Abstract: In a missile which employs a terminal homing seeker and a proportional navigation guidance law the space rate of change of boresight error, i.e., the boresight error slope, is one of the predominant error sources.It has been found that the boresight error slope is proportional to the curvature of the seeker open loop transfer characteristic. Accordingly, the boresight error slope sensor senses the curvature of the seeker open loop transfer characteristic. This is accomplished by intermittently dithering the seeker instantaneous field-of-view about the line of sight at a rate too great for the normal tracking loop to respond. Thus the open loop transfer characteristic is obtained while leaving the normal tracking loop unperturbed. The curvature of the open loop transfer characteristic is then obtained in real time by computing the "second differences" from the measured open loop transfer characteristic.