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Why do two MISR images within the same path and block sometimes cover slightly different regions on the ground?
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There are three reasons that the ground coverage of two MISR images within the same path and block may differ.
- The position and pointing of the Terra satellite can vary by a small amount from orbit to orbit. For instance, the position is allowed to vary by no more than 20 km from one overpass to the next. If two images from the same camera for different overpasses are compared, a slight horizontal offset in image location can often be seen. This offset is usually the result of a shift in position of the satellite.
- The nine MISR camera footprints do not all cover the same exact extent on the ground. This variance occurs because the different camera views are acquired from devices which have different optical designs. In addition, the Earth rotates underneath the satellite in the time that elapses between acquisitions of a particular ground target by two different MISR cameras. Crosstrack boresight offsets are included in the MISR camera pointing angles to compensate, but this does not result in perfect overlap at every latitude. Therefore, two images from the same orbit but from different cameras almost never display precise overlap. A slight horizontal offset between any two camera images is usually visible.
- The MISR cameras are only commanded to acquire imagery over the sunlit portion of the Earth. Therefore a significant seasonal variation of the start block and stop block occurs for orbits within the same path. For example, some blocks acquired in Antarctica in December will not be acquired in June because they are dark during the later month.