A method and apparatus for finding a relative direction of received radiation is described. The received radiation is incident upon a window system at an angle and is transmitted therethrough. The magnitude of the transmitted radiation decreases as a continuous function of increasing angle of incidence, known as Fresnel transmittance. Opposing radiation detectors then detect this transmitted radiation, thereby creating a pair of detection signals, By dividing the difference of the detection signals by the sum of the detection signals, a processor generates a beta angle error curve and finds the relative direction of the radiation. Based upon this beta angle error curve, the processor generates an appropriate error correction signal for guiding an object based upon the relative direction of the received radiation. The method and apparatus are readily applicable to guiding munitions using a laser monopulse to designate a target.