GPS, the MMR, and GPIRS Hybrid Position

The IR of the previous article lives on integration, and its error grows with time. GPS lives on satellites: its error does not grow with time, but it can at any moment be blocked, jammed, or spoofed. The design answer is to marry the two — GPIRS hybrid position: inertial provides the skeleton, satellite provides the calibration, the hybrid solution is computed by the ADIRU itself, and the FMGEC takes it to fix position.

This article covers the whole division of household chores: where the GPS receiver lives (the MMR), how many satellites it tracks, how it catches a lying satellite (integrity), how it pairs with the three ADIRUs, how it re-marries after a single failure, and who reads the "autonomous" solution versus the "hybrid" one.

1. GPS — a 24-satellite public service, two receivers' private subscription

Per FCOM DSC-34-10-15-10:

The Global Positioning System (GPS) is a satellite based radio navigation aid. Worldwide 24 satellites broadcast accurate navigation data that the aircraft can use for the precise determination of its position.

The aircraft has two independent GPS receivers.

Where the receivers live is a feature of this configuration. Per FCOM DSC-34-10-15-10:

Is integrated in the Multi Mode Receiver (MMR). The GPS1 receiver in MMR1, and the GPS2 receiver in MMR2.

The MMR (Multi Mode Receiver) is one box with three tenants: an ILS receiver, a GPS receiver, and a common subsystem (system processor / power supply); GLS is only a provisioned room (ILS article). The "ears for landing" and the "eyes for position" share one box, which decides two later facts: GPS interference can lock the LS function out with it (the residual NAV LS FAULT of the interference article), and because MMR 1 is fed from the static-inverter bus, GPS 1 survives on emergency power. The antenna is active. Per AMM 34-36-00:

The MMR receives RF signals through an active GPS antenna (preamplifier implemented within the antenna).

Satellite signals are a whisper by the time they reach the ground; the amplifier must sit right at the ear so coax loss does not eat the signal-to-noise ratio.

2. One receiver, two faces — the autonomous and the hybrid solution

The IR's GPS partition supplies both solutions at once. Per AMM 34-14-00:

The term hybrid is used to indicate that the navigation solution results from combining the GPS, Inertial and Air Data altitude data. The autonomous navigation solution denotes a solution derived solely from the MultiMode Receiver (MMR) using only satellite data.

For each type of data, hybrid and autonomous, the software provides: - the position - the velocities (Ground, N/S, E/W) - the accuracy parameters - the integrity parameters.

Note the hybrid recipe includes air-data altitude — barometric altitude goes into the pot too (the foreshadowing of "baro-aided" integrity monitoring, §5). The two solutions have distinct destinations. Per AMM 34-36-00:

The hybrid GPIR 1(2) data is used by the FMGEC 1(2) for position fixing purposes. The pure GPS data is used for display on the MCDU 1 and 2.

So the GPS MONITOR page shows the autonomous solution (raw satellite data: MODE/SAT, lat/long, GPS ALT), while the FM fixes position with the hybrid solution — normally almost identical, but under spoofing one may be true and the other false, which is exactly why the interference procedure keeps cross-checking two pages (GPS MONITOR vs POSITION MONITOR). The FCOM entry point. Per FCOM DSC-34-10-15-10:

GPS information are available on the FMS – GPS Monitor Page.

3. The GPIRS hybridisation mechanism — an 18-state Kalman filter

Per FCOM DSC-34-10-15-10:

The GPSSU or the MMR processes the received data, and transfers them to the ADIRU. Then the ADIRU performs the GP-IRS hybrid position calculation. The FMGEC uses this hybrid position.

Hybridisation happens inside the ADIRU, not inside the FM — the FM is only the consumer (the FM chapter covers how it ranks GPIRS, radio, and pure inertial). The filter's identity card. Per AMM 34-14-00:

At the end of GPIR NAV mode ground align processing, an 18-state PR/PD (Pseudo Range/Delta Range) mechanized Kalman filter is engaged to estimate the system error states. The hybrid solution is then corrected by removing these errors estimated from Kalman filter.

In pilot language: the filter does not "take an average" — it keeps an error ledger for the inertial system (18 error states: position, velocity, misalignment angles, gyro/accel biases…), settling the account continuously with the GPS pseudo-range observations. Three payoffs: (1) on a brief satellite dropout the ledger persists, so the hybrid solution dead-reckons on inertial without a break; (2) the inertial drift is continuously "written off," so position stops diverging with time on long routes; (3) it consumes pseudo-range-level raw observations (rather than a pre-computed lat/long), which discriminates a liar far better. There is also a timing line between the ADIRU and the MMR. Per AMM 34-36-00:

The GPS receiver also outputs a time mark discrete signal that tells users of the ARINC 743A data the instant in time when the position solution is valid.

Pseudo-ranges are measured at the speed of light — microseconds are coarse; the Time Mark is the filter's metronome.

4. Pairing and re-marriage — the full supply logic

Normal allocation in one sentence. Per FCOM DSC-34-10-15-10:

The GPS receiver 1 supplies the ADIRU1 and the ADIRU3, and the GPS receiver 2 supplies the ADIRU2.

Underneath, every ADIRU has dual lines to both GPS receivers, differing only in primary/secondary: ADIRU 1/3 primary = MMR 1, secondary = MMR 2; ADIRU 2 the reverse. The re-marriage rule after a failure. Per AMM 34-36-00:

In case of one GPS failure, the three ADIRUs automatically select the only operative GPS to compute hybrid GPIR data.

At the GPS level it is fully automatic. Only an ADIRU failure needs a crew hand: ADIRU 1 fails, FMGEC 1 uses ADIRU 3 / GPS 1 (rotary CAPT ON 3, a naturally same-side link); an ADIRU 2 failure has a twist. Per FCOM DSC-34-10-15-10:

If the ADIRU2 fails, the ATT HDG selector has to be set to F/O 3 in order to maintain Side 1 and Side 2 segregation. In this case, the ADIRU3 will be supplied with GPS receiver 2.

The AMM states the motive plainly. Per AMM 34-36-00:

The primary source of the ADIRU 3 being the GPS 1, it is necessary to select the secondary input port of the ADIRU 3 (GPS 2) by means of the SWITCHING/ATT HDG selector switch (20FP) to preserve side 1/side 2 segregation (GPS 1/ADIRU 1/FMGEC 1 and GPS 2/ADIRU 3/FMGEC 2 architecture).

The segregation philosophy: never let both FM chains feed on the same GPS — otherwise if that GPS sneezes, both FMs catch cold, and the dual-FM comparison (the NAV FM/GPS POS DISAGREE family) loses its meaning. This ATT HDG rotary thus carries a third identity: it switches the IR display source (overview article), it sets IR 3's ADR source (inertial article), and here it is also ADIRU 3's GPS-port selector. The dual-ADIRU fallback: two FMGECs share the surviving ADIRU, which feeds on its own same-side GPS.

5. Integrity — the receiver as its own quality inspector

The GPS's greatest weakness is not "no signal" but "wrong signal, stated with confidence." The receiver's three-layer self-defence is in the AMM. Per AMM 34-36-00:

The GPS receiver simultaneously tracks signals from up to twelve GPS satellites. The signals are processed to generate a three-dimensional position and a precise time.

The receiver generates an estimate of the positional accuracy provided. An integrity alert is activated if an unannounced satellite malfunction is detected. The receiver has the capability of excluding satellites that are malfunctioning. It is able to incorporate external altimeter data to improve the availability of its integrity monitoring functions.

Three weapons: (1) redundant observations — a fix needs only 4 satellites; tracking 12, the extras are witnesses, and cross-examining them exposes the liar (the essence of RAIM) and can vote it out of the solution; (2) self-reported accuracy — HDOP/VDOP (geometry quality), HFOM/VFOM (error-magnitude estimates), and HIL (horizontal integrity limit, 0–16 NM) — HIL is the radius of "I guarantee the true position is inside this circle," which is what ADS-B Out broadcasts (transponder article) and what the FM's accuracy judgement consumes; (3) baro-aiding — treating the computed barometric altitude as a "fifth pseudo-satellite," so integrity monitoring survives when geometry is poor or satellites are few. This answers a common question: GPS and air data seem unrelated — why does the QRH have you use GPS altitude after losing all ADRs, while GPS integrity in turn borrows barometric altitude? The two systems are each other's spare tyre, a designed symbiosis.

Monitoring reports through the three ADIRUs, which watch the GPS status words and their own BITE; a failure is the ECAM NAV GPS 1(2) FAULT (awareness level). The GPS MONITOR reading criteria (NAV mode + at least 5 satellites tracked — 4 in SBAS mode — altitude and lat/long consistent) are the exit-of-interference acceptance test later.

6. Mutual bootstrapping — you give me position, I give you time

A cold-start GPS must search the almanac itself, taking minutes; an IR alignment needs a starting position. Each hands the other a crutch. Per AMM 34-36-00:

In order to reduce GPS initialization time, the GPS 1(2) receives data from the ADIRU 1(2).

Per AMM 34-14-00:

Position initialization of the ADIRU is transmitted to the MMR for its own initialization. In addition to this position, the MMR requires initialization data of date and UTC. The IR portion receives date and UTC from the CMC and transmits the data to the MMR's .

And, as the alignment article covered, the reverse: the alignment position signature is by default performed by the GPS (the "IRS alignment based on GPS position" option). Chicken or egg? Each yields half a step: the IR feeds the GPS its satellite search with "last recorded landing position + CMC clock," and the GPS, once fixed, signs the IR's alignment. The odd interference-scenario procedure "before alignment, ask maintenance to pull the MMR circuit breakers" guards exactly this feeding chain against being poisoned.

7. Boundary — GPS PRIMARY is an ATA-22 word

The GPS PRIMARY / GPS PRIMARY LOST pair, and the "can I still fly RNP with PRIMARY lost" decision tree, belong to FM position management (covered in the auto-flight chapter); this article only supplies the material: the FM dares to flag PRIMARY because this chapter's integrity parameters (HIL etc.) pass threshold. One availability promise for flight planning. Per FCOM PRO-NOR-SOP-02:

GPS PRIMARY availability is demonstrated to be available 100 % of the time with 24 healthy satellites.

Read the conditional in full: "24 healthy" is the premise — for RNP AR, mountainous terrain, or a known depleted constellation, availability is verified procedure by procedure with a ground-based prediction tool.

Key numbers

Self-test

[!note]- Q1. What are the MMR's three tenants, and what knock-on does that "cohabitation" cause under GPS interference? ILS receiver, GPS receiver, common subsystem. GPS interference can lock the MMR's LS function out along with GPS (residual NAV LS FAULT).

[!note]- Q2. Autonomous vs hybrid — which is displayed on the MCDU, which fixes FM position, and why cross-check two pages under spoofing? Autonomous → MCDU GPS MONITOR; hybrid → FM position fixing. Under spoofing the two can disagree (one true, one false), so GPS MONITOR and POSITION MONITOR are compared.

[!note]- Q3. The 18-state Kalman "estimates errors, not position" — explain to a student, and give its three payoffs. It keeps an error ledger for the inertial system and settles it with GPS pseudo-ranges. Payoffs: rides through satellite dropouts on inertial; writes off inertial drift so position does not diverge; consumes pseudo-ranges so it discriminates a liar.

[!note]- Q4. ADIRU 2 fails — why must the ATT HDG rotary be moved, and what architecture principle breaks if it is not? Move ATT HDG to F/O 3 so ADIRU 3 is fed by GPS 2. Otherwise ADIRU 3 (primary GPS 1) and FMGEC 1 share GPS 1 with FMGEC 2's chain — both FM chains on one GPS, breaking side 1/side 2 segregation.

[!note]- Q5. How does the receiver detect an "unannounced satellite malfunction," and what role does barometric altitude play? Redundant satellites (RAIM): extra observations cross-check and vote out the faulty satellite. Barometric altitude acts as a fifth pseudo-satellite (baro-aiding) to keep integrity monitoring available with poor geometry.

Key takeaways

References

• FCOM DSC-34-10-15-10 — GPS description, MMR integration, hybrid computation, supply and switching.
• FCOM PRO-NOR-SOP-02 — GPS PRIMARY availability (24-satellite premise).
• AMM 34-36-00 — MMR architecture, three data types, segregation philosophy, 12-satellite tracking, integrity, active antenna, time mark.
• AMM 34-14-00 — autonomous vs hybrid definitions, parameter families, 18-state Kalman, MMR initialisation chain.

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.