GPS

Please be more specific

Just to possibly remove this from the unanswered questions list, here's a good article to start with:

https://support.strava.com/hc/en-us/articles/216917707-Bad-GPS-Data
 

If you rather mean gps jamming, google says there were some gps disruptions e.g. in the Baltics area reported that day, you can check e.g. the website from this screenshot for another specific date & region: