US Patent 10277321 Acquisition and pointing device, system, and method using quad cell

The Quad Cell permits measurement of four quadrant signal powers simultaneously, metrics which are equal when the laser spot is at the desired zero location, at the center of the cell, the origin of the cells' axes. A control action acts upon the Quad Cell signals to move the laser spot toward the origin of the axes bisecting the four quadrants of the cell, moving the laser spot to the origin to achieve a null in the difference between these four signal levels.