GPS accuracy depends on how many satellites are visible and where they are in the sky. The more satellites, the better it is. Also, where they are in the sky is important. It is best if they are not grouped together in one part of the sky and also if they are not too low to the horizon.
As an example, I tracked my run this morning and there were times when GPS lock was lost. Note some of the distances between points were huge. Typically, they would be about 10m or so.
The satellite skyplot that I generated from the Trimble website show where the satellites were in the sky at the beginning of my run and the projected paths for the next 6 hours. There were 7 satellites visible but two were low in the sky and were setting. Four satellites are required for a 3D fix.
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