Abstract: A method for Schuler cycle error reduction is provided for use in a terrain referenced navigation system. Terrain Reference Navigation filter coefficients are analytically determined using a terrain reference navigation database and a hypothetical Schuler cycle. The optimum filter coefficients are determined assuming that there will be three parallel filters being executed during flight. The first filter is a nominal filter which represents a compromise between sensing zero crossing of a Schuler cycle error and peak changes of a Schuler cycle. The other filters are chosen to be perpendicular to the instantaneous line of flight. One is a peak filter, the other is a zero crossing filter. The peak filter is set to capture the inertial navigation unit when the Schuler cycle is changing rapidly. The zero crossing detector is set to capture the inertial navigation unit when it is zero crossing. The Schuler cycle is detected by algebraically noting similar features of each filter.