young driver insurance specialists?

[Chris Whelan]

Have you read the information in the link I provided as to how Galileo is going to work differently from the GPS that is currently used by sat-nav?

Chris

[tony sayer]

In article , dennis@home scribeth thus

What a fantastic machine works underground. I bet Network rail and DB Schenker would love to hear how it does that then Dennis;!....

And is absolutely accurate of course;)..

[tony sayer]

Yes.. Apart from the standard triangulation of received sats, albeit more of them, there doesn't seem to be anything else in there...

[John Williamson]

Mine does it by assuming you keep foloowing the same road at the same speed as when you went in....

Then it jumps slightly to the correct position when I come out of the tunnel. Except in the long ones through the Alps, where it gets bored and gives up about a kilometre in....

Dennis's might be better, of course, his stuff usually is.

[John Williamson]

What's mentioned as running now (EGNOS) is a wide area variation of differential GPS. It uses a number of base stations to check the accuracy of the received GPS signal, then transmits the variance to users via Inmarsat and a couple of other geostationary birds. Just a bigger version of the technique used by most airports and civil engineering sites now.

Galileo will be a Eurpoean constellation of satellites similar to the US based GPS system which is politics driven, in that some people are scared that the USA will turn off, encrypt, or drop GPS at some point.

[D.M.Chapman]

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

[ARWadsworth]

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

[Bob Eager]

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

[tony sayer]

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

No, seems different...

Well they might..

[hugh]

In message , "dennis@home" writes

I assume they are at or near the points with big notices saying "start of average speed monitoring" and "end of average speed monitoring or whatever the wording is.

[hugh]

How accurate are satnavs when it comes to measuring speed? Hopefully better than they are at measuring altitude.

[John Williamson]

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.

[dennis]

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?).

[Andy Burns]

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!

[John Williamson]

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.

[dennis]

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

[dennis]

or maybe they are at intervals and measure the average between several spots? That's how they are on the motorways so I see no reason they would be different elsewhere.

[Andy Burns]

I don't know which type of camera on which stretch of motorway you're referring to, but the HADECS (Highways Authority Digital Enforcement Camaera System) ones use radar to measure "instantaneous" speed past a single camera *not* number plate recognition and timing between fixed camera positions to calculate average speed ... the temporary cameras for e.g. 50mph in roadworks *are* average based.

[Andy Champ]

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?

's under that office block. It won't matter what frequencies you use, nor how much power. If you're really lucky you'll get a signal reflected from somewhere near the tunnel mouth, and that isn't going to help you find the right platform.

Andy

[dennis]

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).