GPS under the tunnel.

**Ricky327** · 02-26-2004, 06:55 PM

Hi all,

Im wondering how far inside can you go into the tunnel before loosing the lock? Make and model? software used?

Im asking because...I got a Garmin GPS16, Ipaq 3970 running TomTom navigator 2.24. I can go through a 3/4 - 1 mile tunnel and can still show where I am in the map. The heading and position is alittle out but it carried on working.

This is not supposed to happen but its happening on my setup for some reason, am I missing something

Any thought?

**chut** · 02-26-2004, 07:05 PM

how fast are you traveling, do you cover the distance in less than 15 seconds?

**Ricky327** · 02-26-2004, 07:07 PM

About 30mph in 1 tunnel and 40mph in another, both keep the lock almost most of the time. Ill say it take minutes to get to the end.

**unsavory** · 02-27-2004, 03:56 PM

Originally Posted by Ricky327

About 30mph in 1 tunnel and 40mph in another, both keep the lock almost most of the time. Ill say it take minutes to get to the end.

That's very interesting. I lose a lock just entering a parking garage, or going under a freeway overpass in stop and go traffic. I lose GPS shortly after entering a tunnel.

btw wich tunnel? Caldecot?

**Ricky327** · 02-27-2004, 10:37 PM

OK I done another test heres the results...slow driving in the tunnel.

Garmin GPS16
Compaq Ipaq 3970
Mapsonic Navigation software (no road snap)
Blackwall tunnel south bound.
Speed in the tunnel 5mph
9 locked satellite on entering the tunnel
Time is 2:15am
Distance covered under the tunnel = 372 meters

Yup I managed to cover about 372 meters before the GPS stopped working. The strange bit is the s/w is showing zero locked satellite and yet it still show exactly where I am in the tunnel

I guess the speed Im in the tunnel makes alot of difference. I must be driving much faster last time to get through the tunnel without loosing the lock

Also I was using TomTom then, and that snap on to the road therefore not showing the true position.

I read one a newsgroup and one guy is getting a lock inside a house saying >>> :

In fact, I've had the Garmin lock on to satellites in the
house in a room with only one window.

I do get lock on mine in the room too, it is very strange

**GingerPrince** · 02-28-2004, 09:38 AM

Originally Posted by Ricky327

Yup I managed to cover about 372 meters before the GPS stopped working. The strange bit is the s/w is showing zero locked satellite and yet it still show exactly where I am in the tunnel

The SiRF chipset does exactly that. Basically when it loses sight of satellites, it guesses your progress based on your speed and direction when you lost contact. It's called canyon optimization or something, and is what makes some receivers better in high-rise cities than others. My Fortuna-U2 does the same.

Don't think the Garmin has SiRF, but it will probably have a similar feature.

If you drive into a tunnel then stop, you'll find the GPS reports you as still moving

**felix99** · 02-28-2004, 12:13 PM

your garmin shows probably your exact position in the tunnel because it has a integrates compass which doesnt need sats only magnetical earth field to work . the rest of moving data probably by internal interpolation .

sorry for my bad english hope u understand me

**Ricky327** · 02-28-2004, 06:01 PM

Don't think the Garmin has SiRF, but it will probably have a similar feature.

Not sure but I think garmin make their own chipset.

If you drive into a tunnel then stop, you'll find the GPS reports you as still moving

I can understand if the tunnel is straight but the rotherhite tunnel is zigzag and it still follow the position for a while. As I get further into the tunnel the position will show im driving straight and eventually Ill loose the lock.

felix99,

I dont think thats the case here...because garmin would have said it in their spec if there is a built in compass

but you could be right.

Oh well...

**felix99** · 02-29-2004, 04:11 AM

i saw this compass feature recently on a garmin geko 301 - don't know if the gps we are talking has the same but i found thid feature very interesting. but unfortunatly garmin gps is not as cheap as the gps mices so i'll never have enough money to buy such

regards
Felix

**sxpert** · 02-29-2004, 03:06 PM

Originally Posted by Ricky327

Yup I managed to cover about 372 meters before the GPS stopped working. The strange bit is the s/w is showing zero locked satellite and yet it still show exactly where I am in the tunnel

no need to be confused, I have an explanation....
the GPS device uses supposes that the current speed is maintained and the direction too. and guesses that the tunnel is straight.
going through switzerland, with all their notoriously long (and not straight) tunnels, I could see the track going straight while I was going around a curve, then you'd see a jump the the actual location a few seconds after being out the tunnel