NorthStar Navigation EXPLORER 657 Bottom types, Frequency and cone width, Water, Depth, Cone

A

B

C

D

ASingle fish

BLarge school of fish C Small school of fish D Bottom

A

B

C

AKelp / Weed

BSoft bottoms such as mud, weed and sand show as narrow bands

CHard bottoms such as rock or coral show as wide bands

Bottom types

Mud, weed and sandy bottoms tend to weaken and scatter the sonar pulse, resulting in a weak echo. Hard, rocky or coral bottoms reflect the pulse, resulting in a strong echo.

Frequency and cone width

The sonar pulse generated by the 657 transducer travels down through the water, spreading outwards in a cone shape. The cone width is dependent upon the frequency of the pulse; at 50 kHz it is approximately 45°, and at 200 kHz it is approximately 11°.

The differences in the cone width affect what is displayed. See

section 8-3.

Water	Cone width		Cone width
Depth	at 50 kHz		at 200 kHz
25	20		5
50	40		10
100	80	45° cone	20	11°
150	130		30	cone
200	170		40
300	250		60
400	330		80
600	500		120
800	660		150
1000	830		190

Northstar Explorer 657 Installation and Operation Manual 33