Sonar Pulse Range in Water
Dai Farr - A fine summary
Long ago in another job I designed the electronics for hydrographic survey work. Things have changed a lot since then.
Those who have followed this thread have heard that the range of the ULB is likely to be 600 or 800 or 1500 or 2000 metres. We have seen the photo of the ULB locator on the small Chinese boat - with headphone monitoring.
Typically those receivers are tunable and have a bandwidth of about 1 to 3 KHz. The range of detection that has been bandied about is probably about right: .6 to 2 km for that sort of detector.
But with modern technology and a pulsed carrier ultrasonic transmission with reasonable stability one can use a very narrow band detection scheme.
Reducing the received bandwidth to 10 Hz could increase the detection range by a factor of 10 or more (without benefit of any ducting contributions). If the signal from the ULB is sufficiently stable in frequency, going to a coherent detection scheme and slightly narrower bandwidth and gating the receiver at the sonar pulse rate could possibly achieve a detection range of 10-30 km. The major limitation on this sort of scheme - it would only work with a non-moving vessel due to doppler shift becoming a major contribution to the frequency. But that can easily be addressed with modern DSP techniques. Having 500 or 1000 virtual receivers (it's only software) focused on detecting carriers every 2 Hz across the spectrum would be easy. And once the carrier is found, the doppler shift would become a viable cue for deciding which direction to move in search of the ULB.
Pure speculation: this technique is being used right now. I am wondering if the Benthos ULB locator was shown on the small Chinese rubber boat for show only... It would be a nice way to get the report out without disclosing some more serious sonar analysis capabilities.
By complete coincidence my work this last week has been focused on analysis of sonar reflective returns at 40.0 KHz