Page 393 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

14.4 Acoustic noise 385
Tx pulse
Min. accepted pulse length
FIGURE 14.12
Received pulse.
pulse
plus
reflection
Transmitted signal
Frequency
Direct
Direct
pulse Reflection
Received signal
Time
This shows that a surface with 100% reflection will create no resulting signal at the receiver given the distances. The same problem can be caused by ray bending or reflections from risers or a ship’s hull.
Even reflections that are slightly delayed might cause problems. The receiver has certain criteria in order to accept the signal as a pulse. One of them is the pulse length. As Figures 14.12 and 14.13 illustrate, a worst-case condition will be that the direct pulse length is too short to be acknowledged as a pulse before the reflection cancels it. Then the remainder of the reflection is just long enough to be accepted as a pulse.
FIGURE 14.13
Multipath transmission: time and frequency smearing of acoustic signals.