WSPR is a way of sending a tiny amount of data over huge distances using very little power. It’s used by radio enthusiasts to see what propagation conditions are like: receivers all over the world report what they hear to WSPRnet which serves as a real-time map of the radio world. The protocol itself is amazing: it takes nearly two minutes to send just 50 bits of data, and has such clever error-correction that only a tiny amount of received signal is needed to get a clean decode. Seriously, if you’re at all interested in information theory, then read the spec (at Appendix B of that link).
