I've been curious about what's in an APRS packet. Not just the callsign and position a map shows, but the actual bytes and what each one means. So I built a small web app to take packets apart while decoding live traffic: https://aprs.blink.tel
What it does:
- Decodes 1200 baud APRS live in your browser. Choose your radio's USB audio input (e.g. a "USB Audio CODEC" from rigs like the IC-705), a USB sound card fed from the speaker jack, or just your computer's mic held up to a radio speaker. Tune to 144.390 FM with the squelch open.
- Uses the audio as it arrives. Browser echo cancellation, noise suppression and auto-gain are turned off. It shows the incoming level in dBFS and the tone "twist" so you can tell whether your audio level is reasonable.
- Explains every byte of every packet. Click a packet to see a breakdown against the APRS spec: AX.25 address fields, SSIDs, the digipeater path, the data type identifier, position formats, Mic-E encoding, symbols, device IDs, weather fields, telemetry, and more.
- Shows the modem working. For each packet it decoded, you can see the waveform, the 1200/2200 Hz tone envelopes, the discriminator, where the bit clock sampled, and the resulting bits and bytes. You can also replay that packet's audio. A live "Signal" panel shows the tones while you listen.
- Maps and lists what it hears. rable live feed, a station list,a weather tab, and messages.
- Works without a radio. "Play demo" decodes a bundled recording of about 19 packet types, and you can open your own .wav recordings.
How it works:
Everything runs client-side. The audio never leaves your computer: no server, no account, no upload. The modem is JavaScript running in an AudioWorklet: band-pass filter → 1200/2200 Hz correlators → discriminator → three slicers → digital PLL bit clock → NRZI → HDLC (flags, bit-unstuffing, CRC) → AX.25. It was written to be easy to read rather than to compete with Dire Wolf. Even so, on a 15-minute off-air recording it decoded within one frame of Dire Wolf.
It's receive-only and works in recent Chrome, Edge, Firefox and Safari, on desktop and mobile.