Effects of Ship’sVelocity Determined from GPS Data on CurrentVelocity Measured by Shipmounted ADCP

Abstract

It is required to know the ship velocity referred to the earth (ground velocity) in oceanic current measurement using shipmounted ADCPs. At present, the ground velocity of a vessel is usually calculated from the distance between two different points whose positions in latitude and longitude are determined by GPS. This article discusses potential effects of errors in ship velocities determined by GPS upon current measurement by ADCPs. The mean value of distance estimated by GPS location data is to be always larger than the real distance, on the assumption that the GPS location data show a two dimensional normal distribution around the real position and that the zonal (x－direction) and meridional (y－direction) components of the difference between the real and GPS locations have same standard deviations (σx＝σy). This fact results in overestimation in the ground velocities. The overestimated error velocity depends not only on the accuracy of GPS but also both on the time interval of velocity calculations and the ground speed of the ship. The error velocity can be suppressed and negligible through averaging procedure for many velocity data, whereas the individual errors may be too big in current measurement. Further investigation on errors of GPS velocities will be necessary for cases of inhomogeneous GPS location data (σx≠ σy), which has been reported at some fixed points.