How accurate is my GPS?
Errors arise from Noise, Bias and Blunders.
Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter).
Bias errors are from selective availability (deliberate ~100m random errors – currently turned off) and:
Uncorrected S/V clock errors (1 meter)
Ephemeris errors (1 meter)
Unmodeled tropospheric delay (1 meter)
Unmodeled ionospheric delay (10 meters) (Use of L2 reduces this!)
Blunders are human errors: satellite control. Using wrong geodetic datum can give 100m+ errors.
The net result is absent blunders and S/A, the range to a given S/V can be measured to ~15 meters.