Nuvi and dead-reckoning in tunnels

I've had my Nuvi 660 for about a year and travel daily through several
tunnels. The longest one is about 2 miles (3 km) long and the Nuvi have
had no problems showing me my position within the tunnel based on what
must be dead-reckoning using my last known speed or some kind of
average. The manual doesn't say.

Two weeks ago I upgraded to version 2.40 and GPS SW version 2.90. After
that the dead-reckoning performs very erratic. Sometimes I can drive
through 2 miles of the tunnel without any problems, on other occations i
just get halfway when the satellite reception lost warning appears.

Anyone with similar experience or inside info on this ?

Regards,

Per W.

Can't comment on nuvi, but last week was driving with Streetpilot 2610
through the Madrid inner ring road (M30) tunnel which is about 10km
long. Lost signal soon after entrance, but then near halfway point it
picked up a signal and tracked the tunnel for 1km or so, and then lost
it again. I checked the tracking log, and the signal was picked up in
a section where the tunnel passes under a park - there are no obvious
openings in the roof of the tunnel. The tunnel has mobile phone and
radio station reception throughout, so perhaps this was causing it.

The tunnel has mobile phone and radio station reception throughout, so
perhaps this was causing it.
The tunnel must have repeater antennas spaced throuout its length to provide
the cell and radio reception. The repeaters may not carry the frequency
required by the GPS units. As to why a certain section does work, it may be
possible that they have replaced the repeaters in that section with newer
ones that do include the GPS frequencies.

no, GPS signals cannot pass a repeater like radio signals or mobile phone
signals.

If there was a repeater including GPS frequencies, all satellite signals
would pass the repeater system using the same path, so all sat signals would
get the same delay between the outer antenna and the receiver. This would
look like a common clock error to the receiver. At the end, the recevier
would find the position of the outer antenna, not the position of the car.

GPS requires all sat signals coming through the *direct* path.

In this Madrid tunnel, some sat signals will have passed the ceiling of the
tunnel under the park directly.

--

Heinrich
http://www.gartrip.de

True, but it does suggest a possible implementation for the tunnel
authorities if they did want to provide approximate GPS positioning
within the tunnel. Divide the tunnel into sections and in each
section place a retransmitter which is getting GPS signals from an
antenna located outside and directly above the center of that section
of the tunnel. GPS receivers in the cars would then see the position
jump as they passed from one section to the next, but would always
show the car as being in the correct section, albeit at the center of
it rather than at the actual accurate spot. Still better than having
lost position information altogether.

No idea if any tunnels have already implemented such a system.