5. Science Products

5.1 EPIC

Most of the science products (see Table 3) are obtained by combining pairs of radiances measured at different wavelengths to extract information based on their differences. To improve the measurements, the measured Earth radiances are normalized to lunar radiances measured by EPIC on a regular basis. Use of the resulting normalized radiances, I/F, cancels a number of possible instrument errors (e.g., radiometric drift). A similar solar normalization (using a diffuser plate instead of the Moon) is used for TOMS and GOME data processing. Use of the lunar radiances is a complicated problem that is discussed later.

The Triana viewing geometry is different from observations at other angles because of reflections over oceans (sun-glint) and increased effective reflectivity from land surfaces (hot spot phenomenon caused by decreased shadows from plants and rocks when the Sun is behind the observer). For UV wavelengths, the hot-spot increased reflectivity from land is not a problem because of strong Rayleigh scattering in the atmosphere. However, the increased ocean reflection still must be taken into account. For average wind speeds of about 10 km/hour, the ocean albedo increases from about 4% at the edges to about 20% in the center of the sun-glint region (a circle of about 20° of latitude in diameter). As wind speed increases, the albedo decreases from 20% until whitecaps occur. These effects are included in the algorithms through the measured reflectivity and knowledge of the sun-glint region geometry. This technique is currently used in the TOMS data analysis to permit ozone and aerosol amounts to be retrieved throughout the sun-glint region.

In each case, the science quantity is obtained for scenes at a spatial resolution of 8 km x 8 km corresponding to the 2048 x 2048 CCD elements distributed over the image of the sunlit Earth. The expected accuracy is shown in Table 4. Of course, the area projected onto the Earth's surface increases towards the limb. The data reduction algorithms contain routines for geolocation of the measured radiances on a pre-determined latitude by longitude grid.

Table 3 Spectral channels in the Scripps-EPIC. The spectral resolution corresponds to the sub-satellite point at the Earth¹s surface. The 10 wavelengths will be measured once per hour for the entire globe. Spatial resolution corresponds to sub-satellite point. The longest exposure time is 0.1 seconds.

Table 4 Expected Accuracy of Main Data Products