"MGP" . wrote in message ...

"BrianR" wrote in message
...
If both sounders use the same frequency, which they probably will, then
yes they will interfere with each other.
A sounder works by measuring the time it takes for the signal to travel
from the transducer to the bottom and back. If two sounders are
operating
at the same frequency they have no way of determining which signals
return
they are receiving.


If that was the case wouldn't you expect to see interference when sailing
in
close proximity to other vessels with echo sounders running? I never saw
any
sign of this



A quick review of the physics involved:
+ speed of sound in water is roughly 1493m/s in fresh water and 1533m/s in
sea water
+ round trip time for "interesting" depths ( 100m) is less than 150ms
+ the energy of the echo obeys an inverse square law, so the return is much
weaker than
the signal generated by the transducer

Taken together, this means the sounder operates in pulse mode so as not to
drown out the echo and that pulses must be at least 150ms apart to allow for
the round-trip time even in shallow water. For the example earlier in this
thread of 3km charted depth, round trip time is of the order of 4 seconds.
To make the transmitted pulse as efficient as possible (both equipment power
wise and snr), a short duration, high amplitude is required. For sake of
argument, assuming a 5ms pulse, this gives a maximum duty cycle of the
transmitter of about 3% for 100m and 0.13% for km. At this point it begins
to become clear that the interference problem postulated above would require
the sounders on nearby boats (or multiple sounders on a single boat) to be
both in phase and using the same pulse rate if the sounders are using any
form of signal processing (e.g. averaging successive readings and/or range
gating). As the maximum depth resolution of the sounder increases, the duty
cycle decrease further reduces the probability of collision.

Additional refinements ala CSMA (carrier sense multiple access) are left as
an exercise to the reader.

I have personally experimentally verified non-interference in shallow (10m)
fresh water operating a Raymarine ST-60 tridata and a Humminbird ID 120 with
transducers located within 2m of each other on the same boat.