International Beacon Project. The NCDXF, in cooperation with the IARU, constructed and operates a worldwide network of high-frequency radio beacons on:
14.100,
18.110,
21.150,
24.930, and
28.200 MHz.
These beacons help both amateur and commercial high-frequency radio users assess the current condition of the ionosphere.
http://www.ncdxf.org/beacons.html