Before this:What is software-defined radio?
Other signals you’ll meet
Key takeaways Trunked voice is only part of what’s on the air. Because an SDR just delivers IQ samples, the same hardware can decode many other signals with the right software — and GopherTrunk supports a range of them: POCSAG/FLEX paging, AIS ship tracking, ADS-B aircraft tracking, APRS amateur position data, plus signalling like DSC and MDC1200. Most are data, not voice, and many can be plotted on a map. They’re a great way to see results even where voice systems are quiet or encrypted.
You’ve followed the path toward trunked digital voice — but the spectrum is full of other useful, decodable signals. This lesson is a quick tour, each with a pointer to GopherTrunk’s reference page, so you know what else your dongle can do.
Paging (POCSAG / FLEX)
Pagers are far from dead — hospitals, fire services, and industry still use them. POCSAG and the faster FLEX are FSK paging protocols carrying short text and numeric messages. They’re easy to decode and a satisfying first data mode. GopherTrunk decodes POCSAG; you’ll see messages stream in as they’re transmitted.
AIS — tracking ships
AIS (Automatic Identification System) is how ships broadcast their identity, position, course, and speed on VHF marine frequencies (around 161.975 / 162.025 MHz). Decode it and you get a live map of vessel traffic near any coast or waterway. GopherTrunk’s AIS support turns these bursts into plotted ships.
ADS-B — tracking aircraft
ADS-B (Automatic Dependent Surveillance–Broadcast) is the aviation counterpart: aircraft continuously transmit position, altitude, velocity, and identity on 1090 MHz. It’s one of the most popular SDR applications — plot live aircraft overhead in real time. See ADS-B for GopherTrunk’s decoder (note 1090 MHz is within RTL-SDR range, though a tuned antenna helps).
APRS and AX.25
APRS (Automatic Packet Reporting System) is an amateur-radio data network — usually 144.39 MHz in North America — carrying position reports, weather, telemetry, and messages over AX.25 packet. Decoding it shows you nearby ham stations, weather sensors, and mobile trackers. GopherTrunk handles APRS / AX.25.
DSC, MDC1200, and more
Smaller signalling systems are everywhere:
- DSC (Digital Selective Calling) — marine distress and calling on VHF/HF.
- MDC1200 — a Motorola signalling format that sends a unit ID as a quick data burst at the start/end of an analog transmission (PTT IDs).
These add identity and control data to otherwise analog systems, and GopherTrunk can surface them.
Why this matters for your learning
Every one of these uses the exact concepts from this path — a carrier carrying modulated data, captured as IQ, pulled out by filtering, and demodulated and decoded. Learning trunked voice gave you a toolkit that generalises. And practically, these data modes are a great way to see immediate results — a map filling with ships or planes — even when local voice systems are quiet or encrypted.
| Signal | Roughly where | What you get |
|---|---|---|
| POCSAG / FLEX | VHF/UHF paging allocations | Text/numeric pages |
| AIS | ~162 MHz | Ship positions |
| ADS-B | 1090 MHz | Aircraft positions |
| APRS | 144.39 MHz (NA) | Ham positions, weather |
| DSC | VHF/HF marine | Marine calling/distress |
| MDC1200 | On analog voice channels | Radio unit IDs |
Quick check: on which frequency do aircraft broadcast ADS-B?
Recap
- One SDR decodes far more than voice — the difference is software, not hardware.
- POCSAG/FLEX (paging), AIS (ships), ADS-B (aircraft), APRS (ham data), plus DSC and MDC1200 signalling.
- Many are mappable data modes — quick, visible wins.
- They all reuse the same path concepts, so your skills generalise.
That completes Module 5. The final module puts everything to work — starting with finding systems to monitor.
Frequently asked questions
What else can an SDR decode besides voice?
A lot. Common data signals include POCSAG and FLEX pagers, AIS ship-position reports, ADS-B aircraft transponders, APRS amateur position and telemetry, marine DSC calling, and signalling like MDC1200. Because the SDR just provides IQ samples, the same hardware can decode any of these given the right software — and GopherTrunk supports many of them.
What is ADS-B?
ADS-B (Automatic Dependent Surveillance–Broadcast) is a system where aircraft continuously broadcast their identity, position, altitude, and velocity on 1090 MHz. An SDR can receive these and plot live aircraft on a map. GopherTrunk can decode ADS-B alongside its trunked-radio work.
What is AIS?
AIS (Automatic Identification System) is the maritime equivalent of ADS-B — ships broadcast their identity, position, course, and speed on VHF marine frequencies (around 162 MHz). An SDR can receive and plot them, giving a live picture of nearby vessel traffic.
What is APRS?
APRS (Automatic Packet Reporting System) is an amateur-radio data mode, usually on 144.39 MHz in North America, carrying position reports, weather, messages, and telemetry over AX.25 packet. An SDR can decode APRS to see station locations and data.