In order to locate a downed aircraft in water, a faster method might be possible, and cost less for the search. Minor modifications might allow this system to be used on lost aircraft that are not visible from the air.
An array of small, inexpensive repeaters that can be set for various water depths could narrow down the search area quickly.
The device is a plastic sphere with a listening device circuit on board, and a long life LiPo battery. It also has an acoustic transmitter that is more powerful and longer lasting than on-board black boxes.
The devices are deployed by aircraft. Prior to deployment, the depth is set and recorded, and the repeater serial number is also recorded and tied into the GPS derived coordinates from the aircraft, which are entered into a master search database.
Assuming the sensitivity is 2 miles (could be much higher):
The aircraft deploys a 4 mile interval hex grid in the search area.
Each serialized module listens for the BB frequency. If it gets a signal, it sends out 2 signals:
1) Sends out the BB signal just as it is received, but at higher energy and different intervals. This will discern whether it's a black box or a repeater.
2) At a new frequency, it transmits coded BB signal strength and the module S/N.
The surrounding modules will relay this information to each other on channel two.
So if you lay out 50,000 modules, it can cover an area of over 1,000,000 square miles. Assuming you can keep the cost at $200 each module, you can pinpoint the location for $1,000,000 in module costs. Now that's a huge area.
The benefits are that if the BB batteries die, it does not stop the grid signals. It can cover huge areas regardless of the depth or uneven terrain, your search happens very quickly, and the triangulation is instantainous to any helicopter with a hydrophone in a million square miles.
Obviously, the price goes way down if the search area is less than a million square miles.