young driver insurance specialists?

Very rough due to multipath timing errors...

In article , dennis@home scribeth thus

Why did U use one when a boy scout then ?..

Yep Den!, you'll just have to have the law's of physic re writ;)...

No!, Really?...

In article , Gordon H scribeth thus

Not that good enough for railway nav..

Well not that accurate either..

No go, needs Three or more signals to work from and UHF is bad enough just trying to get it through a simple rise in the ground;!.. ..

More a systems problem then much else..

In message , Steve Firth writes

In a local country park which has a canal and a footpath alongside, then a steep drop to a river, I watched a guy with a GPS device on a tripod located over a metal stud in the bank which retains the canal water

He was checking that the bank had not move significantly since the last survey, as leaks had been detected from the canal.

That must have been at least centimetric accuracy, and he was waiting a considerable time whilst his equipment carried out a process of averaging, according to his reply to my question.

I thought they worked by measuring the relative field strengths from whatever they could pick up, and triangulating from that. As you say, a

Millimetres, IIRC. It compares the relative phases of the carriers from a number of satellites to improve resolution.

It also uses signals from fixed point ground stations and a laser to take measurements from other man made and natural fixed points (depending on which model he was using)

Far more accurate than your standard gps. Cost can be as high as £20K dependant on your needs

Firth

The good news is that the US military has conceded that there's no value in keeping the codes for error correction from the public and in the near future consumer GPS could see a drastic improvement in performance.

Thats verging on the impossible in Urban areas and not that simple either in rural ones...

Well we do use "Followus" which on Vodafone can track a mobile phone within some 300 to 400 metres depending on location and the number of base stations in range..

Which won't be that many in rural areas!..

Which are?..

But your still depending on Satellite transmissions which in urban areas can be very difficult to receive..

Humm;!...

Inmarsat:- System Specifications:

1. Tx Frequency;

1. EIRPG33/25dBW
2. Rx Frequency;

1. Channel Spacing;

GPS:-

Agreed, but I think the Galileo operate at a higher power than GPS, though I could be wrong....

Budget cuts, don't you know.

Well the olde Garmin GPS 45 was it?, I have in the car for location in radio networks just did a test of that on the way back from London. Most every bridge over the motorway was marked as a way point and fed into Ordnance survey mapping back here , was spot on. Right carriageway and lane and any errors to the bridge fore and aft down to the time delays on me hitting the marker button.

Course this is in wide open countryside;!...

When I started working in North London in 1983, there used to be a pair of variable message boards on the A414 approaching the Park Street M10 roundabout. I recall seeing two messages capable of being displayed, "QUEUE AHEAD" (there is a brow of a hill which hides any queue for the roundabout), and "TOO CLOSE" which flashed up when it detected tailgating. Not long afterwards, the signs stopped working, but remained there for several years non-functional. (Checked on google streetview, and they've been replaced now by permanent Queue Ahead warning signs.)

That was nearly 30 years ago, so it's clearly possible to do it.

There was a system to detect tailgating on the approach to Windsor Great Park going back for a similar time.

Chris

Railways are quite predictable though, particularly the covered bits, the inertial back ups not going to be any less accurate than the original signal.

It's called RTK, you only need one base station (assuming you can get a= =

radio link to it or a decent data signal) & no lasers. It's not quick =

though & you need too know where you've put the base station. If you're = =

surveying you can work that out after the event, but it's not going to b= e =

much use for navigation.

Right well thats still trying to get awkward frequencies through urban obstructions..

And then if you have or need land based transmitters thats a lot of base stations to penetrate built up areas to mobile powered devices unless use if made of the GSM network...

Well it might but even say 3 dB higher isn't going make that much difference..

Well somewhere this thread was about tracking trains for signalling purposes and getting to such accuracy you'd know what track a train was on;!...

They already use GPS to decide whether it's safe to open the doors. Which led to a story in the press a while ago about how someone had put the wrong co-ordinates into a new train's system for one of the stations it was due to stop at, and nobody could get off.....