Some people have dubbed this activity as the "oddball frequency" exercise. It is presumed the EAM/FDM style messages are being transmitted by Navy ships on a specific scheduled. Call signs tend to the Navy style 3-character alphanumeric variety such as Bravo Nine Mike, of Seven Alfa Yankee. One interesting observation is that the messages themselves seem to be different than the EAM/FDM messages being broadcast on the HFGCS during the same time frame.