GPS readings are based on alot of things, but primarily the signal strength. It takes a minimum of 2 satellites to gather enough information for a position. But the accuracy would be off by hundreds of meters at best. Most civilian units default to reject anything less than 3 satellites, so positioning accuracy is increased, getting down to several meters. Fortunately for us, there are currently 22 public access GPS satellites in geosynchronous orbit that we all can use. At most times of the day, provided the elevation mask allows it, there are 12 satellites that are viewable by any GPS unit. This gets our accuracy down to sub-meter, and depending on where we are, can be within just a couple of inches. There is a thing called PDOP, that stands for Position Dillusion Of Precision, which basically means the signal can be skewed by things like tall reflective buildings, and things that give off RF such as power lines or sub stations. This doesn't take into account the standard deviation that our Government requires all satellites to have. This helps keep the bad guys from just programming bad things to go places we don't want them. But, they do not mess with the timeclock setting that gives us our speed reading.
So, this long winded response is just a way to say that the speed accuracy is good to within one or two tenths of a mile an hour at 60 miles an hour. I have verified mine against radar and marked distance/time method and it is generally .12 miles/hour faster at 60 miles an hour.
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Subject: gps/mph? 
4/29/2010, 9:32 pm
