Southern Africa

Unless you look very closely, you can’t really tell that there is a four hour and fifteen minutedifference between one side of southern Africa and the other in this true-color MODIS image acquired November 7, 2002. This is a composite image ofsouthern Africa taken by both the Aqua and Terra MODIS instruments; the Aqua instrument provided the left half of the image, while the Terra instrumentprovided the right half. Southern Africa is sufficiently wide enough that it is not possible to get its entirety in one instrument’s pass, soimages from the two MODIS instruments were combined.

The calibration of the two MODIS instruments is so accurate that it is possible to stitch together separate passes from each instrument into avirtually seamless image. Were there significant differences in the instruments' calibration, geolocation, or surface reflectance, an image with thishigh of quality would not be possible. That there is no significant difference in reflectance or geolocation for either instrument promotes continuityand demonstrates that both Aqua and Terra data are equally reliable and accurate.

There are only a couple of clues to the fact that this is a composite image. The first is sunglint on the Atlantic and Indian Oceans. In an image bya single sensor, there would only be one glint (silver streak) on the water’s surface. Further, the direction of each glint illustrates whichMODIS instrument took which half of the image. The glint that shines from the upper left to the lower right is due to Aqua’s orbit orientation,whereas the glint that shines from the upper right to the lower left comes from Terra's orbit orientation.

The other clues are in the clouds. Careful observation will reveal that the clouds in the center of the image end very abruptly and form shapes withdistinct horizontal edges. This is not natural, but a consequence of the time difference when the two images were acquired. When Terra acquired itshalf of the image, there were more clouds over central southern Africa than there were four hours and fifteen minutes later when Aqua flew overhead,resulting in the odd cloud patterns when the images were stitched. Finally, the direction of the clouds’ shadows also point out that twoinstruments created this image, and this evidence is only visible in the highest resolutions. The clouds shadows on the Aqua (left) side of the imagelay to the right, while those on the Terra (right) side lay to the left.

Jacques Descloitres, MODIS Rapid Response Team,NASA/GSFC

Published November 7, 2002
Data acquired November 7 - 7, 2002

Source:

Terra > MODIS