[bragr]

My guess would be:

1. Car entered ferry, loses GPS

2. Car entered dead reconning mode used for tunnels and such

3. Car left ferry, acquired GPS

Then either:

4a. Location via dead reconning vasty disagreed with GPS because the car doesn't know about the ferry's movements, triggering some kind of failsafe.

Or:

4b. There's just a plain old bug in the condition to switch back to GPS and maybe people haven't noticed because you don't get as badly desynced in a tunnel.

>the car must have a pretty good dead reckoning system

Yeah all the pieces are there: accelerometers and gyros for stability control, compass for navigation, and the wheel speed sensors give you exact distance traveled.

[toast0]

My local roro ferry drops you off pretty close to downtown. If you don't get a good fix as you get off the boat before you get into the urban canyon, satnav is pretty hopeless for a few minutes.

Doesn't usually take 5 hours to figure out where it is though. At least not on my vehicles, even the one that's always getting confused.

[raverbashing]

It probably doesn't do dead reckoning even in tunnels

And maybe the system sucks to get the GPS almanac if badly desynced

[DonHopkins]

TomTom's have for at least 15 years or so. They have accelerometers to measure the motion when cut off from the GPS satellites. I worked there, knew the guy who developed it, and saw him give a presentation about it.

[mjlee]

I swapped out the satnav in a 2008 Honda for a modern unit and the car had a “speed pulse” wire. I looked it up and that wire is used for dead reckoning.

[rob74]

Dead reckoning shouldn't be a problem for a built-in nav device that has access to the car's odometer (or at least its speed). But as long as the car itself isn't moving, because it's parked in a ferry's car deck, I reckon (SCNR) it shouldn't do any dead reckoning...