What is the meaning of along and across track beam width of Bathyswath?

Sonar specifications often give numbers for along and across track beam widths. What are these numbers for Bathyswath?

Along and across beam width determine the size of objects that can be detected: smaller beam widths allow you to see smaller objects, and large beam widths cause objects on the bottom to be "smudged out". Bathyswath, like all swath bathymetry systems, looks sideways from the vessel, using a beam that is wide in the vertical direction and narrow at right angles to the direction that it is looking (in the direction that the vessel is going: "along track").

The along-track beam width is controlled by the "azimuth beam width" of the transducers. This is 1.1 degrees for the 468kHz and 234kHz transducers, but because these beams are narrow for both the transmit and receive parts, the transmit and receive beams combine to give an effective beam width that is half that: 0.55 degrees.

Across-track beam width is a number that applies to beam-forming sonars, which form separate beams that the receive data is detected in. The more separate receive elements there are, the more beams and the narrower each separate beam is. Interferometers like Bathyswath don't form beams like this; instead, they detect the angle of the incoming sound wave at different times. So, the resolution (the size of the smallest object that can be detected) is controlled by two things:

  • The rate at which angles are measured: using modern electronics and computers, this can be as fast as you like; Bathyswath 2 does this at 100kHz, 100,000 times a second. So this is not a limit to performance.
  • The width of the sonar transmit pulse: the sonar sends out a short "pulse" and listens to the echo. If the pulse is long, then the area covered by each one when it hits the bottom is wider, and the resolution is worse. But longer pulses put more energy into the water, so the echo is stronger and the data quality and range are better. The Bathyswath software controls the pulse length automatically during the survey to maintain data quality (although the operator can over-ride this if required). In the a typical survey, we see that the pulse length was 46 cycles, so the length of the pulse was 46 wavelengths at 468kHz. One wavelength is 3.2mm at this frequency, so each pulse is 15cm long. At a water depth of 10m and a range of 30m, this corresponds to an angle of 0.1 degrees.

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