Decoding a Known Signal

Why start with signals you can verify

Each of these four has an independent check. FT8 decodes carry call signs you can look up. ADS-B gives you aircraft you can see out of the window. APRS positions appear on a public map. A weather satellite produces a picture of recognisable coastline.

That is the point. When a decode is wrong you will know, and you will be debugging a chain rather than guessing.

1. FT8 — the easiest first decode

14.074 MHz USB on 20 metres, or 7.074 on 40.

1. Set your SDR to USB and tune to 14.074.000.
2. Route the audio into WSJT-X — a virtual audio cable, or the SDR software’s own routing.
3. In WSJT-X select mode FT8 and the correct band.
4. Set your computer clock with NTP. This is not optional.

Decodes appear at the end of each 15-second cycle. If the waterfall is full of signals and nothing decodes, the clock is the cause about nine times in ten — which is the E2E answer made concrete.

What to look at once it works: the signal report column. Values around −20 dB are routine, and the mode works well below the level where you can hear anything at all. That is forward error correction and 15-second integration doing exactly what the HF digital lesson claims.

2. APRS — packet you can put on a map

144.390 MHz FM in North America.

1. Tune 144.390 with NFM demodulation.
2. Feed the audio to Direwolf (or APRSdroid on a phone held to a speaker, which genuinely works).
3. Watch for decoded frames.

You will see position beacons, weather stations, and digipeated traffic. Read the path field on each: you are looking at the WIDEn-m notation from the operating module, in live data, and you can watch the second number decrement as a packet crosses digipeaters.

Failure mode: audio level. Packet decoders are sensitive to over- and under-driving in a way that voice is not. If the waterfall shows clean bursts and Direwolf decodes nothing, adjust the level before anything else.

3. ADS-B — the strongest signal you will ever decode

1090 MHz, and it needs no amateur knowledge at all.

1. Run dump1090 (or the software’s built-in ADS-B decoder).
2. Use the supplied whip cut or extended to about 69 mm — a quarter wave at 1090 MHz. Antenna length matters more here than anywhere else in this lab.
3. Open the map view.

Aircraft appear with call sign, altitude, heading, and position. Given your Part 107 background this is the most immediately readable output in the lab, and it is a good demonstration that a $30 receiver plus a correctly sized wire outperforms an expensive receiver with a wrong one.

What it teaches: at 1090 MHz a quarter wave is 69 mm, so being 30 mm off is a large fraction of a wavelength. The antenna lesson’s claim that dimensions scale with frequency becomes obvious when the whole antenna is the size of a finger.

4. NOAA weather satellites — a picture from space

137 MHz, NOAA-15, -18, and -19, using APT.

Frequencies: NOAA-15 on 137.620, NOAA-18 on 137.9125, NOAA-19 on 137.100 MHz.

1. Find a pass with any tracking app — the pass predictions are the Keplerian elements from the satellites lesson, in use.
2. Tune the satellite’s frequency with WFM and a bandwidth of about 34 kHz — that is the signal’s actual occupied bandwidth, so 34–40 kHz on the filter captures it without taking in more noise than necessary.
3. Record the audio for the whole pass, then decode it with SatDump or noaa-apt.

The signal is a 2400 Hz subcarrier amplitude-modulated by the image lines, which is then frequency-modulated onto the 137 MHz carrier. The characteristic steady ticking you hear is that subcarrier — the same “brightness is a property of the signal” idea as SSTV, one layer down.

What to watch: the Doppler shift across the pass, and the signal fading in a regular pattern. The fade is the satellite’s linear polarisation rotating relative to yours — the argument for circular polarisation from the satellites lesson, visible as periodic dropouts in your image.

The general debugging order

When a decode fails, work through this in order rather than randomly:

1. Is the signal there? Look at the waterfall. If you cannot see it, it is an antenna or frequency problem, not a decoding one.
2. Is the frequency right? Apply your PPM correction. At 137 or 1090 MHz an uncalibrated dongle can be far enough off to miss entirely.
3. Is the bandwidth right? Too narrow clips the signal; too wide adds noise.
4. Is the audio reaching the decoder at a sane level? The single most common failure, and invisible on the waterfall.
5. Is the clock right? For anything time-synchronous — FT8, FT4, WSPR, Q65 — this is the first thing to check, not the last.

Check yourself

1. WSJT-X shows a busy waterfall and zero decodes. What is the most likely cause?
2. Your ADS-B setup receives only aircraft directly overhead. What would you change first?
3. Your NOAA image fades in and out in a regular rhythm. What is happening?