As posed, it has nothing at all to do with GPS. It is an inertial guidance system that takes a single GPS fix when you submerge. After that, it relies on forces that move me from place to place.
One question. I splash in my favorite inlet, submerge, and go absolutely neutral in the flow. There are no unbalanced forces on me whatsoever.
Can Ariadna track me?
Sure. No unbalanced forces just means your current speed and direction are completely unchanged since the last force acted on you. For example, right before you became neutral it calculated you were moving at 2 m/s towards 140°. As long as no forces act on you, you'll continue drifting at 2 m/s towards 140°, and the device knows this. So no unbalanced forces = hold speed and direction.
Now this requires knowing how fast you are moving to start with. You either need to be completely motionless when starting the tracking, which is hard when a boat or you are drifting in the current at the surface, so it uses GPS to get your speed and direction until you go underwater and it has to switch to inertial tracking. This may be what you meant, since drifting on the surface also equals no unbalanced forces. So if you were to start tracking while drifting, and told it you were stationary, it wouldn't be able to track you.
Having said that, the following from
their FAQ worries me that this isn't actually how they implemented it.
Q: What about sea-currents?
A: Sea currents can be compensated by activating the sea current compensation function while swimming against the current and maintaining a steady position for a few seconds. The speed and heading of the current are measured and taken into account in subsequent navigation calculations.
And
[ THE DIPS DIVER POSITION SYSTEM EXPLAINED ]
On the surface, the wrist unit uses its GPS receiver to obtain positional information. Upon submerging, where the GPS signal is not available, it relies on the navigation transmitter based on DIPS technology for navigational data. The navigation transmitter uses divers swimming motion capture sensors to measure diver's real-time position, speed, heading and distance information underwater. The resulting 3D underwater route is recorded and shown graphically in real time on the user interface as a continuous trace.
Swimming motion capture sensors? That's not how inertial tracking systems work, unless it is an inertial tracking system and a bunch of marketing BS, but for the case of an inertial tracking system I imagine the sensors would be best suited strapped on a BCD or tank, not on a leg. A video says it tracks similar to a submarine (inertial tracking system) but then says they saw that by tracking the fin movements of a dolphin they could track it's movement, and did a similar thing for SCUBA.
Bottom line, they aren't really clear
how it actually works.
If it works as they claim, seems pretty impressive. They claim 2% error right now, or 10 m off after swimming 500 m.