GNSS Interference: Jamming and Spoofing

GNSS interference is any radio emission that degrades an aircraft's reception of navigation satellites. It takes two forms with very different consequences. Jamming drowns the weak satellite signals so that the receiver loses them, and navigation fails or degrades in a way that is usually announced. Spoofing feeds the receiver counterfeit signals, so that it computes a false position or a false time, possibly without any warning at all.

EASA reports a marked increase in jamming and spoofing, above all in regions around conflict zones, and because GNSS now feeds far more than the navigation display, its effects reach surveillance, terrain warning, data link and timing. This article covers the operational side: what interference does to the aircraft, how crews recognise it and what they fall back on. How GNSS works and the physics of its errors are covered in GNSS and GNSS errors and dilution of precision.

Why GNSS is vulnerable

Satellite signals reach the aircraft at very low strength, so a transmitter on the ground or in the air can overwhelm them within its line of sight. The FAA's Aeronautical Information Manual (AIM) lists unintentional sources: radars, microwave links, ionospheric effects, solar activity, multipath, satellite communications, GPS repeaters and even some systems on board the aircraft. These are generally local and intermittent. Faulty or misused GPS re-radiators, which rebroadcast satellite signals for aircraft maintenance work, have disrupted aircraft receivers and can produce unflagged false positions.

The growing concern is deliberate interference. Military receivers are protected against spoofing by the encrypted Y code; civil receivers use open signals whose structure is published, so the signals can be imitated. And GNSS is everywhere on the modern flight deck. The AIM points out that it serves communication, navigation, surveillance, safety systems and automation, so a single interference event can touch all of them at once, and it asks pilots to be prepared to operate without GPS.

Jamming

A jammer transmits noise, or a signal strong enough to swamp the satellites, on the GNSS frequencies. The receiver loses the satellites and the position is lost or flagged:

• a panel-mounted GPS of the kind fitted to light aircraft may revert to dead reckoning, displaying a position extrapolated from the last fix, groundspeed and track that drifts from the truth within minutes;
• on the A320 the crew sees GPS PRIMARY LOST, and navigation continues on the inertial references with radio updating;
• on the 737, if all GPS data are lost, the FMC position is determined by radio or inertial updating.

Because a jammer works by line of sight, its reach grows with the aircraft's height. The AIM's example of a US GPS testing NOTAM warns that GPS, including WAAS, GBAS and ADS-B, may be unavailable within 468 NM of the test site at FL400, shrinking to 327 NM at 50 ft above the ground. Jamming is at least honest: the crew knows that GNSS has gone, even if the knock-on effects on other systems take longer to appear.

Spoofing

A spoofer transmits counterfeit satellite signals, consistent with one another, so that the receiver computes the position and time the spoofer chooses. The result can look entirely plausible. Receiver autonomous integrity monitoring (RAIM) looks for one satellite range that disagrees with the rest; a spoofer that falsifies all of them consistently leaves nothing to disagree (see GNSS integrity and RAIM). Discussing faulty GPS re-radiators, whose effect it likens to spoofing, the AIM calls RAIM only partially effective against such disruption: the pilot may not notice, and ATC, seeing an aircraft where it should not be, may be the only means of detecting it.

A false position is dangerous in proportion to what is built on it. A hybrid GPS-inertial system that accepts the counterfeit data carries the error into the FMS position, the map and everything that uses them. A false time is just as insidious. Aircraft clocks may jump, and because data link messages are time-stamped a wrong clock can prevent the aircraft logging on to CPDLC; Airbus requires UTC to within 1 second for ATC data link (see controller-pilot data link communications). Effects can outlast the encounter, which is why the FAA recommends recording jamming and spoofing in the maintenance log so that any faults left behind are cleared.

Effects on aircraft systems

The AIM lists the general impacts of jamming and spoofing, and type documentation shows how they appear on particular aircraft:

Terrain warning deserves a closer look. The basic GPWS modes rely on radio altitude, air data and ILS deviation and are unaffected, but the look-ahead functions compare the aircraft's position with a terrain database, and only they depend on position accuracy (see GPWS and TAWS). On the A320, when the FMGC detects low navigation accuracy, the enhanced EGPWS modes are deactivated while basic modes 1 to 5 remain, and a TERR STBY memo appears when the position accuracy computed by the EGPWS is not sufficient for the terrain display and the terrain clearance floor. A spoofed but plausible position is worse, because nothing stands by: EASA has warned of crews reacting to false terrain pull-up warnings with high-rate climbs.

Surveillance suffers in the same way. On the A320 the latitude, longitude, horizontal integrity limit and groundspeed broadcast by ADS-B Out are all supplied by GPS, and on the 737 each GPS feeds the ADS-B position of its own transponder. Interference can therefore remove the aircraft from ADS-B displays or show it in the wrong place, and controllers may simply stop receiving its position messages (see ADS-B).

Recognising interference

The AIM's list of indications that GNSS is being disrupted is the core of most operators' guidance:

• changes in actual navigation performance;
• aircraft clock changes, such as an incorrect time;
• an incorrect FMS position, or a large shift in the displayed GPS position;
• position error warnings on the PFD or ND;
• other aircraft reporting clock problems or position errors, or requesting vectors.

Jamming and spoofing look different. Jamming announces itself through GPS flags and messages, a rising estimate of position error, integrity alerts and the loss of RNP capability. Spoofing tends to show as data that are present but wrong: a position that jumps or drifts away from raw data, a time that changes, a groundspeed or wind that does not fit, a terrain warning where there is no high ground.

The decisive test is independence. The FMS position is compared with raw data from VOR, DME and ILS, which do not use satellites, and, on types that display them, with the separate inertial and GPS positions. On the 737, for example, selecting POS on the EFIS control panel shows the IRS positions, the GPS positions and VOR bearing vectors on the map at the same time (see FMS navigation and position updating).

Crew actions and fall-back navigation

The first rule is unchanged: fly the aircraft, then navigate. The fall-back is conventional navigation, on ground-based aids such as VOR, DME, NDB and ILS and on the aircraft's inertial references, none of which depends on satellites. The AIM tells crews to follow the manufacturer's guidance for their type and avionics, verify the position with conventional navaids, assess which systems depend on GNSS, make sure the navaids needed for the route and approach are available, and be ready to revert to conventional procedures. In practice:

• Navigate on what remains. Without GNSS the FMS falls back on DME/DME and VOR/DME updating and on the inertial references. On the 737, GPS can be deselected on the NAV OPTIONS page so that a suspect receiver no longer updates the FMC. Without any radio updating, an inertial position drifts by a figure usually quoted as 1 to 2 NM per hour (see inertial navigation systems).
• Monitor accuracy. On an A320 approach without GPS PRIMARY, the crew checks the navigation accuracy on the PROG page; if it is LOW, at least one navigation display is set to ROSE LS or VOR so that raw data are in view.
• Tune and identify navaids manually. Autotuning relies on the FMS position and may choose the wrong stations if the aircraft has been spoofed. A navaid used for a cross-check is identified by its Morse code.
• Tell ATC early. Report the anomaly and any lost capability, such as RNP, and request vectors or an amended clearance.
• Choose a conventional approach. An ILS or VOR approach replaces an RNP approach that needs GNSS. RNAV 1 departures and arrivals may remain possible with DME/DME/IRU where the procedure allows it (see area navigation (RNAV)).

European procedures anticipate a loss of guidance during the approach itself. The French AIP requires the operator to define a contingency procedure for a missed approach caused by the loss or failure of GNSS guidance on a PBN approach, climbing to a safe altitude along an obstacle-free path, with the return for another approach then flown on conventional aids or under radar. At Nice, where IFR arrivals must with few exemptions be equipped for RNP APCH, a conventional VOR procedure remains published only as a back-up for unforeseen situations such as a GNSS failure in flight or GNSS interference.

View full schematic

The United States keeps a deliberate reserve. Under its VOR Minimum Operational Network (MON), the FAA retains enough VORs to give nearly continuous coverage at 5,000 ft above the ground outside the western US mountainous area, and to keep every aircraft over the contiguous United States within 100 NM of a MON airport with an ILS or VOR approach. Aircraft with DME/DME can instead continue RNAV through a GNSS outage. The FAA also requires IFR operators under Parts 121, 125, 129 and 135 to keep a non-GPS navigation capability, for example DME/DME, inertial or VOR for en route and terminal flight, and VOR and ILS for the final approach.

Reporting and planning

Before flight, the FAA recommends that operators know the locations where interference is likely, check NOTAMs, plan fuel contingencies, plan to use conventional navaids and approach procedures at the destination, and follow the equipment manufacturer's guidance. When a published GPS test is under way, ATC warns pilots who request or are cleared for a GPS approach that GPS may not be available and asks their intentions.

In flight, GNSS anomalies are reported to ATC promptly. US guidance adds a nuance for announced GPS tests: anomalies inside a NOTAMed test area are not reported in detail unless the crew needs assistance, the degradation is experienced outside the NOTAMed area, or there are unexpected consequences such as suspected spoofing or the failure of a system like TAWS.

A useful report gives the date and time; the position where the anomaly started and ended; heading, altitude, aircraft type and call sign; the avionics in use and the number of satellites tracked; what was observed and for how long; its operational consequences; and what was done about it. Pilot reports matter because ground units may know nothing of the problem. France, for example, notifies ICAO that its aerodrome control, aerodrome flight information and approach control units are not given information on the operational status of the GNSS service.

After flight, the event goes in the maintenance log and into the operator's occurrence reporting system (see safety culture and occurrence reporting); the FAA also collects reports through its online GPS anomaly reporting form.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.