sonar is usually used only entering and exiting ports. Pilotage—in coastal and internal waters, surfaced submarines rely on the standard system of navigational aids (buoys, navigational markers, lighthouses, etc.), utilizing the periscopes for obtaining lines of position to plot a triangulation fix. Voyage Management System—referred to as the VMS, utilizes digital charts with other external sources fed in, to establish the ship's position. Other information may also be entered in manually in establishing a high quality fix or position. Deep water navigation At depths below periscope depth submarines determine their position using: Dead reckoning course information obtained from the ship's gyrocompass, measured speed and estimates of local ocean currents, this could also be considered an estimated position as long as the ocean current is computed in. Inertial navigation system is an estimated position source, utilizing acceleration, deceleration, and pitch and roll for computing. Bottom contour navigation may be used in areas where detailed hydrographic data has been charted and there is adequate variation in sea floor topography. Fathometer depth measurements are compared to charted depth patterns. See also Footnotes References Navigation Submarines