[PSUBS-MAILIST] Underwater Navigation

[River Dolfi via Personal_Submersibles]

That type of system, with several transponders with known location
communicating with the submersible is known as a "Long Base Line" system,
or LBL. That was the state of the art before GPS. Typically, dynamically
positioned drilling rigs would hold themselves on position based on
feedback from LBL transponders installed on the seafloor (often by manned
submersible) providing more reliable and accurate positioning than Loran C
was capable of. Now you would just use GPS.

Today, Universal Short Base Line (USBL) systems are more common. A surface
mounted hydrophone unit transmits an activation ping to an smart beacon
attached to the undersea item to be tracked. Upon receiving the activation
ping, the beacon transmits a timestamp and a depth back to the surface
hydrophone. Using a reference speed of sound in water, the total travel
time of the signal, , bearing of the signal, and the depth reported by the
beacon, the topside unit can calculate the relative position of the subsea
beacon (or multiple beacons) which can be interfaced with a GPS receiver on
the topside unit, and provide a UTM coordinate and depth for the subsea
item to be tracked.

This is all fine and good for tracking an ROV, where the operator and the
surface unit are at the same location. Navigating a sub using a USBL system
requires specialized acoustic modems to transmit the UTM data back down to
the submersible, and then some type of computer to display it all.

There is no need to mention how ludicrously expensive (and heavy, and
complicated, and delicate) all of this equipment is.

One of the biggest unresolved problems on Psubs is navigation, inertial
navigation and anything acoustic based is entirely too finnicky and
complicated.

I think the most practical navigation tools available to amateur
submariners would be a basic heading indicator (magnetic or airplane gyro
compass) and some of the really nice commercially available
fishfinder/sonar units available.

The bathyscaphe crews in the 60's used to sink a number of labelled 55
gallon drums filled with concrete in a grid pattern around the projected
dive area. If you can locate yourself within the grid of barrels using
sonar, you have a reference frame to navigate from.

I propose creating a series of sonar reflectors anchored to the sea-floor.
Attached to the anchor could be a trawl float with a specific number or
pattern of sonar reflectors attached. Think of day markers on a ship. Each
reflector station could be located, and identified, by a fishfinder at a
moderate distance. The position of the sub could be fixed by knowing
heading, bearing to a specific reflector station, and the approximate
distance to the station.

Galvanic timed releases can be obtained for cheap, and used to retrieve the
trawl floats and sonar reflectors after a predetermined period of use.

A typical PSub outing could be covered by 3-4 sonar reflector stations, and
there is no need to standardize on expensive equipment.

-River J. Dolfi
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