[jjwiseman]

I'm using data reported by aircraft equipped with ADS-B. Those aircraft broadcast their positions, and they include information about how much uncertainty they think is in their position estimate. When an aircraft that has been reporting low position uncertainty (< 1 nautical mile) starts reporting high uncertainty (> 4 nautical miles), it seems to correlate well with known areas of GPS interference, so that's what I use to make the map.

This paper describes in more detail some experiments using a GPS jammer and flying an aircraft around Edwards Air Force Base with ADS-B and what happens to the position reporting: https://web.stanford.edu/group/scpnt/gpslab/pubs/papers/Liu_...