2. Meteorological sensor systems on GOES I-M

The earth-oriented GOES I-M enables more efficient data gathering by both the imager and sounder; both yield higher spatial resolution and improved signal-to-noise over that available previously. The separate instruments for sounding and imaging allow full use of both capabilities. The imager and sounder have generally common features (Table 1), although there are some differences (Savides 1992). Specifics concerning each are presented in sections 2a and 2b.

a. Imager

The GOES I-M imager has a five-band multispectral capability with high spatial resolution . This enables improvement in present services and allows for the development of a number of advanced products. The five spectral bands are (a) 0.52-0.72Ám (visible), (b) 3.78-4.03Ám (shortwave infrared window), (c) 6.477.02Ám (upper-level water vapor), (d) 10.2-11.2Ám (longwave infrared window), and (e) 11.5-12.5 Ám (infrared window with slightly more sensitivity to water vapor than the other infrared windows). In comparison, the GOES-VAS imaging mode includes data from one or two atmospheric sounder spectral bands to accompany the routinely available visible and 11.2Ám infrared data (Smith et al.1981). Among the VAS spectral bands frequently included in that multispectral data stream are those centered on 13.3,12.7,7.3, 6.7, and 3.9 Ám. The GOES I-M imager band selections were in part patterned after the Advanced Very High Resolution Radiometer (AVHRR) carried on the NOAA polar-orbiting satellites and in part dictated by the need for continuity of GOES-VAS spectral capabilities. Desired noise characteristics were specified based on experience with the AVHRR and GOES-VAS data and current detector technology.

TABLE 1. Imager and sounder instrument features.

Feature			Imager				Sounder

Optical aperture	31.1 cm				31.1 cm

Type optics		Cassegrain			Cassegrain

Methods of scan		Two axes, continuous		Two axes, step and dwell
			Linear E/W 64 Árad (2.3 km)	E/W 280-Árad steps
			Line step N/S 224 Árad (8 km)	N/S 1120-Árad steps

Spatial resolution	Visible 28 Árad (1 km)		242 Árad (10 km)
			IR windows 112 Árad (4 km)
			H2O band 224 Árad (8 km)

Sampling		Visible 1.75/lGFOV		Four IGFOVs sampled at 
			IR windows 1.75/lGFOV		the same time
			H2O band 3.5/lGFOV

Sampling rate		20í s-1				40 soundings s-l
			183.3 Ásec per pixel(IR)	0.1,0.2,or O.4 s 
			45.8 Ásec per pixel (vis)	per sample

Spectral band
coregistration		+/-28 Árad			Within 22 Árad of IR
							10.7-Ám window

Data output		10-bit quantization		13-bit quantization

Data rate		2.6208 Mb s-1			40 kb s-1

Time between	
space looks		2.2 s				2 min
			(nominally for large frame)
			9.2 or 36.6 s
			(nominally for small frame)

Time between 
calibrations		10-30min			20min

*Instantaneous geometric field of view

The GOES I-M imager provides visible data with about 1 -km resolution as GOES-VAS but with a stable linear response and 10-bit precision (1 part in 1024), improving upon the GOES-VAS variable nonlinear 6bit response (1 part in 64). By using star positions in addition to traditional landmarks, imagery is earth navigated within 2-4 km compared to 3-10 km with GOES-VAS. The GOES I-M imager provides infrared imagery simultaneously in four thermal bands instead of the two or three bands available in the imaging mode from GOES-VAS. For nadir view, the imager's infrared window bands are at 4-km horizontal resolution (water vapor band is at 8 km), while the GOES-VAS infrared window band is at 6.9 km (other bands are at 13.8 km). Onboard calibration provides brightness temperatures with 1.0-K absolute accuracy and 0.3-K relative precision, and noise levels reduced two to three times over GOES-VAS. Table 2 compares expected GOES-I imager characteristics with that of the current GOES-VAS, GOES-7.

TABLE 2. GOES-7 and GOES-I imager characteristics. IGFOV at nadir and SSR are presented in kilometers, and noise-equivalent temperatures for the thermal bands are specified for nominal scene temperatures (300 K for the window bands and 230 K for the water vapor band).

Wavelength		IGFOV (km)		SSR (km)		Noise(Ám)					E/Wx N/S		E/Wx N/S


0.55 0.75		0.75x 0.86		0.75x 0.86		6-bit data
									+ 2 counts
									3 sigma
3.84-4.06		13.8x 13.8		3.0x 13.8		0.25 K @ 300 K,

									6.00 K @ 230 K
6.40-7.08		13.8x 13.8		3.0x 13.8		1.00 K @ 230 K
10.4-12.1		6.9x 6.9		3.0x 6.9		0.10K @ 300K,
									0.20K @ 230K
12.5-12.8		13.8x 13.8		3.0x 13.8		0.40K @ 300K,
									0.80K @ 230K


0.52-0.72		1.0x 1.0		0.57x 1.0		10-bit data▒ 8
									counts 3 sigma
3.78-4.03		4.0x 4.0		2.3x 4.0		0.15K @ 300K, 3.50K
6.47-7.02		8.0x 8.0		2.3x 8.0		0.30K @ 230K
10.2-11.2		4.0x 4.0		2.3x 4.0		0.20K @ 300K, 0.40K
									@ 230K
11.5-12.5		4.0x 4.0		2.3x 4.0		0.20K @ 300K, 0.40K
									@ 230K

The detector instantaneous geometric field of view (IGFOV) or footprint and a derived sampled subpoint resolution (SSR) are presented in Table 2. SSR modifies IGFOV by accounting for instrument response (Gabriel and Purdom 1990) and sampling rate. GOES-I oversamples infrared IGFOVs, 4 and 8 km, along a scan line by factors of 1.75 and 3.5, respectively; the 1-km visible IGFOV is oversampled by a factor of 1.75. GOES-7 oversamples infrared IGFOVs, 6.9 and 13.8 km, along a scan line by factors of 2.3 and 4.6, respectively; the 0.8-km visible IGFOV is sampled contiguously without oversampling.

The visible band, upper-level water vapor band centered at 6.7 Ám, and longwave window band centered at 10.7 Ám on GOES-I are familiar to most GOES-VAS users through their depiction of the earth surface in clear sky, clouds, and upper-tropospheric moisture. GOES-I images in these bands are noticeably sharper through the improved quantization in the visible band and the improved signal to noise and higher spatial resolution in the infrared bands. The band centered at 3.9 ,Ám is useful for the identification of fog at night (Ellrod 1992), discriminating between water clouds and snow or ice clouds during the daytime (Scorer 1989), detecting fires (Prins and Menzel 1992) and volcanoes, and determining nighttime sea surface temperature (Bates et al. 1987). The longwave window band centered at 10.7 ,Ám and the split window band centered at 12.0 Ám in combination are useful for identification of low-level moisture (Chesters et al. 1987), determination of sea surface temperature, and detection of airborne dust and volcanic ash. Differences in emissivity in the GOES-I infrared bands should lead to the development of a variety of applications, especially at night, when the 3.9-Ám band can be used without visible light contamination. Table 3 highlights some anticipated improvements in GOES-I imager products. Section 6 presents simulations of GOES-I imagery and comparisons with imagery from GOES-7.

TABLE 3. Anticipated immediate improvements in GOES-I imager products.

b. Sounder

The GOES I-M sounder has 18 thermal infrared bands plus a low-resolution visible band, compared to the 12 infrared bands plus a visible band on GOES-VAS. The new spectral bands, at wavelengths never obtained before in geosynchronous orbit, are sensitive to temperature, moisture, and ozone. The GOES I -M sounder's design goal, like the imager's, is to provide brightness temperatures with 1.0-K absolute accuracy and 0.3-K relative precision. Table 4 summarizes the spectral band performance characteristics for the GOES-7 and the GOES-I sounders. The full-time availability of the GOES-I sounder enables operational sounding products for the first time; this has the potential for contributing significantly to mesoscale forecasting over the conterminous United States, monitoring thermal winds over oceans, and supplementing the Automated Surface Observing System (ASOS) with upper-level cloud information. The GOES I-M sounders will also allow for the development of a number of advanced products.

TABLE 4. Sounder radiometer spectral channels, bandwiths, and noise equivalent radiance performance characteristics (NEDR in mW ster-1m-2cm). The GOES-7 results are from evaluations of in-flight performance. The GOES-I results are from the prelaunch thermal vacuum tests; the range of values encompasses the four detectors used to detect the spectral radiation. The fourth column indicates the primary purpose of this band.


Figure 2 shows the GOES-I sounder spectral bands together with depiction of the earth-emitted spectra; the carbon dioxide (CO2), moisture (H20), and ozone (O3) absorption bands are indicated. Around the broader CO2 and H20 absorption bands, vertical profiles of atmospheric parameters can be derived. Sampling the center of the absorption band yields radiation from the upper levels of the atmosphere (e.g., radiation from below has already been absorbed by the atmospheric gas). Sampling away from the center of the absorption band yields radiation from successively lower levels of the atmosphere. In the wings of the absorption band are the windows that view to the bottom of the atmosphere. Thus, as a spectral band is moved toward the center of the absorption band, the radiation brightness temperature decreases due to the decrease of temperature with altitude in the lower atmosphere. GOES-I selection of spectral bands in and around the CO2 and H20 absorbing bands is designed to yield information about the vertical structure of atmospheric temperature and moisture.


FIG. 2. Infrared portion of the earth-atmosphere-emitted spectra is shown on top. Brightness temperatures are plotted as a function of wave number. The GOES-I sounder spectral bands (and bandwidths) are indicated below. Qualitative indication of spectral band sensitivity to a given level of the atmosphere is also noted (stratosphere; high, middle, and low troposphere; earth surface).

Initially, the GOES I-M sounder spectral selection was primarily patterned after the High-resolution Infrared Radiation Sounder (HIRS) carried on the NOAA polar-orbiting satellite, which has six bands in the 15 Ám (longwave) band, a split-window pair, two midtropospheric water-sensitive bands (midwave), three 4 Ám (shortwave) bands, and a visible measurement. Noise characteristics were specified based on experience with the HIRS and current detector technology. Subsequently, the sounder was expanded to 18 infrared bands, adding the ozone band and a number of additional shortwave bands (improving low-level vertical resolution), changing the longwave window arrangement to a more accurate split window, expanding the moisture-sensing bands from two to three, and adding a surface-sensing band. These changes were designed to improve vertical resolution for moisture sounding. Table 5 highlights the anticipated improvements in GOES I-M sounder products. Section 6 presents simulations of GOES-I soundings and derived products.