Factors influencing the sonar display

The quality and accuracy of the sonar display can be influenced by a number of factors including vessel speed, target depth, target size, background noise, interference, and transducer frequency. Understanding these factors can assist you in interpreting what the sonar is displaying.

Vessel speed

  • Target shape — The target shape changes, depending on vessel speed. Slower speeds return flatter, more horizontal marks. Higher speeds cause the target to thicken and arch slightly, until at fast speeds the mark resembles a double vertical line.

  • Cavitation — At higher vessel speeds, propellers cause air bubbles to be formed, which appear as noise on the sonar screen, making it difficult to detect targets.

Target depth

Deeper targets will appear smaller and lighter onscreen, compared to targets of the same size that are closer to the surface.

Target size

Generally, target size is relative; the larger the target, the larger the target will appear onscreen.

Background noise

The sonar image can be negatively impacted by returns received from: floating or submerged debris, propeller wash, rain, sea state, or vessel movement. This noise reduces target detection performance.

The Sensitivity settings can be used to help reduce impact of noise. Some settings can also be automatically controlled according to depth and water conditions.

Interference

Interference from other nearby sonar operating in close frequency proximity can negatively impact the sonar image. This interference, known as “cross-talk interference” causes vertical “rain drops” throughout the water column.

Transducer frequency

  • Low frequency (e.g.: 50 kHz) — Lower frequencies can penetrate deeper into the water column than higher frequencies, but at reduced resolution. Lower frequencies are not as effective at detecting smaller targets.

  • High frequency (e.g.: 200 kHz) — Higher frequencies provide better resolution at shallower depths. Higher frequencies are more effective at detecting smaller targets.

  • CHIRP sonar — CHIRP sonar combines both low and high frequency ranges, providing the benefits of both.