Your Gps Speedo Is Not Correct

[vmail]

My speedo overeads by 3MPH all through the range (Checked against Garmin Street Pilot GPS)

That helps when "nudging" the speed limits.

Top speed????

Not known.............. as I have not yet had it on a track...but the accelerator pedal is not very far down at 70MPH ;)

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I have been doing some work on GPS satelites

the 10% speed difference is not correct at 100MPH the GPS calculation read the speed at 96.5MPH (3.5MPH difference)

with 2 sats its only possible to pin point your location by estimating

with 3 sats you can pin point the long at lat (road angle and other sat nav use this method)

with 4 or more sats same as 3 sats but you can also pin point your altitude (this is important for true speed)

I got the above result by reading in the NEMA string from the GPS mouse at real time and putting the data is a julian day formular

[Gareth B]

I have been doing some work on GPS satelites

Thought you'd been gone a while. Bet it was cold up there! :P

[daktari]

I have been doing some work on GPS satelites

Thought you'd been gone a while. Bet it was cold up there! :P

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PMSL.

Welcome back Vmail.

[Kelvyn]

With my iPaq running TomTom navigator 2 I found that my speedo was reading +5 mph at 70 ish. When the speed limiter hit the GPS said I was doing 112 mph but the speedo showed over 125 mph. Personally I'm more inclined to believe the GPS.

[vmail]

it all depends on the refresh rate of the data, tom tom is about 750ms, which would mean that there is some estimation involved, not sure to how many sats it locks on to

[GT4 BOOSTER]

weird topic but arnt speedos out more at the top end when youre breaking the law anyways ?????

my fuel gauge lets me know i'm over the speed limit.

[vmail]

the tests with the GPS speedo was not on the public road, works would not allow me to do that for insurance reasons

[GT4 BOOSTER]

oh of course , silly me

[Moffmo]

Tom tom locks on to 12... i had 7 or 6 most of the time....

Thing is nadim, how did you test it, becuase you dont know what your real speed was, so how could you test it against the incuratty of the speedo and GPS i gone crosseyed

[vmail]

two word matt, "julian days"

by intergrating the GPS loctions into the formula you can get your true speed

also you need to know the correct distance, to make it easy to understand, say town is your start point and town B is your destination, town B is 4 miles away from town A "crows fly". also town B's altitude is 3 miles higher than town A. this would mean if you traveled on the surface you would actually travel 5 miles(most GPS speedos dont use this method)

[SMW]

two word matt,  "julian days"

by intergrating the GPS loctions into the formula you can get your true speed

also you need to know the correct distance, to make it easy to understand, say town is your start point and town B is your destination, town B is 4 miles away from town A "crows fly". also town B's altitude is 3 miles higher than town A.  this would mean if you traveled on the surface you would actually travel 5 miles(most GPS speedos dont use this method)

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Is this the method you applied to get the 124 ?

[GT4 BOOSTER]

what alot of waffle , im going back to sleep .zzzzzzzzzzzzzzzzzzzzzz

[vmail]

no, for insurance reasons, I cant drive faster than 100MPH, but I can with a works car

[Moffmo]

right i undertstand......

But you do understand that it does refresh just under every secound.. so its gonna be more accurate than the speedo :D

[vmail]

I would have to disagree, neither the speedo or the GPS speedos on the market are accurate

[Kelvyn]

with 3 sats you can pin point the long at lat (road angle and other sat nav use this method)

with 4 or more sats same as 3 sats but you can also pin point your altitude (this is important for true speed)

Tom Tom generally seems to run around 6 sattelites so according to your original post the GPS speedo should be accurate. Certainly more accurate than the cars own speedo.

Even if it makes no allowance for altitude variations, in reality the gradient on any road you are likely to drive on will give such a small percentage error that you may as well ignore it. :D :D Of course if you drive off a cliff where if you have no altitude correction then start point A would have the same GPS location as end point B. In that case your GPS speedo would give you a speed of zero whereas in practice you would be travelling considerably faster. :D :D

[jaxx]

so you comparing pen and paper to deathly accurate ex-military designed and built umpteen million pound satellites?