SondeChirp monitor Experimental
An optional, flag-gated chirp-spread-spectrum (CSS) receiver built into SondeFox. It watches a single chirp-modulated channel — the long-range modulation used by popular maker and amateur telemetry gear — with the same Airspy you chase sondes with. Receive-only, off by default, and strictly limited to physical-layer analysis of signals you are authorized to receive.
The chirp monitor does no key extraction, no payload decryption of any kind, no cellular or private-network decoding or interception, and no detection evasion. There is no network or MAC layer in the app at all — it never joins, associates with, or transmits on any network. It recovers PHY-level bits from signals you own, transmit yourself, operate under your amateur licence, run as ISM-band experiments, or are otherwise legally authorized to receive — nothing more. Reception rules vary by jurisdiction: check your local law before enabling it.
What it is
Under the hood it is a single-channel CSS demodulator: a dechirp + 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 — 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–SF12, single channel (performance permitting).
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/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 and antenna setup you chase with.
Off by default — how to enable it
The monitor is doubly gated: a compile-time build flag (off by default) and a runtime toggle under Settings → Experimental, also off by default. Chirp demodulation is CPU-heavy on a phone, so it stays out of your way — and off your battery — unless you deliberately switch it on.
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.
Honest status
The PHY chain — dechirp, FFT symbol decode, preamble detection, payload assembly — is implemented and verified against synthetic chirps (all native tests green with the flag enabled). Over-the-air validation against real, user-owned transmitters is an active part of the beta: real-hardware iteration is ongoing, and beta testers with their own chirp-telemetry gear are exactly who we want feedback from. Report what works and what doesn't to beta@sondefox.com.
Provenance note: several open-source chirp-demodulator projects served as algorithm references only — no code from them is vendored into the build. Full project-by-project credits ship with the app's third-party notices, alongside the acknowledgements.