⁍ Written by Mitchell Hamm, N8XS

In March of 2023, I saw a new rabbit hole that I almost slipped down. My ham club was invited to the National Weather Service station in Wilmington, Ohio and it was a very memorable and worthwhile trip. We learned how they sample the atmosphere with weather balloons launched twice daily and use the data to feed complex models that predict our weather. These latex balloons are filled with hydrogen (much cheaper than helium) at a gas house situated a safe distance away from the NWS office and then it’s a simple process of walking out, releasing the balloon and letting it take the payload (a radiosonde) out of your hand as it rises. The balloon floats on the wind rising steadily until it finally bursts above 100,000 feet then a small, orange parachute brings the radiosonde back to earth.

These three frames show the launch we witnessed:

In the NWS office they had more radiosondes lined up and ready to go:

These little marvels are made by Graw Radiosondes GmbH & Co. in Nuremberg Germany and pack a lot of tech into their 35 grams. Inside they have a transmitter set for the 400-406 MHz range, a ROBLOX GPS chip, two batteries supplying 3.3V DC, and a ribbon header that connects to a sensor pack including a thermistor and humidity sensing element for taking measurements throughout the flight. Hanging off the board is a 180mm wire antenna which is 1/4 wavelength of the transmit frequency. They are encased in Styrofoam to protect the batteries from the -80 F (-62 C) temperatures at 20 miles above the earth.

They can be repurposed for ham radio use with the open source package RS41ng. This works with Vaisala RS41 and Graw DFM-17 radiosondes and allows customization of the transmit frequency, interval and power. You can also select which mode you want: CW, intermittent pips, APRS, HORUS and CATS. This would allow you to launch your own weather balloon or equip a model rocket with a sensor pack or do fox hunt exercises.

Tracking the radiosondes is simple from SondeHub. You can follow your local launches and see accurate predictions of where they will land. The website can show all past landings and will predict a slew of future landings based on weather patterns. This is also where you publish your intentions to chase a balloon and to report planned, attempted, failed, or completed recoveries. All of this data is fed to SondeHub from ground stations around the country that listen to the balloons as they fly and make for very accurate telemetry. This screen cap from SondeHub shows the path of DFM17 25013229 from the morning of 15 June 2025 near Cincinnati. The green line from south of the balloon is the KA8IBZ ground station receiving a transmitted frame from the radiosonde.

Making a ground station only takes a Raspberry Pi, an RTL-SDR and an open source package called radiosonde_auto_rx. It will scan for signals in a defined range and send you an email as soon as it hears a radiosonde after launch. Each flight returns thousands of data points and an archive is maintained on the Pi for post-flight deep dives into the data. This extracted KML shows latitude, longitude and altitude in Google Earth Pro for the radiosonde I retrieved on 15 June.

Antennas are very easy. Even though the radiosondes have low transmit power, the height of their antenna makes them receivable over 100 miles away. An hour in the workshop can turn out a very efficient antenna. But honestly any 2M/70CM will hear them just fine.

My slip down the rabbit hole happened at this year’s Dayton Hamvention. Only two weeks before the event, an iPhone version of a popular Android app called “My Sondy Go” became available. It works with an STM32-based device to lead you directly to any radiosonde you can hear down to a few feet. I found one prebuilt and ready to go in the flea market and $65 later I was ready to hunt.

My hunt yesterday was textbook perfect. I turned on the MY SONDY GO receiver and fired up its app. In just a few seconds I got a lock with the app showing me the balloon’s altitude, ground speed and distance from me. It was still 40-ish miles away and 9 or 10 miles up, but descending (on its parachute I hoped). I found it on Sondehub and touched the screen for its landing prediction then pasted that latitude and longitude into Google Maps on Carplay. I also tuned my dash mounted Yaesu to 403.811 MHz so I could hear the transmitter. It was singing loudly. With my audiobook playing, I headed southeast. Once in the neighborhood, I got a new predicted landing spot from Sondehub and refined the directions in Google Maps. The balloon was within a couple of miles and nearly on the ground. My hope of seeing it descend didn’t pan out as it landed when I was still a mile away under tree cover. I came out of the woods onto a big plain checkered with farm fields and the transmitter started to roar from the Yaesu. It had landed in a big field just ahead on the left and I pulled down a road leading to a honey farm hoping to get permission from them to retrieve it. I could see the orange parachute in the distance. No one was around so I tried the house in front of the field and that turned out to be the land owner. He was quick to OK the walk on his property and off I went. The balloon was laid out nicely like it wanted to have its picture taken. I snapped a few, recorded its landing location (39.152396 N, 84.103464 W), turned off its transmitter (a rookie mistake is forgetting that step) and wound up the parachute, tether and the remains of the balloon which tangles in the tether when it explodes. I began to walk back towards the truck when an interesting twist occurred.

I heard a “Hey there!” and looked toward the land owner’s house. Nothing. I looked at the house across the road. Nothing. As I started to walk again I heard “Hey there!” and saw a man approaching from the honey farm. I turned toward him and once he got within hearing range I wished him a Happy Father’s Day. I pointed to the house and told him I had talked to “the man over there” about entering the field, gave him my name and explained what I was doing. His entire demeanor changed and he started asking questions about the balloon and its payload. Turns out he was a ham! KC8FQP joined the Army back in the 90s and became a radio operator-maintainer (MOS:31 Charlie) at Fort Bragg. He had taken the ham theory classes to better understand HF communications since his Army courses were a bit light on that. He even recalled his instructor’s callsign (KJ4RV) and told me about the radios he used to work with and about Harris Communications bringing in new models for them to test out. It was a nice conversation and made the recovery even better.

After a pleasant drive home, I met up with my lovely daughter for dinner and she, as usual, knocked it out of the park with her handmade card.

To learn more, check out the Facebook group: Radiosonde North America - NWS Weather Balloon

73,

Mitchell Hamm N8XS