Hello Krzysiek,

the balloon are 90cm or 36" party balloons. You can get them at ebay or other suppliers.
The Sonde is an RS41 former radiosondy. We can find them a lot here coming down from the weather services.
The firmware can be flashed accoring to this manual
To track the sonde on ground you can use Windows or Linux tools on PC or Raspberry.

Hi Karol,

thanks for receiving!
-0.5 deg (401km) are excellent values. You must have a very good equipment over there
My balloon decided to land now. But we are very happy that you are still feeding data for the others.

I wish a nice evening to all 4FSK receiving stations! :-)