>>59310>I thought they were made up.
They're very real, there are a lot of them, and some of them sound quite creepy. The mystery around them has been shredded however. In short, they're broadcasting benign military exercise commands and info. Info such as "A plane flying West over position X at 0600 is friendly, do not report or engage" read in a simple but ever-changing code. Position X might be in an area where access to electricity is sparse, so the signal strength is very high so that small compact radios like consumer solar pocket CBs can intercept it. The substance of the information is mixed in with static, strange noise, and scheduled nonesense. Stationary receivers pass the audio output through a series of analogue and digital filters that remove most of the noise, but even if such cleanup isn't available the code can still be heard.
A popular one was triangulated a few years ago in southwest Russia. When some radio enthusiasts noticed it changed locations, they went to the initial position and found a small bunker a few blocks from residential buildings. Inside they found a mostly empty bunker but behind some tables or something they found maps and an old codebook that stated something like "during a certain span of days of the year, the transmitted numbers would correspond to these letters, and the transmitted letters would correspond to these positions and times". If the enthusiasts could find a recording of a broadcast from the dates outlined in the book, they'd be able to decipher it in a matter of minutes.