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.

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toast0 a day ago | parent | next [-]

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.

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raverbashing a day ago | parent | prev | next [-]

It probably doesn't do dead reckoning even in tunnels

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

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mjlee a day ago | parent | next [-]

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.

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rob74 a day ago | parent | prev | next [-]

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...

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DonHopkins a day ago | parent | prev [-]

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.

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rob74 a day ago | parent [-]

That's cool... so I guess this works something along the ways of "calculate the speed via GPS before entering the tunnel, and then try to update this speed using the data from the accelerometer while in the tunnel"? Because as long a the car is moving at constant speed in a straight line, the accelerometer shouldn't register anything...

	▲	netsharc 19 hours ago \| parent \| next [-]
		But tunnels that go up or downhill (or a mixture) would confuse the accelerometer.. well I guess they'll have the road gradient data. I drove in tunnel where the asphalt had undulations today, I'd like to see them try to figure that out!
	▲	DonHopkins 17 hours ago \| parent \| prev [-]
		Well it registers gravity, so you can detect i.e. driving off a cliff. ;) What helps is that tunnels don't usually branch, so once you're in it, your path is usually quite predictable. TomTom maps also have a statistical model of what speed is expected along each stretch of road by hour and weekday/weekend (not 7 individual days, but 2 kinds of days). But I don't know if it uses that to help estimate your expected speed when dead reckoning, it's actually for route planning. One of my co-workers came up with the great idea of gamifying driving: maintain a real time speed leader board, showing the top ten speeders along any stretch of road! So on every road in the world you could compete with other TomTom users who drove it. Kind of like checking in with 4Square, but more fun and dangerous! TomTom legal did not approve. I suggested gamifying and monetizing driving with TomTomagotchi, a virtual pet that gets depressed if you don't drive it around enough, begs you to visit interesting landmarks and sponsored points of interest, like driving through McDonalds to feed it, or through the park to let it take a shit, or driving fast enough to make the leaderboard to entertain it. I'm sure Bandai's lawyers wouldn't approve.

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helf a day ago | parent | prev [-]

[dead]