SondeFox

Closed beta

Getting started

From a boxed SDR to your first decoded RS41 frame. This guide takes you through installing the beta build, the settings that matter on first run, and a start-to-finish chase.

Who this guide is for

SondeFox is in closed beta. If you're reading this, you're probably one of a small group of radiosonde chasers helping us validate the app against real launches; thank you. The RS41 decode chain is validated end-to-end against synthetic signals; what the beta exists to prove is live over-the-air performance across real launches, phones, antennas, and sites. Expect rough edges, and please report them to [email protected].

Not in the beta yet? Request an invite, or poke around the live demo in your browser: no hardware needed.

What you need

  • An Android 10+ phone or tablet with USB-OTG support (no root required)
  • An SDR: an Airspy Mini or R2, or an RTL-SDR dongle (R820T/R820T2/R828D tuners, including RTL-SDR Blog V3/V4)
  • A USB-OTG cable. A powered OTG hub is recommended, since these radios draw significant current
  • An antenna for 400–406 MHz: a quarter-wave whip (~17 cm) is fine to start; a small Yagi helps on the ground

For the full parts list, gain-preset details, and antenna notes, see the hardware guide.

Install the beta build

Because the beta is closed, SondeFox isn't in an app store. Testers receive a signed APK directly from the beta coordinator. If a build is due and you haven't got one, email [email protected].

To install from your phone:

  1. Open the APK from your downloads or the email attachment.
  2. Android will ask you to allow installs from that app ("Install unknown apps"). This is the standard prompt for anything outside an app store. Allow it for your browser or file manager.
  3. Confirm the install. Updates install over the previous build; your settings are kept.

Prefer the command line? With USB debugging enabled, install over adb instead:

adb install -r sondefox-beta.apk

The -r flag reinstalls over an existing build, preserving app data.

First-run setup

Do this once on the Set tab before your first chase; it takes about two minutes.

  1. Pick a gain profile

    In the SDR settings, choose the gain preset that matches your environment: Quiet rural (sensitivity), Urban / interference, External LNA, or full Manual gain if you know your LNA/mixer/VGA numbers. When in doubt, start with Quiet rural.

  2. Check the sample rate

    A fresh install defaults to 6 MHz, which suits the Airspy; Airspy owners can leave it, or drop to 3 MHz to spend less CPU. On an RTL-SDR that default gets clamped to 3.2 MHz, a rate that sits at the USB throughput limit and may drop samples, so RTL owners should pick 2.4M explicitly.

  3. Set your callsign and choose your SondeHub opt-ins

    SondeHub Live is currently available only in the separately installed SondeFox Test debug build. Production v0.14.0 does not expose the Network Live source or authorize its REST/MQTT runtime. In the debug build, selecting the optional foreground preview on the 2D Chase map opens a disclosure and an independent Wi-Fi-only or metered-network choice before it can query the viewed area. Native 3D discovery remains fail-closed until its ground-viewport calibration is approved. The preview does not itself upload telemetry. What you send is up to you: uploading your decoded telemetry is off by default and requires both the opt-in toggle and a callsign, and uploads are currently cleared for RS41 and DFM17 only (other types decode locally but are held from upload). Station position and chase-car position sharing are separate toggles, also off by default. Note that your own position can still leave the phone without them: once telemetry upload is on, each record carries your receiver position as uploader_position whenever you have a GPS fix. Details in the SondeHub guide and the privacy notes.

  4. Plug in the SDR and grant USB permission

    Connect the radio via your OTG cable or powered hub. Android shows a USB permission prompt the first time. Accept it, and SondeFox takes the radio from there. No drivers, no root.

Your first chase

With the radio connected and an antenna attached, the loop looks like this:

  1. Start. On the Chase tab, tap Start Chase and accept the USB permission prompt. The status banner walks from searching to connecting; when it clears, the radio is streaming.
  2. Tune. Open the Spec tab. The radio captures a window around one centre frequency (403.5 MHz on a fresh install). Type your target frequency into the MHz field beside Tune: and tap Go. A fresh install has Scan and decode multiple sondes enabled: it probes sonde-shaped peaks across the captured span and camps on any frequency that produces valid frames. Turn it off on the Set tab if you want single-channel operation or lower CPU and battery use.
  3. Watch the List tab. Decoded frames fill the List tab (its screen title is Sondes) with type, serial, frequency, altitude, and SNR. This list shows what your own radio decodes. To follow a flight that other stations are reporting, add its exact serial or payload callsign under Chase > Saved Flights; SondeFox then syncs that one flight from SondeHub.
  4. Chase. Back on the Chase tab you get a live map with your position, the sonde, and its track. The compass pointer with live distance and bearing appears once your phone has a GPS fix. For a predicted landing point, turn on Predict where it will land (SondeHub) under Set > Landing prediction; it is off by default and needs internet. As you close in, the Signal Hunt heat-map takes over for the final on-foot search: walk toward green, and tap Recovered when you have the sonde in hand.
Beta limitation: tap-to-tune isn't wired yet

In the current beta, tapping a peak on the Spec graph does not retune the radio. Peak-tap tuning is designed but not wired in. Use the MHz field beside Tune: and the Go button instead. It's a known gap, not a bug worth reporting.

When to expect launches

Weather services launch radiosondes on a synchronized worldwide schedule: roughly 00:00 and 12:00 UTC, every day. Sondes typically go up 30–60 minutes before the synoptic hour, so plan to be listening a little early. That's two chase opportunities a day, every day.

Troubleshooting

My SDR isn't detected when I plug it in

This is almost always power. Both the Airspy and RTL dongles draw significant current, and many phones can't supply it over a bare OTG cable: use a powered OTG hub. Also check that your cable actually supports OTG (some are charge-only) and that you accepted the USB permission prompt. If the prompt never appeared, unplug, wait a moment, and re-plug.

The app says the Airspy needs to be re-plugged

Occasionally the Airspy's USB control endpoint wedges: the device enumerates but stops answering commands, which would otherwise show up as a silently dead spectrum. SondeFox probes for this on connect and shows a clear banner instead: unplug the Airspy, wait a second, and plug it back in. That resets the device and capture resumes.

I can see a peak but nothing decodes

Check the launch schedule first: outside the ~00/12 UTC windows there may simply be nothing in the air near you. If a launch should be up, confirm you are tuned to the sonde's frequency (the MHz field and Go), try a different gain profile (a too-hot front end in an urban environment can bury the signal), and check the Decode bandwidth (SNR) setting: a narrow preset such as 6.3 kHz only decodes types that fit inside it, while the 25 kHz default covers all types. Also confirm your antenna is connected and roughly cut for 400–406 MHz.

Stuck on something else? Email [email protected]; beta reports directly shape what gets fixed next.

Hardware guide →