Abstract
Usually, towed hydrophone arrays are instrumented with a set of compasses. Data from these sensors are utilized while beamforming the acoustic signal for target bearing estimation. However, elements of the hydrophone array mounted in the neighborhood of a compass can affect the Earth’s magnetic field detection. The effects depend upon the materials and magnetic environment present in the vicinity of the platform hosting the compass. If the disturbances are constant in time, they can be compensated for by means of a magnetic calibration procedure. This process is commonly known as soft and hard iron compensation. In this paper, a solution is presented for carrying out the magnetic calibration of a COTS (Commercial Off the Shelf) digital compass without sensor motion. This approach is particularly suited in applications where a physical rotation of the platform that hosts the sensor is unfeasible. In our case, the platform consists in an assembled and operational towed hydrophone array. A standard calibration process relies on physical rotation of the platform and thus on the use of the geomagnetic field as a reference during the compensation. As a variation on this approach, we generate an artificial reference magnetic field to simulate the impractical physical rotation. We obtain this by using a tri-axial Helmholtz coil, which enables programmability of the reference magnetic field and assures the required field uniformity. In our work, the simulated geomagnetic field is characterized in terms of its uncertainty. The analysis indicates that our method and experimental set-up represent a suitably accurate approach for the soft and hard iron compensation of the compasses equipped in the hydrophone array under test.
Highlights
The towed hydrophone array (THA) consists essentially of a line of hydrophones mounted inside a flexible hose that is towed by a submerged or surface vessel
As a variation on the standard calibration, which relies on a real geomagnetic field, we expose the sensor to an artificial field
The artificial field has a small degree of non-uniformity, in the soft and hard iron compensation, the important requirement is that the modulus of the stimulus used as a reference be constant during the calibration; the actual value is less important
Summary
The towed hydrophone array (THA) consists essentially of a line of hydrophones mounted inside a flexible hose that is towed by a submerged or surface vessel. It cannot be assumed that the hydrophones lie in a straight line behind the towing vessel. Several digital compasses containing triaxial magnetometers and triaxial accelerometers are mounted in the array to provide heading information along its length. The accelerometer measures the gravitational vector and the magnetometer measures the Earth’s magnetic field vector. Measurements made by the latter can be influenced by any object mounted near the sensor that can affect the Earth’s magnetic field
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