A Simplified (?) Spin on an Underwater Acoustic Positioning System

Hello world!

I've got a bit of a tricky problem I'm attempting to crack here, in the realm of underwater location mapping. I know people have been barking up that tree for a while, and there have been some elegant solutions proposed. Unfortunately, my budget doesn't really allow for elegance; I'm looking for more of a down-and-dirty, cheap alternative. What sets my quest apart (hopefully in a good way) is that I don't necessarily need to be able to locate a specific point in three dimensions, rather, I only need to determine the distance from point A to point B (both points being divers). Furthermore, I don't need to display the distance between them; what I'm looking to do is create what amounts to an "invisible fence" that alerts both divers in the event that one of them strays further than, let's say, 25 feet, from the other.

I've found numerous devices that can tell a diver how far they are from the anchor line/boat, such as the Dive Tracker series from Desert Star, or the L1 Location Transmitter from Liquivision. Most systems I've found use ultrasonic transceivers with band-pass filters and all that fun stuff. My [exhaustive] research has led me to the conclusion that acoustics are the norm, and really the only option, in underwater positioning. I have looked into optical rangefinders, near-field magnetic induction, and using radio frequencies to transmit location to determine distance, but it's looking as though rudimentary SONAR is my best bet (though I could be mistaken).

I have seen several hacks, both on this forum and elsewhere, where folks create underwater rangefinders using the fancy blue/green laser pointers. I have also seen one or two posts on using MaxBotix acoustic sensors to build SONAR systems with varying results. However, I am wondering whether there is a simplified method or type of sensor I have not considered that could help me alert the divers with the fewest parts possible. I am not opposed to having a third device, perhaps attached to a buoy or the anchor line, if it means the devices being carried by the divers could be further simplified. If I am thinking along the right path, each diver would simply need a transmitter, a receiver, and some sort of alert (LED, piezo, whathaveyou), and a third "smarter" device could receive the pings, determine its distance from diver A, determine its distance from diver B, calculate the distance between A and B, compare that distance to a pre-set allowable difference, and if the limit is exceeded, send out an alert to the device each diver is carrying.

If this makes any sense, I'll be highly impressed with myself and your collective comprehension (and patience!). I don't want to re-hash any projects that have already been done, so if I'm describing something that has already been posted and discussed, please let me know and forgive me (I've looked, I promise). I apologize for the lengthy post here, I just wanted to make sure I covered everything, ya know? Long story short, I was hoping to see if I'm on the right track based on what I've described. If that's the case, I came at the right time; I'm at the end of my knowledge rope here and figured that just maybe someone on this forum might be able to advise me as to which sensors I should be looking at in particular.

Thank you all so much for hangin' in there and hopefully helping me out. You're the best. And probably very handsome too.

Two ways come to mind. 1) 25 foot piece of heavy string (works well in cave diving too ) or 2) something along the lines of a transponder they use for aircraft. Your unit would send out a coded acoustic pulse and the other divers unit would respond with his coded pulse. Knowing the processing time in the units, the rest of the time between send/receive would tell you the distance. The advantage of using a coded pulse instead of a simple pulse (which could work for demonstration purposes) is that you can have multiple divers in the water and your unit and your buddies are the only ones that are working between the two of you.