My Forerunner is fairly accurate on speed for an entire run or a mile, etc., but if I look at it to see my speed right then, the numbers are all over the board.
I was talking to my engineer brother about powering up the watch and how sometimes on a clear day it can take 10 minutes just walking around on my driveway with no building or tree overhangs to get a reading. He explained that a GPS gets readings from more than one satellite so the best thing to do is to set it down and don't touch it because even the slightest movement could cause it to start over with another satellite. With that in mind, I don't expect much as far as instant readings, but over a distance it's more reliable.
Hope that helps.
Deb



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