The patch test: a comprehensive calibration tool for multibeam echosounders

The patch test procedure is the first stage required in any multibeam echosounder survey. The patch test is designed to precisely determine the static configuration of the sonar head (roll, pitch, yaw) and the latency remaining between the reception of the GPS fix and its integration by the acquisition system. An imprecise knowledge of these four values introduces bias when computing the real 3D geographic location of each beam. In the proposed patch test program, these four values are sequentially computed, according to tailored track acquisitions. An extended set of measured information is involved, such as the dynamic attitude, position and heading of the supportship, the geometric offsets of each acquisition unit (MRU, GPS, MBES, compass) and some environmental features (sound velocity profile). The computation mainly relies on performing a matching between specific sea-bottom profiles extracted from each overlapping track. A first rough computation is performed to help the user quickly select the range and step of values to search. Then the user can refine its search as required with successive iterations, during which both the error measures and the error curve for all the previous tested values are displayed. The computation is thus fully automatic, searching the best value within the range. The impact of selecting such at value is immediately shown on the global display of the surveyed area, which provides three representations to enable the user to evaluate the accuracy of the current result: a color-coded real location of the beams, the iso-depth contours extracted from each track, and synthesized depth grids which perform direct comparisons between the track depths.