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Reading Topic: My Speedometer is off
I have been using handheld GPS for a few years now. I had a Honda Civic that would always indicate 2mph faster than the GPS. I even cross-checked a different handheld with similar result. Here is what I know from studying this topic.
The GPS equimpent you own is the limiting factor in the discrepancy, although I do believe that some speedometers are off a bit too. The variables that affects any GPS the most are the speed of light and you moving. Before I explain, I must remind you that there must be at LEAST three satellites within line-of-sight in order to triangulate your geoposition, direction, speed, altitude, time-to-go or whatever. Otherwise your GPS would not even work at all.
As "pchaves" mentioned, the military is indeed capable of more accurate satellite service than the public. This is due to more expensive and advanced GPS equipment which corrects for the errors I am will explain below. The last I heard, the military has GPS accuracy down to a sqare foot or so, while the public is limited to however many square METERS. Oh, did I mention that you get what you pay for too ?
GPS is made possible by light communication with the satellites. Those satellites orbit hundreds of miles high depending on where they are in relation to the horizon (on horizon would be much further than directly above). The amount of time it takes for your GPS to transmit its blip, then for the light to travel to all satellites involved (often as many as 9), those satellites to process and return the pulse of light, and your GPS to process the [up to] 9 incoming signals takes a few moments. Keep in mind, both you and the satellites are in motion so there will be inherent errors from this fact alone. The send/receive process between GPS and satellites is continous and very frequent.
Now, in that extra second of your GPS waiting for replies from all the satellites, you have moved a few extra feet. The satellites receive the signal thinking you are exactly where the signal was sent when in fact that point is slightly behind where you truly are. While the light returns from the satellite you move a little more, and the time delay compounds as the send/receive process continues. The result is the satellites and your GPS think that you are constantly a short distance behind your true position - hence showing a slower than actual speed.
You will notice this effect if you slam on your brakes...pay attention to the speed on the GPS. Even when you reach a complete stop, your speed will show 5 mph or whatever for a second longer. That's a perfect example of how long the send/receive process really takes.
Does this make any more sense now, or did I confuse the heck out of you guys? I hope it isn't too lengthy. See ya!
2004 Mazda 6s MTX