Every decoder in SondeFox runs inside libsonde, a portable C++17 core with no radio, USB, or UI dependencies — the exact same code powers the Android app, the host-side sonde_replay CLI, and the desktop SDR++ plugin. This page explains the decode pipeline and gives an honest status for each sonde type.

How decoding works

From antenna to telemetry, one frame of a radiosonde signal passes through this chain:

1. IQ capture

   The Airspy streams 12-bit samples over USB OTG; the capture bridge synthesizes complex IQ and pushes blocks into a lock-free ring for the DSP thread.

2. Channelize & demodulate

   A DC blocker, frequency translator, and decimator isolate the sonde's channel (about ±5 kHz for RS41), then a narrow-FM/GFSK demodulator recovers the baseband waveform.

3. Bit sync

   Symbol-timing recovery locks onto the bit clock (4800 baud for RS41), and matched-filter header correlation identifies the sonde type and finds frame boundaries.

4. Frame decode

   The frame is de-whitened and split into its sub-packets — for RS41, TLV packets carrying serial, GPS, and PTU data inside a 320-byte frame.

5. Error correction

   Forward error correction repairs channel damage — two interleaved Reed-Solomon RS(255,231) codewords for RS41, Hamming(8,4) for DFM — and per-packet CRC-16 checks gate what counts as a valid frame.

6. Telemetry

   The parser converts raw fields into position, velocity, altitude, and UTC time (ECEF → WGS84 for RS41), plus temperature and humidity once enough calibration data has accumulated. Results feed the map, the chase pointer, and (opt-in) SondeHub.

Because the core is hardware-agnostic, every decoder can be exercised offline: sonde_replay plays recorded .cf32 IQ or .wav files through the identical pipeline, which is how new decoders are developed and regression-tested before they ever touch a live signal.

Status matrix