The Boeing E-6 mercury is an airborne command post and communications relay platform. You can read more about it on Wikipedia. The E-6B missions often involve flying into Travis AFB, here in California. Occasionally the E-6B will be active somewhere in the Military Airband 225-400MHz with either PSK or FM modulated signals.
Shown below is a very strong PSK signal that I have confirmed on numerous occasions is emanating from the E-6B.
The E-6B uses the callsign GOTO FMS on Mode-S with full position data (so it can be tracked).