Date: 201808
Local time of day: 0601-1200
Location: ZLC.ARTCC, UT
Aircraft: Citation Excel (C560XL)
Operator: Corporate
Operating under: Part 91
Flight plan: IFR
Mission: Passenger
Flight phase: Initial Approach
Airspace: Class E ZLC
ATC / advisory: Center ZLC; Tower SUN
Reporter organization: Government
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Human-Machine Interface; Situational Awareness
Anomaly: ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Published Material / Policy; Inflight Event / Encounter CFTT / CFIT
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance
Contributing factors: Airspace Structure; ATC Equipment / Nav Facility / Buildings; Company Policy
Primary problem: Company Policy

Narrative: During this time there was widespread GPS jamming going on. Almost every aircraft was reporting with GPS outages. Two other SUN arrivals had advised GPS malfunctions; but reported their GPS back online prior to the initial approach fix. They flew the RNAV approach from PRESN without incident. We contacted SUN Tower after the first approach to make sure they were unaffected and the tower said the pilot didn't report any issues. When Aircraft X was approximately 4 miles south of the PRESN intersection the pilot reported his GPS capabilities had returned. The R-Side cleared the aircraft to cross PRESN at or above 9;000; cleared RNAV approach.After transferring communication to SUN Tower; [R-Side] noticed the aircraft had taken a more easterly turn than we normally see on the approach. With the abundance of smoke in the area; and the TFR very near the RNAV track; we wondered if the tower had broken off the approach; or if something different was happening. I called the tower and told them Aircraft X was off course; and if they knew what was going on. They said he had just reported over PRESN at 11;000. The RADAR showed that Aircraft X was about 12nm North West of the PRESN intersection; nearing (about 3 miles from) a 10;900 TAV mode C indicating 10;700. [R-Side] told me to issue a turn to the south. I issued; to the tower; a 155 heading and 10;000 altitude. We had traffic south of Aircraft X at 11;000. The lowest altitude we saw Aircraft X was 9;600 in either a 9;000 or an 8;000 TAV; the turn seemed to be timely enough to keep him out of the 10;900. Had [R-Side] not noticed; that flight crew and the passengers would be dead. I have no doubt. The military must practice - I understand. GPS jamming (or 'Testing' as they call it) is part of that. What I hear; third party; is that the military doesn't believe it affects civilian aircraft to a noticeable extent. They need to understand that it does. That the workload is exponentially increased with this type of exercise. We're willing to help as much as we can - but there's a limit; where loss of life can happen because ATC and a flight crew believe their equipment are working as intended; but are in fact leading them into the side of the mountain with numerous aircraft active. That's a pretty reasonable limit; and I think we could actually pull that line back a bit.




Synopsis: ZLC Controller reported an aircraft flew off course due to GPS jamming and went below the minimum terrain clearance altitudes.