Crusie control..... What speed am I actually going??

[Anonymous]

I am new to the wierd, frightening yet fascinating world of crusie control.... it is amazing (the simplist pleasures ay!!)

However, when i set my cruise to 71on the computer... on the round dials it actual indicates 73!!?? It generally is 2-3 mph out... so, what speed am I actually going??!!

Does this happen to anyone else's??

 

[Anonymous]

My analogue dial doesn't match my digital speed either. Like yours the dial reads higher than the digital display. I drive exclusively on the digital speed readout, but having said that, neither are very accurate compared to my GPS speed. Both are reading high compared to the GPS speed. So, if your digital display reads 70mph and your analogue display reads 72mph, you're really only doing around 68 mph so you're still OK.

 

[Anonymous]

On my hobby horse here. Your GPS speed is wrong.

GPS is subject to a whole load of errors that if anyone is really interested in I will explain in detail. As a Commercial Pilot it is a subject I am very involved in.

But in a nutshell it is subject to a whole bunch if timing errors that are caused by atmospherics, reflections and spectrum interference. The satellite uses an atomic clock and the receiver uses a standard clock which can have different cycles. This causes positional errors which in turn effects the calculated speed and position. Even when using WAAS or fixed correction there will be errors. The GPS also uses a WGS84 geodetic model that assumes the world is a perfect sphere which it is not it is actually slightly oval thinning out towards the poles. This error in the grid also gives positional errors which will have a slight effect on the positional and speed data.

They are not huge with modern parallel receiver units but they do exist and are one of the reasons your speedo and your GPS have different figures. Your speedo is probably closer to the mark and I would not use GPS to judge speed, especially around speed cameras!

Modern speedos are very highly calibrated devices and while they have an error of margin built in to account for tyre wear etc are in fact very accurate.

With a GPS derived speed you can usually expect a couple of MPH difference depending on the accuracy of the position fix.

 

[Anonymous]

The GPS will be more accurate. Manufacturers have to make their speedometers read higher so you don't get caught for speeding as easily

 

[Anonymous]

On my hobby horse here. Your GPS speed is wrong.

I don't wish to take this thead off-topic but I beg to differ with regard to GPS Speed. I worked as an engineer in the GPS industry for over 15 years and I've applied the results to navigation solutions on land, sea and air as well as surveying applications. You are correct that there are lots of causes of positional errors and in the early days these were significant. These days positional accuracy have been significant improved and the results are much more reliable. However, speed isn't measured using positional measurements, it's measured using Doppler shift so the positional errors you described dont apply. The result is that a good consumer GPS system can quite easily achieve reliable steady state speed accuracies of 0.2 kph. By steady state I mean that the vehicle needs to be traveling at a constant speed and direction rather than accelerating or decelerating. Thats quite easy to achieve on any straight road and only takes a few seconds. I've tested this with police and military radar systems and the steady state speed on the GPS always matches that on the radar to the nearest 0.5 kph. By comparison the vehicle almost always reads higher than the GPS indicating the speedo has been set to ensure you're always slightly under the indicated speed. The speedo accuracy is I believe is supposed to be within 10%.

With my own driving, I regularly drive on the highway to the GPS speed rather than the speedo and I've never had a radar based speeding ticket - even in Victoria (Australia) where they will book you if your speed exceeds +2 kph of the posted limit.

 

[Anonymous]

We will have to agree to disagree here I think. As I have said I am very actively involved in this subject on behalf of the aviation industry to get NPA vertical guidance appraoches approved. We have some of the finest minds in on GPS in Europe on the working group.

GPS is indeed highly accurate but not as accurate as you claim for some of the reasons I have stated above. The constant variation in accuracy is the nature of the beast and the reason we require differential correction for aviation use.

Not getting done is not justification for your argument!!

But I think it bets we leave it at that point as I this is an RCZ forum.

 

[Anonymous]

I agree with you completely with regard to positional accuracy - especially with vertical navigation where errors can be magnified 2 or 3 fold. :thumbup:

However speed isn't calculated using position measurements. By using Doppler shift, GPS speed accuracies in the order of tenths of a mph are easily achieved even on standard off-the-shelf consumer GPS units. Even manufacturers like Garmin publicly quote their steady state speed accuracy as being within 0.1 mph.

 

[Anonymous]

GPS Speed:

GPS receivers display speed and calculate the speed using algorithms in the Kalman filter. Most receivers compute speed by a combination of movement per unit time and computing the doppler shift in the pseudo range signals from the satellites. The speed is smoothed and not instantaneous speed.

HOW ACCURATE IS THE SPEED READING?


From the NAVSTAR GPS User Equipment Introduction document Section 3.7:

GPS receivers typically calculate velocity by measuring the frequency shift (Doppler shift) of the GPS D-band carrier(s). Velocity accuracy can be scenario dependent, (multipath, obstructed sky view from the dash of a car, mountains, city canyons, bad DOP) but 0.2 m/sec per axis (95%) is achievable for PPS and SPS velocity accuracy is the same as PPS when SA is off.

Velocity measured by a GPS is inherently 3 dimension, but consumer GPS receivers only report 2D (horizontal) speed on their readout. Garmin's specifications quote 0.1mph accuracy but due to signal degredation problems noted above, perhaps 0.5mph accuracy in typical automobile applications could be possible.

Getting the accuracy to within .5mph requires a number of dominoes all to be lined up. It is often difficult for this to be achieved and as a result GPS speed accuracy can vary quite a bit.

I am not denying that they are capable of great accuracy, my life depends on them. However don't be lulled into believing the GPS speed is 100% accurate 100% of the time......

I would not be relying on one to save me from a speeding ticket..... :dance:

 

[Anonymous]

... its a Peugeot.. I dont think we can be lulled into believing anything is working 100% accurately 100% of the time...

Heck even I aint working 100% Accurately 100% of the time... and im freaking awsome.

Ill go back to my kennel now :lol:

 

[Anonymous]

Is the difference in the arguments something to do with the medium if the vehicle? Mr shiner is referring to air vehicles which operate in a 3dimensional sphere and fir gps to accurately predict their speed (although velocity may be a mire accurate description?) they will need a good satellite fix to calculate position both vertically and horizontally, this leaves them open to many more errors than a car on a purely horizontal plane? I'm guessing it's much easier to predict speed when there are fixed reference points such as junctions etc?

No expert by any means!

 

[Anonymous]

No the vehicle makes no difference. In an aircraft we are also able to derive vertical speed using the same process as horizontal speed. We also have WAAS to correct the signal errors and of course we have no geographical interference or RF interference so our signal is actually much more accurate.

The issue around the accuracy is around the way the vehicle sees the GPS signals and then process them. If you were out in the desert with no interference then the data is potentially accurate to a fraction of a mile an hour. A soon as you get into areas where geography and RF interference come into play the errors increase exponentially. So in a built up area for example where the buildings shield the line of sight to the satellites and you get reflections the accuracy degrades very quickly. You don't see this on your average car GPS because the data is averaged and smoothed before display. What you can find is that the car speedo and the GPS then start to get further apart. Now the problem is that you don't really see this as you are not watching the display all the time as you are driving. This means to your minds eye the speed is fairly constant but in fact it may be fluctuating due to the errors mentioned earlier.

As I have said some of this is semantics hence my comment about my hobby horse. I am merely making the point that you should not be 'calibrating' against GPS but rather the other way around. To avoid speeding tickets if you observe the vehicle speedo you cant go far wrong!!

At this point it is probably a subject best let to lie now as it has the potential of getting out of hand in arguments!!