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THE ECHOTRAC E20 IN ACTION
6.3.4
Interference from Other Echosounder Systems
Interference from other echosounder 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
Echotrac E20 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. The most effective approach
is to remove the source of interference entirely, by either moving the equipment away from the
acoustic sensor of the Echotrac E20 or turning it off.
6.3.5
Speed
Hydrodynamic flow noise can be a concern when operating at higher speeds. Externally mounted
acoustic sensors should have fairings that are mounted as flush (or near flush) to the ship hull as is
practically possible. This way the hydrodynamic flow-induced vibration noise should be minimal.
A poorly designed sonar fairing can degrade the system performance by a factor of 4 for speeds
above 8-10 knots. This noise source is a common problem on all echosounder installations.
6.3.6
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 acoustic sensor and the water environment.
If air bubbles are present between the acoustic sensor 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.
Air bubbles mask some or all of the view of the acoustic sensor where the affected area will completely
blank out. Common causes for bubble masking include:
o
The vessel props when thrusting into reverse and pours a stream of bubbles over the acoustic
sensor.
o
The acoustic sensor has not been deployed deeply enough and there is bubble wash from the
sea surface.
o
Bubble ingestion as the vessel pitches into the sea.
6.3.7
Environment
A high-reverberation environment where echoes of the previous ping are contaminating the current
ping may prove to be difficult to overcome.
Echotrac E20 Operator's Manual8
Version 8
p. 36 of 60
Uncontrolled Technology Subject to Restrictions Contained on the Cover Page.
November 25, 2020
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