AIS Channel A and B Frequencies Explained

[Pavel]

Short answer: AIS channel A is 161.975 MHz and channel B is 162.025 MHz in the common international allocation. Ships alternate or use the channels according to AIS operation; a receiver should monitor both for complete local traffic.

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: Set the SDR center between the channels, commonly 162.000 MHz, and choose sample rate plus filters that cover both complete signals.
2. Step 2: Verify frequency calibration with a stable reference so narrow channel filters do not clip one side. Correct PPM before compensating in the decoder.
3. Step 3: Route separate channelized streams or use a decoder designed for a wide I/Q source. Label outputs so duplicate or channel-specific diagnostics are understood.
4. Step 4: Compare valid-message count on A, B and combined over an hour. Large differences may reflect traffic scheduling, antenna or frequency/filter setup.

Concrete example​

The two centers are 50 kHz apart. A complex sample rate comfortably above that separation can contain both, but the usable bandwidth and transition filters must also leave margin at each edge.

How to judge the result​

Both channels are configured correctly when bursts appear at the expected offsets and the decoder produces valid messages from each without filter-edge loss.

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.
• Monitoring only one AIS channel can make traffic appear intermittent or incomplete even though receiver sensitivity is adequate.

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.