A navigation system contains a modified Kalman filter processor which continuously receives both TERCOM and SITAN control information so that the operation of the SITAN processing is effectively continuously optimized. The system employs an over flight terrain data storage map to which position and altitude signals are coupled for extracting both elevation and slope information from the stored map. The extracted elevation and slope data are coupled, together with the outputs of baromatic and radar altimeter sensors and estimated altitude and position data outputs from the navigation unit, to a correlation/modified Kalman filter processor. Elevation profile data accessed from the map is correlated with elevation profile signals derived from aircraft on-board sensors, in order to find that flight path on the stored map, parallel to, but displaced from the flight path indicated by the navigation unit, for which successive elevation data values correspond most closely to the elevations measured by the altimeter sensors. In this correlation processing operation the "most likely" path is selected by defining a performance index associated with each path and selecting that path with the best performance index. The result of the correlation processing provides a position fix to be combined with the position estimates in the modified Kalman filter.