GPS Drift

I don't know what the hell is going on but there has been a LOT of GPS drift recently. I set up DGPS with 3 sensors connected to my APRS station. I was shocked to see the amount of artificially introduced GPS errors on L1 and L2 channels.
I have noticed several times that it appears selective availability was used in my area.
Notice the the 'changes' in altitude as well as location 'change'.
This is a fixed APRS node with DGPS (3 receivers) positioned 32ft above the ground.