Manual Humminbird 998C SI (page 155 of 160) (English)

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Power Output The amount of sound energy emitted into the water by the

transducer’s transmitter. Power output is measured using either RMS (Root Mean

Square) or P-T-P (Peak-to-Peak) measurement systems. Either method is acceptable,

but it is important when comparing power outputs, to make sure that the same

measurement system is being used for both outputs, because P-T-P numbers are 8

times higher than RMS numbers. Greater power output allows the sonar signal to

penetrate through weeds and thermoclines, reach deeper depths and operate more

effectively in noisy environments, such as when the boat is running at high speed.

Pulse Width (Pulse Length) The length of time that a sonar sound burst is

transmitted into the water. Shorter pulse widths provide better target separation, but

cannot travel to great depths. Longer pulse widths provide better depth penetration,

but result in poorer target separation. Humminbird® varies pulse width based on depth

to optimize both target separation and depth performance. See

Target Separation.

Second Return Describes the appearance of a second sonar return below the

primary sonar return (bottom contour) at exactly twice the true depth. The second

return is caused by the same sonar energy bouncing off the bottom once, rebounding

to the water surface and then traveling back down to the bottom to be reflected again.

Second returns are more common in shallow water and over hard bottoms; it is

possible to see a third sonar return under some circumstances. The second return

provides useful information to help determine bottom hardness, as areas with harder

bottoms will generally create a second return. The second return can be used as a

guide to set Sensitivity when in shallower water.

SONAR (SOund and NAvigation Ranging) Sonar technology uses precision sound

bursts transmitted underwater to determine the distance and other attributes of objects

in the water. Distance can be determined because the speed of sound in water is

constant, and the time for the signal to return is measured. Sound also travels very

quickly underwater, making sonar a responsive, cost-effective tool. Sonar is the basic

technology behind all recreational and commercial fishfinding and depthfinding devices.

Sonar Update Rate The number of times per second that the transducer’s

transmitter/receiver sends and receives sonar signals. A very fast sonar update rate

collects more information and provides a more detailed image of the bottom, fish, and

structure. Many Humminbird® units operate at up to 40 times per second when in

single frequency operation. Due to the limitation of the speed of sound in water, the

update rate begins to slow as depth increases to deeper than 50 feet. In very shallow

water (less than 10 feet), however, update rates as much as 60 times per second can

be achieved.

Speed The rate at which the boat moves through the water. Boat speed can be

measured as Speed Over Ground or Speed Through Water. Speed Over Ground is

provided by GPS, and is the measurement of the boats progress across a given

distance. Speed Through Water is provided by a speed paddlewheel, and is the

measurement of the flow past the boat, which may vary depending on current speed

and direction. Speed Through Water is most critical for anglers using downriggers, as

it impacts the running depth of the down riggers. Speed Over Ground is optimal for

navigation, as accurate destination times can be derived from this measurement.

Humminbird® products allow for input and display of both sources.

Structure A general term for objects on the bottom that present a discontinuity and

are a likely attractor for fish. This includes bottom contour features (drop-offs, humps,

and holes), standing structure (stumps, timbers, brush piles), and a wide range of

other potential objects (sunken boats, reefs).

Surface Clutter A phenomenon where sonar returns are reflected off of tiny objects

near the surface of the water, including algae and even air bubbles. Typically, saltwater

environments have significantly greater surface clutter than freshwater due to

continuous wind and wave action that causes aeration at the surface.

Target Separation The measurement of minimum distance that a fishfinder needs to

be able to recognize two very close objects as two distinct targets (i.e. two fish

hanging very close, or a fish hanging very close to structure). Humminbird® fishfinders

Glossary - Sonar Terms

Glossary Sonar Terms

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Humminbird 998C SI Manual

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