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 blackbody 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 GOES-7 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 GOES-I 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 @230K 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.
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.