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Speed accuracy on GPS systems: Navman, Road Angel

Discussion in 'A4/A4 cabriolet/S4 forum(B6 chassis)' started by ashleyminors, Dec 28, 2004.

  1. ashleyminors
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    ashleyminors New Member

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    [Dec 28, 2004]
    Perhaps not really an item for this post, but its the section of the site I use most often, so anyway:

    I've just bought a Navman 510. (Awesome by the way) When travelling at any speed, the Navman speed always shows 3 mph slower than the actual speed I am travelling, as indicated by my car's speedo.

    I recently noticed the same thing when travelling in a taxi. The guy had a Road Angel and this showed approx 6 mph slower than the car's speedo.

    I could understand a bit of intolerance when speeding up or slowing down because it must take a second or so for the GPS system to relay back to the satellite, but when travelling at a fixed speed, i.e. with cruise on, why is there a difference? Which is correct, car or Nav system?

    I would have thought that the car's speedo is more accurate only because of the distance the satellites are from the Navigation system, and I assume potential inaccuracies. /ubbthreads/images/graemlins/confused.gif

    Has anybody ever tried for instance measuring their cars speed against a radar gun or any other system for accuracy??

    Just a topic for conversation, but would be interested for your thoughts.
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  3. imported_GezzerS4
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    imported_GezzerS4 Guest

    [Dec 29, 2004]
    The speed showing on the Nav equipment via GPS is 100% Near as makes no difference when travelling at a constant speed, there is a slight delay when changing speed, due to calculations and signal travel, the area where GPS is less accurate is in determining Altitude due to satellite position with regards to triangulation calculations, but the systems we use in our cars do not use this anyway, but it could in theory distort your Speed over Ground (SOG) if you are descending or climbing a long steep hill at high speed, more sophisicated systems used in high end commercial and military uses take this into account but for our purposes ignore it.

    /ubbthreads/images/graemlins/crazy.gif
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  4. scoTTy
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    scoTTy Active Member

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    [Dec 29, 2004]
    My S4 is always within 1-2mph of the gps.
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  5. a4sline190
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    a4sline190 Member

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    [Dec 29, 2004]
    got the tom tom go and is around 1-2 mph slower than speedo, guess thats pretty accurate as all speedos generally over estimate speed to allow for a margin of error.
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