USING DGPS TO MEASURE THE HEAVE MOTION OF HYDROGRAPHIC SURVEY VESSELS

One quite significant error source encountered by hydrographers is wav e induced vertical motion of their survey vessel (heave). In heavy swe lls, uncorrected heave noise will degrade the accuracy of the surveyed soundings upon which mariners rely for safe navigation. Heave motion can be measured using inertial technology thus enabling the raw survey ed soundings to be corrected to calm water conditions. Unfortunately, the high cost of inertial heave compensators has prohibited their wide spread use. This paper documents a test carried out by the Canadian Hy drographic Service in which very accurate relative position derived fr om GPS phase observations were used to determine heave corrections for a hydrographic survey vessel. The algorithm is simply a high pass fil ter acting on the unused DGPS vertical position record already being o bserved on the vessel. An inexpensive pitch and roll inclinometer is u sed to correct for the lever arm effect between the GPS antenna and th e sounder's transducer. The experiment indicated that decimetre heave compensation accuracy was obtained.