Scuba Diving Range to boat (subsurface)

I have had an idea in my head for almost 20 years and it's time I attempt it. I'm looking to have a conversation on the design. I am skilled enough to build and program it but maybe together we can think of some clever design idea.

Use Case:
Nearing the end of an open water scuba dive you need to return to the boat. Surfacing isn't always safe as boats don't know what the flag means and swimming on the surface is terrible.
1: Swim to within 50 feet of boat without surfacing. From distances as far as 1000ft / 300 meters.
2: Know the distance to the boat in any relative unit. 1-very close ... 10-very far
3: Know the direction to swim or ability to figure it out
4: Divers tool should be more passive to allow it to run at or under 9volts for a few hour total run time
5: Boat tool runs on 12volt house battery, probably active

My concept:
Boat uses house batteries and a subsurface antenna to transmit RF in the range of 20Khz to 100Khz, in 5K blocks (for example) on a 1 second, per 5k, repeating pattern, 20,25,30,...100K,20,25,30... This could provide 16 'ranges' each RF band would attenuate to zero in progressively longer or shorter distance, yet non-directional as I'd need it to be to find the boat, at least the first time.

I (the diver) would have a receiver that looks like a flashlight. The receiver would be recessed and lightly shielded on 2 side. Shielding would add extra attenuation so that I could pan 'left and right' and pick up or drop off any weak range RF. This will provide a relative direction to the strongest sign as every range but the very longest will be shared by many frequencies and hence ranging accuracy and direction can be distilled.

A simple line of LEDs in the classic Red, Yellow, Green will indicate range as each of the 16 LEDs will respond to only one 5Khz RF band. While I swim I can continue to pan for the strongest signal and eventually all the LED will be lite and I should be within range of the boat and safe to surface.

I need to test what frequencies to use to get the 15m to 300m range and work our the Decibels to start with so they fall off evenly as I don't expect they will.

I'm open to ideas but active sonar ranging is far to complex and I don't want use a surface mounted radio on a float wired down to me, too much to break and store.

Ideally I'm looking for wisdom from those with the proper education to add something I haven't read in the other 'underwater RF' posts as they wanted to communicate. I just need datum to navigate from. I have also, mostly read, the ARRL handbook so I think the way water eats RF is my best and simplest design.

For what it's worth, that's my idea. Thanks for reading.

Spend a few days designing the antennas for the frequencies you are suggesting.

Paul

to add something I haven't read in the other 'underwater RF' posts

Help us out by summarizing what you have read, and what steps you have taken toward executing this very ambitious project.

Hello jremington,
The other posts that I found here about UW RF were all about command and control over sonic, electromagnetic, or photonics. All have there space, however, the only one I think is pedestrian enough for ranging is to leverage the well documented attenuation of electromagnetic signals in water and simply layering what is already defined. I don't need or care to encode data in the signal so we can rule out baud rates, defined protocols and all the other comment in the other post.

For as my knowledge, that's going to be difficult to summarize. I'm a profession programmer and my electrical engineering is usable but I do not have a BS in EE. Lets leave it at that.

If you consider it very ambitious that is of no concern to me. I need what I need regardless of it's challenges. The human endeavor knows no limits. Try we must.

I'd like your wisdom, however, it too isn't required.

What steps? I've posted a well defined concept to a forum filled with members that are better EE then I in search of that which I do not now to ask.

What steps?

See reply #1.

If I were determined, I would start by making a single frequency transmitter and receiver, with appropriate antennas, and seeing what range the setup gets under water.

Keep in mind that the antenna engineering books are generally written for antennas in air or vacuum. In air, a 50 kHz half wave resonant dipole antenna is about 2900 meters from tip to tip.

The OP also needs to investigate which frequencies are available for him to use. ALL RF frequencies are controlled by various US government agencies, such as FCC.

If it were this easy, the U.S. Navy would have done this long ago. They haven't.

First, you can discard all antenna and resonant circuit formulas out the window because they are based on being in open air.

Next, RF doesn't penetrate more than a few feet in water. The Navy experiments with VLF radio underwater uses kilowatts of transmit power and is limited to just a few bits per second to a few hundred BPS depending on the frequency. (Reference).

It's possible that you can use the functional theory of the VOR navigation system, but using audio frequencies.

Paul- if the RF is all underwater, then there's no issue with the FCC. There aren't any FCC monitoring stations underwater, and probably no other users there to be bothered with interference.

Take a look at commercial offerings. I have seen transmitter units tied to the anchor line, but I suspect that the receiving side gives approximate direction, not distance.

I’m not sure if this falls into the safety critical box , in which case it’s not a suitable project .

hammy:
I’m not sure if this falls into the safety critical box , in which case it’s not a suitable project .

Only if it goes wrong.

My experience is that I have watched a lot of submarine movies and I think every episode of The Silent Service.
I have also watched some of James Cameron documentaries and, if you are of similar age, The Incredible Mr. Limpet, and also Flipper. I also watched every episode of Sea Hunt with Lloyd Bridges.

As you can tell, I am well qualified to offer for free, an opinion and observations.

We have side scan sonar as well as regular sonar. sonar can tell the distance to target and outline a shape.
This is done by reflective beams.

Dolphins and the navies of the world use sonar and a listening device. you should be able to make a such a listening device that straps to your wrist that has open ports on either side. the one closest to the 'ping' will react first. the one further away will take a bit longer.

Maybe you could have one on each forearm so the separation is half a meter.
maybe on the tank harness.

If this works, you would be able to know what quadrant you are in from the boat. compass and sensor might need to be combined. you do a slow spin and get a bearing. plot that with compass direction for a start.

Distance could be a return that your set gets upon receipt of the ping from the boat. You make a 'ping' the boat gets that, then responds sending an acknowledgement. Time from your ping to receipt of the ship ack ping would give distance.

In Sea Hunt, Mike Nelson would tap his tank with his knife. if the ship can pick that up, then so could your receiver. Tap your tank. the ship picks that up, responds with one ping Vasili, one ping only. time your receiver gets from the tap on the tank to the boats response is calculable to distance.

As for accuracy, I do believe that "within a dozen meters" or "within 45 degrees" is really all you need.

as for knowing how to generate or receive a ping from a boat and how to make a ping on your tank. well, I'll leave that up to people whose brains did not turn to mush from watching too much TV.

hammy:
I’m not sure if this falls into the safety critical box , in which case it’s not a suitable project .

I cannot fathom how it could be safety related.
the diver has other stuff to know when to breath, how long to stay down, when to decompress, etc.
if the diver swims in exactly the opposite direction, then they will surface and swim to the boat. It would be hard to correlate a swimmer, with fins, needing to swim a bit further as a safety issue.
An impaired diver would use a rope line on the anchor and just reel themselves back to the chain, then go up the chain.
A free diver is prepared to be swept pretty far from their boat and have to swim back. I know that dropping your weights and wearing a wetsuit makes you really boyant and protects you from exposure.

SteveMann:
If it were this easy, the U.S. Navy would have done this long ago. They haven't.

that we know about. also, it would violate the first rule of stealth to be sending out signals

Paul- if the RF is all underwater, then there's no issue with the FCC. There aren't any FCC monitoring stations underwater, and probably no other users there to be bothered with interference.

I think that most FCC regulations stipulate distance. Every microcontroller radiates EMF on certain frequencies, but the distance is to short it falls well under regulations. As you noted, for most frequencies, the distances would be really short.

There are good reasons why whales, dolphins and Mike Nelson of Sea Hunt use sound, rather than radio, to communicate underwater.

The FCC is not one of them.

dave-in-nj:
I cannot fathom

Ha! I see what you did there.

I just run up a DSMB, and wait for the boat to come to me.
DSMB even has a light stick hooked to it, for those dark dives.

Again, thanks for the time to reply. Lots of stuff here.

  • First, it's not critical safety. Well, no more than SatNav in your car. Like SatNav, it will enable me to return in a path I do not have memorized. I pull a dive flag per the law to warn off boaters but they are clueless untrained seamen that have actually tied a boat to my flag thinking it was a mooring and left the motor running with me stuck under the boat.

-When it matters, like shipwreck diving, cave diving or other 'must exit' environment, I use cave reels but anything past 500 feet and it becomes snagged so you have to swim slow as you reel it all back up and clear the snag.

  • This product exists already, I saw one many years ago, I'll try and find it.
  • The Navi already has Diver Detection System to protect ships in harbor be sabotage. They are very complex because they need to filter out seals and other animals as none threating.

Antennas, yes, that is a messy one. I'm using mostly navy research from declassified documents from the 50's and some IEEE docs from 2010. Besides a few days reading antenna design, HAM, Navy RF electronics pdf published in 2008, to get my head around the the issues in air I haven't done much in way of designs.
re-evaluation of rf electromagnetic communication in underwater sensor networks
The link is very good, yet it to is communication, still mostly usable.
I don't have an electronic link but if you can find
"U.S. NAVAL UNDERWATER ORDANANCE STATION NEWPORT, RHODE ISLAND"
"ELECTROMAGNETIC RADIATION IN SEA WATER"
They are researching how to control torpedo's with radio. spooler, they recommend not doing it.

I can change the frequencies to anything that works. I used 20K to 100K as a simple talking point and to indicate lower is better for water. I will not need 1 sec band gaps, that too made the description simple as the reads are unlikely to be divers and wanted everyone to understand the idea more than the detail.

Active Pings, maybe, I used something similar to one posters stereo receiver a long time ago to find the EPIRB on downed aircraft. I was very much like an animal tracking collar system. Given the speed of sound is 4x I wasn't sure how well could get direction without the half meter dipole getting snagged in all the weeds we have in Minnesota.

For those playing along some grim facts. So my 20-100K window is looking more realistic however the antenna is a problem as expected. A normal whip antenna for 100Khz looks like 2.5 meter but that is for a normal wavelength of some 3000 meters, 1/4 wave 750 and add loading.... So I need to build and test based on the documented 6 meter wavelength and design a 1/4 or 1/2 wave from that at about 1.5/3m. That works for the boat vertical polarized, now I need to receive. Consider the the 2.5 meter air antenna must have been loaded to correct the reactance from 750m to 2.5 a factor of 300, I should hope to be able to load my diver antenna to 500cm.

The real killer is 8dB per meter. That's not going to work at all. A chart reports propagation of 183meter for 100dB attenuation at 100Khz. I can get 578meter if I drop to 10KHz. So I may have to live with a shorter range. A compact omnidirectional loop antenna is sited in the footnotes if I can get access to the document.

Navy charts
Wavelenth in Sea-water in meters.
100MHz is 200cm
10MHz is about 600cm
1MHz is about 2 meters
100KHz is about 6 meters

Attenuation Nepers/meter (db)
10MHz is about 11 (95.5)
1MHz is about 4 (34.7)
100KHz is about 1 (8.6 dB)

Impedance of Sea water in Ohms
10MHz is about 4.5
1MHz is about 1.5
100KHz is .4

arcforge:
The real killer is 8dB per meter. that's not going to work at all.

Do also factor in that a 'whip' antenna that is significantly shorter than a 1/4wave at the frequency of choice will be massivly inefficient as a radiator.

How may killowatts are you planning for the transmitter to need ?

srnet:
Do also factor in that a 'whip' antenna that is significantly shorter than a 1/4wave at the frequency of choice will be massivly inefficient as a radiator.

How may killowatts are you planning for the transmitter to need ?

I'm still sneaking up on that, but I get the joke. I may need a nuclear plant at 100KHz, however the chart from the IEEE doc shows this very thing at 200meter. That will do.