GP1670F_1870F Operator's Manual - Furuno USA

7. FISH FINDER OPERATIONSPlanktonA plankton layer appears as a large amount ofgreen or blue dots, and is a possible location forfish. A plankton layer moves down in the day andup at night.School of fishPlanktonCurrent ripWhen two ocean currents of different speeds directionsand water temperatures meet, a currentrip occurs. A current rip appears on the screen asshown in the right illustration.Current ripSurface noiseWhen the sea is rough or the ship moves over awake, surface noise can appear at the top of thescreen.Surface noiseAir bubbles in the waterWhen the sea is rough or the ship makes a quickturn, there can be blank spots in the bottom echo(see the right illustration). These blank spots arecaused by air bubbles which stop the movementof the sound wave. These air bubbles can occurwith the low frequency ultrasonic waves.Ultrasonic waveblocked by air bubblesUnstable bottom echoThe bottom echoes can have a saw-tooth appearance.This occurs in heavy weather becausepitching and rolling changes the direction of theultrasonic pulses and the vertical motion of theship causes the distance to the bottom to change.Unstable bottom echo7-25