Other sonars, wakes and your own hull can all paint echoes that are not fish. This lesson covers how to find the source, how to reject it, and which false echoes to expect.
At a glance
- Turn TRANSMISSION off to find the source
- INT REJECT 0 to 3, NOISE LIMITER and TX CYCLE help
- A wake shows as a thick line and blocks the sound beyond it
- Sidelobe false echoes: try VER BEAMWIDTH WIDE
About 7 minutes. Part of Tune the picture in the CSH-8L MK2 course.
Find the source
Turn TRANSMISSION off. If the noise stays, it is arriving from outside, such as another sonar or equipment on your own boat. Switch equipment on one at a time to find it.
Reject it
INT REJECT runs from 0 (off) to 3. NOISE LIMITER removes the weakest signals, and TX CYCLE controls the ping timing. Lowering TX CYCLE can reject interference rings from other sonars.
Wakes
The wake of your boat or another vessel shows as a thick, continuous line. The wake blocks the sound, so what lies beyond it is poorly covered.
Sidelobes and dome projection
Over shallow, rocky bottom, sidelobes can paint false echoes. Setting VER BEAMWIDTH to WIDE helps. Furuno also notes that poor projection through the dome can do the same, so persistent false echoes are worth a word with your technician.
After a cable change
Furuno's installation notes say false echoes after a transducer cable has been extended point to the junction box connectors, so check them first.
Check yourself
Three questions
Sources
- CSH5LMK2 CSH8LMK2 Operator's Manual: Sections 1.4 and 6
- CSH8LMK2 Installation Guide: Interference and cable notes
This lesson summarizes Furuno's public documents in our own words. Limits and settings differ by model and software version, so your own manual and your installer have the last word. OmniCSH is an independent project, not a Furuno program.