Microwave holography differs from optical holography in three distinct ways. First, in microwave holography the reference field may be eliminated and replaced by correct phasing of the local oscillator within the microwave receiver. Second, the reference wave need not coexist in time with the signal or object field. Third, it is not necessary that the object field be measured simultaneously across the array. Aside from these fundamental differences, which give the microwave holographer additional freedom in system design, another practical difference should be noted. Because of the long wavelength used in microwave holography, the aperture must be huge to obtain images with fine resolving power. Such apertures are often mechanically unstable. By introducing adaptivity or self-cohering into the creation and measurement of the reference field, it is possible to ‘straighten out’ a distorted microwave aperture. An instrument which performs these operations is called a radio camera. A twodimensional radar map of a few blocks of a small town in Pennsylvania illustrates the process.