“If the band is open and nobody is transmitting, can anybody here it?”
Amateur radio operators have been exploring radio propagation for decades. They have come to utilize reflection from the ionosphere, tropospheric ducting, tropospheric scatter, reflection off the moon, and various other modes in which radio waves are transmitted beyond line of site to achieve communications.
Some DX hobbyists like myself enjoy receiving distant stations beyond the distances normal conditions and methods allow. This can involve optimizing equipment, developing techniques for detecting weak signals buried in noise, knowledge of various propagation modes and when they are likely to occur, and just plain patience and luck.
Amateur radio operators have come up with a new way to exploit digital technology and the Internet to allow mapping of radio propagation at various wavelengths in real time. The technology consists simply of beacons that broadcast digitally encoded messages on fixed frequencies at regular frequent intervals and receiver stations that listen for these beacons and when they receive one report back to a central internet server that then records that reception and maps that propagation path in real time making it possible to develop maps, updated in real time, of radio propagation at various frequencies.
Propnet.org is a the result. Presently the mapping is pretty sparse, however, as more people become aware of this project and join it will become more complete. You need an amateur radio license to participate as a transmitting beacon but anyone with the proper receiver equipment can participate as a receiving station.
In addition to this mapping project there are also links to five day tropospheric ducting forecasts and radio and television DX’ing.