Effects of possible scan geometries on the accuracy of satellite measurements of water vapor

For future satellite instruments, two scan geometries have been proposed: c ross-track and conical scanning. With the mixtures of possible instruments, a future sounding suite may consist of all cross track, all conical, or a combination with temperature channels on one and moisture channels on the o ther This paper evaluates the effect of scan angle on the accuracy of moist ure soundings. It is found that, as the scan angle increases from nadir, th e accuracy of the moisture soundings near the surface decreases because les s of the surface signal reaches the satellite. At the same time, the accura cy of the upper-level sounding increases because the weighting functions be come sharper as the angle increases. If a mixed system is required where th e temperature and moisture channels have different scan geometries, the bes t accuracy is obtained if the temperature channels are on a cross-track sca nner. The scan angles also affect the number of measurements that are too c loud contaminated to provide accurate meteorological information. A second study shows that the number of radiances that exceed a given error level in creases with scan angle. This results in a decrease in coverage for a conic al scanning instrument.