Terrain navigation which has been studied the last years within FOA has proven to be alternative to GPS aiding of submarines inertial navigation systems (INS) and it fulfills the Swedish Navy´s requirements for an absolute reference system as a complement to the submarines INS-system. The terrain navigation sonar not only gives an accurate position but also a 3D-picture of the bottom under the submarine and by this making it easier for the submarine to acquire a position in a bottom depression with low risk of revealing. The good result that has previously been recorded in simulation and sea trials has theoretically been verified. The ability to accurately determine a unique position depends exponentially on the size of the correlation area. A suitable navigation sonar is a modern bathymetric sonar with large opening angles. Most suitable are so called 3D-sonar since they can determine the position in only one very short ping, approx. 30 µs. Due to the short pulse time and the high frequency, 300 kHz, the risk of detection could in practice be neglected. The measurement of the topography by laser does not seem to be an alternative to hydro acoustic measurements due to the short range.