Frequently asked questions

A radiosonde is a small instrument package carried aloft by a weather balloon. As it climbs, it broadcasts its position, altitude, temperature, and humidity on the 400–406 MHz band as part of the public meteorological service. Weather agencies launch them worldwide roughly twice a day, around 00 and 12 UTC. Eventually the balloon bursts and the sonde falls back to earth — which is where the chase begins.

SondeFox scans for these signals, decodes the telemetry, maps the flight with a predicted landing point, and guides you to the sonde on the ground.

In most places, yes — weather services generally don't collect landed sondes, and finders keepers is the norm among chasers. Rules do vary by country, though, so check your local guidance before making a habit of it. And wherever you chase: never trespass. If a sonde lands on private property, ask permission first.

Generally no. Radiosonde telemetry on 400–406 MHz is unencrypted, broadcast for the public meteorological service, and receiving it is legal in most jurisdictions. SondeFox is receive-only — it never transmits.

Laws vary, so check your local regulations before going further than listening — in particular before uploading telemetry or chasing. See license & legal for the longer version.

Yes. SondeFox is free and open source under the GPL-3.0 — that covers the Android app, the portable libsonde C++ decoder core, a host-side replay CLI, and an SDR++ desktop plugin, with third-party provenance documented openly. During the closed beta the app is distributed by invitation; see the beta program for how to get in.

When beta validation completes — there's no date to announce. The RS41 decode chain is validated end-to-end and live Airspy reception works on real hardware; the closed beta exists to prove it across more devices, antennas, and launch sites before a wider release. If you'd like to help get it there, join the beta — or poke around the live demo right now, no install needed.

Three things: an Android 10+ phone or tablet with USB-OTG support, an Airspy Mini or Airspy R2 SDR connected over an OTG cable, and an antenna for 400–406 MHz — a quarter-wave whip (~17 cm) works, a small Yagi works better. A powered OTG hub is recommended, since the Airspy draws significant current. Full details in the hardware guide.

No. SondeFox drives the Airspy Mini and Airspy R2 directly over Android's USB host API, and those are the only supported receivers. RTL-SDR dongles are not supported.

Any Android 10 or newer device with USB host (OTG) support. Because the Airspy draws significant current over USB, some phones need a powered OTG hub — we recommend one for long chases regardless. A foreground service keeps capture and decode running with the screen dimmed, so the app survives a pocket.

No. SondeFox talks to the Airspy through Android's standard USB host APIs — no root, no kernel drivers, no custom firmware. Plug in, grant the USB permission prompt, and you're receiving.

RS41 The Vaisala RS41 is the working decoder: validated end-to-end, with live Airspy reception working on real hardware. Wider field validation is ongoing — that's exactly what the beta is for.

DFM06/09/17 · M10/M20 Graw DFM and Meteomodem M10/M20 decoders are implemented and tested on synthetic signals, but still in validation against real RF.

iMet · LMS6 · Meisei · MRZ · MTS01 On the roadmap — not decoding yet.

The decoder support page tracks status in detail.

Yes. Decoding is fully local — no cloud in the signal path — and map tiles are cached, so the chase map keeps working without coverage. The last known target and landing prediction are cached too, so a chase in progress continues offline.

SondeHub features (seeing what other stations hear, community landing predictions, uploads) naturally need a connection; if you've opted in to uploading, telemetry still within the upload window is flushed when you reconnect.

No, and none is planned. SondeFox exists because Android's USB host API lets an app drive an SDR directly over OTG with no root and no drivers — iOS offers no equivalent path for this kind of hardware access.

Not unless you deliberately switch it on. Fetching public SondeHub data is passive. Uploading decoded telemetry is off by default and requires an explicit opt-in plus a configured callsign — and even then, uploads carry only the sonde's position, never yours. Your own chase-car position is uploaded only if you enable a separate, clearly labelled chase-car mode toggle, which is off by default.

One honest caveat: anything you do upload joins the public SondeHub stream and may be redistributed. Details in the SondeHub guide and the privacy notes.

No. The experimental chirp monitor is off by default and recovers raw PHY-level bits from chirp-spread-spectrum (CSS) transmissions only — it's meant for signals you're authorized to receive: your own devices, amateur-radio telemetry under your license, ISM-band experiments, and other legally authorized public telemetry.

It contains no key extraction, no payload decryption of any kind, and no private-network interception — that's a deliberate design boundary, not a missing feature. See Chirp monitor.