The National Oceanic and Atmospheric Administration (NOAA) operates a system of Geostationary Operational Environmental Satellites (GOES) to provide frequent visible and infrared images of the Earth as well as quantitative meteorological data products, such as estimates of atmospheric temperature and moisture profiles, winds, and precipitation. The imagery is well-known to the general public through televised weather reports. The GOES-8 and -9 satellites, launched on April 13, 1994, and May 23, 1995, respectively, are currently stationed at 36,000 km above the equator at 75W and 135W longitudes, respectively. Both carry an Earth-atmosphere imager and an atmospheric sounder^1-3. These instruments observe the Earth in a total of 24 spectral intervals ("channels") at wavelengths between 0.6 µm and 14.7 µm. The imager makes observations in five channels (Table 1), which are isolated by stationary filters in its optical chain. Each channel utilizes a separate linear north-south array of detectors--eight detectors in channel 1, two in channels 2, 4, and 5, and one in channel 3. The sounders observe in 19 spectral intervals (Table 2).

The spectral intervals in the infrared are isolated by a rotating wheel of filters. The filters are placed on the wheel in three concentric circles, which correspond to the channels of the three wavenumber regions--longwave (channels 1-7), midwave (channels 8-12), and shortwave (channels 13-18). Adichroic beamsplitter and a fixed filter isolate the visible channel. There is a north-south array of four detectors for each of the three infrared wavenumber regions, and a similar four-detector array for the visible. Each detector has a field of view (FOV) of approximately 8 km. Imager outputs are transmitted to the ground station as 10-bit words, sounder outputs as 13-bit words.

Before the launch of each satellite, the radiometric performance of the instruments was characterized in an extensive program of pre-launch tests conducted by their manufacturer (ITT, Ft. Wayne, IN) and by the GOES I-M prime contractor (Space Systems/Loral, Palo Alto, CA). The form of the in-orbit infrared calibration equation and some of the coefficients were determined from these tests. Despite the extensive testing before launch, the data from orbit revealed unforeseen performance anomalies, most notably a variation in the emissivity of the instruments' scan mirrors with east-west scan position that necessitated a change in the in-orbit calibration equation and procedures. Operational processing by the new calibration equation was initiated months after the launches of the GOES-8 and GOES-9 satellites and at different times for different instruments.

The purpose of this memorandum is to document the complete calibration processing, including the post-launch modification for the scan-mirror emissivity variation, which NOAA carries out in its ground-system computers. This is covered in sections 3-6, which follow a short description of the calibration data in section 2. Section 7 describes the digital data that users receive, which, in infrared channels, are scaled radiances, and, in visible channels, normalized instrument output relative to the level of space. Appendix A presents the procedure for users to transform the scaled radiances to physical radiances and brightness temperatures.

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