Submitted by Martial Haeffelin
The TERRA spacecraft passes over the Chesapeake Lighthouse in daylight once or twice per 24 hours. If the CERES instrument scans in the cross-track direction only one footprint will contain the Chesapeake Lighthouse at each overpass. The rotating azimuth capability of the CERES instrument can be used to adjust the scan plane such that the Chesapeake Lighthouse remains within the scan swath as the spacecraft moves along its orbit. Figure 1 shows the azimuthal direction of the scan plane, represented by arrows directed towards the Lighthouse, as TERRA proceeds on its descending orbit. The direction is shown once per minute. The colors represent separate days. On day3 and day5 CERES is able to observe the Lighthouse from two consecutive orbits. On average the Lighthouse can remain in the CERES scan swath for about 7 minutes per orbit, which, with proper adjusments to the CERES scan pattern could yield more than 100 observations each day!CERES Science Team meeting 2001/05 Presentation.
Figure 2 shows the distribution of viewing zenith and relative azimuth angles sampled during one week of operation. Note that the relative azimuth angles are uniformely sampled. Large viewing zenith angles are preferentially sampled because they are used for all overpasses. Small viewing angles can only be obtained when the subsatellite point of TERRA is close to the Lighthouse.