Why ? How?

[Hall]

Well as I posted earlier I have a new GPS. Have noticed that when I travel through a tunnel the map still scrolls along with the arrow moving along. Not only does it move along it seems to move at the pace I do as in if I travel slow and the map still exits the tunnel the same moment as I do. The signal is lost as the speed stops at what I entered the tunnel at and resumes when I exit. Also when I park and stop the motor ( in my ute it is a pull stop so the twelve volt is still on ) the unit asks if I want to shut down. Only happens when I stop the motor. Will be good if it makes me a coffee :)
Cheers Hall

[Blknight.aus]

its got 2 primary things working for it.

1. predictive routing
2. Error probablity.

newer GPS's calculate out what speed you're doing and where you were pointed, when the signal starts to get vague they use that as part of the error correction (if it works out that you could be in one of 5 places and 4 of them you are not reasonably capable of doing it displays the last option as your current position) If you start going through the tunnel and the maps pathways only let you travel within the tunnel it predicts your course from rough dead reckoning as it looses signal, when the signal is completely gone it mostly gives up BUT it remembers what the geometry of everything was when it lost the plot.

When it starts to get signals back it starts using that to work out whats where and does a bit of jiggery pokery with its guestirouting, last known position and what its expecting to see as the error correction for the fix goes from "you're somewhere on planet earth" down to "your position is here +/- 60 cms. As soon as both get to the point where its accuracy is good enough (ie the GPS position might be at the "you are somewhere on football field X" level but because the permissable routes within that area mean you must be in position Y) it will give you back your position and actual speed even though a proper fully calculated GPS position is not yet established

[OffTrack]

its got 2 primary things working for it.

1. predictive routing
2. Error probablity.

newer GPS's calculate out what speed you're doing and where you were pointed, when the signal starts to get vague they use that as part of the error correction (if it works out that you could be in one of 5 places and 4 of them you are not reasonably capable of doing it displays the last option as your current position) If you start going through the tunnel and the maps pathways only let you travel within the tunnel it predicts your course from rough dead reckoning as it looses signal, when the signal is completely gone it mostly gives up BUT it remembers what the geometry of everything was when it lost the plot.

When it starts to get signals back it starts using that to work out whats where and does a bit of jiggery pokery with its guestirouting, last known position and what its expecting to see as the error correction for the fix goes from "you're somewhere on planet earth" down to "your position is here +/- 60 cms. As soon as both get to the point where its accuracy is good enough (ie the GPS position might be at the "you are somewhere on football field X" level but because the permissable routes within that area mean you must be in position Y) it will give you back your position and actual speed even though a proper fully calculated GPS position is not yet established

It seems to be entirely down to the Nav software. I've just moved from iGo8 to iGo Primo on my cheap chinese GPS unit. With iGo8 I'd get the "signal lost" message and the position would freeze until I got out the other side. I went through a tunnel today running iGo Primo and the position marker kept following the road at the speed I was travelling when I entered the tunnel. It got a little bit ahead of me by the exit but corrected seamlessly.

[Tombie]

It seems to be entirely down to the Nav software. I've just moved from iGo8 to iGo Primo on my cheap chinese GPS unit. With iGo8 I'd get the "signal lost" message and the position would freeze until I got out the other side. I went through a tunnel today running iGo Primo and the position marker kept following the road at the speed I was travelling when I entered the tunnel. It got a little bit ahead of me by the exit but corrected seamlessly.

Works quite funny in Sydney tunnel near the airport where you can turn off underground..

Keeps going forward and then misses the turn and corrects on exit.