VDL Mode 2 vs Classic ACARS

[Pavel]

Short answer: Classic VHF ACARS uses narrowband audio-frequency-shift-keyed messages on AM channels, while VDL Mode 2 is a higher-rate digital VHF data link with different modulation, wider channel handling and different decoders. Software for one will not decode the other.

Before you start​

• Confirm that reception and message handling are lawful in your location, and avoid publishing personal or sensitive message content.
• Use the correct channel plan, modulation and decoder for AIS, classic ACARS, VDL Mode 2 or an unknown signal; they are not interchangeable audio decoders.

Step-by-step method​

1. Step 1: Identify the assigned channel and expected protocol from current regional information rather than visual appearance alone.
2. Step 2: For classic ACARS, use a suitable AM audio path into an ACARS decoder. For VDL2, use complex baseband and a decoder designed for its modulation and channel bandwidth.
3. Step 3: Set sample rate, PPM and filter width for the selected protocol, then verify signal shape and decoder synchronization separately.
4. Step 4: Compare valid-frame counts over the same observation period and keep message payloads private when posting diagnostics.

Concrete example​

A strong VDL2 channel can look like continuous dense digital energy, while classic ACARS appears as shorter narrow bursts. That visual clue helps selection but does not replace frequency plan and decoder validation.

How to judge the result​

The protocol choice is correct when the expected decoder synchronizes and produces valid framed messages without forcing extreme audio levels or arbitrary bit settings.

What to record​

• Record center frequency, sample rate, occupied bandwidth, symbol or burst timing and a short redacted I/Q sample for identification work.
• For AIS and ACARS, compare valid decoded messages over equal periods rather than raw waterfall intensity.
• A signal that stays at the same display offset when the receiver retunes may be an internal spur rather than an over-the-air transmission.

Common mistakes​

• Changing several hardware, software or RF variables at once, which removes the controlled comparison needed to identify the cause.
• Treating one autoscaled screenshot or one unusually good result as proof without recording the settings and repeating the test.
• Sending demodulated AM speaker audio to a VDL2 decoder discards the complex signal information it needs.

Final check​

Replay only a lawful redacted sample with its original metadata and confirm the decoder or identification result. Remove message content and sensitive identifiers before any public upload.