A Doppler navigation radar is described which can reduce sea-bias errors to a negligible degree. The technique also reduces all other errors which result from asymmetrical distortion of the echo spectrum. Spectrum distortion is caused by modulation effects (altitude holes), by angle-sensitive reflectivity of certain types of terrain, and by asymmetric receiver responses. Error reduction is achieved by the use of monopulse-type antenna patterns, which are directed toward the earth fore and aft of the aircraft velocity vector. The Doppler frequencies corresponding to the null planes of the split beams are tracked. Because the positions of the null planes relative to the aircraft velocity vector are dependent only upon the antenna configuration, and not upon outside factors beyond the control of the radar designer, the system is practically insensitive to the class of errors listed above. A particular system configuration illustrating the null-tracking principle is described. Initial acquisition of the ground track is made by a conventional lobe-tracking system. Rapid switchover between null tracking and conventional tracking is provided in order to permit picking the best mode for the particular conditions encountered. The added complexity required by the null-tracking feature depends somewhat upon the antenna design, but does not appear prohibitive for either lens antennas or planar arrays.