Why radar doesn’t work underwater — and what that forces
Salt water eats radio in centimetres. No radio means no GPS, no radar, no telemetry link when submerged. Every design choice downstream is a consequence.
Underwater there is no radio, no GPS, and no second chance to explain yourself. Seawater is conductive; it attenuates electromagnetic waves so fast that radar and GPS are simply off the table below the surface. This one physical fact writes most of the architecture for you.
If you cannot get a position fix from the sky, you have to estimate it from the inside — dead reckoning on an IMU, corrected by whatever you can measure locally: depth from pressure, velocity from a camera, heading from the AHRS. If you cannot stream telemetry home, the vehicle has to be right on its own.
A vehicle either understood what it was looking at, or it did not — and it will not tell you which.
That is why the interesting engineering is not the happy path. It is what happens when the vehicle cannot do something. The answer, on a well-built stack, is never a silent workaround that keeps the demo running. It is a loud refusal and a written reason.