Ask HN: Why is there no GPS in the ocean?

The ocean can’t have GPS because GPS relies on time of flight calculations for radio waves (aka light) traveling at the speed of light through air. The ocean, to the surprise of many, is not composed of air, and is really, really, exceptionally good at scattering light. What you could have, instead, is an acoustic positioning system that would rely on fixed transponders that would allow time of swim calculations for sound waves traveling through water. This would be phenomenally problematic, as you’d need a near perfect understanding of confounding factors like salinity, temperature, depth, etc to accurately model the incoming reply… you’d also drive the whales and fishes to consider well justified total warfare against the really loud and insensitive terrestrial creatures they vastly outnumber.

GPS works on the surface of the ocean. But the radio waves cannot penetrate water very far, so it fails if you are submerged (few things can which is why subs are so well hidden compared to aircraft.

Unless a nuclear sub was specifically sent to the area to look, it is very very unlikely to just be "in the area". Even if it were there, it is very hard to find a small metal box on the bottom of the ocean. There is no real sensor for that. Most military subs are not designed to dive beyond about 500m depth (there is no need, 100m is more than enough to be functionally undetectable from the surface). The titanic is at about 3800m. So they likely could not get anywhere near close enough.

GPS is a one way system. A GPS receiver can only tell where it is, it does not transmit its position. To transmit position you would use a EPIRB. A EPIRB however cannot work underwater as its radio signal would be blocked by the water just as a GPS signal is blocked.

They are looking for this submersible by sonar. You don't need a nuclear submarine for sonar as it can done with ships ,sonobuoys dropped by aircraft or RV subs with sonar.

Adding some points: Radio waves (very high frequency) of GPS cannot go deep in the ocean. Very low frequency can, but their wavelenght is very big, so unsuitable for a GPS. The only ways to "see" undersea are acoustic and magnetic. Nuclear sub navigation rely on inertial systems, and i think they can give a position to civilians for rescue.