originally posted by auv-ee ...
Note, however, that the frequency of the pingers is badly controlled, likely by an RC timer. The spec says the frequency is 37.5 +/-1kHz. That leaves plenty of room to separate one pinger from another, even with Doppler obscuring the result, unless the two pingers happen to be close in frequency.
Yes, the spec is a bit broad. I was thinking that for an R/C oscillator entombed in parafin wax within a very solid steel/titanium case, the external temperature would eventually have an effect on its frequency and that a xtal/piezo filter may be in the R/C loop. Thinking in terms of manufacturing to meet the nominal frequency +/- 50Hz, I'd move away from an R/C circuit and use something less affected by temperature and voltage. The stuff I can find just quotes the tolerances without any schematics to back it up.
As there are three timing issues involved, 37.5kHz [output frequency], 1Hz [period] and 375/37500Hz - 10m/s [duration]; simple xtal divider logic would seem more practical.
With regard to the Doppler shift, my feeling is that differentiating that from multi-path would be a nightmare. Perhaps careful reference to the local bathymetry may help sort it out. Best of luck to them!
mm43