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 methods correcting roll and pitch biases of multi-beam bathymetric system
Compared with the traditional single beam system, multi-beam bathymetric system measuring water depth in the form of belt can adjust parameters such as real time velocity of sound and vessel posture. The pitch and roll biases during transducer installing are important error sources in the bathymetric system. The effects of these two parameters on precision of bathymetry have been analyzed. According to the error properties in terrain profile, parameter testing methods by both real time measurement and overlaid profile are discussed, especially, a visible operating interface for overlaid profile is designed for the convenience in adjusting field and indoor parameters. The pitch and roll biases obtained from testing this multi-beam system are used to adjust real time water depth. In addition, field data can also be reprocessed in indoor data processing based on parameter corrections to improve data quality.
Signal Processing for Swath Bathymetry and Concurrent Seafloor Acoustic Imaging
Whether they use the multibeam or sidescan geometries, swath bathymetry sonar systems aspire to provide the widest cross-track coverage with the highest spatial resolution possible. Although these two requirements are often contradictory, a fair compromise can be achieved through a combination of array design and signal processing techniques. To this end, a version of the split-aperture correlator is used in sidescan sonar systems to obtain estimates of differential phase as a function of time of arrival of seafloor echoes. These estimates are subsequently converted to sequences of angle of arrival versus time and then to bathymetry. The same technique is used in beamformed systems to detect the time of arrival of the echoes at the zero crossing of the differential phase sequence. However, this technique is unreliable in the near-specular direction and other beamformed echo detection methods working in the time domain or the spatial frequency domain are considered. FFT beamforming techniques offer some echo detection implementation advantages allowing a choice between any of the aforementioned techniques. For each ping, once signals backscatterred by the seafloor have been processed for bathymetry, the magnitudes of the returns can be positioned at their corresponding horizontal distances in an amplitude cross-track profile. Accumulation over many pings of such profiles as lines of a raster image forms a sidescanned acoustic image of the seafloor. This article reviews the signal processing issues related to these echo detection techniques, and their implication for bathymetric resolution and swath coverage.