Air-to-ground ranging using electronic roll stabilization of monopulse data

A radar operating in an air-to ground ranging mode obtains precise range measurements from the antenna to the point of intersection of the antenna boresight with the ground. If elevation monopulse data is used, the boresight range is characterized by the range at which the sign crossover occurs in the data. In mechanically scanned arrays, the monopulse axes are fixed with respect to the array. Unless roll-stabilizing gimbals are provided, the elevation monopulse channel is increasingly corrupted by azimuth data as the airframe rolls. The AN/APG-67 multimode radar, in addition to its sum channel, has a difference channel which uses quadrature multiplexing to simultaneously receive and process both elevation and azimuth monopulse data. By comparing the difference channel signal with that of the sum channel, both azimuth and elevation errors are extracted from the same set of data. The inertial elevation monopulse data component can be extracted from the multiplexed difference channel by phase rotating the multiplexed data opposite to the inertial twist of the antenna array. The resulting multiplexed data is roll-stabilized and can be used for air-to-ground ranging regardless of the airframe´s orientation