GOES Sounder overview

GOES Sounder overviewJeanine E. Murphy and Sanford W. HinkalNASA, GOddard Space Flight CenterGreenbelt, Maryland 20771ABSTRACTThe GOES Sounder is similar in appearance, size, and design to the GOES Imager. It provides independentradiomen-ic sounding of the atmosphere from GOES-East and GOES-West (GOES 8 and 9), which was accomplishedon previous GOES by a single instrument with responsibility split between the functions of imaging and sounding.With 19 channels ranging from 0.70 m to 14.71 m, the Sounder probes the atmosphere to measure radiance atdifferent depths (altitudes) while also monitoring surface and atmospheric temperatures, ozone, water vapor content,and cloud cover by means of the calibration of radiance data at selected wavelengths.Like the Imager, the Sounder scans the full earth, and can be commanded to scan local regions of interest.The scan mirror steps across the disk of the earth in synchronization with a dead zone on a filter wheel. The filtersdefme 18 of the channels as they rotate on the wheel in the path of light split into three beams traveling to detectorsdivided into longwave, midwave, and shortwave arrays. For visible (0.70 m) sounding of cloud cover, energy is splitoff ahead of the filter wheel and passed to the 19th channel and also to star sensing detectors used for instrumentnavigation.Keywords: GOES, sounder, weather, satellite2. INTRODUCTIONThe GOES Sounder is one of the two primary instruments on the Geostationary Operational EnvironmentalSateffites (GOES). This instrument is a radiometer similar to the GOES Imager but with 19 distinct bandwidths orchannels and with 13 bit resolution. Eighteen of these channels are selected by a rotating filter wheel ahead of threearrays of detectors which select the seven longwave (LW) (12.0 to 14.7 tim), five midwave (MW) (6.5to 1 1.0 im),and six shortwave (SW) bands (3.7 to 4.6 gm). One visible band (0.7 1um) is split off ahead of the filter wheel. Thesechannels are chosen for the functional relationships between the energy radiated in each band and the atmospherictemperatures and moisture content at different altitudes, surface and cloud temperatures, and ozone distribution.This paper presents an overview of the instrument from an instrument manager's perspective. The opticaldesign of the instrument is described with an overview of the mechanical and electronic layout. Data products aredescribed, with a discussion of some of the uses of the data. Calibration methods are summarized. The instrument hasundergone continuous improvement and the effect of this is briefly discussed in terms of the data products of GOESSounders now on orbit, presently undergoing assembly, and planned for the future.3. DESIGNThe Sounder instrument consists of the sensor module (Fig. 1), which occupies a 1.2 X 0.8 X 0.7 metervolume within the 6.4 meter cube envelope of the spacecraft body, the electronics box, and the power supply. Thishardware contributes 129 kg of the total spacecraft mass of 977 kg excluding fuel and pressurizing gas. The sensormodule is mounted on the north end of the spacecraft surface which faces Earth, with the radiant cooler pointed northand the Sounder optical port facing the earth. The electronics box and the power supply are mounted within thespacecraft (Fig. 2) on the north panel.Light enters the Sounder optical port (see Fig. 3) and falls on the lightweighted elliptical (0.51 m x 0.32 m)beryllium scan mirror, directing radiation into a 0.31 m aperture Cassegrain telescope which focuses it on the optical174/SPIE