Decoders in RTL-SDR mode: what they read and how to use them
Decoders turn signals into text, tables, maps and pictures: Morse into letters, aircraft replies into positions on a map, a weather satellite’s pass into an image. They run in the browser on the same stream you are listening to. Open the Decoders tab in the Output bar (or press D), choose Add decoder, and pick one. Up to four can run at the same time.
Before the very first decoder starts, the app shows a short notice. Whether you may receive, record, export or share a given signal depends on the law where you are and on your situation, for example whether a transmission is meant for you. You decide how you use the decoders, and you are responsible for it.
How a decoder finds its signal
Each decoder reads one of three things. Some read a VFO’s audio and need that VFO in a given mode: FT8 wants USB, CW wants CW. Some listen at a frequency inside the capture and set up their own channel there. A few read the whole capture and need a minimum sample rate. The add menu says, per decoder, what is missing: the wrong mode, a frequency outside the capture, a sample rate that is too low. On the spectrum, Decode here offers the decoders that usually live at that frequency.
What is in the list
| Group | Decoders | Good to know |
|---|---|---|
| Amateur radio | CW, RTTY, PSK31, FT8, FT4, WSPR, SSTV pictures, APRS, AX.25 packet 9600 baud, NCDXF beacons | FT8 and FT4 read USB audio; FT8 works in 15-second UTC slots, so keep the computer clock right. APRS positions show on a map. |
| Aircraft | ADS-B, ACARS, VDL2, VOR and ILS ident | ADS-B reads the capture around 1090 MHz and needs at least 2.0 MS/s, best at 2.4. VOR is marked “Not for navigation”. |
| Ships and the sea | AIS, DSC (VHF channel 70), DSC (MF/HF), NAVTEX | AIS listens to both channels near 162 MHz in one run. |
| Weather and satellites | Weather fax (HF), Meteor-M LRPT pictures, SAME/EAS alerts (North America) | LRPT pass predictions use orbit data you fetch on request and the locator you saved; the picture builds up line by line. |
| Broadcast | DAB+ digital radio | Needs exactly 2.048 MS/s. You can listen to a service; encrypted services are marked and not decoded. |
| Tones and paging | CTCSS and DCS tones, DTMF, Selcall, aviation SELCAL, two-tone paging, POCSAG pagers | The tone decoder can set the squelch tone it heard on the VFO for you. |
| Time signals | DCF77, MSF, WWVB, JJY, WWV/WWVH, e-CzasPL | Tune the station's exact frequency. The first time appears after two to three minutes, once two minutes agree. The long-wave stations need an HF path. |
| Sensors and beacons | ISM sensors, 406 MHz distress beacons, 121.5 MHz distress tone | Weather stations and other small sensors show up as rows in a table. |
| Digital two-way radio | DMR, TETRA, dPMR, D-STAR | Signalling only: talkgroups, IDs, network information. No voice. |
What decoders never do
- No voice from digital radio. DMR, TETRA, dPMR and D-STAR show signalling only; the app has no voice codec.
- No decryption. Encrypted traffic is marked as encrypted and left alone, on every decoder.
- No transmitting. The stick only receives.
Reading and keeping the results
Each running decoder gets a card with its status (Listening, Decoding, Outside the capture), a message rate and a choice of views: messages, a table, an image or a map, depending on the decoder. The menu exports what it has: CSV or JSON for most, KML for those with positions (and GPX for APRS, AIS and ADS-B), ADIF for FT8, FT4 and WSPR, PNG for pictures. Add to log copies a decoded station into the logbook.
Decoders stop when the stick disconnects. Several of them run in a background worker so the page stays responsive; a browser that cannot run workers cannot start those. Decoders also work on an IQ file, which is a good way to try one on a recording before you sit through a live pass.
Recommended gear
- 1090 MHz ADS-B antenna (SMA male, magnetic base)AntennaA 1090 MHz antenna is the single biggest step up for ADS-B aircraft reception.
- RTL-SDR Blog multipurpose dipole antenna kitAntennaThe dipole kit doubles as a V-dipole for weather satellites around 137 MHz.