Thanks to information provided by Trevor VK8TH, I am now receiving data from Radiosondes. In my carefully cataloged equipment storage facility (junk box) I had a spare Pi-3 and RTL-SDR . I am using software called Radiosonde Auto-RX developed by Mark Jessop VK5QI.
Here is a Youtube video from Mark Jessop VK5QI explaining how it all works.
I am feeding the received data to aprs.fi
Radiosondes are usually in the air just before 00.00z and 12.00z.