GPS vs Speedometers

[Paul B]

Which is likely to be accurate ??

I have the Tom Tom 2 package in a PDA and it says that I am doing 70mph when the speedo in my tin top says 76mph.

Anyone know how reliable the speed indication will be from the GPS system ?

Paul B

[Steve]

I used my GPS to dial in my speedo for SVA. It was spot on

My BMW reads very high, my wifes minivan is almost spot on.....Speedo's seem to vary quite a bit.

[alsuth]

Hi there,
GPS should be more accurate. The easy way to see how accurate is to see how many satellite's your PDA is receiving from. Most will do up to 12 these days. Three is ok as you then get a triangulated position. Any more narrow downs the accurace to a couple of feet. As the americans have been kind and removed their error code out of the system you can't go wrong.
Cheers
Al

[robert]

Originally Posted by Paul B

Anyone know how reliable the speed indication will be from the GPS system ?

Very reliable, more reliable than your speedo, speedos are always out. :thumb:

[Paul B]

Does that mean I can use it as an excuse if I get pulled by the plod ?????

It is great that with this portable unit I can check the speedo accuracy without needing a rolling road etc. After the SVA test, none of us really have a very accurate way of checking the speedo calibration when we change wheel or tyre sizes etc. Pity I haven't found any PDA software that could record my 0-60 times as well ! (unless anyone knows different ?)

[Grease Monkey]

Couple of things to bear in mind:

There are different types of GPS for different applications. I have one for sailing, which is no good for flying (or driving at high speed) as its not designed to refresh quickly enough.

They are only accurate in a straight line. If you were to drive around a roundabout at 30mph, it would say your speed was zero, as your position would be pretty constant.

They only update speed as an average over the last pew positions so to record accurately you'd have to be driving at a constant speed for a good few seconds.

[Andy302]

Interesting comments.... Ask yourself one question though, "Why do all mainstream in car navigation systems still use the vehicle speed pulse as well as satellite and gyroscope data?"

Answer:- Because GPS data alone is not sufficient to give 100% accurate navigation and systems based on the three data set approach are more accurate and reliable. "Why? 1) From GPS the Nav gets vehicle position data which it updates from so many times in a given period, 2) the speed pulse data tells it how far its gone and how fast, it uses the GPS data to double check this and carry out self calibration. 3) The gyro tells it which way it has turned.

If the system looses any of the three it will start to give erratic navigation until all sensors are back online. Nav units without speed pulse and gyro data will be a bit buggered in long tunnels!

So in answer to your question, I would say that GPS data is accurate but is probably only as reliable as the vehicles speedo, remember the vehicles speedo is not effected by tall buildings, mountains, bad weather conditions and tunnels!!

JIMHO.

Cheers

[robert]

This post is copied from a US site that was discussing data loggers and video. Andy seems to be very fair in his comments, and helpful. Rob

[quote] Hi Everyone,

My name is Andy; I'm the software/GPS guy from Race Technology whom manufactures the DL1/AX22 products. Ørjan invited me to join in with the discussion, I'm not here to advertise honest(!)... just answer a couple of the points that have been raised.

First GPS accuracy, I could write a book on the subject as there is lots to it, but briefly....

Accuracy

We quote 3m CEP, which is about right for a "typical" tree lined circuit with buildings blocking clear views to the horizons. However, at a clear track (e.g. and airfield) then the accuracy comes down to well under a meter, conversely with poor GPS conditions, i.e. surrounded by buildings it can also go up. We choose to be conservative with the specifications and quote 3m CEP as it much better to get people saying it better than expected, than visa-versa! There is lots of rubbish quoted about GPS accuracy, 3m CEP is as good as it gets for a standalone GPS receiver without taking about a subscription to the very expensive differential services ($k's a year), or having a trackside differential correction system. The 3m error is limited by the physics of the GPS problem, there is "very" little error added by the GPS receiver itself. Most GPS manufacturers quote accuracies under ideal conditions and these can be misleading to say the least! Also: never confuse accuracy and resolution as the 2 are unrelated, our GPS system has a resolution of 0.1mm!

Causes of GPS errors

With a clear view of the sky the GPS positional errors are caused by atmospheric phenomena and inaccuracies in the satellite orbits… these problems typically cause the GPS position to wander around quite slowly, over the period of 10’s of minutes (it’s not seconds, but it’s not hours either). With buildings/trees in close proximity, positional errors are also induced from the reflected GPS radio signals (AKA multi-path), this can cause quite fast positional errors. Multi-path is a big problem when using you sat-nav system around town, but fortunately they don’t normally build skyscrapers by race tracks!

Combining accelerometers and GPS

Combining accelerometers and GPS does not normally improve the overall positional accuracy a great deal (in practical terms we get about 20% improvement), however it does allow use to every effectively "filter" the GPS data to fill in any dropouts and eliminate and large, short term GPS errors – it basically cleans the data up in an intelligent way.

Comparison of GPS and lap beacon wheel speed systems.

Optical lap beacons have a beam spread of about 2-3m (radio transponders are about the same). So if you have good/average GPS conditions then GPS based lap timing is a bit more accurate, with poor GPS coverage then an optical lap beacon is a bit more accurate – in practice neither can "promise" 0.01s accuracy. Assuming your on doing 100mph on the straight then 2m takes "about" 0.04s. Line up 10 optical systems at the side of the track and 10 lap timers in the car and I can promise that is about the spread you will see (we’ve done it!). But that isn’t the end of the story…. A lap beacon system only "knows" where you are once per lap, so you get a valid lap time but the intermediate sector times are based on the highly inaccurate wheel speeds – so they are basically all over the place! In contrast GPS knows where you are the whole time so the lap and sector times are all accurate. There is also the practical problems of using optical beacons to consider…..

Speed errors

Speed and position are calculated almost independently – the accuracy of speed is a completely different topic to position. Errors in speed are better defined than position – typically speed errors are in the range 0.05mph to 0.3mph. In contrast to position these errors are mainly due to errors in the GPS receiver itself (it’s the carrier noise), the receiver in the DL1/AX22 has a "carrier noise" around 0.6mm which is the currently as good as it gets using today’s semiconductors. I’ve read on other manufacturers websites speed accuracies of 0.01mph from GPS, I’ve no idea where they get this figure from but it is currently fundamentally impossible regardless of cost! The speed error is so low, the error can normally be neglected altogether.

And now a quick word on the video overlay stuff….

The new v5.1 is on the web site which is a significant improvement over v5.0 in terms of the video export functionality. If anyone would like to make suggestions about how we could improve things further then now would be a good time to suggest it – we are well stuck into v6.0 of the software and welcome all feedback!

Blimey… that’s enough typing for now, better get back to work ;o)

[/quote:e29dc0485c]

Hope this help. :thumb:

[robert]

Originally Posted by Grease Monkey

They are only accurate in a straight line. If you were to drive around a roundabout at 30mph, it would say your speed was zero, as your position would be pretty constant.

Sad as this is, I have just been out and tested this, strapped my Garmin to the bike and the garmin measured 48mph round the roundabout, I was banked over and knee down, but it didn't reaad zero, and the speedo was reading about 45. :thumb:

[Dutch Paul]

Originally Posted by robert

Sad as this is, I have just been out and tested this, strapped my Garmin to the bike and the garmin measured 48mph round the roundabout, I was banked over and knee down, but it didn't reaad zero, and the speedo was reading about 45. :thumb:

What about doing do-nuts?
Speedo will say 80mph - GPS nothing
Fu###ng technology confuses me