The FAA has issued a warning to all flights in the Southwest, especially Southern California, that during 6 days in the month of June, there will be GPS Interference Testing lasting for 6 hours a day that may interrupt flight navigation and controls in aircraft that rely heavily on GPS for flight stability controls, and it may affect simulcast radio systems as well.
Since modern trunking systems also rely on GPS signals, and that the notification lists interference down to the 50 foot level in parts of Northern California, it may be wise to notify system administrators to be on the lookout for irregularities and unexplained errors with their radio systems.
The testing will take place for 6 hours each day, from 09:30 to 15:30 hours, on these days:
7 JUN 2016
9 JUN 2016
21 JUN 2016
23 JUN 2016
28 JUN 2016
30 JUN 2016
You can view the FAA NOTAM here.