We have two Garmins. Garmin A is a basic walker’s device called an “eTrex h” (the “h” is for high sensitivity). Garmin B is a nüvi something (and what’s that umlaut all about?) for our cars. A friend of mine has one of the earlier eTrex (no “h”) models which is quite prone to losing contact with the satellites when walking under any moderate tree canopy. Mine seems better so the “h” has apparently done some good. However, both GPS devices have trouble when I turn them on inside our house; they go into “acquiring satellites” mode and usually fail. The eTrex h even helpfully asks, “are you indoors?”
Now, the French are quite keen on building tunnels so that they can pile roads, particularly autoroutes, through mountains that have inconsiderately been thrown up by the forces of nature in the road engineers’ way. I appear to be able to drive through a 1km tunnel with hundreds of feet of rock above my head without the nüvi (what is that umlaut all about?) losing touch with the satellites; my track remains well plotted on the road.
If Garmin nüvi can’t see thrü (if you can’t beat ‘em, join ‘em) my skimpy roof tiles at home, how does it manage to see through a mountain?
Jüst cürioüs.
I suspect it’s some dastardly computer trickery that takes into account the known length and direction of the road in the tunnel, and current vehicle speed to calculate position on the road.
The way to test this theory would be to track it through something like the Chunnel, but change trains and reverse direction halfway through 🙂
Interesting idea – a quick dash through the service tunnel to the return line. Excellent!
A quick google shows that some car sat navs have some sort of dead reckoning. This forum reference is quite old and was complaining that the nuvi did not have the feature.
http://www.mobinauts.com/topic/95670-garmin-nuvi-mxt-for-tunnels/
Hmmm, dead reckoning would require a knowledge of current speed which would also require satellite tracking to be working (since it isn’t hooked in to my speedo). I didn’t try slowing down so I suppose it might just extrapolate assuming constant speed. What we need is a traffic jam inside a tunnel. On second thoughts … 😀