Field Calibration Methodology for a Multibeam Echo Sounder Using a Split Beam Sonar System and a Standard Target
|Title||Field Calibration Methodology for a Multibeam Echo Sounder Using a Split Beam Sonar System and a Standard Target|
|Degree and Program||Master of Science|
|Number of Pages||156|
|University||University of New Hampshire|
|Keywords||Thesis; Field Calibration; Multibeam echo sounder; Split beam sonar system|
A method to determine the combined transmit/receive beam pattern for a ship-mounted multibeam system was developed and tested at distances of up to 8 m using a RESON 7125 MBES inside the fresh water calibration tank of the University of New Hampshire. The calibration method employed a tungsten carbide sphere of 38.1 mm diameter as the target and a SIMRAD EK60 split-beam sonar system to provide athwartship and alongship angular information of the target sphere position.
The multibeam sonar system was configured for 256 beams in equi-angle mode with an operating frequency of 200 kHz; the split-beam system was set to work passively at the same frequency. A combined transmit/receive beam pattern was computed for athwartship angular ranges between −6°and +6° and alongship angular ranges between −1 ° and +3°. The target sphere, with target strength of −39 dB at 200 kHz, was suspended in the water column by a monofilament line and manually moved along the range of athwartship and alongship angles. The limited angular range of the measurements is due to the −3 dB beamwidth of 7.0° in the alongship and athwartship direction of the split-beam sonar system coupled with the alongship offset of 1.6° between the maximum response axes (MRA) of the two systems. Possible acoustic interference caused by the monofilament line was found in the measurements for alongship angles smaller than −1°.
Beam pattern measurements for the combined transmit/receive beam pattern at a distance of 8 m show a −3 dB beamwidth of 1.1° in the athwartship direction and a −3 dB beamwidth of 2.0° in the alongship direction for the most inner beams. The dynamic range for the measurements was approximately −40 dB, limiting the ability to resolve side-lobes.
Tests of the accuracy of the target angle estimates from the split-beam system were also conducted. Errors for athwartship angles were smaller than 0.1° for the most inner angular positions (athwartship and alongship angles closer to the split-beam MRA), increasing to 0.3° for athwartship and alongship angles close to + 4° and −4°. Alongship errors were found to be smaller than 0.1° for the most inner positions increasing to around 0.5° for athwartship angles close to + 4°and −4° and alongship angles close to +4°. Alongship errors for alongship angles less than −1° were much larger (on the order of 5°), which compromised the beam pattern measurements for that angular region. Further tests suggested that interference from the monofilament line used to suspend the sphere may have compromised the angle estimates for alongship angles less than −1°. The acoustic interference from the monofilament line suggested by the tests may be due to the particular configuration of the transducers used here, where the MRAs of both sonar systems were pointed parallel to the water level (horizontally) and approximately perpendicular the monofilament line. Different results may be observed for ship-mounted transducers where the measurement geometry would be different than the one used in the tests described here.