Satnav accuracy

[DavidW1]

My Satnav - a Navman 630 - shows consistently slow by about 5mph against the car speedo. I have used it in a variety of vehicles in uk, a Vectra, Shogun, two Isuzu's and a Matiz and they all over read the Satnav by about the same amount - ie.when the Satnav says 30mph the car says 35.

When towing I like to sit just faster than trucks so they are not constantly overtaking and my speedo reads about 63ish and the Satnav 58 which would suggest the Satnav is correct?

Interestingly when I used it in the States recently in a Dodge hire car both speedo and satnav were spot on.

Anybody had the same problem or know why this is ?

Navman say positioning is accurate to 5 metres and I have found distances compatable with road signs.

Don't anybody tell me to time distances between motorway markers either cos I can't be bothered with that - Just want to know which indicator to believe so's I don't have a dispute with the constabulary's yellow roadside tax machines.

 

[Guest]

Yesterday on the E70 we past a Volvo towing a Lunar with 62mph indicated on the Cars speedo and 60 mph on my Garmin that I prefer to the X5's in built model. At a service point the Volvo stopped along side us and as we chatted the gent said that his TomTom showed him at 57mph whilst the speed showed 60mph. He had upped his speed to 60mph on the Tom Tom and found he was keeping pace with us.

On flat ground I'm told that sat nav gives the most accurate speeed check you can attain. With triangulated multiple satellite contact even on hills the satellite reading is better than the cars speedo.

My wifes cars speedo is 3mph fast campared to sat nav. Sad but we have driven both cars as per sat nav speed and we stay together, at an indicated 70 I slowly pull away with sat nav reading 68mph and hers reading 67mph.

According to my techie son inlaw this is regular debate on Sat forums and trust should be with the very accurate Satellite reading not the cars speedo.

If you get different sat navs in a car at the same time, they all read the same apparently as does my cars and my Garmin.

 

[joeby]

a few speedo's show discrepencies as well ,one week i took the Mondeo to Sunningdale Berks from Swansea ,a distance of 176 miles in the Mondeo , the following week i took our Nissan Terrano in following the axact same route ,and it was 3 miles less

 

[popupJohn]

I always thought that most satnavs showed the average speed over the last minute or so. Not the actual speed at that second.

I could well be wrong tho

 

[PC_5119929]

Most Satellite Navigation Systems can display actual speed.

Some do not show speed whilst navigating but most are able to do so depending on mode being used.

Even basic inexpensive units show as near to true speed as you can get due to the way they work.

Sat Nav speed accuracy is said to be better than some law enforcement systems in current use.

 

[rob_jax]

Hi David,

Satellite Navigation systems determine speed with an accuracy much better than they can display - if they say 60 mph then it really is 60 - and you don't need to know your speed to three decimal places.

Car speedometers on the other hand usually over estimate - by between 2% and 10%. My C270 is less than 2 mph off at 60, but a Ford Cortina of mine in the 70's was out by 10% (mile posts and a stopwatch)

And the car gets increasingly inaccurate as the tyres wear down.

Also manufacturing tolerances between different tyre manufacturers don't help - nor does getting the tyre pressure wrong.

Sat Nav right - car speedo over estimates - But don't try to use the Sat Nav to draw out that last legal mph.

Robert

 

[JamesFrance]

Hi David,

The automotive satnav is based on a GPS system devised by the US military. They used to make it intentionally unreliable, however now with differential GPS it is extremely accurate.

I have a sailing boat fitted with a GPS chart plotter which can be programmed to follow a course round rocky headlands by controlling an autopilot.

If the chart is accurate it is 100% reliable.

James

 

[DavidW1]

OK so far as it goes guys - understand the principles, triangulation and all that stuff, which plots how far you've been against a timebase which of course equals speed and I definately have no quarrel with the navigation aspect which works fine.

Can't get my head round the 15% difference between car speedo and Satnav in europe though and why was the US car speedo so in tune with the satnav?

Thought about tyres too - my Bighorn tyres are almost new and the other cars have varying depths of tread but all have the manufacturers original tyre equipment and all with consistantly over reading speedos.

Wildcard aspect - is it anything to do with the GPS system being US and all the satellites having been launched at Cape thingy and having an orbit biased to US users? Aircraft nav systems such as TACAN and ILS use local ground based beacons and beams and are megga accurate which is a bit different than relying on fixes from space, some of which can be almost over the horizon.

So still not convinced that Satnav speed indication is as reliable as a speedo but open to more opinion and facts.

I think it could to go to dopplers or gyros on this one which is probably the world cup equivalent of penalties!

 

[PC_5119929]

It is said that vehicle speedo's can be upto 10% , hence for years Police were said to have given some leeway for your speedo being inaccurate.

Fact is that your sat nav is more accurate that your cars speedo, this is not to stay that you can not get a speedo that can be spot on, but most are not.

If you accept that sat nav is very up to date technology compared to somewhat antiquated speedo basics surely you should be on to realising that sat nav is the one to trust.

A network of satellites all sat at just over 20000KM above earth provide accuracy of around 3 - 5 metres yet you are doubting the speed facts. Strange!

A simple search on this question or a visit to a sat nav forum will show that we are correct with our statements that sat nav speed is the most reliable.

Majority of speedos read between 3 - 5 mph over, some Italian ones have been found to be around 10 mph out maybe it's a latin speed thing.

 

[PC_5119929]

If this helps, quote from one source.

"Sat nav more accurate ? - Honestjohn new Sun 18 Sep 05 10:09

Satnav is 100% accurate because the position of the car is crosspatched by several satellites at the same time. True speed in 'Autocar' road tests is read by GPS, not by timing or measuring devices."

ps , try borrowing a couple of other sat navs and put them in your windscreen with yours, you will find they all say the same.

We have assessed a variety of brands one against the other, the one thing they all do together is show the same speed.

 

[ProfJohnL]

DIGITAL SPEED INDICATION BY GPS IS NOT INFALLIBLE! .......

As with all things digital that are monitoring an analogue variable, any scientist will tell you that you can never accept the least significant digit on the instrument at its face value. There are many errors including Scaling, Off set, None linear, Bias and many other artefacts that will compromise the accuracy of any conversion between analogue to digital.

Most of these errors are accounted for when the manufacture gives a statement of the accuracy you can expect. They usually add some phrase to the effect of the error can be as great as + or - X counts of the Least Significant Digits (LSD) If no statement is issued you should assume that at least +/-1 count of the LSD is inaccurate

With GPS:

Consider when the GPS is indicating 30Mph (or any other speed) How much faster or slower do you have to go to actually get it to change to by 1 mph?

You could actually be travelling at 30.9999(rec) mph and it still it shows 30Mph, yet a speed camera may have a less accurate display and it may show you as 31mph. That could be enough to trigger a speeding ticket.

Based on other respondents who have stated that gps units have a positional accuracy of 3 to 5 meters. Most GPS units tend to sample at 1 second intervals, So for two successive positional readings 1 second apart the GPS may have a cumulative error of + or - 10M over the actual distance travelled.

To keep the maths relatively simple I will use 50km/h which is close to 30mph. At 50Kmph you would be travelling at 13.9 M per sec (M/s)

If the GPS over indicates the distance you have travelled (i.e.13.9m+10m) 23.9m/s this equates to 86 Km/h you would slow down, but if it under reads (i.e. 13.9m -10m) 3.9m this equates to a speed of only 14 Km/h. You may think it is safe to speed up by 10m/s and travel at 23.9 m/s (86Km/h or 51mph!).

It is likely that over only a relatively short distance these errors tend to diminish, but it only takes the driver to glance at the GPS at the wrong moment to gather the inaccurate information and to make adjustments to the cars speed.

As a suggestion and unless other road conditions prevail, I recommend that for any given speed limit, you subtract 1mph and and use that as your target speed for Sat Nav and cruise control.

Just because its digital doesn't mean its accurate or right!

 

[WatsonJohnG]

I have recently changed the X-Trail Sport for the X-Trail Columbia.Same car with different badge so identical engine/gearbox apart from some tuning.

With my Sport the Inforad sat. speed camera detector bleeped at a car indicated 35 mph in a 30 zone.The Columbia has bigger wheels and the Inforad now bleeps at 31mph.

On a trip to Vannes in Brittany from Exeter my mileage read 6 miles less than my friends Freelander over 160 miles and we covered exactly the same route.

The American car must have had a more accurately calibrated speedo than the ones we get in UK.

Its to the manufacturers advantage to have it reading faster as its better to appear to have higher mpg --keeps the punters happy !!

 

[PC_5119929]

Aplogies to John L, the original question was asked re accuaracy between speedo and sat nav.

Factual eveidence says that even a very basic sat nav is one of the most accurate measures of speed. Over fixed measured distances basic sat nave units costing less than

 

[JamesFrance]

David, I can't remember the details and I do not have the info here, but certainly GPS used to be much less accurate than now.

The modern technology uses ground stations also, with systems called waas for the USA and egnos in Europe as far as I can remember. These do compensate for local inconsistencies.

James

 

[ProfJohnL]

Hello PC

My point was to show the often held view that 'Digital instruments are always right' is flawed. I then demonstrated using supplied figures how potentially inaccurate a Sat-Nav could be, given worst case scenario.

The sad fact is we have little chance of knowing how flawed the the instrument can be at any given instance as we can's see the signals that the GPS uses.

I chose Kmph for the calculations because the quoted errors were in metric units. From understanding the maths involved, I knew that the potential error would have maximum effect at lower speeds, and to keep it within the range of real life values I chose 50Kmph which I stated was close to, not the same as 30mph.

Fundamentally GPS only produce positional fixes. The Sat-Nav software does the rest including the calculations for speed. For the reasons I gave and explained GPS has a much larger potential for error between successive position samples when travelling at low speeds. This error reduces with increasing distance moved (speed).

Your point about comparing different models of Sat Nav, simply shows they have a high level of consistency - this is not too surprising because virtually all the Sat Nav system manufactures use the same hardware decoding protocols for the GPS signals. This does not confirm or verify the accuracy of the Sat-Nav speed calculation. The greater the distance travelled between sampling points will reduce the speed calculation error this would be achieved by travelling faster or reducing the sample rate.

I do not know, but I suspect that some Sat-Navs will uses sample averaging techniques. This can reduce the effect that an single reading of maximum error has on the averaged result. So in practice the speed indication will improve its accuracy by several factors as the journey progresses.

My other point about the accuracy of the speed calculation and its display is still valid. You do not know at what point in the analogue speed the digital LSD will increment up or down. For expedience you should assume the worst case, which for drivers is that it requires you to assume that Sat-Nav reading will at least 1 count pessimistic, i.e. you are travelling at least one count faster than is indicated.

 

[DavidW1]

Well, having read some of the thread comments and still confused I ventured on to the net and confused myself some more!

To sum up though, I have learned that pure GPS, using satellites only, depends on hugely accurate clocks and expensive computing power for accuracy, a system called Differential GPS (DGPS)is used commercially. This includes additional ground reference stations at known positions to compensate for error.

I suspect this is the reason for varying performance in different parts of the world - in UK for instance we use a ground station based in Sutton Coldfield and run by a commercial FM radio company, in the States they have a system called WAAS which is federally regulated, has more stations and is more sophisticated.

At any rate, with DGPS the errors in positioning are still no better than 1 or two metres and can be as great as 10 and that is static remember - my satnav has a diference of speed between the car speedo at 30 mph of 5mph. 5 mph is 2.2 metres per second so that is not a bad accuracy but I think it is worse than the car's speedo even wih worn tyres!

So - answering my own original question, I think that Satnav should not be relied on as an accurate speed indication and consensus from the thread suggests that they read slow by varying amounts.

Car speedos probably read a tad fast but at least that's OK for the dreaded speed cameras.

The explanation for the Dodge Caravan/Explorer being more lined up with the speedo was probably the better performance of WAAS differential referance points.

I shall therefore continue to rely on the speedo in future - unless of course somebody convinces me otherwise.

 

[PC_5119929]

Hello PC

My point was to show the often held view that 'Digital instruments are always right' is flawed. I then demonstrated using supplied figures how potentially inaccurate a Sat-Nav could be, given worst case scenario.

The sad fact is we have little chance of knowing how flawed the the instrument can be at any given instance as we can's see the signals that the GPS uses.

I chose Kmph for the calculations because the quoted errors were in metric units. From understanding the maths involved, I knew that the potential error would have maximum effect at lower speeds, and to keep it within the range of real life values I chose 50Kmph which I stated was close to, not the same as 30mph.

Fundamentally GPS only produce positional fixes. The Sat-Nav software does the rest including the calculations for speed. For the reasons I gave and explained GPS has a much larger potential for error between successive position samples when travelling at low speeds. This error reduces with increasing distance moved (speed).

Your point about comparing different models of Sat Nav, simply shows they have a high level of consistency - this is not too surprising because virtually all the Sat Nav system manufactures use the same hardware decoding protocols for the GPS signals. This does not confirm or verify the accuracy of the Sat-Nav speed calculation. The greater the distance travelled between sampling points will reduce the speed calculation error this would be achieved by travelling faster or reducing the sample rate.

I do not know, but I suspect that some Sat-Navs will uses sample averaging techniques. This can reduce the effect that an single reading of maximum error has on the averaged result. So in practice the speed indication will improve its accuracy by several factors as the journey progresses.

My other point about the accuracy of the speed calculation and its display is still valid. You do not know at what point in the analogue speed the digital LSD will increment up or down. For expedience you should assume the worst case, which for drivers is that it requires you to assume that Sat-Nav reading will at least 1 count pessimistic, i.e. you are travelling at least one count faster than is indicated.

Dear John L

I just found it funny that comment on accuracy had somewhat inaccurate conversion. No slight on your intentions was meant.

My brother and I had two matching motorcycles ordered and bought together with consecutive frame numbers. The speedos read 5mph different.

This is a common fault or reality.

US military used to distort the accuracy of Sat Nav signals, that has now changed and with WAAS and EGNOS enabled units we have very good accuracy and vary fast response re data. Allthough accuracy re position is said to be around 5 metres that takes into account the actual mapping and its pictorial accuracy that is scrolling across the Sat Navs screen. With a reasonable level of Satetllite signals the sat navs probably have you on the very spot you are on most of the time even with speed you are travelling at any time as the units calculate average speed factor into its system.

Accurate testing has shown Sat Nav to be the most accurate speed calculator available to Joe Public Drivers, you can of course have a rogue unit I guess as you can have rogue speedo's even with their well known basic inaccuracy to start with.

Quote from sat nav expert re race technology timing follows. This also relates back to equipment that was using older slower processors and chipsets I understand and checked against the most accurate fixed distance timing equipment available.

"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."

This is the same sort of results found in this country, what are considered to be quite elderly sat nav units now still give the same sort of speed accuracy of modern up to date units as well.

 

[PC_5119929]

Dear John L

I just found it funny that comment on accuracy had somewhat inaccurate conversion. No slight on your intentions was meant.

My brother and I had two matching motorcycles ordered and bought together with consecutive frame numbers. The speedos read 5mph different.

This is a common fault or reality.

US military used to distort the accuracy of Sat Nav signals, that has now changed and with WAAS and EGNOS enabled units we have very good accuracy and vary fast response re data. Allthough accuracy re position is said to be around 5 metres that takes into account the actual mapping and its pictorial accuracy that is scrolling across the Sat Navs screen. With a reasonable level of Satetllite signals the sat navs probably have you on the very spot you are on most of the time even with speed you are travelling at any time as the units calculate average speed factor into its system.

Accurate testing has shown Sat Nav to be the most accurate speed calculator available to Joe Public Drivers, you can of course have a rogue unit I guess as you can have rogue speedo's even with their well known basic inaccuracy to start with.

Quote from sat nav expert re race technology timing follows. This also relates back to equipment that was using older slower processors and chipsets I understand and checked against the most accurate fixed distance timing equipment available.

"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."

This is the same sort of results found in this country, what are considered to be quite elderly sat nav units now still give the same sort of speed accuracy of modern up to date units as well.

ps. re Davids comments, if you rely on the average speedo re speed limits you will not be over the limit for sure.

You asked re accuracy, yet you seem to ignore the facts that sat nav is still in fact a far more accurate speed indicator than your or the average speedo.

On a road where there are accurate distance marks for speed checking I understand that basic cheapest sat nav unit has still proven more accurate than any other equipment re speed.

 

[DavidW1]

Dear John L

I just found it funny that comment on accuracy had somewhat inaccurate conversion. No slight on your intentions was meant.

My brother and I had two matching motorcycles ordered and bought together with consecutive frame numbers. The speedos read 5mph different.

This is a common fault or reality.

US military used to distort the accuracy of Sat Nav signals, that has now changed and with WAAS and EGNOS enabled units we have very good accuracy and vary fast response re data. Allthough accuracy re position is said to be around 5 metres that takes into account the actual mapping and its pictorial accuracy that is scrolling across the Sat Navs screen. With a reasonable level of Satetllite signals the sat navs probably have you on the very spot you are on most of the time even with speed you are travelling at any time as the units calculate average speed factor into its system.

Accurate testing has shown Sat Nav to be the most accurate speed calculator available to Joe Public Drivers, you can of course have a rogue unit I guess as you can have rogue speedo's even with their well known basic inaccuracy to start with.

Quote from sat nav expert re race technology timing follows. This also relates back to equipment that was using older slower processors and chipsets I understand and checked against the most accurate fixed distance timing equipment available.

"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."

This is the same sort of results found in this country, what are considered to be quite elderly sat nav units now still give the same sort of speed accuracy of modern up to date units as well.

ps. re Davids comments, if you rely on the average speedo re speed limits you will not be over the limit for sure.

You asked re accuracy, yet you seem to ignore the facts that sat nav is still in fact a far more accurate speed indicator than your or the average speedo.

On a road where there are accurate distance marks for speed checking I understand that basic cheapest sat nav unit has still proven more accurate than any other equipment re speed.

No I didn't ignore accuracy - it's just that none of the sources I could find had any info on speed computation, just positional accuracy. I thought it safe to surmise that positional error would translate into a some speed error given that speed is the function of time over distance.

 

[PC_5119929]

Davis. Sat Nav is aimed at navigation and speed reading in cars is just a kind of extra.

Re navigation accuracy, you have a map thnat comes up on the unit being used, and differnet suppliers of thiose maps so the positional accuracy is based partially on the map in front of you on the unit.

Chances are that with good satellite lock on a number of them you are nearer to 1 foot accuracy but positianaly on the drawn map the accuracy may be a little out.

As an example my present Sat Nav was initially set-up on my back garden bench as it has clear view to the skies above and could be left to get its first fix. Set as "home" even months later the unit when zoomed in fully will show a difference of 1 foot from its original position all the over the 300 metres to the road I believe the drawn map is slightly out as position when joining the road from our drive appears a few feet out. But position of the maps soon seems ok as I move from the drive along the road.

I hope that makes some kind of sense. As a point of interest race cars tracked over accurately measured and timed distance with no speedos are shown to have as near accurate speeds loged on sat nav systems. A sat nav in the cockpit is an easy way to get an instant speed check on a new track when testing I understand.

 

[DavidW1]

Davis. Sat Nav is aimed at navigation and speed reading in cars is just a kind of extra.

Re navigation accuracy, you have a map thnat comes up on the unit being used, and differnet suppliers of thiose maps so the positional accuracy is based partially on the map in front of you on the unit.

Chances are that with good satellite lock on a number of them you are nearer to 1 foot accuracy but positianaly on the drawn map the accuracy may be a little out.

As an example my present Sat Nav was initially set-up on my back garden bench as it has clear view to the skies above and could be left to get its first fix. Set as "home" even months later the unit when zoomed in fully will show a difference of 1 foot from its original position all the over the 300 metres to the road I believe the drawn map is slightly out as position when joining the road from our drive appears a few feet out. But position of the maps soon seems ok as I move from the drive along the road.

I hope that makes some kind of sense. As a point of interest race cars tracked over accurately measured and timed distance with no speedos are shown to have as near accurate speeds loged on sat nav systems. A sat nav in the cockpit is an easy way to get an instant speed check on a new track when testing I understand.

No sorry you are not making much sense - I have my house set as a favourite and it chimes destination at varying distances from the house when returning on differing occasions but that's not my point, I am in fact quite happy with that.

Do you actually know how a Satnav derives it's speed indication? I don't and if there is an error in positioning then I fail to see how a speed reading can be reliable, consitant or correct - please feel free to enlighten me.

 

[JamesFrance]

Davis. Sat Nav is aimed at navigation and speed reading in cars is just a kind of extra.

Re navigation accuracy, you have a map thnat comes up on the unit being used, and differnet suppliers of thiose maps so the positional accuracy is based partially on the map in front of you on the unit.

Chances are that with good satellite lock on a number of them you are nearer to 1 foot accuracy but positianaly on the drawn map the accuracy may be a little out.

As an example my present Sat Nav was initially set-up on my back garden bench as it has clear view to the skies above and could be left to get its first fix. Set as "home" even months later the unit when zoomed in fully will show a difference of 1 foot from its original position all the over the 300 metres to the road I believe the drawn map is slightly out as position when joining the road from our drive appears a few feet out. But position of the maps soon seems ok as I move from the drive along the road.

I hope that makes some kind of sense. As a point of interest race cars tracked over accurately measured and timed distance with no speedos are shown to have as near accurate speeds loged on sat nav systems. A sat nav in the cockpit is an easy way to get an instant speed check on a new track when testing I understand.

Hi David,

To get back to your original question, surely car speedos have always been optimistic so you feel that they have a better performance than they really have.

Also mileometers overread to make you think your fuel consumption is better than it is.

I do think PC understands the technology and is talking a lot of sense.

 

[ProfJohnL]

Hi Dave W,

I am sorry but we do seem to have sidetracked the thread a little into technical issues.

In answer to your original question, there is an EU ruling that requires car manufactures to ensure that speedometers are never allow the driver to be travelling faster that the indicated speed, in other words the allowable is I believe zero to +7% of actual speed.

This must be maintained for the life of the vehicle, so allowing for manufacturing tolerances and wear and tear, most speedo's will indicate faster than the actual motion.

 

[ProfJohnL]

Now PC!

I am unable to full understand the logic behind your argument that says despite apparent positional errors that affect GPS, the speed indication of s Sat Nav is highly accurate.

The only way a Sat Nav can calculate speed is to take two or more positional samples at known times and by the calculation of distance over time. If the positional fixes are inaccurate then the calculated speed is also inaccurate.

Now I accept that the GPS positional errors are likely to be significantly less than the +/- 5M, but there is always that potential between successive readings.

I also understand that if a rolling average of GPS samples is used, then the average speed calculation will tend to improve, as the inaccuracies of the GPS will tend to cancel out.

As a result a reasonable accurate speed could be calculated after several positional samples have been taken.

This however does nothing to relieve the quantitative errors that afflicts A to D conversions and all digital displays that monitor an analogue measurand. So there has to be a potential error of at least +/- one count of the least significant digit.

At their worst (which admittedly is very infrequently) Sat Nav speed indications can be worse than the average vehicle mechanical speedometer. At best they can produce far more accurate results

Part of the problem is that how do we know when they are accurate and not?

 

[PC_5119929]

The signal from satellites reach the unit at 186000 miles per second or in around 0.06 of a second from around 20000 K's above us. Signals are processed together in a split second I understand.

Taking it that you accept speed data from laser equipment etc, why the problem with accepting accuracy from sat navs.

Re positioning accuracy, your position is shown on the maps small unit screen.

If I were to draw a map of the UK and put it into my sat nav I doubt that the position of my home would show as being as accurate as Tom Tom or Garmins mapping. Your position is shown on a drawn map, so accuracy is based partially on the units map.

So if I satnd on the white line in the middle of my road and the unit shows me on the pavement, chances are that it is the map that is slightly out, not that the satellite has me in the wrong place.

Re position accuracy, the manufacturers will not want to be blamed for you walking off a cliff as you relied on your sat nav.

If the speed accuracy is said by staff of police, military, aircraft industry and the likes of top motor sport teams. That does for me.

A sat nav guided weapon that can be put through a window and timed over great distance also does it for me. Joe publics sat navs now use very high tech chips and systems on the same systems as all sat nav.

The units use a clever trick where they know when the signal left the satellite to calculate the position and as various technical authorities accept the units as the most accurate speed calculator, why doubt!

Can't say I fully undestand how my speedo's work but I'm prepared to rely on them to keep me out of trouble.