young driver insurance specialists?

Errr.... it continued to plot your position all the way through the tunnel despite having no signal?

Was the tunnel perfectly straight?... and you had a perfectly constant speed?

Darren

The tunnel was in Dennis World, where anything is possible.

D.M.Chapman wrote: [snip]

Not necessary if the GPS has a connection to the vehicle speedometer and has a "snap to road" option. Many cars have such a system, and it was an option on some of the stand alone Garmin models, presumably using a transmitting sensor attached to the speedo.

Of course someone stupid might confuse such dead reckoning as being the GPS continuing to function in a tunnel.

Perhaps dennis added a backup inertial platform, knocked up in his shed.

Umm ... that still requires a satellite signal path to the end user does it not?..

No, seems different...

Well they might..

In message , tony sayer writes

Usually there is a wild leap as the signal is regained, as I have noticed when I have checked the track after leaving a GPS switched on in the boot of the car.

Cave or subway rescue people would be very interested in a UHF device which could perform underground.

Having said that, I don't know what the cutting edge technology is now, but there was inadequate communication in the aftermath of 7/7.

Two, in fact, on different frequencies.

The current Differential GPS starts with a receiver in a known position, and transmits the difference from that position to end users using a local low power transmitter. This can give a position to within centimetres, over an area the size of Heathrow. EGNOS uses half a dozen widely spaced base stations with known positions and transmits the errors to end users using satellites as relays, which gives a position to within a metre or so over an area the size of Europe.

Only if they can replace it with something better, their military rely on it too much. Or if they go broke and can't afford to replace the satellites, as they're now down to about half the spares they started with, IIRC.

No, of course not. Any fule know that. The system loses the signal and carries on inertially. This doesn't last for long, there are too many errors possible, but it hacks it for a while.

Unlikely, its a cheap tomtom start. Better than most things three years old of course as is most new stuff using the latest gps chips (its about time for a new generation isn't it?).

There's usually tumbleweed on the A14 when I'm using it, today for example. Actually, there are fewer cameras than I remember on that stretch, but not remembering where they are, means they do keep my speed down anyway (to 70 + camera spotting distance), I think one of them might act as a marker for turning off onto the A509 for Arfa's Burgers!

So it works as a GPS until it loses the signal, then starts guessing, just like everybody else's.

SIRFStar 4 has been around since late 2009, and some units can now use cell stations and some wifi hubs to give a rough location where GPS signals can't be received.

I have an inertial platform built into one of my gps devices, not the one I was talking about though.

No, I hadn't. And I still haven't - the only link from you I can see is about EGNOS. But IIRC the primary difference is that Galileo runs on multiple frequencies which helps it to avoid atmospheric distortions.

But have you ever seen New Street?

Andy

Maybe, but it still worked 25m into the tunnel. There was a time they wouldn't work in trees. It may even have still had a 2D lock but tomtoms only show lock when they have a 3D lock.

It is possible for a gps to see 3 sats through the tunnel entrances even if its unlikely.

I have one with three axis accelerometers and other sensors that can even tell you which way its pointing and superimpose augmented reality info onto the screen depending on where it is and where its pointing (even indoors where I have tried it).

GPS had been in operation for many years before they turned off encryption so civils could get full res from it. Up to that time the civil one had degraded resolution and only US military had access to the full resolution. That's how the system was designed. Presumably they could go back to encrypting the signal again if they wanted.

I think they only dropped the encryption because the commercial products were getting good resolution anyway. They had come up with various ways to improve accuracy before the encryption went.

There were local services which transmitted the error, coining the term Differential GPS:

Built-in Sat-Nav's sometimes have the ability to monitor your speed and direction. My old Nissan's Sat-Nav did this. Gets increasingly less reliable the longer you're without a GPS lock though.

Even now only the military have access to "full resolution". Consumer GPS is not able to compensate for ionospheric delays in transmission because consumer GPS does not have access to the encryption key used to decode the error signals transmitted on two separate frequencies to permit the determination of the delay (which varies depending upon ionospheric conditions).

It is possible to work around the problem but the solution is slow and is only used on surveying instruments at present. The difference in accuracy between surveying instruments and consumer GPS gives a feel for how good military GPS is, it can give surveying accuracy but at the speed of a consumer GPS fix.