The ABI-Based Combined Sounder/Imager (CSI)

A persistent challenge for weather forecasters is the generation of timely and accurate severe weather warnings. More frequent measurements of pre-storm atmospheric stability and moisture gradients, both horizontal and vertical, are needed to move to effective warning times of greater than an hour. Collecting the required rapid sampling information with high spatial resolution and continental scale coverage is an ideal application for satellite-based hyperspectral observations from geosynchronous orbit.

Currently, no existing or planned United States observing system can meet the unfilled needs of the operational geostationary hyperspectral sounding mission. A space-borne system to answer this challenge was highly recommended in the 2007 Decadal Survey. The Survey strongly supported implementing an advanced imaging sounder that would provide high resolution spectra from which high vertical resolution temperature and water vapor profiles could be obtained on a 500 km square imaging grid of 4-5 km pixels in 12 seconds. The 2007 Decadal Survey Executive Summary stated that “NOAA should restore several key climate, environmental, and weather observation capabilities” including the development of “a strategy to restore the previously planned capability to make high-temporal- and high-vertical-resolution measurements of temperature and water vapor from geosynchronous orbit.”

To meet both the future observational and budget needs of NOAA, Harris is developing a hyperspectral Combined Sounder/Imager (CSI) for future geostationary weather platforms. CSI operates in two different modes. While in the Imaging Mode, the CSI produces ABI spec-compliant data products (16 spectral channels, 0.5-2 km spatial resolution, and moderate spectral resolution). While in the Sounding Mode, the CSI produces hyperspectral sounding data (~2000 spectral channels, very fine spectral resolution) for regions of the earth that are identified as worth interrogating, based on the Imaging Mode data. This would include hurricanes, severe weather outbreak regions, or other areas of active weather. Imaging and sounding scenes can be seamlessly interleaved just like ABI interleaves Full Disk, CONUS, and mesoscale collections. Hence, one payload can address both missions.

CSI is heavily based on the ABI instrument, and takes advantage of the highly modular design of the ABI aft optics to achieve hyperspectral capabilities while minimizing development cost. The standard ABI aft optics assembly is removed and replaced with a Hyperspectral Aft Optics (HAO) unit containing three IR FPAs and a filter wheel assembly that feeds a visible/near-infrared FPA. The IR FPAs cover ABI channels from 3.9 through 13.3 microns, while the 8-position filter wheel with a single Visible FPA covers the ABI channels from 0.47 through 2.2 microns plus a Day/Night Channel (like that used on VIIRS) and a true-color Green channel.

The CSI retains an earth coverage rate similar to that of ABI, while adding the ability to perform hyperspectral soundings in areas of active weather. While in Imaging Mode, the instrument views an earth position for 0.5 seconds. This instrument then steps to the next position. During the 0.5-second stare time, the interferometer performs a “short stroke” (¬+0.2cm-1 OPD travel) to provide imaging data at moderate spectral resolution for the IR channels, while the filter wheel rotates to provide data in all eight visible and near-infrared channels. While in Sounding Mode, the interferometer performs a full stroke (+0.8cm-1 in OPD travel) to provide sounding data at very high spectral resolution, and stares at each earth location for 4.0 seconds to maximize the SNR of the sounding data.

This paper describes the CSI design, its concept of operations and its expected performance.