Reson SeaBat 7125 SV2, Operator'S Manual

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ACTIVE SONAR USAGE
SeaBat 7125 SV2 Operator‟s Manual
Page 114 July 25, 2011
Version 3
shipboard equipment should be conducted to determine if there is a potential for
machinery interference prior to conducting at-sea testing. For means of compensating
for this, see subsections in
6.2 above .
6.7.3 Interference from Other Sonar Systems
Interference from other sonar systems can be seen as radial lines, typically moving away
the minimum to maximum range scale as these pings are not correlated with the ping
repetition rate of the SeaBat system. The most typical source of interference of this type
is navigational sonars (often 50kHz systems) and Doppler velocity logs.
Synchronization of acoustic systems is one means to mitigate this effect (see
section
4.2.5.2 Trigger Output ). The most effective approach is to remove the source of
interference entirely, either by moving the equipment away from the SeaBat or turning it
off.
6.7.4 Speed
Hydrodynamic flow noise can be a concern when operating at higher speeds. Externally
mounted acoustic sensors should have fairings that are as flush (or near-flush) mounted
to the ship hull as is practically possible. This way the hydrodynamic flow-induced
vibration noise should be minimal. For means of compensating for this, see subsections
in
6.2 above .
NOTE
A poorly designed sonar fairing can degrade the system performance by a factor of 4
for speeds above 8-10 knots. Typically, this noise source is the most common problem
on all sonar installations.
6.7.5 Air Bubbles
Bubble sweep-down can be another potential noise problem. The significance of this
problem is associated with the noise generated as the bubbles cascade along the hull
after being ingested in the bow wave of the ship and, more importantly, with the baffling
produced by the entrained air layer between the face of the sonar sensor and the water
environment.
If air bubbles are present between the sonar and the water, the system will not function
properly. This phenomenon is transient in nature occurring only for a few seconds per
wave period. It is typically most prevalent on shallow draft ships and increases in
intensity at higher sea states and ship speeds.
Modeling of this problem only indicates that air bubble noise may be present by
determining the transmission vehicle of potential bubbles. Actual at-sea measurements
are usually required to fully determine the presence and severity of bubble sweep-down.
At-sea diver viewing has been very successful in the past to capture the bubble sweep-
down characteristics of a particular ship hull on film.