Humminbird 998c SI - Manual (Page 155)

145
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