Artemis BP00707, Руководство пользователя

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7 UNDERSTANDING SONAR IMAGERY
Interpreting sonar imagery may seem challenging at first, but with practice and some
knowledge of how the sonar works, it doesn’t take very long for an operator to
understand what the sonar image represents.
Many people try to look at the pictures and understand them as you would a
photograph, but this however is not strictly the case. In the following sections, we will
look at how the sonar produces its images, and see how information can be obtained
from them.
Basic Operation
The Sonar used by Artemis is a “forward looking, scanning sonar”. In simple terms the
acoustic beam produced by the sonar is much like that of a torch beam having a
vertical and horizontal beam width.
The sonar will build up an image of echoes it receives from targets in front of it by
mechanically scanning a narrow (approx 3° horizontal width) wide acoustic sound beam
horizontally through a 90° arc, performing “pings” at set intervals.
When an object is ‘illuminated” within the sonar beam, the sound energy reflects off
the target and is presented on the Artemis sonar display. The size of the object on the
display is a function of the size of the target itself and the range at which it is
observed.
The operator should hold the Artemis console (and sonar) horizontally, and stationary
for a few seconds while a full scan completes. If the sonar is moved or rotated during a
scan, the resulting image will appear smeared and distorted.
For relocation of large objects, wrecks, jetties, underwater swimmer delivery vehicles
and the like, it is possible to use the maximum range of 100m.
Once the object has been detected, the diver can close the range, changing the Range
Scales as the target range is closed.
For relocation of smaller objects such as mines, depth charges, etc it is recommended
that the diver is placed as close as possible to the datum position (within 50m
preferably).
Range & Gain Controls
The sonar has two main controls the help optimise the image; Range and Gain .
The range control allows the operator to choose the maximum range targets will be
shown at on the display. As sound travels at a fixed speed through water, increasing
the range means the sonar must wait longer to collect echoes from more distant
targets and so the scan rate will slow down, while shorter ranges will cause the sonar
to scan faster.
The gain control can be thought of like a “volume” control – increasing the gain will
amplify fainter echoes making the entire display appear brighter (and possibly saturate
some target detail), while decreasing the gain will dim the display (possibly hiding
fainter targets, but making it possible to detect texture and detail on others).
Gain is mainly used to reduce the background clutter reflections in a high clutter
(rocky/boulder) environment. By reducing the gain, the clutter and seabed
reverberations can be reduced on the Sonar display making the initial detection of the
target easier to see.
When scrutinizing a target at close range (within 3m range) in turbid or nil visibility,
the gain is particularly useful in establishing the shape, constitution and dimensions of
the target being prosecuted.