Hyperspectral microwave atmospheric sounding from geostationary orbit: The GeoMAS concept

Recent advances in microwave device technology permit high-performance radio-frequency (RF) receivers to be miniaturized and tightly integrated with intermediate-frequency (IF) electronics, thus reducing mass, power, and volume of spaceborne and airborne spectrometers. These advances also enable microwave receiver arrays for synthetic sparse-aperture systems. Alternative receiver arrays can multiplex multiple broad frequency bands into more than ~100 spectral channels, allowing hyperspectral operation. For example, multiple antenna feedhorns could sample a given point on the ground with each antenna sampling a slightly different set of spectral response functions. Thus an arbitrarily large number of spectral channels can be synthesized to substantially increase the radiometric information content per field of view. This increase can improve both the vertical and horizontal resolution of the retrieved atmospheric profile. A simulation study using approximately 100 channels sampled near the 118.75-GHz and 183.31-GHz absorption lines demonstrates that such frequency multiplexing techniques substantially improve temperature and moisture profiling accuracy, especially in atmospheres that challenge conventional non-hyperspectral millimeter-wave sounding systems because of high water vapor and cloud liquid water content. Hyperspectral millimeter-wave operation at geostationary altitudes is particularly appealing, since practical antenna reflector sizes less than 2 m in diameter can be used. Global simulations over ocean and land demonstrate that the temperature and water vapor profiling performance of a modest hyperspectral millimeter-wave system with only 16 receiver arrays (eight antenna feeds) exceeds that of a nominal ~900-receiver 10-channel 60/183-GHz synthetic thinned aperture radiometer (STAR) system throughout the entire troposphere (including the surface), even in very moist atmospheric cases with integrated water vapor approaching 100 mm and integrated cloud liquid water approaching 1 mm. This notional system (the Geostationary Millimeter-wave Array Spectrometer, or GeoMAS) yields RMS errors in 2-km layers in cloudy, non-precipitating atmospheres of 0.5-1.25 K (temperature below 20 km) and 15-25 percent (water vapor mass mixing ratio below 10 km, relative to a priori). Precipitation retrieval accuracies of GeoMAS also exceed those of the STAR system. Thus, GeoMAS could fulfill all the requirements of the proposed NASA Precipitation and All-Weather Temperature and Humidity (PATH) mission while exceeding the performance of a STAR system in all products throughout the troposphere.

This work was sponsored under Air Force contract FA8721-05-C-0002. Opinions, interpretations, conclusions, and recommendations are those of the authors and not necessarily endorsed by the United States Government.