Operations III: GPS Interference and Polar Navigation

GPS interference has gone from "war-zone news" to line routine: crews who have flown certain airspaces know the map suddenly drifting on the ND, an inexplicable PULL UP, jumping time predictions. This article packs the GPS-interference supplementary procedure (flight preparation → before the area → within → after → parking, five stages) and the FM/GPS POS DISAGREE decision tree into a memorable framework; the second half is another "beyond GPS" world — the polar region: the latitude band where the magnetic compass fails, the switch to a TRUE reference, and the operator's "expected polar phenomena."

1. Know the enemy — jamming and spoofing

Per FCOM PRO-NOR-SUP-GPSINT:

‐ GPS Jamming: Type of radio frequency interference that degrades the GPS reception capability. The GPS data used by the aircraft systems can be degraded or lost. ‐ GPS Spoofing: Type of radio frequency interference that makes the GPS signals erroneous, and that can result in erroneous GPS data used by the aircraft systems.

Tell the two words apart and you tell the danger levels apart: jamming = blindness (data gone, the system knows it is gone, the MMR integrity system raises its cards); spoofing = hallucination (data present and "valid," integrity monitoring may not catch it — a wrong position feeds straight into the FMS, EGPWS, ADS-B). A blind aircraft turns conservative; a hallucinating one flies wrong with confidence. Two lines from the official effect lists deserve emphasis. Per FCOM PRO-NOR-SUP-GPSINT:

The ADS-C and CPDLC should be considered inoperative due to erroneous time stamp on position report that may affect the controlled aircraft separations.

TAWS warnings may be considered cautionary during daylight VMC conditions provided that the cause of the warning can be identified immediately.

The latter is the manual's harshest authorisation for "PULL UP may not be pulled" — with no such option at night or in IMC (the full-climb discipline of the EGPWS article stands).

2. Reconnaissance — the GPS MONITOR page is the health report

Per FCOM PRO-NOR-SUP-GPSINT:

When the flight crew suspects GPS interference due to the cockpit effects, the flight crew can check the GPS data.

Look at three things: is the position dashed/absurd, is the MODE/SAT tracked count below 5 or zero, are altitude and time reasonable. The dashes rule. Per FCOM PRO-NOR-SUP-GPSINT:

In the absence of data available (dashed) in the GPS MONITOR page, the flight crew should consider the GPS capability of the affected equipment inoperative. Depending on the number of GPS impacted, the aircraft condition may no longer be compliant with the minimum navigation equipment required for RNP operations.

3. The five-stage procedure — one pillar action per stage

Flight preparation — four things to align. Per FCOM PRO-NOR-SUP-GPSINT:

‐ The possibility to identify the entry and exit points of the interference area ‐ The possible effects of GPS interferences on flight operations ‐ The availability of alternate routes ‐ The availability of routes, procedures and approaches that do not require GPS (e.g. ILS, VOR/DME) for the affected area

(The radio-nav "veterans" are recalled to duty here — they are the interference-area approach backup.) Two special actions with spoofing already seen on the ground: (1) GPS deselected but kept until after take-off. Per FCOM PRO-NOR-SUP-GPSINT:

Keep GPS deselected on ground and during takeoff to enable takeoff position update (Refer to DSC-22_20-10-20-10 Takeoff Update). This ensures a precise aircraft position during departure.

(2) Alignment against poisoning (the "IRS alignment based on GPS position" option is fitted — a fail-safe normally, a vulnerability under spoofing). Per FCOM PRO-NOR-SUP-GPSINT:

Erroneous GPS data may result in an incorrect alignment of the IRS. Before the alignment procedure, ask the maintenance personnel to de-energize the MMRs (pulling the associated circuit breakers).

(If the inertial system's "birth certificate" is signed by a false GPS, it is wrong throughout — better to return to the old manual-coordinate method.) Before the area — a time insurance: CLOCK → INT. Per FCOM PRO-NOR-SUP-GPSINT:

‐ If the clock was previously synchronized with the GNSS, the INT mode of the clock maintains all other ATC datalink applications for 24 h.

(Datalink reads the time stamp; let the clock run on its own and a false GPS cannot change your watch.) Within the area — three pillars. Per FCOM PRO-NOR-SUP-GPSINT:

The flight crew must regularly monitor the aircraft position using non-GPS references.

Suspected spoofing or FMS position drift → GPS DESELECT, whose three lines are the article's most important mechanism. Per FCOM PRO-NOR-SUP-GPSINT:

‐ FMS navigation modes revert to IRS/DME/DME, IRS/VOR/DME or IRS only ‐ The GPS deselection only affect the FMS position computation. Other aircraft systems will continue to use the GPS data if available (aircraft clock, TAWS, ADS-B out, etc.) ‐ The GPS deselection on one MCDU side applies to both FMS.

(So a false TAWS alert does not disappear after deselection — the EGPWS has its own GPS port — which introduces the third pillar.) Expected false TAWS alerts → the measure of TERR OFF. Per FCOM PRO-NOR-SUP-GPSINT:

In cruise above MSA, the flight crew can set GPWS TERR pb-sw to OFF. In other flight phases, the Operator must perform a safety assessment based on their knowledge of operations to adapt the procedures and pilot briefings to specific routes.

(The same bloodline as the 15 NM discipline: turning off the prophet is not the captain's call alone.) The flying: one reliable side of FMS → use its AP/FD; both wrong → HDG/TRK as the floor. Plus a diplomatic duty: notify ATC of the observed effects. After the area — three recovery acceptances. Per FCOM PRO-NOR-SUP-GPSINT:

‐ NAV mode with at least 5 satellites tracked (or at least 4 satellites tracked in SBAS mode) ‐ Altitude information consistent with the aircraft altitude. ‐ Latitude/longitude information consistent with the aircraft position.

Only after acceptance re-select GPS and restore the TERR/SYS keys; the clock returns to GPS only when GPS 1 recovers NAV mode (the clock listens to GPS 1 only). The EGPWS "old wound" guarded separately. Per FCOM PRO-NOR-SUP-GPSINT:

The GPWS predictive modes may be unreliable after exposure to erroneous GPS altitude. The GPWS basic modes remain fully operational below 5000ft AGL.

On approach, expecting NAV GPWS FAULT to recur and false predictive alerts → consider TERR OFF, and below 5000 ft ensure SYS ON — keep the veteran, dismiss the ailing prophet. Parking — against a wrongful conviction and for evidence. Per FCOM PRO-NOR-SUP-GPSINT:

In the case of GPS spoofing, if FMS position is affected, the POSITION MONITOR page may provide erroneous IRS drift resulting in undue ADIRU removals. The IRS MONITOR page provides data independent from GPS.

The flight crew must report any kind of interference encountered in flight as maintenance actions may be required before the next flight.

4. The FM/GPS POS DISAGREE decision tree

The trigger threshold first (the higher the latitude, the denser the meridians, so the longitude threshold widens). Per FCOM PRO-ABN-NAV:

This alert triggers when the FMS and GPS positions differ by more than: ‐ A longitude threshold that depends on the latitude: ‐ 0.5 min between 45 °N and 45 °S ‐ 0.7 min between latitudes 45 ° and 60 ° ‐ 1 min between latitudes 60 ° and 70 °. ‐ A latitude threshold of 0.5 min anywhere.

The QRH tree's three branches: climb/cruise/descent — check accuracy on the PROG page; estimated worse than required →

CONSIDER NAV MODE AND ND ARC/ROSE NAV

accuracy adequate → HDG/TRK + fly raw data; then POSITION MONITOR to adjudicate: one FM agreeing with its onside GPIRS → use that AP/FD; neither agreeing → GPS DESELECT + raw data. ILS/LOC approach —

NAV MODE: DO NOT USE

RNAV(GNSS)/RNP approach —

If visual references not sufficient: GO AROUND

VOR/NDB approach — HDG/TRK + raw data. The whole tree in one line: guidance follows "do you still trust the position" — trust the FM, use NAV; trust the beam, use ILS; trust neither, use raw data and go around. A cut on the spoofing-scenario approach discipline. Per FCOM PRO-NOR-SUP-GPSINT:

GLS, RNAV(GNSS) or RNAV(RNP) approaches should not be considered.

The FMS uses the GPS-based position as primary source, also for approaches such as VOR, DME or NDB.

("The station is old, the guidance is new" — without deselecting GPS, a VOR approach is a GPS approach.)

5. The polar region — a world where magnetic north fails

The domain in which magnetic heading is available is drawn by the magnetic-variation table (magnetic heading = true heading + table variation). With all three ADIRUs carrying the same table. Per FCOM LIM-NAV:

In NAV mode, the IR will not provide valid magnetic heading and magnetic track angle: ‐ North of 73 ° North, between 90 ° West and 120 ° West (magnetic polar region) ‐ North of 82 ° North ‐ South of 60 ° South.

When flying at latitudes beyond these limits, the TRUE reference must be selected.

(Configuration note: the DSC-34 description writes the western edge of the magnetic polar region as 92°W, while LIM writes 90°W; the operating limitation governs.) With one ADIRU carrying a different table version, the rule jumps from "change reference" to "prohibited." Per FCOM LIM-NAV:

In NAV mode, the IR will not provide valid magnetic heading and magnetic track angle: ‐ North of 60 ° North, between 30 ° West and 160 ° West ‐ North of 75 ° North ‐ South of 55 ° South.

Flying at latitudes beyond these limits is prohibited.

Why so severe? Different tables = three IRs resolving different magnetic headings at the same point, driving the comparison logic (NAV HDG DISCREPANCY) mad with no "who is right" — so the whole high latitude is ceded. This is why the dispatch check requires the magnetic-variation table version to match across the three units. The ground-alignment latitude limit is on the same page. Per FCOM LIM-NAV:

IR alignment in NAV mode is possible on ground only. Ground alignment of the IRS is possible in latitudes between 82 ° North and 82 ° South.

(The other half of "above 82° do not turn off the ADIRS": it is not just slow — it cannot align at all.) The reference switch is triggered by an ECAM caution. Per FCOM PRO-ABN-NAV:

This alert triggers when the aircraft enters a polar area. The crew must select true heading reference.

The magnetic heading is replaced by the true heading on EFIS and DDRMI .

(The ISIS NORTH REF blue light illuminates in step; the standby compass still points to magnetic north — the last "magnetic-world resident," used only as a rough reference in the polar region.)

6. Expected polar phenomena — the "normal abnormals"

The operator's polar procedures forewarn three "false alarms" that polar flight will meet, and the mechanism is worth stating in full. Act 1 — near the polar boundary: the three IRS switch to TRUE mode automatically but not simultaneously — for the few minutes of transition the three headings do not agree, so the comparison caution NAV HDG DISCREPANCY sounds; it self-clears once all three have switched. The handling word is "delay the ECAM action." Act 2 — directly over the pole: the three IRS cannot cross the pole in the same instant, so the heading flip from 360° to 180° happens one after another — again a few minutes of "argument," raising CHECK HDG on ND/PFD with the same NAV HDG DISCREPANCY; delay the ECAM action, and both clear within minutes. Act 3 — departing a true-north-referenced airport: a CHECK NORTH REF amber message on ND/MCDU is to be disregarded after engine start / before entering the terminal area (its appearance and the ND runway orientation depend on the airport's TRUE/MAG north-reference coding in the database). The common handling for all three is "delay the ECAM action" — these polar cautions are geometric and physical inevitabilities, not faults; recognise them so you do not take a real action against a self-clearing ECAM at 85°N. This page is also the seed of the MEL: it makes "operation inside / outside the polar region" a dispatch boundary (a failed NORTH REF pushbutton is dispatchable, but not into the polar region).

Key numbers

Self-test

[!note]- Q1. How do the cockpit "faces" of jamming and spoofing differ, and which may trigger no ECAM at all? Jamming = blindness (data lost, integrity flags). Spoofing = hallucination (data present and "valid"), which may pass integrity monitoring and trigger no ECAM — the more insidious.

[!note]- Q2. After GPS DESELECT the map stops drifting, but PULL UP sounds again — contradiction? Why? No. Deselect affects only the FMS position; the EGPWS keeps using its own GPS port, so a spoofed altitude still drives false predictive alerts. Hence the TERR-OFF pillar.

[!note]- Q3. In the area, FMS 1 agrees with its onside GPIRS, FMS 2 has drifted 3 NM — which AP, per which page? Use AP/FD on the reliable side (FMS 1), per POSITION MONITOR (QRH 23.06A).

[!note]- Q4. A VOR approach at a spoofed airport with FINAL APP guidance — what is wrong, and the right order? The FMS guidance still uses GPS-based position as primary, so it is a GPS approach in disguise. Deselect GPS first, then use the guidance (or fly raw data).

[!note]- Q5. The three recovery acceptances, and why the clock waits longer? NAV mode + ≥5 satellites + altitude and lat-long consistent. The clock returns to GPS only when GPS 1 specifically recovers NAV, because it listens to GPS 1.

[!note]- Q6. Same table on all three ADIRUs vs one different — which word changes the polar rule, and why so much? "Prohibited" vs "TRUE required." A different table makes the three IRs resolve different magnetic headings, so the comparison monitor loses credibility — the whole high latitude is ceded.

Key takeaways

References

• FCOM PRO-NOR-SUP-GPSINT — interference definitions, effect lists, five-stage procedure, deselect mechanism, TERR authority, recovery acceptance, IRS MONITOR.
• QRH 23.06A — FM/GPS POS DISAGREE decision tree.
• FCOM PRO-ABN-NAV — NAV EXTREME LATITUDE, DISAGREE thresholds.
• FCOM LIM-NAV — alignment latitude limit, dual magnetic-table regions, TRUE required and prohibited.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.