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Global Satellite Navigation System Radio Frequency Interference

Signals from the Global Navigation Satellite System (GNSS) are one of the main inputs used for aircraft positioning or time reference for Communication, Navigation and Surveillance functions on-board most of the Airbus aircraft. Operators report an increasing number of events related to the loss of GNSS signals due to Radio Frequency Interference (RFI) during operations in some areas of the world. This article explains the causes of RFI, the effects on the aircraft systems and provides recommendations for flight and maintenance crews.   RADIO FREQUENCY INTERFERENCE (RFI) A low power signal sent from space The GPS signal is a low power signal. It is comparable to the power emitted by a 60W light-bulb located more than 20,000 km away from the surface of the earth. This means that the signal could easily be disturbed by any ground source located near an aircraft and emitting in the GPS L1 frequency band (1575.42 MHz +/-10 MHz), leading to the loss of GPS data GNSS Spoofing Some of the known RFI sources are reportedly capable of emitting signals that mimic GNSS signals. Objectives for such spoofing include providing GNSS positioning service within hangar with repeaters, preventing GNSS receivers to compute position over prohibited area or triggering geo-fencing responses as part of anti-drone measures. There are no reported events of GNSS spoofing leading to wrong aircraft position and timing on any modern aircraft to-date. However, Airbus constantly monitors the emerging threats and launched investigations to further evaluate GNSS spoofing threat and its possible consequences. EFFECTS ON AIRCRAFT SYSTEMS AND ASSOCIATED COCKPIT EFFECTS Impact on the aircraft position computation     GNSS RFI can cause the loss of GNSS position and timing. Even if GNSS plays a major role in the aircraft positioning system, Airbus aircraft are designed to be robust to GNSS signal loss. The use of other sources of data (IRS, VOR and DME) enables the aircraft systems to maintain a position computation capability. A loss of GNSS inputs does not lead to a map shift or to an erroneous position computation by the Flight Management Systems (FMS). In the case of a loss of GPS signal, the FMS switches from the mixed GPS/IRS position to an IRS-DME/DME position or IRS-VOR/DME or pure IRS in order of priority. Refer to the FCOM description of the FMS position computation for more detail.  MAINTENANCE CONSIDERATIONS     At the end of a flight impacted by a transient loss of GNSS, a confirmation should be done to make sure that the effects encountered were due to RFI and not to a system or equipment malfunction. Transient loss of GNSS in an area with known RFI At the end of a flight affected by transient GNSS loss within an area with known RFI, Airbus recommends that maintenance personnel reset the system and test both Multi-Mode Receivers (MMR). To ensure that there was no system failures, Airbus also recommends a system test of any equipment affected by a loss of GNSS signal based on the cockpit effects observed during the flight. Should any system test fail, maintenance personnel must perform troubleshooting in accordance with the associated Trouble-Shooting Manual (TSM) task. Refer to the “GNSS loss and GNSS interference on Airbus aircraft” ISI article ref 34.36.00049 available on the Airbus World portal for more details and a list of the related AMM/MP tasks for system tests. What if the interference is still present on ground? It the GNSS is still impacted by RFI on ground, the aircraft should be moved out of the RFI area. A dispatch under MEL conditions should be considered if this is not possible to do so. Transient loss of GNSS in areas not known for RFI At the end of a flight affected with transient GNSS loss within an area without known RFI issues, Airbus recommends that maintenance personnel confirm the root cause of the GNSS loss by studying all potential sources: aircraft system failure, GNSS constellation anomaly, environment masking, multipath or space weather events such as ionospheric scintillation. When all these potential causes are eliminated, RFI can be suspected. In this case, aircraft data should be sent to Airbus for further investigation.