Suspected Spoofing
GPS spoofing is different from jamming in that spoofing signal mimics the true GPS signal and can be received and processed by the GPS receiver. If the GPS receiver is partially spoofed, only a portion of the tracked signals are affected. The receiver may be unable to track some or all GPS signals and may have ranging errors on other signals. If the GPS receiver is completely spoofed, all tracked signals being used by the receiver are affected by the spoofer. Spoofing may be suspected if any of the following occur:
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FMS Degradation (see previous section on degradation indications when GPS is lost).
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Sudden, erratic, or unexplained changes of aircraft location on the moving map.
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Unexpected autopilot commands or oscillations during straight and level flight.
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Unexpected SVT and forward looking terrain alerts.
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Ground track angle, track vector, or ground speed indication inconsistent with actual aircraft heading and true airspeed, accounting for expected winds.
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ADAHRS reverting to No-GPS Mode.
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Incorrect computed wind speed or direction.
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System messages “GPS[1/2] CHECK POSITION” may be displayed.
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Incorrect automatic system date or time update, or incorrect indication of date/time on GPS status pages.
If the aircraft is not within range of a suitable NAVAID, or if the only NAVAIDS within range have been excluded ('Disabled NAVAIDS' listed under NOTAM NAVAIDS), the pilot should use alternate navigation methods such as VOR, LOC/ILS, or an ATC-assigned heading, if possible, instead.
The pilot may also choose to disable SVT, and inhibit nuisance terrain alerts, if desired. SVT is discussed in the Flight Instruments Section. Traffic and Terrain alerting and discussed in the Hazard Avoidance Section.