My GPS Seems to track inside tunnels

I have a Garmin Nuvi which I used this weekend. I was driving through
the tunnel from New brighton into liverpool and was expecting my GPS
to report that it had lost GPS reception. Interestingly, it seemed to
track my position all the way through the tunnel. Is this because I
have WAAS turned on (and if so, do they build something into the
tunnels for this (Where can I get more info about this?)) or was my
GPS getting it's signal up the tunnel? I guess the tunnel is about a
mile or more long.

David

For short tunnels it will just continue to appear to be trackking at
the rate you were going before you entered the tunnel.

Bitstring

Yep, normally they just assume constant speed for some long time before
admitting they were guessing. For car navigation models which are
basically stuck in 'lock to road' mode this can be pretty accurate
(hiking models where you can go any-which-way-you-like have more trouble
guessing what you actually did), assuming the roads maps are good.

For models which record a tracklog you can even watch them get it wrong
sometimes (you and the road both make a 90 degree turn, but the logged
points keep going straight ahead for a while - assuming 'lock to road'
is off).

I don't know if the Nuvi is smart enough to know there is a tunnel there
and thus EXPECT to lose satellite lock, or if it has any extra smarts to
cope (e.g. inertial sensors, which are pretty cheap these days). If
definitely isn't receiving through the tunnel roof though, and no, they
don't rebroadcast the signal (wouldn't make sense, since you'd only get
the position reading of the point they were collecting it from).

--
GSV Three Minds in a Can
11,020 Km walked. 2,118 Km PROWs surveyed. 38.3% complete.

On Fri, 28 Mar 2008 22:49:10 +0000, GSV Three Minds in a Can wrote:


Another explanation may be that they have repeaters inside the tunnels.
When I visited the Gateway Arch in St.Louis, my satellite radio and GPS
maintained lock, even in the underground parking garage. I checked the
Sat radio to see if it was picking up a local repeater (it was not), and
the GPS reported an accurate speed and direction while I was moving. This
was with a Garmin Etrex Vista.

But you can't do it without having repeaters all over the place, since a
repeater only repeats ITS location, not where YOU are. And if you're
moving towards a repeater you are obviously moving 'towards' any/all
satellites it is repeating (that should confuse a GPS chipset pretty
good).

However the satellite signals will get through a small amount of cement,
or through a reasonable sized gap (they diffract around corners pretty
good).

--
GSV Three Minds in a Can
11,020 Km walked. 2,118 Km PROWs surveyed. 38.3% complete.