A system that is mounted on a vessel that has a net average motion will measure the sum of the vessel velocity and the ocean currents. Thus to get information about the ocean currents only, the vessel velocity must be subtracted from the instrument's reading. This requires information from the ship heading (gyro) and navigation system (DGPS). The SurveyVM software is designed to provide realtime monitoring of current speed and direction at different depths below a moving vessel. What the SurveyVM software does, is to combine “raw” current profile data from the sensor with speed and heading data from external inputs like DGPS/Gyro. The corrected values, or difference between the two, represent the net ocean current relative to ground. Without external information the result is more or less meaningless for the intended use of this software.
SurveyVM displays data in ENU (east-north-up) coordinates, where a positive value indicates north direction and a negative value indicates south. For XYZ coordinates, a positive velocity in the X-direction goes in the direction of the X-axis arrow (marked on the instrument head). The X-axis points in the same direction as beam 1. Use the right-hand-rule to remember the notation conventions for vectors.