesis approach implemented with GeoSTAR finally overcomes that obstacle, and the large number of microwave receivers and associated electronics required is made possible with the new technology that has now been developed and fully tested. Low-risk mission development can start as soon as funding is available.

An interesting option to implement such a mission, which might normally cost on the order of $500M, is to host the instrument on a commercial communications satellite, which could reduce the cost by a factor of 3-4. Plans to demonstrate such a mission have been developed at JPL for proposal to NASA’s Venture program. Called “GeoStorm”, it is focused on observing severe convective storms – tropical cyclones, mesoscale convective systems, and extratropical cyclones – with a goal of improving our understanding, modeling and prediction of these destructive phenomena. GeoStorm can equally well be configured as an operational mission, where the goal is to collect data for immediate assimilation into regional forecast systems, provide “now-casting” as the storms unfold, and to support post-disaster relief and recovery efforts. Many focused applications are possible, particularly pertaining to aviation, transportation and marine operations, in both the civilian and defense domains. While efforts are currently under way to explore whether constellations of cubesats and other small satellites can provide the same functionality as GEO systems at lower cost, it is far from certain that such an approach will succeed. In the meantime, GeoSTAR, with its proven design and technology, is ready to be implemented. With the growing threats from severe weather, a geostationary microwave sounder is timely indeed.

Copyright 2016 California Institute of Technology. Government sponsorship acknowledged.