Im thinking about something similar to ......
Three beacons lying on the bottom. Each one transmits a particular signal, lets say, a song (binary sequence), that starts at a given UTC time. (GPS like, but acoustic).
One of the beacons is the Master one.
Lets say, master beacon is number 1, and the other two are 2 and 3.
Vectors 1-2, and 1-3, define a 2D vectorial space, which base matrix is design by them.
Vector 2-3, defines the error between theoretical and measured lectures.
Hence, two beacons are Tx/Rx, while the third could only be Tx.
Master beacon, from theoretical(geometric) datas and error, modifies the vectorial base, sending the correction data beside its own signal.
The ROV, receives at least two beacons (master + another). From each particuar songs Rov software obtains:
Beacon identification-Time from transmission start, and hence, by means of each message phase lag, the distance to each one.
Time stamp can be substituted by a Known message transmission time.
Beside its own message, master beacon sends the correction to the vectors forming the vectorial reference base.
ROV software, solves for the base change matrix, returning the desired kind of possition coordinates.
Which coordinates ? It will all depend on the software as its just a simple matter of maths.
As an information:
Ais systems, send the whole navigation and safety information of a ship, packed in a 168 bits chain.
By the way, I've alredy developed that coding software for another project, and would be pleased to share it with the ROV project if it can be useful.
AIS parsing example:
' type 1, 2, 3
(first bit is 1 )
1) Message ID, 6 bits 1-6
2) Repeat Indicator, 2 bits 7-8
3) User ID, 30 bits 9-38
4) Navigation status, 4 bits 39-42
5) Rate of turn, 8 bits 43-50
6) SOG, 10 bits 51-60
7) Position accuracy, 1 bit 61
8) Longitude, 28 bits 62-89
9) Latitude, 27 bits 90-116
10) COG, 12 bits 117-128
11) True Heading, 9 bits 129-137
12) UTC second when report generated, 6 bits 138-143
13) Regional Application, 4 bits 144-147
14) Spare, 1 bit 148
15) RAIM Flag, 1 bit 149
16) Communications State, 19 bits 150-168
Of course thats the IMO/ITU standard, but we could use the same coding but using message blocks for our particular needs.