So – I’m thinking GPS or radio can’t work through water; gyroscopic reference might but you have ocean currents and it can’t be too precise; sonar mapping would seem to require a lot of processing and a previously mapped ocean floor. Plus the Z coordinate complicates typical X-Y map navigation.
Also, is the drones exploration path programmed and run without communication to the surface?
In: Engineering
In addition to what the other guy said about GPS buoys with a datalink, if that’s not possible then inertial navigation is still used, taking up not a lot of space or computing power.
Having 3 axes to work in can actually make them more accurate, since you always know your depth from a pressure sensor and accurately calculating one axis reduces the error in the other two (through clever maths). Ocean currents don’t create an error with this since it’s calculating your position and speed relative to the earth, not relative to the water around you. Typically this will get less accurate over time, either limiting your time or forcing you to narrow down your position, either with GPS or by comparing with seabed depth if you have an echosounder fitted.
Somewhat accurate charts are always needed because you don’t want to, well, crash into the seabed.
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