How to Receive AIS Ship Tracking with an SDR

[Pavel]

Short answer: Use an SDR that covers marine VHF, a vertically polarized antenna, both AIS channels and a decoder that accepts direct I/Q or clean discriminator audio. Reception is receive-only, but message display and sharing must follow local privacy and legal requirements.

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: Install the SDR and verify a nearby VHF signal, then position a vertical marine-band antenna as high and clear toward the water as practical.
2. Step 2: Tune a sample rate wide enough to include AIS channel A at 161.975 MHz and channel B at 162.025 MHz, or configure two channel filters.
3. Step 3: Use an AIS decoder with unclipped I/Q or audio and disable squelch that could cut short bursts. Adjust manual gain for valid messages rather than maximum signal height.
4. Step 4: Open the local map, verify positions are plausible and redact identifiers if sharing troubleshooting material. Feed external services only from the stable local decoder output.

Concrete example​

A center frequency of 162.000 MHz with enough complex bandwidth can include both channels 25 kHz below and above center. The decoder still needs correct channel extraction and GMSK handling; ordinary voice audio is not sufficient.

How to judge the result​

Success means sustained CRC-valid AIS messages and plausible vessel positions over a fixed period without clipping or frequent device 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.
• A narrow NFM audio recording with aggressive squelch may remove burst edges and fail even when the AIS signal is visible.

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.