Dynamic calibration for multi-beam bathymetric system based on characteristic data processing
The geometric installation error of the multi-beam bathymetric system will affect measure accuracy of the depth and location, which must be faced and resolved in the engineering survey urgently. The most important three kinds of the installation error are roll error, pitch error and yaw error. Through quantitative analysis of impact of installation error to the measure result, a dynamic calibration method has been proposed in the paper. In the method, installation errors can be obtained by analyzing bathymetric result from the characteristic terrain seafloor and the errors will be used as the parameters to calibrate other bathymetric result. Validity of the method has been proved by processing the measure data from Songhua Lake test.
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.
Bottom detection for multibeam sonars with active contours
In multibeam sonar bathymetry, a receiving array is used to form a fan-shaped beam sector. Bottom echoes of near-normal incident angles are usually detected based on the amplitude of the received signal. To detect bottom echoes of oblique incident angles, it is common to use the phase information obtained by subarray interferometry. The type of the bottom echo is not known a priori. Hence, which of the two bottom detection methods to use for a particular beam is determined by ad hoc criteria. Localization errors of the bottom are often the consequence of choosing the wrong bottom detection method. In this paper, the authors propose to use a bottom image transform and an active contour model to integrate the amplitude and phase information, so the bottom echoes from all directions can be detected by a single algorithm. Promising results were obtained in the authors' evaluation using Simrad EM3000 sonar data recorded from shallow water.