Acoustic Doppler Shift
The reason the pinger detectors are looking for signals over an extended range of frequencies is because the speed of sound in water varies due to pressure changes with depth as well as salinity changes.
The speed varies up to 10% depending on conditions.
This in itself is no problem if the detector and sender are at the same salinity and pressure. However if you have a detector in a different pressure and salinity layer to the source the detected frequency will be altered - doppler shifted if you will - by up to 10% either side of nominal. Pulse duration will also be affected but pulse interval won't be altered.
This also means that a signal detected by a deep towed detector will be different frequency to a shallow towed detector.
Last edited by Mahatma Kote; 10th Apr 2014 at 02:10.
Reason: depth effect