[PSUBS-MAILIST] OTS

[Sean T. Stevenson via Personal_Submersibles]

Rick, ultrasound behaves quite a bit different than does radio as far as the attenuation relationship to distance. Things like shadowing, (where a diver's body / equipment or the hull or superstructure of your sub gets in between the direct line of sight between transducers) will have a much more profound affect on transmission effectiveness than distance alone. Similarly, thermoclines and haloclines can act as "surfaces" that refract or reflect the ultrasonic transmission, so it is important for the surface crew to lower their transducer into the same operating layer as the diver / sub. As a diver, I always report the presence and depth of any thermocline encountered on descent to the surface support for this reason. Also, the surface transducer needs to be suspended in the water column and not bottomed or lost in surface sea clutter, and the transducer needs to be weighted and boat speed limited so that it doesn't trail behind at an angle which is ineffective for transmission.

The rating in the published specifications for the OTS SSB-2010 (a 5 Watt unit) is greater than 1000 meters in a calm sea, and ~200 meters in sea state 6 (Beaufort), but I presume that these are ideal scenarios whereby the surface and dived transducers are suspended at the same depth, in the same (vertical) orientation, with clear line of sight and no interfering reflections, and also with no squelch applied. Real-world conditions are obviously somewhat different, and the environment can actually be somewhat noisy at typical channel frequencies, requiring the application of some squelch and the consequent loss of range.

Rated frequency response is ~300 Hz to ~3000 Hz, which covers most of the vocal range, but will not reproduce true timbre of voice. It is fine for intelligible speech, but not for e.g. music.

I have used this unit as a diver only, so my impression may be also due to the influence of the earphone / microphone assembly in my mask. Choice of mask makes a difference (shape and size of gas cavity), as does proximity to the mic. You need to be almost kissing those OTS diver mics to get decent sound. That said, my experience with the SSB-2010 was decent, but that was in the context of a dive team separated by a few meters at most, and at relatively shallow depths (limited distance from surface unit) where helium based breathing gas was not required.

I can't speak to long range effectiveness, other than at Flathead last year I was surface support for Cliff in the R-300. He was using a SSB-2010, but the surface unit we had was something different - one of the surface specific boxes, and I don't recall the model (Cliff?). There were times at which we lost him and had to reposition the boat to reacquire him, and others where we could hear him but he couldn't hear us, but I can't speak to the exact ranges, depths or boat speeds that caused those issues. It was never such a sustained loss of comms that the dive had to be called. Likely the result of relative orientation or depth, or shadowing of the transducers.

Sean

-------- Original Message --------
On Oct. 24, 2020, 12:10, Rick Patton via Personal_Submersibles wrote:

> wanted to hear from those who have and have used the OTS comms system. I was wondering how you liked the system and what your max clear audible range was.
> Thanks
> Rick
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