Some fellow hams recently got me interested in radiosonde tracking, with SondeHub being one of the sites for tracking them.
What’s a radiosonde, you might ask? They’re transmitters attached to weather balloons, as documented by NOAA here. They have a GNSS receiver, along with temperature and humidity sensors, plus a radio transmitter in the 401-406 MHz range.
They can be tracked with an SDR using something like radiosonde_auto_rx. Or, an esp32 board and rdz_ttgo_sonde.
Seems like something to set up alongside my ADS-B receiver!