The declining accuracy of my Garmin FR210

I'd add 3% just to be safe

Today's run (street): 2.53 miles (Gmaps measured)

I'm not sure why, but my Garmin FR 210 has been under-performing lately. It was especially bad this morning. I generally run the same route every day at 4:00 AM and, after careful measurement, I know that the distance is exactly 2.53 miles. The Garmin's margin of error is generally -3%, which means that it under-counts to that degree fairly consistently. Since it is consistant, I accept that variance and correct for it in my pace calculation.

It's no mystery why this happens. Looking at my run captured in Garmin Connect, I can see that the watch will vectorize corners and straighten out curves. This is due to the capture time between GPS signals (approximately one sample per second). If the signal was captured continuously, the course would be displayed accurately, with no corners cut, etc. When it's cloudy, the signal can get interrupted. When that happens, the watch interpolates the distance between signals as a straight line. Enough of those and your accuracy really suffers.

Lately my FR210 has been under-reporting by about 5% and this morning it came in at -7%. It was cloudy, but clear enough to see some stars. My run could only be described as slow, and with the under-counted distance, my watch recorded my pace as glacial. Correcting for the true distance, it was still the slowest 4:00 AM run in memory.

I'm puzzled as to why my Garmin's GPS has become less accurate of late. Perhaps it's just going through a bad spell. The GPS accuracy tends to be better when I run a mostly straight course like at Bethpage. I'll see how it does tomorrow.