Underwater propagation ?

[rchateauneu]

Did someone tried to evaluate under water the propagation of the sound emitted by APRSDroid ? There might be an interesting use of this software for divers : In dark and muddy water, the only transmission media is sound: Everything else is blocked. Therefore, APRS signals emitted from an underwater tablet ( like this one http://www.dive-xtras.com/pages/accessories/underwater_tablet.asp ), relayed from one machine to another, might be helpful especially in cave diving.

Given the slow speed of sound, it might even be possible to evaluate the distance between two machines by counting the time taken by a response trip.

[ge0rg]

This is a very interesting idea, however I am not sure if APRSdroid is the right app for that. I would rather look into making something specially for that purpose.

Several practical aspects come to my mind:

• is the barometer sensor usable under water, to determine the current depth?
• is AFSK1200 robust enough? maybe you can test it out with two Android devices... otherwise, PSK31 or some other more robust encoding might be used
• not sure how easy a round-trip measurement would be to implement with audio, as processing adds a significant latency/jitter (currently in the order of 1s to make it more efficient)

[rchateauneu]

"is the barometer sensor usable under water, to determine the current depth?"

Great idea! Knowing the depth is necessary for a diving computer. If the case is flexible, the internal and extern pressure should be the same. On the other hand, the atmospheric pressure range is much narrower than underwater ones (One bar for every ten meters). Wonder if the accelerometer is sensible enough to detect slow movements of a diver. This would be ideal to set up the exact 3D path of a dive (Which is impossible now). Also, the orientation sensor is definitevely a plus for night divers who lose vertical visual references ( http://en.wikipedia.org/wiki/Night_diving ). But this has nothing to do with radio of course.

"how easy a round-trip measurement"

Indeed. A solution would be to send a signal while receiving it, thus evaluating the local latency, which can later be forwarded to remote machines

Anyway voice communication is used by professional divers ( http://en.wikipedia.org/wiki/Diver_communications#Voice_communication_protocol ). APRS adds something completely new by relaying information such as:

• the presence of a diver (Simply by listening, see for example mheardd http://manpages.ubuntu.com/manpages/hardy/man8/mheardd.8.html ).
• messages from and to the surface.
• Maybe medical information such as the breath or heartbeat.

APRS is convenient for this because it is distributed, fault-tolerant and designed to relay short message without router configuration.

In the case of cave diving ( http://en.wikipedia.org/wiki/Cave_diving ) and given the relatively low price of Android machines, one might even imagine to leave digipeaters at tunnel corners or every X meters.

"is AFSK1200 robust enough"

No idea. My bathroom is not big enough :) There is an AX25 extension with FEC, called FX25, compatible with AX25 : http://en.wikipedia.org/wiki/FX.25_Forward_Error_Correction

Remi F4ECW