Simrad SC90, Руководство по установке

Страница: 83 / 544

419050/B
81
Electrical self noise
Electrical or electronic self noise is picked up or generated in any other part of the
equipment than the transducer.
Hum picked up by the transducer cables or picked up from the power supply is usually
the most common source of electrical self noise. At higher frequencies – where rather
wide bandwidths are necessary – the noise from components, transistors or other
analogue electronic may be a limiting factor.
Reverberation
When you transmit an acoustic pulse into the water to search for a target, echoes will also
be returned from any other objects in the beam, and some times even from the seabed.
The energy reflected from these "unwanted" objects is referred to as
reverberation .
Reverberation is generated when the echoes from your SC90 transmission are reflected
not only by fish and valid targets in the water, but also from the seabed and from the sea
surface. Reverberation is often regarded as noise. There are two kinds of reverberation:
•
Volume reverberation
: This is caused by minute plankton and air-bubbles in the water.
This reverberation often has daily cycles in intensity.
•
Surface and bottom reverberation:
The surface reverberation is dependent on the
weather condition, while the bottom reverberation depends on the roughness of the
bottom.
Reverberation is often a limiting factor, especially if you are searching for smaller
targets. The echoes from the bottom will depend on the bottom properties; how uneven it
is and what kind of bottom you have under your vessel (rock, mud, sand). Your current
tilt setting will also have an effect on the reverberation. The echo from the sea surface
will vary with the current sea state and your current tilt setting.
The illustration demonstrates how the bottom and the sea surface return echoes back
to your SC90.
A
In this situation you will be able to identify the upper school of fish, but the lower
school will be hidden in the shadow of the seabed rock formation.
B In this situation you will see several weak echoes from the waves and from the rock
on the bottom, but you will still be able to identify the school of fish.
Preparations