real_welshboy

I was wondering, if you purchased a road angel or cyclops gps speed device, this is obviously registered to you and is highly accurate. What s stopping police knowing exactly when you exceed speed limits by obtaining information from the GPS companies??? They can easily and accuratly know if you have done 60mph in a 30 couldnt they??

Neil Smalley

No, GPS devices are'nt type approved so they can't do you.

real_welshboy

sorry to show my dullness but what does that mean? it is possible isnt it?

Neanderthal

iTrader: (6)

Do GPS recievers 'transmit' anything then?

real_welshboy

dunno, must do as the sattelite sends the signal down so knows where u are.

GU5

The satilite doesn't know where you are, the GPS unit knows where you are from the signal being referenced back to the maps in the onboard memory of the unit

OllyK

The Road Angel etc does not transmit, it recieves. It needs to get a signal from at least 3 satellites and then it calculates where you are via triangulation. That's a very simple description, there is rather more to it than that, but that covers the basics, if you want more detail then do a google.

End result, when you use Road Angel etc, people do not know you are using it or where you are or how fast you are travelling.

If you have a tracker however, that's a different story. They either send out a radio signal that is picked up by various masts to send back info on where the veicle is or they have a mobile hone built in using GRPS to send data back to the company that supplies the device.

Now the Poilce "could" ask for that information, maybe to determine where a car was at a given time to eliminate you from a crime (but some trackers need to be activated first, i.e. the vehicle stolen, so this won't help much).

As Neil Smalley said the GPS units are not type approved or subject to regular calibration so the speed would not stand up in court. Having said all that, there have been discussions on here where this technology has been discussed and is being trialed for putting in your car to actually apply the brakes if you travel too fast and send you a congestion charge bill if you use roads at certain times. How far it will actually go is still a matter of debate.

HTH

GC8

It cant happen; t'internet is full of explanations as to how GPS location works but very simply: all satellites know the time and transmit it. The GPS receiver knows roughly where it is in the world (which is why a 'cold' start can take so long) from the differences in the time signals it receives it calculates its exact location, ie. nearer to this satellite and further from that one. Radio waves always travel at 186,000 miles per second, the speed of light.... As you see, the satellites are passive and do nothing more than transmit a time signal.

Simon

OllyK

The staellites don't know where you are - they don't send a narrow directed beam out it is more of a shotgun approach - send out a wide beam signal that anybody with the correct kit can pick up. Bit like a hi-tech light house

Jay m A

iTrader: (2)

Its a receiver, not a transmitter, the satelite doesn't know where you are to 'beam down' - the satelite just transmits over a specific area, as do at least one other satelite. The reciever picks up both satelites signals, does a quick calculation and then it knows where you are and (relative to its last calculation) how fast you are going. I suppose the Road Angel can store info, then download it via your modem when you update the maps. But I doubt that would be in Road Angels interests to do that to their customers

unclebuck

Interesting that you should mention that. My belief is that by buying devices such as the remarkable 'Tom Tom Go' owners are unwittingly carrying out trials of exactlythe technology the Government intend to use when it becomes compulsory to have the 'official' GPS devices fitted to all motor vehicles. Notice that it not only logs your position but your current speed as well.

This data could easily be logged to determine road usage fees and trigger speeding fines. Some are suggesting that it may actually be used to apply brakes and prevent owners from travelling on roads they are not authorised to by disabling the vehicle.

Frighteningly, all this is almost now possible nationwide. But far worse than that is that we have a Government in place who already have an agenda to introduce control of this sort.

Be afraid....

UB

hedgehog

Some are not suggesting that they will be applying the brakes and controlling the throttle, the administration have actually already tendered for the supply of devices to do just this.

Leeds university are currently at the "leading edge" of testing this intelligent speed adaptation technology so the case is being made at that level, but the actual infrastructure is already nearly in place.

Last I looked at the progress document the mapping of the UK for the system was due to be complete next September. After that it is likely that insurance companies will want ISA and various logging systems on performance and high risk cars and within a few years the administration will have legislation that means it must be fitted to all cars.

You and I probably have about 2 insurance renewals before we have to fit a device that controls our speed and reports our position and other information.

This also explains why the administration want to make laser and radar detectors illegal and yet have no objections to GPS based camera detection systems: it is because they want to use the GPS based systems themselves within the next few years.

Say night night to freedom.

carl

No. It calculates your position from the satellite signals. It then calculates your speed as the first differential of the position (rate of change of position). I would guess that for the speed it averages it over the last 3 or 4 position updates.

However, GPS was developed for things like aircraft, so what the receivers calculate is ground speed and something like climb rate. The "speed" shown on Tomtom is the ground speed, and isn't accurate when you're going up or down hills (it drops by a couple of mph) or round bends (because it assumes you've taken a straight line between the two sample points). I can't see how this "GPS speed limiting" will work -- you pootle through a town with the GPS limiter on at 30 mph, yet when you get to a bend you can do 32 or 33 mph (because the GPS still thinks you're doing 30). Surely it's safer to exceed the limit on the straight bits?

unclebuck

UB

hedgehog

Many GPS devices, though I can't comment on the Tomtom system, use a doppler system for determining velocity and so are very accurate no matter the path of the object.

While the ISA system, to limit and control the speed of your car, will at times allow the maximum speed within a given speed limit for much of the time it will be operating to a lower limit as the paperwork has already indicated that the posted speed limit will very much be seen as a maximum but that reduced limits will apply where the road conditions are deemed dangerous, such "dangerous" areas will include bends, hospitals, schools, areas with cycle lanes etc. etc.

carl

Huh? How does that work? Bear in mind that GPS continues to work if you lose the carrier -- how can you measure Doppler shift with no carrier? And the Doppler shift will vary with the elevation and azimuth of the satellite -- it's not like Doppler radar where everything's pretty much coming straight at you in the same plane.

hedgehog

That I know of many GPS systems use doppler for velocity calcs, this was just the first easy examples I could come upon that are car based:

http://www.aft-werdohl.de/Deutsch/Ma..._L3_VBOX_D.htm

http://news.managingautomation.com/fullstory/23852

The accuracy using Doppler is in the region of a cm per second. Even my old handheld uses Doppler but as it is multichannel there is a sort of historical use of the information as well which means that you could stretch the point to claim that it uses Doppler to calculate distance over time from which it then calculates velocity.

There is a reasonable explaination of that here, but please note that this was written in the days of SA and that makes the water somewhat more muddy, to say the least:

http://www.aprs.net/vm/gps_cs.htm

carl

The Racelogic and VDO Dayton boxes are for measuring speeds, 0-60 times, standing quarters, etc. Normal in-car GPS doesn't use the Doppler shift, AFAICS.

[Bear in mind, I stopped dealing with GPS "professionally" in around 1998]

matty01

http://www.parliament.the-stationery...t/21017h01.htm
The facts,(hope you like reading )

matty01

To xp users, click on "edit" and "find on this page" type in 'speed' and it'll pick out the appropriate sections for you

hedgehog

Even my rather old Garmin handheld bases it's velocity upon Doppler shift, honest, and I believe that all current Garmin engines have Doppler information available as part of the equations they use. (They have dx and dy available but discard dz as I recall) It is possible that in some implementations they use the time - distance thing to determine velocity but this would seem unlikely when the GPS system by its operational nature makes a better system available. If you read the final link you can see that you are partially correct in that more modern RXers may use what is, sort of, historical data to refine their current information and so they sort of use "knowing where they've been" to assist them with determining velocity. However, Doppler is the major factor and the historical data is not much more than averaging to remove any gross errors.

When SA was on the positional accuracy was to about 100m but yet you could travel much less than that and get a pretty accurate speed measurement. The reason this could happen was just because the unit was using Doppler rather than positional information to determine velocity. If it had used positional information then you would have to travel several hundred meters, on an average day, before it would have given you a reasonable speed reading. I realise that with SA off the accuracy is now about 15m but even at that you would probably have to travel 50m to get an accurate speed reading. My unit will give an reasonable estimate of my velocity if I walk a few strides with it in my hand.

I don't, however, dispute that you have encountered devices that used the time - distance method but the Doppler system is available and implemented on common and very low cost receivers. It might be an interesting exercise to ask the Tomtom folks which system their units actually implement.

carl

Indeed. I was involved in military GPS receivers (yes, we got to use PPS and the P-codes/Y-codes) where velocity information wasn't actually that important (700 kts isn't much different from 720 kts, particularly when you have other systems on board).

hedgehog

I can absolutely see how in an aircraft the time - distance method could be reasonable and perhaps more robust. It is also possible that in an aircraft your speed on a track would be much more useful to know than your velocity. I know nothing about aircraft and don't fully believe they can stay up there so that little problem will give me something to think about :-) Sad I know.