SATCOM

SATCOM (satellite communication) is the third leg of external communication: VHF is near-range, HF is long-range but noisy, and SATCOM goes up to a satellite — covering the globe except the poles, with steady quality. It carries both voice (cockpit or cabin) and data (ATSU/ACARS), and is the most reliable bearer for oceanic datalink (CPDLC/AOC).

One thing to state up front so this article's brevity is honest: SATCOM functions are heavily operator-customised (programmed via the ORT), and the FCOM itself declines to detail them. So the focus here is not a menu to memorise, but the invariant mechanisms — the architecture (how the satellite link is built), the crew interface (ACP set-up + MCDU dialling), the priority-tiered directory (why it splits EMERGENCY/SAFETY/…), and the relationship to the datalink (SATCOM takes over data only when VHF 3 cannot).

1. Architecture

                 satellite (L-band 1525–1660.5 MHz)
                            ▲   ▼
                 ┌──────────┴───┴──────────┐
                 │  top high-gain antenna   │  beam-steering unit:
                 │  + diplexer/LNA          │  tracks the satellite
                 └────────────┬─────────────┘
                              │ RF
     ┌────────────────────────┴──────────────────────┐
     │ SDU (Satellite Data Unit) — manages the        │
     │ satellite-side RF link protocols               │
     │ + high-power amplifier / high-speed data unit  │
     └──────┬─────────────────────────────┬───────────┘
            │ voice                         │ data
       ┌────▼─────┐  set up / terminate ┌───▼──────────────┐
       │ ACP      │                     │ ATSU / ACARS     │→ ATA-46
       │ (SATCOM  │                     │ (normally VHF 3; │
       │  keys)   │                     │  SATCOM when VHF │
       └────┬─────┘                     │  3 unavailable)  │
       ┌────▼─────┐  dial / number / priority
       │ MCDU     │  SAT menu: MAIN / DIRECTORY / MANUAL DIAL / STATUS
       └──────────┘

Note the antenna: unlike VHF/HF omnidirectional antennas, the SATCOM high-gain antenna is directional and beam-steered to track the satellite — which is why the poles (satellite elevation too low) are the coverage limit.

2. The satellite link — L-band, high-gain antenna, SDU

The AMM states SATCOM's essence. Per AMM 23-28-00:

The function of the SATCOM system is the reception and processing of signals via satellites providing aeronautical services in the L-Band (1525-1660.5 MHz). The system provides a capability for all aeronautical satellite communication requirements external to the aircraft, including passenger telephone and data services.

Its core box is the SDU (Satellite Data Unit). Per AMM 23-28-00:

The SDU uses the SATCOM antenna and related Radio Frequency (RF) components to give air/ground communications through satellites. The SDU controls the RF link protocols on the satellite side and gives the applicable interface with the communication management avionics.

The configuration covered here is a high-gain system offering Classic Aero multichannel voice/fax/datalink plus SwiftBroadband high-speed data. The top-mounted high-gain antenna is beam-steered onto the satellite in real time.

3. Voice and data channels, and the datalink relay

The two channel types, per FCOM DSC-23-30-20-10:

It provides both voice and data channels: One or several channels are used for voice transmissions (cockpit or cabin voice). The cockpit voice function must be activated, in order for it to be available. [...] One channel is used for data transmissions (ATSU or ACARS). ACARS or ATSU communications normally transmit via VHF 3. They automatically switch to SATCOM when VHF 3 is not available.

[!note]- Two-tier datalink bearer: VHF 3 first, SATCOM as backstop Article 4 noted VHF 3 normally runs the datalink; here is the other half — when VHF 3 has no coverage (e.g. beyond VHF line-of-sight over the ocean), the datalink automatically switches to SATCOM. So oceanic CPDLC is really "VHF 3 over land, SATCOM over water, switched automatically without crew action". This is the most elegant expression of "external radios carry both voice and data" (article 1). How the datalink messages themselves are read and answered belongs to ATA-46.

4. Cockpit voice — ACP set-up plus MCDU dialling

Making a SATCOM call splits across two panels. Per FCOM DSC-23-30-20-10:

The cockpit voice interface is controlled by the Audio Control Panels (ACPs) for call set-up and call termination, and by the MCDU for the call number selection. It allows the crew: To initiate air to ground calls and to receive ground to air calls; To select the call priority, in case of air to ground calls; To use manual dial or pre-recorded phone numbers.

In practice (with article 3): select a number on the MCDU SAT menu → press the ACP SATCOM transmit key → the AMU initiates the call, green bars flashing twice per second (air-to-ground call establishing) → steady when connected. To use VHF during a call, use the SATCOM HOLD function (green bars to once per second, article 3). The MCDU MAIN MENU shows channel status: READY TO CONNECT / NOT AVAILABLE / DIALING / INCOMING CALL / CONNECTED / CALL FAILED.

5. The priority directory — why four tiers

The SATCOM directory is not a simple phone book — it is priority-tiered, and that has safety meaning. Per FCOM DSC-23-30-20-20:

This page provides access to 4 phone number lists, where phone numbers can be memorized, according to their priority. 1L : EMERGENCY for Priority 1 - Reserved for emergency and distress phone numbers only. 2L : SAFETY for Priority 2 - Reserved for regulatory and flight safety phone numbers only. 3L : NON-SAFETY for Priority 3 - Reserved for non flight safety phone numbers. 4L : PUBLIC for Priority 4 - Reserved for personal phone numbers.

[!note]- Priority means channel pre-emption Satellite channels are limited. These four tiers are not for tidiness — a higher-priority call pre-empts a lower-priority channel: an EMERGENCY (distress) call being established will drop a PUBLIC (passenger personal) call in progress. This is an ICAO requirement for aeronautical satellite communication (distress/safety always ahead of commercial). On the CATEGORY NUMBERS page, protected numbers show green and unprotected numbers show in blue brackets — the protection guards operational/safety numbers against deletion.

6. Status monitoring

To judge whether SATCOM is usable, read the MCDU STATUS page: the LOG-ON status (LOGGED-ON / LOGGED-OFF / SATCOM INOP); the signal level (HIGH or LOW) for voice channels SAT 1/SAT 2 and the DATA channel; and the BITE status. One line: LOGGED-ON + signal HIGH = usable; LOGGED-OFF or SATCOM INOP = do not rely on it. The pre-oceanic SATCOM check (article 11) reads exactly this page.

7. ORT customisation and the boundary

Finally, why SATCOM "looks simple". Per FCOM DSC-23-30-20-10:

SATCOM functions are programmed through the Owner Requirement Table (ORT), according to airline needs. Due to the highly customized programming, the SATCOM functions may vary for different airlines and are, therefore, not described in detail.

So the specific menus, stored numbers and safety-service details follow the operator's ORT — this material teaches only what does not vary with the operator. The datalink application layer (ATSU/CPDLC) it bears belongs to ATA-46 (chapter boundary, article 1).

Self-test

[!note]- Q1. What band does SATCOM use, why is its antenna different from VHF/HF, and what coverage limit follows? L-band (1525–1660.5 MHz). Its high-gain antenna is directional and beam-steered onto the satellite, unlike omnidirectional VHF/HF — so the poles (low satellite elevation) are the coverage limit.

[!note]- Q2. Which radio normally carries the datalink, and when does it switch to SATCOM? Does the crew notice? Normally VHF 3; it switches automatically to SATCOM when VHF 3 is unavailable, with no crew action.

[!note]- Q3. In a SATCOM call, what does the ACP handle and what does the MCDU handle? The ACP handles call set-up/termination; the MCDU handles number selection (and priority).

[!note]- Q4. What is the practical effect of the four directory tiers? What do green versus blue-bracketed numbers mean? A higher-priority call pre-empts a lower-priority channel (distress ahead of passenger calls). Green = protected numbers; blue brackets = unprotected.

[!note]- Q5. How do you tell whether SATCOM is on line right now? The MCDU STATUS page: LOGGED-ON with HIGH signal = usable; LOGGED-OFF or SATCOM INOP = not.

Key takeaways

References

• FCOM DSC-23-30-20-10 — voice/data channels, cockpit-voice activation, VHF 3 → SATCOM switching, ACP/MCDU interface, ORT customisation.
• FCOM DSC-23-30-20-20 — MCDU pages (MAIN/DIRECTORY four tiers/STATUS); green/blue numbers.
• AMM 23-28-00 — L-band, satellites, passenger telephone + data; SDU role; high-gain antenna and beam steering.

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.