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Performance Analysis Using an Uncertainty Model for HydroChart 5000
Sonar Propagation Model Allows Estimation of Uncertainty In Depth Measurements, Meets IHO S-44 Requirements

By Yuhui Ai
Principal Systems Engineer
Marc Parent
Vice President of Engineering
L-3 Communications Klein Associates Inc.
Salem, New Hampshire

Shallow-water bathymetry is important for many survey applications, some of which include port and harbor navigation and inspection, environmental assessment and mapping, underwater construction and rapid area assessment. Many of these surveys are conducted in water depths of less than 20 meters and require accurate, high-resolution data with the ability to detect small objects while providing full bottom coverage. Survey data that require adherence to bathymetric standards such as the International Hydrographic Organization (IHO) Publication S-44 must meet strict statistical data quality requirements for accuracy, resolution and coverage of the processed data.

Swath sonars utilizing phase-difference bathymetric processing are particularly well suited for these shallow survey environments and offer distinct advantages over multibeam echosounders, including increased portability, lower costs and wider swath coverage. In 2010, L-3 Communications Klein Associates Inc. introduced the HydroChart 5000 (HC5000), which combines a dynamically focused multibeam side scan sonar with a high-resolution phase-differencing bathymetric sonar (PDBS) to produce simultaneous and coregistered acoustic backscatter imagery and seafloor bathymetry that meets IHO S-44 requirements.

This article will describe the HC5000 system and then focus on the development of a sonar propagation uncertainty model and its application to survey data with results meeting IHO S-44 requirements.

HC5000 System Description
The HC5000 system has two primary components. The submerged component is a lightweight, portable sonar head unit designed to be mounted to an over-the-side fixture (via standard pipe flange) that suspends the sonar in the water. The topside component, the transceiver processing unit (TPU), interfaces with the sonar head unit, the data acquisition and display computer, and the motion sensor (for correction of ship-induced motion). The sonar head contains a set of transducers providing measurements for dynamically focused, multibeam side scan imagery as well as phase-difference bathymetry, sonar electronics, an altimeter and a sound-speed sensor.

The sonar electronics include a sonar transmitter capable of ensonifying the seabed with a frequency-modulated chirp or continuous wave pulse; a pair of multichannel receiver modules for analog signal processing of the received bottom backscatter; a multiplexer module that performs digitization of sonar backscatter and facilitates bidirectional data telemetry; and a power distribution subsystem. Putting the sonar electronics in close proximity to the sonar transducers helps to improve the overall performance of the system.

Accurate time tagging of sonar data, ship motion and navigation is important for accurate motion correction and sounding position. Systems trying to perform this function in software can run into significant latency issues, reducing the accuracy of the data. In the HC5000, the time tagging of this data is done by hardware in the TPU, reducing latency to tens of microseconds. One pulse-per-second and NMEA-0183-compliant serial interfaces are provided on the TPU to facilitate this function.

The HC5000 can interface with a variety of third-party motion sensors, allowing for accurate motion correction of acquired bathymetric data. The HC5000 can also be interfaced to a variety of sound-speed sensors. The data acquisition and processing computer is provided as an option to the system. It connects to the TPU via an Ethernet interface and runs L-3 Klein's SonarPro software, a Windows-based application used to acquire, display and log real-time side scan, bathymetry and sensor data. Several real-time displays are provided specifically for the bathymetric component of the HC5000 to facilitate sonar quality control during survey operations.

Dual-Use System
Because the HC5000 has similar core components to its cousin, the S5000 V2 multibeam side scan sonar, it is also capable of performing high-speed, high-resolution side scan sonar surveys while deployed in an over-the-side configuration. This provides an added capability to suit applications that require rapid area assessment. Acquired bathymetric data can then be used to facilitate the measurement of local bottom slope for more accurate target positioning, and/or for aiding side scan sonar image interpretation.

Bathymetric Data Processing
Processed HC5000 data are compatible with a number of third-party hydrographic post-processing packages. To facilitate this post-processing, L-3 Klein provides a separate batch-processing graphical user interface, the BatchProcGUI application, to compute both the beam-formed side scan solution and the bathymetric solution for a batch (or folder) of selected files.

For bathymetric processing, the application allows the user to apply a variety of options to the solution, including ship geometry (lever arm correction), motion sensor source, sonar calibration and a sound-velocity profile. Processed data can be stored in L-3 Klein's SDF format or industry-standard formats such as XYZ or GSF. To continue this article please click here.

Yuhui Ai is a principal systems engineer with L-3 Communications Klein Associates. He specializes in sonar signal processing, underwater acoustics and sonar systems design. He has published more than 20 peer-reviewed papers in these fields.

Marc Parent is vice president of engineering for L-3 Communications Klein Associates. He has more than 20 years' experience in the development of sonar systems and acoustic instrumentation for the marine industry.

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