Techniques ror Change Detection

The problem of change detection presents itself for imaging systems that view the same scene repeatedly. Current research programs based on the processing of side-looking radar imagery show that spatial alignment of the various parts of the image must be highly accurate if noise in the difference picture is to be reduced to acceptably low levels. Typically, the spatial alignment accuracy must be better than one-fourth of the diameter of the smallest resolvable feature in the imagery, and this often requires several hundred degrees of freedom in the performance of the map warp for images that are of the order of 10⁷picture cells (pixels) in size. Gray scale rectification of conjugate sampling points is less difficult, requiring typically only 10 to 20 percent as many degrees of freedom. Point by point adjustment for differences in mean transparency and contrast is employed. Recently developed equipment provides a continuous pipeline processing capability. With this equipment, each picture element of the second image is transformed with four degrees of freedom (two spatial and two gray scale). The digital correlator is capable of processing 4×10⁵six-bit picture elements per second when used in conjunction with a CDC 1700 computer.