Date: 201704
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Flight conditions: VMC
Light: Daylight
Aircraft: Embraer Phenom 100
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Aircraft component: GPS & Other Satellite Navigation
Component problem: Malfunctioning
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Human factors: Distraction; Confusion; Communication Breakdown; Situational Awareness; Workload; Troubleshooting
Second reporter function: First Officer; Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Other / Unknown
Detector: Automation Aircraft RA; Person Flight Crew
When detected: In-flight
Result: Flight Crew Became Reoriented; Flight Crew Returned To Clearance; Flight Crew Took Evasive Action
Contributing factors: Aircraft; Human Factors
Primary problem: Aircraft

Narrative: On a flight to ZZZ we had been cleared to descend from FL380 to FL360. As soon as we leveled at FL360 we began to have GPS issues. The CAS message displayed 'both on GPS 1'. Shortly thereafter we completely lost GPS signal. The autopilot reverted to pitch and roll mode. As the Pilot Flying (PF) was resetting the autopilot to altitude hold and heading mode ATC gave us a traffic call. The call was; 'Traffic at 12 o'clock at FL370 and FL350'. The PF (busy resetting the autopilot) did not hear the full radio call; only the altitudes. The Pilot monitoring (PM) thought he heard; 'Traffic at 12 o'clock at FL370; descend to FL350'. The PM told the PF to start the descent to FL350. A descent was initiated and almost immediately the PF spotted the traffic at FL350. The PF immediately commanded the autopilot to climb back to FL360. As we leveled at FL360 the B737 at FL350 reported that they received a TCAS RA; and reported that they showed that we were 400' low. ATC asked us to verify our altitude. We told them that we were level at FL360; but that we were having some GPS/autopilot issues we were working on. Controller advised us that they never saw us leave FL360. Several minutes later GPS signal was restored; and the rest of the flight was completed without incident.Moving forward; more attention will be given to verifying clearances during high workload times. Additionally; any time both crew members do not hear the full clearance; we will ask ATC for clarification.


Second narrative: After leveling off at FL360 we received a GPS 1 alert. Shortly after the first alert almost simultaneously we received a full GPS outage and autopilot kicked into roll and pitch mode. While monitoring the GPS page and programming the Automated Flight Control System (AFCS) panel ATC gave the crew traffic advisories for traffic at FL350 and FL370 both opposite direction. Both crew members were immersed resolving the issue the PM thought he heard descend to 350 for traffic at 370. Very shortly after starting the descent we spotted the traffic at FL350 realized that we had misunderstood the controller and immediately returned to FL360. The traffic at FL350 reported to ATC that he had received a RA. ATC reported that they never received any indication that we had vacated our assigned altitude. Shortly after this exchange the GPS reception returned back to normal. Once in a non-critical phase of flight the crew reflected on the event while it was fresh on our minds and discussed the things that could have been done differently. Most importantly confirm any instructions that the crew is unsure of.


Synopsis: An EMB-500 flight crew reported their Crew Alerting System (CAS) alerted GPS OUTAGE at which time the autopilot reverted to PITCH and ROLL. The crew misunderstood ATC and mistakenly began a descent to FL350 while dealing with the autopilot but TCAS alerted RA at which point they returned to FL350. Shortly thereafter; GPS returned to normal.