Decoding Digital Signals
From ADS-B to FT8 — a beginner explanation of what digital signals are, which tools you need on Windows, and 3 complete step-by-step tutorials with embedded videos.
What does "decoding digital signals" mean?
Analogue signals like FM radio you hear directly — the speaker turns the signal into sound. Digital signals are coded data packets: text, numbers, positions, images. A decoder program "translates" this data stream into readable information. Tools: RTL-SDR v4 as the receiver · SDR++ or SDR# as the waterfall display · specific decoder software for each signal type.
Analogue signals like FM radio you hear directly — the speaker turns the signal into sound. Digital signals are coded data packets: text, numbers, positions, images. A decoder program "translates" this data stream into readable information. Tools: RTL-SDR v4 as the receiver · SDR++ or SDR# as the waterfall display · specific decoder software for each signal type.
Which digital signals are there — and what do you need?
ADS-B
1090 MHz
Aircraft positions — no decoder needed, straight into dump1090
APRS
144.800 MHz (DE)
Amateur radio position reports — Soundmodem + SDR#
FT8
14.074 MHz (20 m)
Weak-signal DX — WSJT-X + Virtual Audio Cable
NOAA APT
137 MHz
Weather images — noaa-apt / WXtoIMG
AIS
161.975 / 162.025 MHz
Ship positions — AIS-Catcher or AIS plugin
POCSAG
150–170 MHz
Fire-brigade pagers (unencrypted) — multimon-ng
What do I need in general? (Windows setup)
| Component | What | For what | Download |
|---|---|---|---|
| RTL-SDR v4 | Hardware dongle | Receive the RF signal | rtl-sdr.com |
| Zadig | Driver tool | Install the WinUSB driver | zadig.akeo.ie |
| SDR++ or SDR# | SDR software | Waterfall, demodulation, audio output | github / airspy.com |
| VB-Cable | Virtual audio cable | Route audio from SDR# to the decoder | vb-audio.com (free) |
| Decoder for each mode | e.g. WSJT-X, Soundmodem, dump1090 | Decode the data stream | depending on the tutorial |
VB-Cable (Virtual Audio Cable): The most important aid for decoding on Windows. SDR# or SDR++ outputs audio to a virtual cable — the decoder program reads this cable as its input. That way the programs communicate without a physical cable. Alternative in SDR++: some plugins decode directly without an external tool.
3 Windows tutorials — step by step
Tutorial 1
ADS-B — aircraft on the map Beginner
RTL-SDR v4
SDR++
dump1090 (Windows)
Browser
ADS-B is the ideal start: no VB-Cable needed, no licence, instant success. Every airliner transmits position, altitude, speed and call sign on 1090 MHz — unencrypted, intended for the public.
📺 Tech Minds (Matt, M0DQW) — ADS-B antenna build + setup with RTL-SDR · Dec 2023
Step by step (Windows)
1
RTL-SDR v4 driver: Zadig → WinUSB → Interface 0
(see the installation guide)
2
Download dump1090 for Windows:
github.com/flightaware/dump1090 → Releases → Windows binary (.exe)
3
Start dump1090:
Detected aircraft with positions appear in the terminal immediately.
dump1090.exe --interactive --net --gain -10Detected aircraft with positions appear in the terminal immediately.
4
Open the map in the browser:
http://localhost:8080 → an interactive map with all aircraft in real time
5
SDR++ alongside: Optionally a waterfall at 1090 MHz (AM, 2 MHz bandwidth) — you see the ADS-B bursts as characteristic short spikes in the spectrum.
# Frequency: 1090 MHz (no SDR program needed — dump1090 talks to the RTL-SDR directly)
# Typical terminal output:
# Hex Flight Alt Speed Lat Lon Track Messages
# 3C4B55 DLH123 9500 450 52.5200 13.4050 270 1234
# 4B1805 EZY456 FL350 480 52.3100 13.2200 180 987
Tip: As an alternative to dump1090, you can also use VirtualRadar Server (Windows, free) with a better map interface. Or directly in the browser: flightradar24.com shows what your dongle could theoretically receive too.
Tutorial 2
APRS — decode amateur radio positions Fair
RTL-SDR v4
SDR#
VB-Cable
Soundmodem
APRS-IS client
APRS (Automatic Packet Reporting System) is the digital backbone of amateur radio: stations, vehicles, weather balloons and even the ISS transmit position data on 144.800 MHz (Germany). With SDR# and a soundmodem, these data packets can be decoded and shown on a map — completely without a transmit licence (receive only).
📺 APRS iGate with SDR# and Soundmodem — complete Windows setup
Step by step (Windows)
1
Install VB-Cable: vb-audio.com/Cable → Download → run Setup.exe as administrator → restart
2
Start SDR#: Frequency:
144.800.000 Hz · modulator: NFM · bandwidth: 12,500 Hz
3
SDR# audio output: Settings → Audio Output →
CABLE Input (VB-Audio) select
4
Download Soundmodem: uz7.ho.ua/packetradio.htm → soundmodem.zip → extract
5
Configure Soundmodem: Settings → Devices → Input Device:
CABLE Output (VB-Audio)
→ select AFSK 1200 mode
6
See APRS packets: Decoded APRS packets appear as text in the Soundmodem window. For a map view: open APRS.fi in the browser, or install YAAC (Yet Another APRS Client).
# Typical Soundmodem output (decoded APRS packets):
# DL3XYZ-9>APRS,WIDE1-1,WIDE2-2:!5228.50N/01324.30E>073/025/A=000285
# → Call sign: DL3XYZ-9 | Pos: 52°28'N 13°24'E | Course: 073° | Speed: 25 km/h
# DB0ABC>BEACON,APRS:;DB0ABC *111111z5215.00N/01323.00Er Digipeater Berlin Nord
# APRS on a map: aprs.fi shows all stations across Germany live
Transmit licence: Receiving and decoding is allowed without a licence. Anyone who wants to transmit their own APRS packets (iGate, own position reports) needs the amateur radio licence class E or A — info here.
Tutorial 3
FT8 — decode worldwide weak-signal DX Fair
RTL-SDR v4 (direct sampling)
SDR#
VB-Cable
WSJT-X
FT8 is the most used digital amateur radio mode worldwide. Signals 20 dB below the noise are still decoded correctly — from all of Europe, Africa, America, Asia. With the RTL-SDR v4 in direct-sampling mode (built-in HF circuit), HF signals can be received without an additional upconverter. Suitable band: 20 m (14.074 MHz).
📺 FT8 with RTL-SDR v4 + SDR# + WSJT-X + GridTracker2 — February 2025 (current)
Step by step (Windows)
1
Enable RTL-SDR v4 direct sampling: In SDR# → Source → RTL-SDR →
Direct Sampling: Q-Branch select (for HF below 30 MHz)
2
Set the frequency:
14.074.000 Hz (FT8 20 m band) ·
modulator: USB (upper sideband) · bandwidth: 3,000 Hz
3
Audio output to VB-Cable: SDR# → Audio Output → CABLE Input (VB-Audio Virtual Cable)
4
Install WSJT-X: physics.princeton.edu/pulsar/k1jt/wsjtx.html →
Windows installer → run
5
Configure WSJT-X: File → Settings → Audio →
Input Device:
CABLE Output (VB-Audio) · Mode: FT8
6
Synchronise the PC clock: Important! FT8 works with 15-second windows — the Windows clock must be accurate to ±1 second. Windows time → Sync now, or install
Meinberg NTP for permanent synchronisation.
7
Start receiving: Click "Monitor" in WSJT-X. After 15 seconds the first decoded call signs appear in the window. Green lines = good signals, grey = weak but decoded signals.
# Typical WSJT-X output (decoded FT8 signals):
# 074500 -3 0.3 1234 CQ DL3ABC JO52
# 074515 -12 0.1 890 DL3ABC SP5XYZ KO01
# 074530 -20 0.4 456 SP5XYZ DL3ABC +03
# Format: time · SNR(dB) · time offset · frequency · message
# SNR -20 dB = only just decodable — on analogue radio that would be pure noise
# Most important FT8 frequencies:
# 14.074 MHz (20m — best DX band, daytime)
# 7.074 MHz (40m — evening, Europe)
# 3.573 MHz (80m — night, local)
GridTracker2: A companion program that shows all received FT8 stations on a world map and calculates the decoded distance. Download: gridtracker.org — free, runs alongside WSJT-X.
Antenna for HF: The RTL-SDR v4 in direct-sampling mode needs a longer antenna for 14 MHz. Best: a dipole of 2 × 5 m wire, or a long wire (10+ m) out of the window. The supplied mini whip antenna is barely usable on HF.
Tutorial 4
AIS — ships on the map in real time Beginner
RTL-SDR v4
AIS-Catcher (Windows)
Browser
AIS (Automatic Identification System) is the maritime equivalent of ADS-B: every ship over 300 GT transmits position, course, speed and name on 161.975 and 162.025 MHz — unencrypted, intended for the public. With AIS-Catcher for Windows, a complete decoder including a map interface runs in the browser, without VB-Cable or additional tools.
📺 AIS-Catcher — AIS reception with RTL-SDR on Windows, browser map view
Step by step (Windows)
1
Download AIS-Catcher:
github.com/jvde-alan/AIS-catcher → Releases →
AIS-catcher-Windows.zip
(no installation needed, extracting is enough)
2
RTL-SDR driver: Zadig → WinUSB must already be installed
(as described in installation guide )
3
Start AIS-Catcher — the simplest method:
The program detects the RTL-SDR automatically and starts receiving both AIS channels (161.975 + 162.025 MHz) at once.
AIS-catcher.exe -vThe program detects the RTL-SDR automatically and starts receiving both AIS channels (161.975 + 162.025 MHz) at once.
4
Enable the browser interface:
→ open http://localhost:8100 → an interactive map with all ships in real time
AIS-catcher.exe -v -N 8100→ open http://localhost:8100 → an interactive map with all ships in real time
5
View alongside in SDR++ (optional):
Frequency:
162.025 MHz · modulator: NFM · bandwidth: 12.5 kHz. In the waterfall you see the characteristic GMSK bursts — short, regular bright stripes at a 25 ms rate.
# AIS-Catcher basic commands (Windows):
AIS-catcher.exe -v # start receiving, terminal output
AIS-catcher.exe -v -N 8100 # + browser map on port 8100
AIS-catcher.exe -v -N 8100 -g 40 # + set gain manually to 40 dB
# Typical terminal output (decoded AIS messages):
# MMSI: 211234560 Name: BERLINER BEAR Speed: 8.2kn Course: 273
# MMSI: 244123456 Name: AMSTEL Speed: 12.4kn Lat: 53.545 Lon: 9.980
# MMSI: 636012345 Name: MSC EUROPA Speed: 18.1kn Type: Cargo
# Forwarding to OpenCPN (navigation software):
AIS-catcher.exe -v -U 127.0.0.1 10110 # UDP output for OpenCPN
Location tip: AIS signals have a range of about 30–70 km over water. In Berlin (inland shipping on the Havel, Spree, Oder-Havel Canal) inland vessels with AIS can also be received. At the coast or near a port: considerably more traffic. No results? → A raised location (roof) helps, since AIS needs line of sight.
Online comparison: marinetraffic.com
shows what should be receivable in your region — a good check of whether the antenna has sufficient range.
Decoder software — Windows overview
| Signal | Frequency | Decoder | VB-Cable needed? | Licence? |
|---|---|---|---|---|
| ADS-B | 1090 MHz | dump1090, VirtualRadar | No | No |
| AIS | 161/162 MHz | AIS-Catcher (Windows) | No | No |
| APRS | 144.800 MHz | Soundmodem + YAAC/APRS.fi | Yes | Receive only |
| FT8 | 14.074 MHz | WSJT-X + GridTracker2 | Yes | Receive only |
| NOAA weather | 137 MHz | noaa-apt (Windows) | Yes | No |
| POCSAG pager | 150–170 MHz | multimon-ng (Windows port) | Yes | Public |
| ACARS (aircraft) | 131.550 MHz | ACARSD or Jaero | Yes | No |
All-in-one alternative: Cascade-SDR (2025, free) bundles many decoders in one interface: ADS-B, AIS, APRS, NOAA, POCSAG — with an integrated map view. Good for beginners who don't want to set up several tools. Download: github.com/cascade-sdr
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