A320 Radio Navigation and GPS

The Airbus A320 navigates on a blend of sensors. Two GPS receivers and three air data and inertial reference units (ADIRUs) provide the position the flight management system flies, while VOR, DME, ADF and ILS receivers supply the conventional radio navigation that the crew still uses for approaches, for raw-data cross-checks and as a fallback. In normal operation the flight management and guidance computers (FMGCs) tune the receivers themselves.

This article describes the receivers and how they are tuned, displayed and monitored. The descriptions follow the Airbus A320 flight crew operating manual (FCOM); equipment varies between aircraft, which may have one or two ADFs, GPS receivers in separate units or in the multi-mode receivers, and GLS or MLS as options. The flight management side is covered in A320 FMGS and MCDU, the inertial side in A320 ADIRS and standby instruments, and the navaids themselves in VOR, DME, NDB and ADF and instrument landing system.

Navigation sensors on the A320

The antennas sit where each system needs them. The GPS antennas are on the upper forward fuselage, with an unobstructed view of the sky; the DME 1 and DME 2 antennas are under the forward fuselage; the VOR antenna is at the tip of the fin. The radio altimeters, with their own antennas under the rear fuselage, are part of the same ATA chapter but serve height, not position.

VOR, DME and ADF receivers

The two VOR receivers feed the NDs and the DDRMI, a radio magnetic indicator with DME distance counters. On each EFIS control panel two ADF/VOR switches put a VOR 1 or ADF 1 pointer and a VOR 2 or ADF 2 pointer on that pilot's ND; ROSE VOR mode presents the VOR raw data around a full compass rose. The DDRMI shows VOR 1 and VOR 2 bearings only while its heading signal is valid, and with the ATT HDG selector at NORM that heading comes from ADIRU 1, which also feeds the captain's PFD and ND. Its dashed pointer is VOR 1 or ADF 1 and its double pointer VOR 2 or ADF 2.

The two DMEs need no frequency of their own. Each is set automatically to the channel paired with the frequency selected on the VOR, ILS, GLS or MLS; under the frequency pairing plan, as the AIM notes, automatic DME selection assures azimuth and distance from a common source. Where the distance appears depends on its partner: DME associated with the ILS, GLS or MLS is shown on the PFDs, and DME associated with a VOR on the NDs and the DDRMI. The DDRMI counters read whole nautical miles beyond 20 NM and tenths within it.

An aircraft has one or two ADFs. On the radio management panel the BFO (beat frequency oscillator) key helps identify an NDB: with most ADFs the identifier is heard with the BFO on, with some only with it off. Identification matters more than it seems, because the FMGC, not the pilot, chose the navaid; the AIM notes that an aircraft spoofed off its path may find that autotuning no longer selects the nearby navaid. On the audio control panel, the VOICE key filters the VOR and ADF identifiers out of the audio when they are not wanted.

ILS and the multi-mode receiver

On aircraft fitted with multi-mode receivers (MMRs), each MMR contains the landing system receivers: ILS 1, and GLS 1 and MLS 1 where installed, in MMR 1; ILS 2, GLS 2 and MLS 2 in MMR 2. The MMRs can also house the GPS receivers, GPS 1 in MMR 1 and GPS 2 in MMR 2.

The ILS displays are crossed. PFD 1 and ND 2 show ILS 1; PFD 2 and ND 1 show ILS 2. With an ND in ROSE LS mode, each pilot therefore sees both receivers at once, one on the PFD and the other on the ND, which makes a disagreement between them easy to spot.

Two further details are often examined. To hear the audio of the DME paired with an ILS, the LS pushbutton on the EFIS control panel must also be selected. And the flight director warns of lost guidance: if the MMR cannot compute the vertical deviation above 100 ft RA, the pitch bar flashes; if it cannot compute the lateral deviation above 15 ft RA, the roll bar flashes.

The localizer also serves outside the approach. Airbus lists it among the aids for confirming the takeoff runway, since the localizer deviation should be centered after line-up. On an autoland the PF checks at 350 ft RA that LAND is engaged and that the ILS course on the PFD matches the runway: if the course pointer and the runway track differ by more than 5°, the crew go around, or land manually if the visual references are sufficient. How the AP/FD flies the beams, and the FMS-based FLS function used for other approaches, are described in A320 auto flight: approach, landing and go-around.

Navaid auto-tuning and manual tuning

The A320 has three tuning modes: automatic, manual and back-up.

• Automatic tuning (navaid auto-tuning). In normal operation each FMGC tunes navaids automatically, controlling its own side's receivers. If one FMGC fails, the remaining one controls the receivers of both sides.
• Manual tuning. The crew use the MCDU, on the RAD NAV page, to override the automatic choice and select a particular navaid for display. This does not affect the automatic function of the FMGC, which continues unaffected.
• Back-up tuning. If both FMGCs fail, navaids are tuned on the radio management panels, as described below.

Manual tuning is how the crew make sure the right raw data is in front of them. When an approach is based on radio navaids and GPS PRIMARY is not available, the FCOM requires the reference navaid and the corresponding airborne receiver to be serviceable, tuned and monitored throughout the approach. Selecting the approach VOR or NDB on the RAD NAV page, and identifying it, meets that requirement without affecting the FMGC's automatic tuning.

RMP radio navigation back-up

The radio management panels (RMPs) are primarily communication tuning panels, two on the pedestal and a third overhead, interconnected so that each reflects the selections made on the others. They also back up the FMGCs for navigation. The guarded NAV key engages RMP radio navigation back-up: it takes control of the VOR, ILS, GLS, MLS and ADF receivers away from the FMGCs and gives it to the RMPs, and its green light comes on. Pressing it again returns control to the FMGCs.

The rules of the back-up mode are typical exam material:

• selecting NAV on one RMP removes navaid tuning from both FMGCs, while the communication radios remain selectable as usual;
• the captain's RMP controls VOR 1 and ADF 1;
• either RMP can tune the ILS, GLS or MLS, provided STBY NAV is selected on RMP 1 and RMP 2;
• RMP 3 plays no part: its NAV key has no effect;
• the STBY/CRS window shows a course as well as a standby frequency, and the transfer key swaps the active and standby values, tuning the receiver to the new active frequency;
• when an ILS/DME, GLS/DME or MLS/DME is tuned from an RMP, the PFDs do not display the DME distance;
• in the emergency electrical configuration only RMP 1 is powered.

The panels themselves are described in A320 communications and datalink, and the MCDU back-up navigation function that keeps a flight plan when both FMGCs fail in A320 FMGS and MCDU.

GPS and the GPIRS hybrid position

The A320 FCOM describes GPS as a satellite radio navigation aid based on 24 satellites broadcasting navigation data. The two receivers, separate GPS sensor units (GPSSUs) or part of the MMRs, process the satellite signals and pass the result to the ADIRUs. Each ADIRU then computes a GPIRS position, the GPS/IRS hybrid, which the FMGCs use as their position.

The wiring is fixed: GPS 1 supplies ADIRU 1 and ADIRU 3, and GPS 2 supplies ADIRU 2. If one receiver fails, all three ADIRUs use the one that remains. When the FMGCs are using the hybrid position the ND and MCDU show GPS PRIMARY, and when they are not, GPS PRIMARY LOST. For RNP 1, RNP 2 and RNP 4 operations and for RNAV (GNSS) and RNAV (RNP) approaches, the integrity monitoring (RAIM/AIME) has been demonstrated to be available 100% of the time with 24 healthy satellites.

GPS feeds more than the FMGCs. The ADS-B OUT function transmits the latitude, longitude, horizontal integrity limit and ground speed supplied by GPS, and when all three ADRs are switched off, the PFDs show the backup speed scale and GPS altitude in place of the barometric altitude. A GPS fault or jamming therefore reaches surveillance and the GPS altitude shown with the backup speed scale as well as navigation (see GNSS interference, jamming and spoofing).

The inertial units start from a known position. Airbus recommends gate coordinates for the manual initialization of the IRS position; AIP France, for example, lists stand coordinates as INS checkpoints in AD 2.8 (see aerodrome charts and airport diagrams).

Navigation accuracy and display

The FMGS estimates the error of its own position, the estimated position error, and compares it with the accuracy required for the phase of flight: NAV ACCUR DOWNGRAD appears when the estimate exceeds the requirement, and the MCDU PROG page shows the accuracy as HIGH or LOW. The general principle is described in FMS navigation and position updating.

GPS PRIMARY simplifies the monitoring:

• with GPS PRIMARY available, no navigation accuracy monitoring is required on approach;
• with GPS PRIMARY LOST, or no GPS installed, the PM checks on the PROG page that the accuracy suits the phase of flight; with the appropriate RNP checked or entered and HIGH accuracy displayed, the navigation remains good enough for RNP operations;
• with LOW accuracy on approach, at least one ND must be in ROSE LS or ROSE VOR, depending on the approach, so that raw data is on view;
• an RNAV (GNSS) approach may be flown only with GPS PRIMARY available.

With GPS PRIMARY, the FCOM limitations give a navigation accuracy of 0.3 NM on approach; the full table, with the en-route and terminal values, is in A320 FMGS and MCDU. GPS PRIMARY also enables the enhanced localizer capture, a pre-capture zone beyond 2.3 dots of localizer deviation, used when the intercept angle lies between 25° and 115° and a single-turn capture is possible. When the FMGC detects low navigation accuracy, the enhanced terrain modes of the EGPWS are deactivated, while the five basic GPWS modes remain (see A320 EGPWS and weather radar).

The ND turns these sensors into a picture in five modes: ROSE LS, with the landing system deviations from the opposite-side receiver; ROSE VOR, with a VOR course display; ROSE NAV and ARC, the FMS map with bearing pointers overlaid on demand; and PLAN. A sound habit follows from the architecture: the FMS map shows where the FMGC believes the aircraft is, and the raw data shows where the navaids say it is. Keeping one ND on raw data during a navaid-based approach, and comparing both ILS receivers across the PFD and ND, is how the crew catch the rare case in which the two disagree (see A320 EIS displays and EFIS controls).

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.