Technical Description - Interferometry

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A “swath-sounding” sonar system is one that is used to measure the water depth in a line extending outwards from the sonar transducer. Such systems are generally arranged so that the line of depths, or “profile”, lies at right angles to the direction of motion of the transducer. A series of these profiles are known as a swath.

The term “interferometry” is generally used to describe swath-sounding sonar techniques that use the phase content of the sonar signal to measure the angle of a wave front returned from a sonar target. Systems using this technique are also known as Phase Differencing Bathymetric Systems, or PDBS. This technique may be contrasted with “beamforming” multi-beam echosounder sonars (MBES).

These generate a set of receive beams, and look for an amplitude peak on each beam in order to detect the sea-bed (or other targets) across the swath. See below for a comparison between beamformers and Bathyswath.

Interferometers themselves fall into several categories. All of these use similar transducer geometry: two or more horizontal arrays (or “staves”) arranged one above the other. Each array is equivalent to a “normal” sidescan array, producing a beam that is narrow in azimuth (that is, viewed from above), and wide in elevation (viewed from the side). One of these arrays is supplied with a pulse of electrical energy at the sonar frequency, producing a narrow “shell” of sound that moves outwards from the transducer. Where this shell meets the seabed there is a small “ensonified” patch, which moves across the seabed as the sound travels outwards. The ensonified patch scatters sound energy in all directions. When this scattered sound is detected back at the interferometric transducers, the angle it makes with the transducer is measured. The range is calculated from the travel time there-and-back. The range and angle pair enables the location of the ensonified seabed patch to be known relative to the sonar transducer.

Bathyswath measures the phase of the measured signal at each of the transducer staves relative to a reference signal at the system’s sonar frequency. The phase difference between the staves is derived by subtracting these phase measurements from each other. The phase is derived from a simple and robust electronic method, which directly provides a digital measurement of phase. The electronics are thus kept simple and therefore small and reliable. Wavefront angle is calculated from a simple formula relating phase and transducer spacing measured in wavelengths.

In order to measure the angle accurately, more than one pair of staves must be used. Narrow spacings give an unambiguous measurement of angle, but are more susceptible to noise and give poor resolution. Wide spacings give good resolution and noise immunity, but any one-phase measurement from them can decode to several elevation angles. To overcome these restrictions Bathyswath uses a range of spacings to obtain the best results. The combinations of spacings are used in a manner similar to that used by a mechanical vernier measuring device.