Listening before transmitting is the most important rule of radio usage. Make sure that no one else is already using the frequency. And don't be fooled by momentary silence; some transmissions are silent for a full 4 seconds between bursts. Listen for a while. It can be difficult to determine through the interference if the frequency is in use. Obviously, if you hear voices, it's busy. If you hear a chirping noise, it's a Pactor III transmission; if you hear a rasping noise, it's a Pactor II transmission. All mean that the frequency is busy. And, obviously, if you hear the familiar dits and dahs of a Morse-code transmission, go find another frequency. The SailMail Association sums up the cardinal rule of SSB operation perfectly: "Not listening first or attempting to connect on a frequency that is already in use is counterproductive, lubberly, pointless, rude, un-shipmatey, and violates international telecommunications law (as well as common sense)."