Ok. I was driving throuh the tunnelsystem under the City where I live, and the satnav kept track of where I was. How does it do that?

There are several different routes to take in the tunnels, so it doesnt just estimate my position based on speed. It even got wich route I took correctly.

My old handheld Garmin did not do this, so what magic is working inside the Columbus?

Car has a compass, could be using that. If it knows your direction and speed, it knows where you are. It's not a new one, it's how they made it to the moon!

Ok they used a gyro not a compass but the principle is the same

Aint the compass based on the GPS?

I guess it could use the know speed and steering angle sensor to keep track of the cars direction.

As a programmer who has interfaced a microcomputer with a GPS + accelerometer/gyro/compass a few times, I'd say it's a pretty easy task if the system knows where you were headed before the signal was lost. Should be even easier in a car as the steering angle and data from ABS sensors can be taken into account. It just takes the last data it had from the GPS and then uses the data it gets from other sensors (whatever it has, it should be enough for a while) to find your new direction. These kinds of systems usually get pretty inaccurate after some time but as there shouldn't be a lot of extremely long tunnels that have different routes.

There could be Ariels in the tunnels.  Your mobile phone might also work. We have them at work in long tunnels so the GSM-R Radios work

Was there a route set - mine will assume you are on your route at the speed your going until it recognises other was.

My old mondeo 2010, used to do this in tunnels also (Mersey tunnel was one I recall it working through), so don't think anything special with the Columbus - a colleague actually was heading for Costco in Liverpool one time which is built over the Mersey tunnel and his Audi sat have sent him under the tunnels so it was targeting the postcode and told him he'd reached his destination part way through the tunnel!

My £100 TomTom does this too.

There could be Ariels in the tunnels.  

 

I didn't know that little mermaids made SatNavs work!!   

 

I thought it was magic moonbeams.

The inbuilt satnav does not just use GPS to determine its position, it also receives data via Canbus from the steering angle sensor and ABS wheel speed sensors. When no GPS signal is present the car can use these other signals to have a pretty good guess at your current position.

I didnt have a route set, just noticed it on nav screen.

It Sounds reasonable that it can read speed and steeringinput to give position.

The tunnels aint that long, but it is 5 different exits and 3 roundabouts in it.

There is a true digital compass inside so it knows which way the car is facing.

 

It then uses the cars speed to know how far you have travelled into the tunnel. It won't be totally accurate but it's enough to get you through the tunnel till you're out and it can get a signal again.

 

Phil

It's a "best guess" if you crash in the tunnel it won't know until you emerge. It will keep on thinking you carry on at same speed.

It's a "best guess" if you crash in the tunnel it won't know until you emerge. It will keep on thinking you carry on at same speed.

No, it adjusted my position according to my speed. It was more accurate than a guestimate.

My garmin handheld did estimate based on entryspeed, and it was usually way off.

Edited 22 May, 201511 yr by Gromle