Abstract: 1. A real time signal correlator for use on high resolution mapping radar apparatus aboard a moving aircraft having a ground speed indicator and comprising on an optical axis in the order named: A time modulating light source, a first cylindrical lens, an optical filter having at least one optically apertured time variable function and located to be substantially at the imaging plane of said first cylindrical lens, a second cylindrical lens optically parallel to said first cylindrical lens, and an optically sensitive film substantially at the imaging plane of said second cylindrical lens normal to the optical path and movable in a direction parallel to the time variable function on said optical filter, the linear velocity of said film being synchronized to the ground speed indicator on the aircraft so that when the time function of said modulating light source is identical with the time variable function of said optical filter the two time variable functions are correlated and impressed upon said film.