Quote:
Originally Posted by bweybright
Just so you are aware, GPS is government controlled satellite navigation supported for military use. Therefore the signal, hence accuracy can vary significantly from day to day, hour to hour, etc. I don't know that I would expect my GPS to be more accurate than my vehicle on any given day.
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Quote:
Originally Posted by avanti
This is not accurate. The feature you describe (called "Selective Availability") is technically possible, but was discontinued in May of 2000 at the direction of President Clinton. The next generation of GPS will not even have this feature.
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Quote:
Originally Posted by LB_3
Position accuracy is irrelevant in determining speed which is calculated from the Doppler shift in the GPS carrier wave.
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Mostly but not entirely true. A GPS computes its position several times a second. When you first fire up the GPS while your vehicle is at a standstill in Park, the GPS is calculating the same position over and over, and can get a very accurate fix. When you start moving, your previous position versus your new position is used in the speed computation - but so is the doppler shift from the movement of the various satellites it's tracking. And every time you come to a stoplight or a stop sign, the GPS again has a chance to calculate the same position over and over and get a more accurate fix.
The speed indicator on your automotive GPS will be accurate to about half a mile per hour, more accurate if you're on flat terrain so there's no real elevation change factored in.
For the sake of safety— and for moving-violation avoidance— you should always assume that if you see multiple speed indications— analog dial, digital readout in the message center, and/or GPS— whichever one shows the highest speed is right. The highest indicated speed may not actually
be right, but assume it is and drive accordingly.
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