Turning a camera into a velocity sensor
No DVL, no GPS. A downward camera and the ground can tell you how fast you are moving — if you trust the geometry more than the number.
A Doppler velocity log is the obvious way to measure an underwater vehicle’s speed over ground. It is also expensive, and we did not have one. So the question became: what do we already have that can measure motion?
The answer was the downward camera. If you know the height above the floor and the camera’s field of view, then optical flow — how far the texture on the floor slides between two frames — is a velocity. The geometry is simple. The trap is that every part of it can lie plausibly.
The verification that actually worked
The only verification that has ever worked here is looking at the frames. We ran three 30 cm slides across a known distance and checked the implied height against a tape measure: 0.72 / 0.69 / 0.70 m against a 0.72 m tape, worst-case position error 1.09 cm. That is a real measurement — it has a method, conditions, and a bar it had to clear.
A number with no bar cannot be wrong, so it never gets checked — it just gets quoted, and eventually shipped.
What makes this trustworthy is not that the estimator agreed with another estimator — that only measures agreement. It is that truth was known: a tape on the floor, a slide of a fixed length. Construct the case where you already know the answer, then see if the machine finds it.