I have an amateur built experimental airplane, and on my first flight, GPS wasn't working. It was working on the ground, so I figured a bad coax cable or something from the vibration. When I landed it started working again, but later in the day it wasn't. A few days later I found out that the nearby Air Force base was testing GPS jamming, and there was even a NOTAM about it.
There's a site that tracks GPS jamming: https://gpsjam.org/