Whisper

Joe, K1JT has developed a new software called WSPR. It is pronounced "whisper", and stands for "Weak Signal Propagation Reporter". It implements transmitting and receiving functions for a digital soundcard mode called "MEPT_JT", which stands for "Manned Experimental Propagation Tests, by K1JT". WSPR generates and receives signals using structured messages, strong forward error correction, and narrow-band 4-FSK modulation. Its principal design goal is reliable copy at very low signal levels. In practice it works well at signal-to-noise ratios down to -27 dB in a reference bandwidth of 2500 Hz says Joe on his web site.

In the screenshot above you can see that I have received and decoded OY3JE´s (Faroe Islands) transmission at -23 dB: His call OY3JE, transmit frequency 10.140087 MHz, QTH locator IP62 and his transmit power 33 dBm (2 Watt).

WSPR is a one-way mode to explore how far away very low power signals can be received. On MEPT Spots the received signals are announced. I am sure of that other applications will happen.