Sat Nav in Dartford Tunnel

[Panic Button]

I tried out my new satnav today, and went through the Dartford Tunnel.

I assumed that once in the tunnel it would show me as not moving, as it couldn't see the satellites.

However it showed me as moving all the way through, and was in the correct location on the map when I emerged.

How does it do this?

[rojash]

From what I can gather, it simply extrapolates your position based on speed when entering the tunnel, and re-syncs when you exit. I suspect that if you were forced to stop for even a short time in the tunnel you'd quickly become aware that it's not actually tracking you.

[sp1214]

I also notice that in some tunnels you do not lose your radio signal. I believe this is something to do with special cables laid in the tunnel.

[Lakitu]

Special cables for what, sp?

[sp1214]

Maybe something like this

http://www.roadtraffic-technology.com/contract ors/tunnels/broadcast-solutions/

[Lakitu]

But the SatNav is 'followed' by a satellite in Space?

[sp1214]

So how do you receive satelite stations by a cable? I believe they are collected by a giant receiver and then fed by cable to peoples homes. Maybe someone with a technical bent would explain it.

[Lakitu]

LOL, I wasn't disagreeing with you, it was a genuine wondering on my part, I really don't know about this stuff.

I always thought that if it was satellite received then there was no need for cables. Like when I was a kid you could only have cable TV if the cables ran in your area, otherwise you were snookered but today you have satellite TV with a dish outside. I thought it was just like..........waves really.

Shall I stop talking now? LOL

[ChuckFickens]

rojash is correct..

Your sat nav does not work in the tunnel. Even heavy tree cover can stop it working. What is happening, is that you are not in the tunnel long enough for it to complain because sat nav systems are quite clever and expect to be continually losing and regaining signals if they lose signals they keep extrapolating your journey based on the direction and speed you were going when they lost the signal and wait up to a minute or 2 before they complain

And the radio signals are picked up and re-transmitted down the tunnel so you don't lose the radio signal while in it, they only started doing this fairly recently and it was a bit of a shock when they first started as I was so used to losing the radio in the dartford tunnel when it suddenly worked all the way through it was a slight WTF moment :)

[rojash]

"rojash is correct." Thank God for that - I just coudn't face another 5:52PM August 15th 1981 again....

[mibn2cweus]

Many factory installed systems also process speed and directional information obtained from sensors fixed to the vehicle.

[sp1214]

My Sat Nav does not calculate when in a tunnel. How do I know this. Because when counting down the mileage to go remains the same and only on exit does it recalculate the mileage.

[Panic Button]

Thanks y'all.

A few months ago I borrowed an old Tom Tom, and that stopped me moving and complained when I went through the Dartford Tunnel.

This new Garmin showed me moving all the way through, so I guess this is the continuing advance of technology.

It's not perfect though - when I tested it for my journey home, asking for the quickest route it cetainly did not choose the quickest. You can't put full faith in them.

[Paul22118]

I knew a chap once, since deceased, who worked on the building of the Dartford Tunnel way back. He told me that it was a technology leader in that the builders installed wiring that allowed the reception of radio signals, indeed as it does today still. Go through the Rotherhithe or Blackwall to prove his case.

Perhaps the answer to the question is buried in this fact?

[soldierboy001]

The Dartford tunnel has what is called a Leak Off cable that transmits all the radio waves so your radio will still work in the tunnel, if something happens in the tunnel it will then interrupt the radio signal to give you update on to what has happened and also give you instructions on what to do. I.E. leave the tunnel by walking etc..