SondeChirp monitor Experimental
An optional chirp-spread-spectrum (CSS) receiver built into SondeFox. It watches chirp-modulated channels (the long-range modulation used by popular maker and amateur telemetry gear) with the same SDR you chase sondes with. Receive-only, off by default, and intended only for signals you are authorized to receive.
The monitor recovers chirp-modulated signals and includes Meshtastic packet decoding. Meshtastic payload decryption uses the public default key or a key supplied by the user; it does not extract keys or bypass unknown private keys. The app does not join or transmit on radio networks. Reception and payload use must be authorized under the rules that apply to you.
What it is
It is a CSS demodulator: a dechirp plus FFT symbol decoder with preamble detection and payload bit assembly. Point it at a chirp-modulated channel and it recovers the raw symbol stream and assembled payload bytes from the spread-spectrum signal. This is the same signal-level view you would build on a desktop SDR, running on your phone.
- Symbol decode: dechirp against a locally generated reference chirp, then an FFT picks the symbol bin.
- Preamble detection: locks onto the upchirp preamble before attempting payload assembly.
- Bandwidths and spreading factors: 125 / 250 / 500 kHz, SF7 to SF12 with blind spreading-factor detection, and multiple channels watched from a single SDR capture.
Recent beta builds have extended it further:
- APRS-over-CSS decode: recognizes and decodes chirp-modulated APRS traffic, the tracker format many chasers already run in their cars.
- Region presets: one-tap frequency and parameter presets for the common regional ISM allocations chirp gear uses.
- Custom presets: define your own frequency, bandwidth, and spreading-factor combinations, including bandwidths below 125 kHz.
Why it exists
Sonde chasers tend to carry more than one radio hobby into the field. Many run chirp-modulated APRS trackers or mesh-radio nodes in the chase car, and it is genuinely useful to point the SDR you already have at your own kit: confirm your tracker is actually beaconing, sanity-check its parameters, and watch your own telemetry come through while you wait for the next launch. The chirp monitor gives you that with no extra hardware, using the same Airspy or RTL-SDR and antenna setup you chase with.
Off by default
The monitor ships in beta builds as the LoRa tab, but it does nothing until you deliberately switch it on. The toggle is Set > Experimental > Experimental LoRa PHY Monitor, off by default, and the monitor also needs a running chase: start a capture from the Chase tab first. Chirp demodulation is CPU-heavy on a phone, so it stays off your battery until you enable it.
Expect rough edges: this is the least mature part of SondeFox, and parameters, presets, and UI may change between beta builds. If it misbehaves, turning the toggle off returns the app to its normal radiosonde-only behavior.
Current status
The PHY chain (dechirp, FFT symbol decode, preamble detection, payload assembly) is implemented and verified against synthetic chirps; the native tests pass. Over-the-air validation against real, user-owned transmitters is an active part of the beta. Beta testers with their own chirp-telemetry gear are exactly who we want feedback from. Report what works and what does not to [email protected].
Public distribution of the retained LoRa/Meshtastic components remains subject to independent provenance, licensing, and patent review. Implementation and synthetic test results do not establish legal clearance.