IntroductionSatellites traditionally used for weather monitoring have proven to be useful in environmental monitoring. Thus, the ability to compare the measured radiances from different instruments has become increasingly important. The Cooperative Institute for Meteorological Satellite Studies (CIMSS) has been intercalibrating five geostationary satellites (GOES-8, -10, METEOSAT-5, -7, GMS-5) with a single polar-orbiting satellite (NOAA-14 HIRS and AVHRR) on a routine, automated basis for over a year using temporally and spatially co-located measurements. The primary focus of this effort has been in comparing the 11-µm infrared window (IRW) channels. Similar efforts are being made for the 6.7-µm water vapor (WV) channels. The numerical methods for conducting the routine intercalibration are being updated. This includes updating the radiative transfer forward model used to estimate the differences in calculated brightness temperatures based on spectral differences between satellite instruments.
ApproachThe technique for intercalibration has been developed at CIMSS over the past several years (Wanzong et al., 1998). Collocation in space and time (within +/- thirty minutes) is required. Data is selected within 10 degrees of nadir for each instrument in order to minimize viewing angle differences. Measured means of brightness temperatures of similar spectral channels from the two sensors are compared. In the IRW channel, data collection is restricted to mostly clear scenes with mean radiances greater than 80 mW/m2/ster/cm-1, and no additional effort is made to screen out clouds from the collocation area. In the WV channel there is no clear scene restriction applied. Data from each satellite are averaged to an effective 100-km resolution to mitigate the effects of different field of view (fov) sizes and sampling densities; HIRS under-samples with a 17.4 km nadir fov, AVHRR Global Area Coverage (GAC) achieves 4 km resolution by resampling, GOES imager over-samples 4 km in the east-west by 1.7 (Menzel and Purdom, 1994), and METEOSAT-5, METEOSAT-7, and GMS-5 have a nadir 5 km fov. Mean radiances are computed within the collocation area. Clear-sky forward calculations (using a global model for estimation of the atmospheric state) are performed to account for differences in the spectral response functions. The observed radiance difference minus the forward-calculated clear-sky radiance difference is then attributed to calibration differences.
For comparing a geostationary satellite to HIRS,
where GEO indicates geostationary, HIRS indicates the HIRS instrument, mean indicates the mean measured radiance, and clear indicates the forward calculated clear-sky radiances. Conversion to temperatures for a comparison between a geostationary satellite to HIRS is accomplished by,
where B-1 indicates converting radiance to brightness temperature using the inverse Planck Function. An identical method is used for calculating the temperature difference between a geostationary satellite and the AVHRR instrument.
Last Updated: April 12, 2002
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