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Noisy GPS day doubled length of Ultratrac activity

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Had another one of those absurd days where my FR255 had really poor quality GPS fixes and over estimated distance.

I walked the same course three days in a row using Ultratrac. It is very close to 3 miles and the first two days did well enough, measuring roughly 2.9 miles with little visible deviation in the map trace.

The third day, I got 1 mile auto-lap alerts way too soon and it ended up measuring over 6 miles. The trace even on the watch screen shows crazy deviations. Like a drawing of the map from someone suffering severe tremors/seizures. Lots of back-and-forth and lateral scribbles.

These were in suburban SF Bay Area in California, so nowhere near any contested zone where you expect GPS denial etc. I wonder what is the most likely explanation for such huge day to day difference in quality...

FWIW, I was doing these to assure myself all was well with the watch after updating the firmware over the weekend. Since it worked normally the first two times, I doubt it has anything to do with the firmware update.

  • +1

    UltraTrac Mode is designed to extend battery life by reducing the GPS recording rate, and as a result, track detail and distance accuracy can be reduced compared to standard GPS recording. For additional information, visit Setting a Garmin Watch to UltraTrac Mode for Longer Battery Life

    GPS drift and other factors can also affect GPS signal quality and impact recorded tracks and distances, as explained here: Top FAQs About GPS Distance, Speed, and Pace Accuracy With Garmin Devices

    When GPS positions become unusually noisy, the reduced sampling rate used in UltraTrac Mode can increase the effect on the recorded activity. The map behavior you described is consistent with poor GPS positioning during the recording.
  • 0 in reply to Garmin - KS

    I'm aware of these basics and "theory".

    To be more explicit, I am wondering if anybody knows what in practice leads to such day-to-day qualitative change in the amount of GPS drift or inaccuracy for the individual location fixes.

    I walked the same route each day, with the same houses, yards, tree cover, etc. None of the explanations like urban canyons etc can account for the difference between one day and the next.

    And, the error seemed consistent through the entire walk, with 1 mile auto laps alerting steadily. Not some individual glitch or local source of interference like a house or vehicle.

    Is there some kind of global GPS jamming or spoofing happening these days? Or is there some other more natural explanation? The bad results was recorded yesterday evening 7/8 in local California time.