How to Identify an Unknown Signal from a Waterfall

[Pavel]

Short answer: Identify an unknown signal by recording frequency, occupied bandwidth, time pattern, modulation clues and receiver tests—not by matching one waterfall image. First prove it is over the air, then narrow candidates using local allocations and signal structure.

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: Record center frequency, calibrated PPM, sample rate, gain, UTC time and approximate country or region. Measure occupied bandwidth using the waterfall scale.
2. Step 2: Retune the receiver while watching the signal. A spur that stays at the same display offset or vanishes with the antenna disconnected may originate inside the SDR or computer.
3. Step 3: Observe burst length, repetition, drift and whether energy is analogue, narrow digital, multi-carrier or frequency hopping. Avoid demodulating private content.
4. Step 4: Save a short nonclipped I/Q sample with format and sample-rate metadata. Check an authoritative local band plan before assigning a protocol name.

Concrete example​

A 12.5 kHz-wide burst repeating every minute is not enough to call DMR, telemetry or paging. Add center frequency, symbol structure, duration and allocation; many protocols share similar waterfall shapes.

How to judge the result​

A useful identification includes evidence excluding an SDR spur and enough RF metadata for another person to reproduce bandwidth and timing measurements.

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.
• Guessing from colour palette or waterfall brightness is unreliable because FFT settings and gain change appearance without changing the underlying modulation.

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.