SAT NAV in tunnels

Submitted: Thursday, Jan 10, 2008 at 21:03
ThreadID: 53311 Views:1940 Replies:9 FollowUps:3
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I went to Sydney over the break and was intrigued that my Sat Nav system would still plot my travel, and give me turn off instructions etc. when I was driving in the tunnels. Can someone tell me how this works - the satellites are obviously not visible.
Thanks,

IAN
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Reply By: ZukscooterX90 (Qld) - Thursday, Jan 10, 2008 at 21:10

Thursday, Jan 10, 2008 at 21:10
Ian,you have answered your own ?.However you nwill also find that you will lose reception when going through relatively not so dense forest.i.e tree tunnel on roadway as my gps does on occasions.Must have three sats to give a reading.
Cheers Bob.
AnswerID: 280837

Reply By: keith & louise - Thursday, Jan 10, 2008 at 21:10

Thursday, Jan 10, 2008 at 21:10
if you have an in car system that has a feed of the vehicle speed to the GPS then it will still work, but if it is a stand alone system then when it looses the satellites it assumes you are in the same position even when you are traveling.
Robert Peppers book "how GPS works" will explain the use of a GPS.

Keith
AnswerID: 280838

Follow Up By: Bonz (Vic) - Friday, Jan 11, 2008 at 21:18

Friday, Jan 11, 2008 at 21:18
It actually assumes constant velocity and direction until it picks up satellites again or until the no satellite situation times out and it cacks itself.
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Reply By: Member - Ian M (QLD) - Thursday, Jan 10, 2008 at 21:15

Thursday, Jan 10, 2008 at 21:15
Thanks Keith and Loiuse
Yes it is an in car system - bloody clever actually - I assumed it would lose me in the tunnel when it lost the satellites

IAN
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Reply By: Bob Y. - Qld - Thursday, Jan 10, 2008 at 21:42

Thursday, Jan 10, 2008 at 21:42
Mirrors, Ian.

It's all done with mirrors!

hooroo,
Bob.
Seen it all, Done it all.
Can't remember most of it.

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AnswerID: 280850

Reply By: Go Fishing - Thursday, Jan 10, 2008 at 22:27

Thursday, Jan 10, 2008 at 22:27
The Sat Nav is guestimating your location based on the direction the car is travelling and speed, then when its in range of the Satellite, its switchs back over to Sat.

Happens when you drive amongst tall buildings as wel.
AnswerID: 280868

Reply By: HowdyDoody - Friday, Jan 11, 2008 at 12:33

Friday, Jan 11, 2008 at 12:33
My tomtom does the same when I go through the Melb tunnels. I assume it is estimating my position based on it's last speed reading from the satellite etc.
AnswerID: 280961

Reply By: David N. - Friday, Jan 11, 2008 at 15:50

Friday, Jan 11, 2008 at 15:50
There are two systems which "work" in a tunnel -for a time.
The cheaper ones will assume you will follow the road in the tunnel for a certain amount of time at your existing speed. If you stop in the tunnel it will not know. Eventually they will come up with an error message when they don't find another satellite lock-on.

Some better and yes much more expensive ones have an "accelerometer" built in and can tell if you stop/start or even slow down and turn within reason, till they get another satellite signal.
(This is how inertial nav systems work on aeroplanes, and how the Apollo and LM orbiter were navigated to the moon.)

These will obviously work better in poor signal areas.
Cheers
AnswerID: 281007

Follow Up By: Mike Harding - Friday, Jan 11, 2008 at 16:41

Friday, Jan 11, 2008 at 16:41
In recent years accelerometers have been put onto integrated circuits (Analog Devices) in a brilliant piece of engineering but I was unaware that anyone was using them in commercial GPS' Perhaps you could point me in the direction of a web reference?

Mike Harding
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Follow Up By: Member - Ian M (QLD) - Sunday, Jan 13, 2008 at 18:38

Sunday, Jan 13, 2008 at 18:38
I have no idea how it works but it the tracker icon definitely does appear to follow the tunnel route and is almost perfect on the timing for the end of the tunnel - it must be getting data from the car in both speed and direction. I even "veered" when instructed and it followed the route. It is a Blaupant (spelling!) OEM, not a portable unit.
IAN
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Reply By: Member - Davoe (Yalgoo) - Friday, Jan 11, 2008 at 19:05

Friday, Jan 11, 2008 at 19:05
the missus MIO just says signal lost when going through the polly farmer
AnswerID: 281052

Reply By: Member - Mike DID - Sunday, Jan 13, 2008 at 19:05

Sunday, Jan 13, 2008 at 19:05
It makes the usually safe assumption that you are in a car rather than a tunnel boring machine, so it assumes you will continue travelling along the tunnel.
AnswerID: 281436

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