Weather balloons, also known as radiosondes, are one of the most exciting targets for SDR (Software Defined Radio) hobbyists. These automated weather measurement devices are launched hundreds of times daily worldwide, collecting atmospheric data and transmitting it via radio signals to ground stations.
With SDR technology, it’s possible to decode these signals at home with a simple receiver and an antenna — allowing us to track weather balloons in real time as they ascend up to 10-30 km altitude, transmitting temperature, humidity, pressure, and GPS position.
In this article, you’ll learn:
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What is a radiosonde and how does it work?
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How to decode its signals at home using SDR
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What hardware and software are required
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What frequencies they use
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Practical examples and tips — suitable for beginners!
What is a radiosonde?
A radiosonde is a small electronic device carried into the stratosphere by a hydrogen or helium-filled weather balloon. While ascending, the radiosonde continuously measures:
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Temperature
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Relative humidity
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Air pressure
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GPS position (latitude, longitude, altitude, speed)
The radiosonde transmits this data continuously via radio to ground stations.
Radio amateurs and SDR hobbyists can also receive these signals and decode them live.
What does a typical radiosonde look like?
Main components:
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Plastic or foam enclosure
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Sensors (temperature, humidity, pressure)
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GPS module
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Radio transmitter (typically 400-406 MHz or 1680 MHz)
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Power supply (AA batteries or lithium)
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Antenna (usually a simple wire or spiral antenna)
How does it fly?
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Typically launched once or twice daily by weather stations (e.g. Budapest, Pápa, Debrecen).
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Ascent rate: 5-6 m/s
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Max altitude: 25-35 km (before balloon burst)
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Descent: parachute or balloon remnants.
Why is radiosonde tracking fun with SDR?
Because:
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You can monitor live meteorological data!
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You can track the balloon’s ascent and drift.
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You can display flights on tracking maps.
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You can recover radiosondes after landing (radiosonde hunting).
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It’s a great way to practice antenna and SDR tuning.
What frequencies do radiosondes transmit on?
Common in Europe (and Hungary):
1. 400 – 406 MHz UHF band
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The most common band.
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Typical radiosondes: Vaisala RS41, RS92, M10, DFM09, DFM17.
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Example frequencies:
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403.000 MHz
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404.000 MHz
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405.500 MHz
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2. 1680 – 1700 MHz L-band
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Less common, but used in some countries (e.g. Southern Europe, USA).
Important:
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Frequencies vary by country and station.
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In Hungary and most of Europe, the 400-406 MHz band is most widely used!
What hardware do you need?
1. SDR receiver
For beginners:
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RTL-SDR v3
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Perfect for radiosondes.
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Price: ~$30-40 USD.
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For advanced users:
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SDRplay RSPdx
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Airspy Mini / R2
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LimeSDR
→ Advantages: higher sensitivity, wider bandwidth, better noise filtering.
2. Antenna
Minimum:
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400 MHz-tuned dipole antenna.
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Discone antenna (wideband).
Ideal:
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Quarter-wave ground plane antenna.
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Yagi antenna (directional, high-gain).
Tip:
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If you’re serious about radiosonde tracking, a directional Yagi antenna tuned to 400-406 MHz gives excellent performance.
3. Accessories
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USB extension cable (to keep the SDR away from computer noise).
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High-quality coaxial cable (e.g. RG58 or better).
What software should you use?
1. SDR control software
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SDR# (SDRSharp) — Windows
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SDR++ — Windows/Linux/Mac
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HDSDR
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CubicSDR
2. Radiosonde decoding software
a) SondeMonitor (paid)
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Professional solution.
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Highly detailed data visualization.
b) RS41 Tracker (free, Windows/Linux)
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For Vaisala RS41 sondes.
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Simple but effective.
c) Sondehub + auto_rx (Linux, Raspberry Pi)
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Automated tracking and upload to Sondehub map (https://sondehub.org).
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Very popular in the international radiosonde community.
d) SondeMonitor fork / RS41ng — community-developed options for Linux/Windows.
How to decode — step by step
1. Set up your antenna
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Place outdoors or near a window.
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The higher, the better reception.
2. Configure your SDR receiver
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In SDR# or SDR++, set:
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Frequency e.g.: 404.000 MHz
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NFM or USB mode (depending on sonde type).
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Sampling rate: 2-2.4 Msps (ideal for RTL-SDR).
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3. Start the decoder
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Example: RS41 Tracker or SondeMonitor.
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Connect via virtual audio cable or direct sampling.
4. Track the flight
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The decoder will automatically detect and decode:
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Altitude
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GPS position
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Temperature
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Humidity
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Pressure
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Speed
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Landing location after descent!
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Where to find current radiosonde flights?
Online maps
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https://sondehub.org
→ Live worldwide radiosonde map! -
Local SDR groups (e.g. Hungarian SDR Hungary Facebook group)
→ Up-to-date frequencies and launch info.
FAQ
How difficult is it to start radiosonde tracking with SDR?
→ Very easy! An RTL-SDR + dipole antenna + free software can be set up in just a few hours.
Can I track radiosondes mobile (from car or in the field)?
→ Yes! Laptop + RTL-SDR + car antenna = mobile radiosonde hunting.
How often are radiosondes launched in Hungary?
→ Example: Budapest Ferihegy station:
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Twice daily (morning and evening).
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Similar schedule in other cities.
How can I “catch” a radiosonde?
→ If the sonde transmits landing position, you can locate and recover it in the field!
→ There’s a great community of sonde hunters who do this regularly.
Summary
Radiosonde decoding with SDR:
✅ Low cost — can start with an SDR under $50
✅ Visually exciting — live maps, real-time tracking
✅ Useful — real meteorological data
✅ Great way to practice antenna and SDR tuning
✅ Fun — and can become an adventurous hobby (hunting)!
If you haven’t tried it yet — I highly recommend it! 🚀🎈📡