How low can you go ?
This is commonly done in three ways:
2) Automatic surfacing and a radio link (such as wifi)
3) Acoustically, f is less than 50 Khz. Higher f = lower range. (electromagnetic waves don't propogate well thru water)
I have been working on an inexpensive way to get a drone in the water since the early 90's and I have found that tethered is the best way to go. Depending on what you want to do and your budget. I work for the local fire dept. and built an drone for under water search and recovery of bodies. I have a tethered drone that I built several years ago that I tow behind a boat and it is also maneuverable from a stationary point.The tether is 200 feet with an IR camera so it compensates for light at depth. once you get over 100 feet, the voltage drop in the wire is quite high so you will have to over come that. The next problem was keeping water out so a pressure compensating surface supply was installed with a fail safe CO2 system in case of power loss it will surface by its self. At 200 meters the pressure at depth will be around 300 psi or so and to keep water out at that depth will be a trick. I found that it doesn't matter how good you think your seals are, they will leak.
Hope that helps a little.
Thanks a lot for the info,
I still in the plans stage and didn't built nothing yet.
Tethered sound like the place to go with.
Actially, the best way might be optical communication, at least that's something to consider. You can make a great and surprisingly fast comm link with a strobing light! If I can find this one talk I once saw by this one inventor... I think he was on TED.com and talked about it.
Edit: Didn't find it yet, but I came across this: http://www.slideshare.net/guestcd295/short-range-underwater-communi... - maybe it could be helpful for brainstorming.
Ultrasonic is about the only way, if you don't want wires.
Speak to some boating/fishing type people, I know one frequency is 40 KHz, the other 400 KHz ???
Slow data rate though.........
Way back when I was involved in a Submarine race competition.
Our team used a device called the buddy phone from Ocean Technology Systems. It was a two way radio used with scuba. I worked fine but we used up our air supply twice as fast. We ended up using just the reciever in the water and the ground station to talk to the divers. Here is a link to the explination of how their system works.
The military uses a VLF system, very low frequency, something less than 100KHz. The subs tow a very long wire antenna, as in 1000's of feet. The frequency is very low and hence so is the bandwidth, that is the speed of data will be slow.
Search VLF submarine and you will find lots of info.
Open Acoustic-modem protocol. Sample code in Matlab & C.
One idea that I have been kicking around is to actually use the audible spectrum at very low frequencies (i.e., 20-30 khz), using submersible speaker systems pushing at very high decibels and using a hydrophone (microphone) receiver on the UAS. The 10-30 Mhz spectrum is what whales "sing" on, and the sound waves can propagate over 100's of miles)
The system would require the following:
1. Submersible speaker system capable of generating high decibel outputs at desired frequency
2. Hydrophones on UAS to record signals output by speaker system
3. On-board processing system to interpret recorded commands - this would essentially be the same as the voice processing software the google and siri use to translate your words, but for pre-determined, repeatable sounds. This can easily be programmed up using Matlab.
This system has essentially been tested in nature - you can dip your hydrophone in the water where whales are singing, record their "output" and then have your PC manipulate that output into basic commands.
However, you would be severely limited in terms of bandwidth - i.e., you can only output one command at a time... so you'd either have to settle for limited capabilities or use pre-programmed sounds to perform a fixed set of operations.
The concept is relatively simple, but as in most cases, the logistics is a nightmare. Things you would have to contend with include:
Background noise (whales, boats, drilling platforms, etc), wave noise if near shore, signal attenuation over long distances, and the fact that anyone could record your output and use it to steal your UAS - which means that you would need a rather sophisticated encryption system of some sort.
Thanks for the very detailed answer,
At the moment I put this project aside but sure will get back to it someday.