a method for calibrating and correcting circuit errors induced in an aircraft ils receiving system is disclosed herein . the spectral components of the input signal are processed through bandpass filters and then rectified and integrated in order to produce an output signal representative of the deviation from the desired centerline course . first and second test signals are generated corresponding to first and second known positions . these test signals are sequentially processed in the same manner as the operational input signals in order to develop first and second position errors which represent distortion and circuit induced errors within the processing system . a corrective function is then derived for a range of operational input signals by interpolating from the first and second position errors . all subsequently processed signals are adjusted by applying the corrective function . in this manner , the various distortions that are introduced through the processing system will be measured and then subtracted from the output signals in order to define a corrected representation of deviation from the desired centerline course . sampling periods are utilized which contain an integer multiple of cycles of each of the major frequencies present in the operational input signal .