A sonar that circles a patch of seabed sees it from every side, so the depth plane given to its beamformer becomes a true plane of focus. A point on the plane is sharp; a point target 3 m above it is drawn as a ring 0.7 m in radius and loses about 35 dB of peak.
Tour
Controls
SAS image of the patch
8 m × 8 m, north up · × where the point stands · ○ predicted ring
Point target, 4 m
North-south cut
East-west cut
Method
The sonar is the site's standard array: 36 receivers 30 mm long at 300 kHz with 60 kHz of bandwidth. It is flown clockwise around a circle of 30 m radius, 8 m above a flat sand bed, looking inward, with consecutive pings sharing two phase centers. The seabed is simulated with ApertureLab's point-scatterer engine at 20,000 scatterers per square meter, with the wedge, truncated-cone and cylinder meshes of ApertureLab's dataset and a point target. Each image is a time-domain back-projection of all 370 pings on a 1.25 cm grid, formed on a flat plane at the height and with the sound speed that the controls set. The path error adds 5 mm of sway, and heave of about half that, over each 6 m of track to the path given to the beamformer.
The point target's own response is isolated by simulating each scene a second time without it and subtracting the two images; the subtraction is exact because the imaging chain is linear. The point-target panel and the readout are measured on that response, in decibels against a perfectly focused point, or against the point's own peak when Scale to own peak is ticked. The seabed image is scaled to its own median brightness so the rings stay visible. The straight-track counterpart is The surface of focus, the imaging chain is described on Image formation, and the effect of faults in the sonar's hardware on an image is shown on Hardware faults.