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

2 points by keepamovin 2 years ago | 13 comments

I'm not an electrical engineer so please forgive my ignorance. I guess I could just ask ChatGPT but figured there may be a more interesting response here. Why can't the ocean have something like GPS? Also, if a nuclear submarine (those ones whose exact positions are classified) identified the location of this submersible, would they tell the relevant authorities? Or would they not, in order to protect their classified sensors and position?

yawpitch 2 years ago |

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.

LatteLazy 2 years ago |

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.

richardjam73 2 years ago |

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.

paymycodes 2 years ago | |

These are all good answers by the way thank you. Isn't there like an "emergency transponder beacon" or something (like aircraft have?) that ships and submersibles must have? Why can't this be used?

yawpitch 2 years ago | | |

That’s what the EPIRB is… problem is it has to reach the surface to broadcast; they’re designed for finding floating vessels / life boats, not submerged objects (in this case, tombs). To be useful it would need to be spooled out on a VERY long line that you could follow back down. In this case that means the EPIRB would need to be buoyant enough to float both itself and 4000+ meters of heavy line … which would be great, although also a threat to navigation.

effed3 2 years ago |

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.