Weather what?#

Internationally, weather balloons are used to collect all sorts of data such as temperatures, humidity and pressure whilst transmitting those metrics (and their location) in the ~400MHz band, to be picked up by anyone. Of course, the balloons themselves do not transmit this data, instead, a little radiosonde is tied to the balloon. These little transmitter critters come in a variety of shapes and sizes which makes them perfect for a ‘catch ‘em all’ collection. They are (usually) made out of styrofoam presumably to prevent these things from knocking someone out on their way down.

Once launched from one of their countless sites, the balloon along with the sonde will climb up to ~30km (that’s ~18.5 miles for all my imperial enjoyers) and eventually pops due to the expansion of the gas inside them. It is then up to the wind to carry it wherever it pleases.

You hunt these things?#

Yes, and not just me. There’s a constantly growing community of weather balloon hunters out there doing their best to recover as many as they can. You could too! With only an SDR (~€40), a laptop and a way of getting from point A to point B, you can start getting all sorts of weather balloons.

The whole process can be easily broken down into a few phases as listed below.

1. Preparation

   Unless you’re doing what we call a ‘cold hunt’ (which is finding an already landed radiosonde that doesn’t transmit anymore), you’ll most likely need some way of tracking down the balloon. Most likely you already have a few people nearby that run radiosonde-auto-rx. What this program does is it listens to a set frequency band, usually between 400MHz and 406MHz for any sondes and, if found, publishes their data to SondeHub, a database of realtime data from listeners all over the world.

   projecthorus

   /

   radiosonde_auto_rx
   Waiting for api.github.com...
   00K

   0K

   0K

   Waiting...

   Following balloons this way only works if it is landing very close to any of said listeners. Usually trees, buildings, terrain elevation and other obstructions block the signals of radiosondes as they descend below a hundred or so meters. In this case you’ll need to run radiosonde-auto-rx along with chasemapper yourself on a laptop or Raspberry Pi to get the signals while being closer to the transmitter. While radiosonde-auto-rx stands in for the collection of data, chasemapper will plot these on a map and optionally makes local predictions for the balloons. There are also alternative hardware and software solutions like rdz_ttgo_sonde and mysondygo that make it even more portable (no laptop necessary) to hunt balloons.

   TIP

   I also recommend bringing a telescopic pole as this could be a lifesaver if a sonde lands in a small tree or a lake.

   I won’t go over how to set up any of these solutions but there are numerous resources available online. My personal radiosonde-auto-rx stack can be found below.

   bddvlpr

   /

   chasemapper-stack
   Waiting for api.github.com...
   00K

   0K

   0K

   Waiting...

2. Planning

   Start by identifying any potential sites that might be in range. Visit SondeHub to check out potential launch sites nearby. Look for small gray circles on the map. Pressing on them reveals their usual launch times (though in my experience, these can be inaccurate) and which types of sondes they launch.

   By pressing on the Generate Predictions button of one of these launch sites, SondeHub will display predicted paths using tawhiri (a prediction model). These predictions could range from almost spot on (~1km off) to devastatingly off (~15km off). This is why once a balloon launches, it’s best to watch the live predictions and decide from those whether it’s worth chasing.

3. Chasing

   Once you’ve decided on a balloon, start by going towards its general predicted landing area. It’s best to keep the live predictions in mind and plan as they change, don’t drive out to the exact predicted landing position as this is never accurate. Once the balloon pops, the predictions become quite a bit more accurate as the exact descent time can never be predicted. Try to place yourself between the balloon and it’s predicted landing zone to maintain a good signal while still being able to maneuver as its path changes.

   WARNING

   Depending on the manufacturer, model and parameters set by a launch site, balloons might use a burst timer. This timer automatically shuts down the transmission process of a radiosonde after a set amount of time that is configured by the launch site. Keep this in mind when hunting already landed radiosondes.

4. Hunting

   As the balloon gets close to landing try getting line-of-sight of the balloon as this makes it a lot easier to find them. This shouldn’t matter too much in open fields when there’s plenty of GPS reception but closed off woods could interfere with its position data making it harder to find. Using your navigation software and the last known coordinates try to locate the weather balloon. Please do not trespass on private property or industrial sites; always ask someone for permission.

That’s basically all that there is to it. I’ll probably release a more in-depth guide in the future but for now this small summary will do. All pictures are either mine or Marleen’s (DF4RBM), big thanks to her.

Some tips#

• If possible, bring a backup SDR as these things can be pretty volatile.
• When working with large distances, preamp + SAW filters do wonders. I personally use Uputronics’ 403MHz filtered preamp which does an amazing job at increasing my signal strength and filtering out noise.
• Drive safe and responsibly. This is not a treasure hunt and should always be treated as a hobby.
• If you are the first to arrive at a landing site, please please please stay there for the next 5-10 minutes. Most hunters should arrive within this time and spares them a lot of time searching for a sonde that’s already gone. While you’re waiting, you might as well mark the sonde as recovered in SondeHub.
• Any radiosondes with a bigger, very noisy cube attached to them are ozonesondes. This cube contains instruments used to measure ozone concentration from the surface through the mid-stratosphere. While you do not need to return radiosondes, these cubes contain the necessary paperwork to be returned to their launch sites as these do get reused. Here in Belgium, the full shipping price is covered by the national meteorological institute and they may even send a small reward your way. :)