Thread: GPS reception in tunnels

Originally Posted by p38arover

The in-built GPS in our 2010 Subaru Forester works in the Sydney M5 tunnel. I assumed it had some sort of inertial system to maintain track. It's accurate, too.

At first glance mine does too however it is just the software in the GPS knowing what road you are on, that you are in a tunnel and continues to calculate and display your position based on the speed it has programmed for the tunnel - all is OK normally unless there is a traffic jam and you have to stop. If there is a turn off in the tunnel and you have it programmed in your route unless you have had to stop the exit command from the GPS will be close enough for Government work.

Garry

Originally Posted by OffTrack

There is work being done on indoors gps which I believe is reasonably advanced. From what I understand it relies on the gps signal being received by an antenna on the roof off the building being retransmitted on individual floors. I wouldn't be surprised if the technology was applied to tunnels at some point.

Googling reveals that at least one company has commercialised this technique for various uses including tunnels.

GPS Repeater

But if you read further, what it is actually doing is re-transmitting the GPS data for the location of the initial receiver on the roof which will remain static even though your car is moving within the tunnel. So the reality is that speed measurement will likely fall to zero while you are in the tunnel and then jump as your receiver starts having direct line-of-sight to the actual satellites.

What it will do is reduce the time to acquisition when exiting the tunnel and why it is important for Police, ambulance and fire authorities.

Originally Posted by OffTrack

There is work being done on indoors gps which I believe is reasonably advanced. From what I understand it relies on the gps signal being received by an antenna on the roof off the building being retransmitted on individual floors. I wouldn't be surprised if the technology was applied to tunnels at some point.

Googling reveals that at least one company has commercialised this technique for various uses including tunnels.

GPS Repeater

But if you read further, what it is actually doing is re-transmitting the GPS data for the location of the initial receiver on the roof which will remain static even though your car is moving within the tunnel. So the reality is that speed measurement will likely fall to zero while you are in the tunnel and then jump as your receiver starts having direct line-of-sight to the actual satellites.

What it will do is reduce the time to acquisition when exiting the tunnel and why it is important for Police, ambulance and fire authorities.

Remember that a GPS requires a minimum of three satellites to calculate position in three dimensional space.

I thought there were a number of GPS systems that read speedo input. A fallback for temporary GPS drop outs which works well for tunnels. Perhaps some use magnetic compass as well?

brilliant, another bit of electronic junk in a tunnel that will on a hickup close down every tunnel in oz

Originally Posted by manic

I thought there were a number of GPS systems that read speedo input. A fallback for temporary GPS drop outs which works well for tunnels. Perhaps some use magnetic compass as well?

OEM In Car nav systems maybe but not after-market ones.

The other problem is that my D4 speedo over-reads by ~ 6KPH so while in the tunnel I won't know whether I was reading GPS +/- 0.5KPH or speedo -6 KPH.

There was an article in azimuth or position magazine i cant remember which about a land based positioning system. Basically the same as gps but the 'satelites' are sitting on top of hills/buildings/in tunnels etc. and because they aren't moving there are no complex calculations to determine their position, as well as a hell of a lot less atmosphere to interfere with. The result was that you could, just from your phone be able to locate yourself to the chair you are sitting in, in your house. A few large mines already use similar systems, and i think a few euro countries have or are setting up systems. i will try and find the article and scan it. but basically this solves the gps tunnel issue as the 'satelites' can be mounted anywhere.

cheers
Tangus

I do know that GPS used to have an amount of fuzz added to the system (by randomly changing the time signal on particular satellites) to create positioning errors and prevent the enemy using the system for accurately positioning it's troops against the US military. The US were able overcome the error by knowing when to get an accurate fix from the satellites and use inertial navigation between fixes. The US could also turn off the satellites at will and intermittantly did so over the middle east during Gulf War Mk1.

The whole fuzz system came undone when the CSIRO testing automated pilot systems for Port Phillip Bay fitted a GPS receiver in the Point Lonsdale Lighthouse. From the known location of the lighthouse they could calculate the error in the GPS signal and then transmitted a correction signal to the Port Phillip Bay area. Once this research was published and the world knew of the workaround the fuzz became redundant and the US Military removed the error generation.