Benthos 865-A, Instructions For The Installation, Operation And Maintenance

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Model 865-A Deep Sea Acoustic Release/Transponder M-855-009
Benthos, Inc.
1
APPLICATION NOTE #500-001
SUBJECT : Minimum Detectable Signal (MDS) of Underwater Transponders
Benthos specifies the MDS to be less than 85 dB re 1 uPascal for our underwater transponders. Most units typically
test out to be between 75-80 dB re 1 uPascal during our standard acoustic pool test. The noise level in our pool
approaches that of the deep ocean, i.e. approximately 50-60 dB re 1 uPascal at 10 kHz.
What does this mean in terms of ranging capability? Let's take an example at 10 kHz.
First, assume the Source Level Output (SLO) of the surface unit is 192 dB re 1 uPascal at 1 meter. Then, the
transmission loss at maximum range is simply the difference between the SLO at the surface and the MDS on the
bottom:
Transmission Loss = 192 dB - 85 dB = 107 dB
From transmission loss tables, this figures corresponds to a maximum range of about 28 kilometers. This is more than
adequate sensitivity, since the limiting factor in ranging capability is the reception at the surface, where the noise
environment may be worse than the deep ocean by a factor of 30 dB or more, due to ship and surface noise.
Therefore, further improvement of the sensitivity of the underwater receiver is not only unnecessary, but in fact,
somewhat detrimental. As the sensitivity is brought closer and closer to the ambient noise level, the False Alarm Rate
goes up resulting in unnecessary transponds that will drain the battery and confuse the surface receiver. Thus, by
specifying the MDS to be less than 85dB re 1 uPascal at 1 meter, Benthos has insured adequate ranging capability while
maintaining a negligible False Alarm Rate.
It should also be noted that all Benthos receivers will maintain a Constant False Alarm Rate even if the ambient noise
environment changes. This is known as a Constant False Alarm Rate or CFAR receiver. All Benthos deck units also
have sensitivity controls that allow the operator to maximize the sensitivity for a particular noise environment. This
essentially allows the operator to choose an acceptable False Alarm Rate, which is then held constant at that level by the
electronics.