Two-component Electromagnetic Ship's Log

Abstract

NAVIGATION on the deep ocean, away from accurate shore-based radio aids, is an important problem for the oceanographer. The use of the US satellite navigation system allows accurate position fixes to be obtained at intervals depending on how many satellites are operational, but there remains a need for a system to interpolate between these, particularly when complex manoeuvres are being carried out. To fill this need this institute is developing an improved dead-reckoning system, in which the ship's track is calculated from its velocity through the water based on measurement of fore-and-aft speed, sideways speed, or “leeway”, and ship's heading. An electromagnetic log is available commercially which measures the fore-and-aft speed to an accuracy of about 0.1 knot, but to obtain leeway as well we have developed a new type of electromagnetic log. The prototype has been tried successfully at sea, and though full calibration has not yet been possible, there is every indication that an accuracy of 0.1 knot will be obtainable. The chief problem in calibration is that account has to be taken of the varying pattern of flow around the hull of the ship for different combinations of fore-and-aft speed and leeway.