underwater locator beacon: why not chirp?

[apinger (Guest)]

Hi

in light of the recent aircraft accidents, i was wondering why the 
locator beacons are fixed frequency at 37 kHz. Wouldn't it be better if 
they emit a known time structure e.g. Chirp or the like. That would not 
only make the range much better but also virtually remove the 
possibility of spurious signals. Any ideas why they don't do that?

[Bernhard (Guest)]

The ultrasonic transducer is probably narrow band with a bandwidth of 
around 1 kHz. That is why you can not transmit any broadband pulse like 
a chirp.

[mscho (Guest)]

apinger wrote:
> Any ideas why they don't do that?

I guess its due to the fact that the overwhelming amount of the pingers 
will never be used in all their lifetime - so they try to keep this as 
cheap as possible but still functioning. Making the transducer resonant 
to a single frequency keeps it loud but still energy efficient enough to 
work for a month.

[Lurchi (Guest)]

Resonant emitters are usually more energy efficient. There could be 
still a slow modulation like a pulse structure (e.g. 1 pulse of 0.5 
seconds every 10 seconds) if the power is limited by the battery and not 
by the transducer.

For me the question is a little why they choose such a high frequency. 
Lower frequency sound has less damping and thus a longer range.

[mscho (Guest)]

Lurchi wrote:
> For me the question is a little why they choose such a high frequency.
> Lower frequency sound has less damping and thus a longer range.

Its still low enough to not be confused with echo transducers from 
ships' depth measuring equipment in the 100-200kHz range and high enough 
to be no natural sound. It also requires larger transducers to emit 
deeper sounds efficently and both size and weigth is limited in aircraft 
equipment.

[William (Guest)]

mscho wrote:
> Its still low enough to not be confused with echo transducers from
> ships' depth measuring equipment in the 100-200kHz range and high enough
> to be no natural sound.

These are flat water echo sounders only.

For deep water echo sounders are using frequencies in the 50 kHz range. 
Have a look to Raymarine or what ever.
Echosounder for fishery requirements use a broad range of transducer at 
38 kHz and many other frequencies.
SimRad 
https://www.simrad.com/www/01/nokbg0240.nsf/AllWeb/3E3ABDF14A22990DC12570AD0034AD3B?OpenDocument

[apinger (Guest)]

38 kHz seems like a bad choice for fish finders. one fishing vessel in 
proximity and boom fake signal. The power of these sonars is immense and 
sind can be guided over long distances between water layers.

[apinger (Guest)]

Lurchi wrote:
>
> For me the question is a little why they choose such a high frequency.
> Lower frequency sound has less damping and thus a longer range.

Newer ones come in sub 10 kHz also and are mandatory now.