Getting Started¶

Aviation datalink is easier to understand if you read it from the outside in:

1. Radio frequencies, i.e., where signals are transmitted.
2. Bearers, i.e., the physical/link technologies: VHF ACARS, VDL Mode 2, HFDL, SATCOM
3. Wrappers and network layers, i.e., ACARS frames, AVLC, X.25, ARINC 622, ATN B1
4. Applications, i.e., CPDLC, ADS-C, ATIS, OOOI, weather, maintenance, and airline operations.

datalink is a receiver/decoder for this stack.

Communication vs. Surveillance¶

ADS-B and Mode S are mostly surveillance systems. They answer questions like: where is the aircraft, how fast is it moving, and what transponder address identifies it?

Datalink answers a different class of questions:

• what did ATC instruct the crew to do?
• what did the crew acknowledge?
• what surveillance contract is active over oceanic airspace?
• what route, waypoint, or next-report information did the aircraft send?
• what airline operational events happened: out, off, on, in, ATIS, weather, maintenance?

Frequencies and receiver tools¶

Different datalink systems live in different parts of the spectrum.

They may need different receiver setups. The key take-away is that, unlike ADS-B or AIS, you cannot receive everything at once with a single RTL-SDR setup.

(*) also C-band feeder links

For reference, here are frequencies for other surveillance systems:

Selected VHF ACARS channels¶

VHF ACARS channels are region-dependent.

When no explicit channel list is configured, datalink first tries to choose known channels that fit inside the source bandwidth. If bandwidth metadata is missing or no known channel fits, it falls back to a compact default scan list.

Known VDL Mode 2 channels¶

VDL2 channels cluster around 136–137 MHz.

datalink uses the same idea: choose known channels that fit the source bandwidth when possible, or fall back to default scan channels.

Others¶

HFDL is different: frequencies change with station, propagation, time of day, and operational use. A receiver often monitors one or more known HF channels rather than a compact VHF band.

The docs.rs page contains a list of known HFDL frequencies. https://docs.rs/crate/datalink/latest

Bearers¶

A bearer is the physical/link technology that carries the data.

VHF ACARS / POA¶

VHF ACARS is the original "plain old ACARS" (sometimes written as POA) link.

It carries ACARS messages directly over a low-rate VHF signal (MSK modulation).

VHF signal → demodulator → ACARS frame → label/application decoder

It is simple compared to other bearers: the ACARS frame is the main container.

VDL Mode 2¶

VDL2 is a higher-throughput VHF (D8PSK modulation) digital link. It carries bursts that decode into AVLC frames.

VDL2 burst → demodulator → AVLC frame → payload

The AVLC payload can take two important paths:

AVLC I-frame → ACARS prefix → ACARS frame → ARINC 622 / ACARS applications
AVLC I-frame → X.25 packet  → CLNP/COTP/ULCS → ATN B1 applications

This is why VDL2 output can contain both ACARS-style traffic and ATN B1 CPDLC traffic.

HFDL¶

HFDL uses HF radio for long-range datalink, especially where VHF line-of-sight coverage is unavailable.

HF signal → HFDL demodulator → HFDL frame → decoded payload

HF reception depends heavily on propagation. Native HFDL work in datalink is experimental, but decoded HFDL messages are still normalized into the same output envelope.

SATCOM and Iridium¶

Satellite ACARS bearers are important in the real world because they provide oceanic, remote, and polar coverage. They are part of the conceptual stack, and Airframes.io may expose decoded satellite-origin messages, but native SATCOM/Iridium RF reception is not currently a datalink target.

Wrappers and containers¶

Once a bearer has produced bytes or frames, the next question is to identify the container.

ACARS frame¶

ACARS is both a legacy system and a common message container used across bearers.

mode · registration · ack · label · block_id · text · checksum

The label is the first application router:

The text field may be human-readable, airline-specific, or another structured envelope.

AVLC frame¶

AVLC is the VDL2 link-layer wrapper.

It includes source and destination addresses (as 24-bit ICAO addresses, the same as in ADS-B), link control, payload, and frame check sequence.

VDL2 → AVLC(dst, src, control, payload, fcs)

AVLC tells you who is talking to whom at the VDL2 link layer: aircraft, ground stations, all-stations broadcasts, XID management frames, flow-control frames, or I-frames carrying user data.

HFDL frame¶

HFDL has its own frame structure and ground-station network. It is not simply “ACARS over VHF on HF”; it has its own link behavior before any application payload is interpreted.

ARINC 622 envelope¶

Many high-value ACARS messages use ARINC 622 inside ACARS text:

/ATSU.IMI.REGISTRATION payload checksum

Typical IMIs:

ATN B1 over X.25¶

VDL2 can also carry a non-ACARS network path:

VDL2 AVLC → X.25 → CLNP → COTP → ULCS → ATN B1 CPDLC

This is structurally very different from FANS-1/A over ARINC 622, but the application result may still be a CPDLC message such as "CONTACT EGTT 129.605".

Applications¶

ADS-C¶

ADS-C means Automatic Dependent Surveillance – Contract.

It is different from ADS-B, where B means Broadcast; ADS-C is a negotiated contract between a ground facility and an aircraft.

ADS-C reports can contain:

• position, altitude, and timestamp;
• flight id;
• next and next-next waypoint predictions;
• ground speed, track, and vertical rate;
• heading, airspeed, Mach;
• wind and temperature;
• airframe identifier;
• event or emergency report context.

CPDLC¶

CPDLC means Controller-Pilot Data Link Communications.

It is structured text communication between controller and pilot.

A typical session is:

• connect request
• connect confirm
• uplink/downlink messages
• disconnect

Messages include clearances, altitude instructions, route clearances, contact instructions, WILCO/UNABLE responses, position reports, and logical acknowledgements.

CPDLC can arrive through two major paths:

Both CPDLC implementations follow slightly different standards.

Others¶

Most traffic observed on datalink is not very meaningful application payload but messages for maintaining the communication channel (ping, acknowledgement).

Examples of other useful applications include:

• Airline Operational Control (AOC) messages
• ATIS requests and deliveries;
• OOOI timestamps: Out, Off, On, In;
• weather and METAR bundles;
• ETA and slash-field reports;
• maintenance and performance messages;
• link-state/media advisory messages;
• airline-specific free text or binary payloads.

These messages help enrich a trajectory with an operational timeline.