2024-04 · NASA ASRS report 2115100
B747 flight crew reported GPS Spoofing resulted in a false GPWS alert on approach to LLBG.
Received a spurious GPWS Hard Warning: TERRAIN TERRAIN Pull Up!" on approach to LLBG. Conditions were day VMC with the runway in sight while being fully established on the ILS Localiser and Glideslope. The "GPWS Response" or "CFIT Escape Maneuver" was not performed for these reasons. Additionally; this threat was addressed thoroughly when briefing this arrival into LLBG due to known "GPS Spoofing" events. When identifying threats for this approach; our focus was on the high number of recent "GPS Spoofing" events in the LLBG area. "GPS Spoofing" being quite different to "GPS Jamming" or "GPS Blocking" was discussed; as were the different flight deck effects of each. With many ground-based navigation aids in the area; we elected to disable GPS updates to the FMC in Nicosia airspace (approx. 200NM from destination); with a strategy to more closely monitor the ANP (Actual Navigation Performance) of the aircraft in the FMC. As expected; this was adequate for the rest of the approach and landing; due to regular DME-DME updating of the FMC position. However; as stated in Manual; the "Look-Ahead Terrain Alerting System" utilizes a terrain database; coupled with the GPS position to generate alerts. Importantly; the system only reverts to IRU position "if GPS position is intermittently unavailable". This is where the distinction between "GPS Spoofing" and "GPS Jamming" comes into play; as the system does not recognize a "Spoofing" event as an intermittently erroneous GPS input. It simply applies what it thinks is the correct GPS position over the Terrain Database and generates an alert. In our event we quickly assessed the warning as "nuisance" for all these reasons; and elected to inhibit the alerts by pressing the GND PROX TERR OVRD switch. The approach was continued uneventfully in day VMC.Documenting these events helps disseminate information to other crews to enhance awareness. Collection of this data might help manufacturers formulate more specific guidance to crews on how to configure aircraft systems to better prepare for these events and mitigate the threats that they pose."
On short final to Runway 12 we received Terrain GPWS aural warning. We had Visual contact with the runway and visibility was unlimited so we continued to land. The landing was uneventful. During our briefing; we spoke about GPS spoofing and possible erroneous GPWS warnings. Given previous experiences as well as information from other crewmembers we understood this to be a distinct possibility. Weather was completely VMC with unlimited visibility. at approximately 100 feet above minimums; the GPWS issued a terrain aural warning; the pilot flying had visual contact with the runway and with the terrain. We determined it to be an erroneous warning; and continued safely with the landing. The FAA observer in the jump seat was also briefed before descent about this possibility.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.
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