User guide

FS 20/25 Operators Manual 901-10037001/1.1
Maintenance and Troubleshooting Page 5.1
3
Kongsberg Simrad Mesotech Ltd.
Port Coquitlam, BC - Canada
♦ Cable Gain Calculations
To illustrate how you can evaluate these “Cable Gain” numbers, let’s assume that
your system gave the above as Cable Gain numbers for the FS20 Vertical Head
(LF1) on an 8’ service test cable…………Cable Gain 6,44
Number A = 6
Number B = 44
A general rule of thumb formula for establishing real time cable attenuation (for
which the system is compensating) is as follows:
Cable Attenuation (dB) = 20 log [ (Number A x Number B) / 36]
In the above example, the cable gain numbers would produce the following result:
Cable Attenuation (dB) = 20 log [(6 x 44)/36]
= 20 log [7.33]
= 17.3 dB
The key thing to note here is that the calculated attenuation is < 40 dB (which is one
of the system specifications). If over a period of time, you notice that the cable gain
numbers are getting larger, the calculated attenuation will start to approach 40 dB.
Once it goes beyond 40 dB, the system will not be able to compensate and you will
start getting telemetry errors or outright system failures. There is a direct relationship
between a degrading 3
rd
wire and the cable gain.
By periodically checking the cable gain compensation numbers and comparing it to a
previously recorded value, the condition of the trawl cable can be monitored.
5.4.4 System Error Messages
In the normal operation of the FS Trawl Monitoring system, commands are sent from
the processor unit down the trawl cable, to the trawl unit electronics and
acknowledgments are then sent back up to the processor unit.
Telemetry errors can result from noise induced on the trawl cable due to
interference, or problems with the trawl cable or trawl unit electronics. Though a few
telemetry errors are quite common and acceptable, an excessive number of errors
will adversely affect the sonar operation and may indicate a problem that will
eventually cause a system failure.