Why WSPR stands for Weak Signal Propagation Reporter, a mode designed specifically for propagation beaconing. Stations transmit a short, highly compressed message (callsign, grid square, power level) using very narrow 4-tone FSK over a two-minute window, then listen and upload decodes to a central database. Because the mode works far below the noise floor, operators can run a watt or less and still see which paths are open worldwide.
Watch out MFSK16 and PSK31 are keyboard-to-keyboard conversational modes, not automated beacons, and SSB-SC (suppressed carrier single sideband) is an analog voice technique, not a digital mode at all.
WSPR sounds like 'whisper': a tiny signal whispering around the world to map propagation.