A Corporate pilot reported GPS interference.

Narrative

The GPS navigation system worked perfectly on leg one;ZZZ-ZZZ1. The ForeFlight tracking moving aircraft symbol; pilot and copilots iPads; both showed center of airways on leg one. On leg two; ZZZ1 -ZZZ; A few miles west of imperial (IMP) VOR; both the Captains and Copilots ForeFlight tracking symbol would vanish from our screens and reappear for a few seconds only to vanish again. This symbol is normally used to confirm position on airway or track. The Captain asked the Copilot to look at the J2 fixes and read them back to him in the order of progression heading east. The Captain verified the data in the UNS nav unit.At some point the aircraft very slightly to the south which pointed it toward the restricted area. The Copilot remarked that he could not see the aircraft position on the airway because the symbol was missing. Than the controller came on and said to turn to heading 040; which we did immediately. After passing GBN ; the aircraft symbol returned on the chart; and normal operation the balance of the flight.it seams as there was some form of interference. With the CPS signals in the area behind us.this interruption has never happened before in the years the pilot and copilot have been using ForeFlight EFBs.