A radiant energy detection system includes a first and second pairs of radiation detectors, the detectors of each pair facing in opposite directions to each other, the first and second pairs being positioned in a mutually orthogonal relationship. The detectors may be joined together in a cube structure which is fixedly mounted on a support. A chopper mechanism is mounted over the detectors; this chopper mechanism being supported on a motor shaft for rotatable drive thereby around the detector assembly. The signals arriving at the detectors from a radiant energy source are chopped by the chopper to produce a periodically varying output signal from each of the detectors. the outputs from each detector pair are fed to a separate differencing amplifier which generates a signal in accordance with the difference therebetween; the outputs of the amplifiers being phase demodulated to provide signals having peak amplitudes representing the cosine and the sine respectively of the angle of the radiant energy source (target) from the detector assembly. The signal in accordance with the sine of the angle, and the signal in accordance with the cosine of the angle, are fed to an analog processor which generates output signals in accordance with the azimuth angle of the target, the angular quadrant in which it lies, and a signal which appears at the time maximum energy is being received by the detectors.