posted on Apr, 22 2014 @ 07:03 PM
4724 is part of the GHFS system (Global HF System). It is a worldwide network of 13 stations that provide air/ground communications between ships and
aircraft. It is used for phone patches, HF Radio Teletype (RTTY), command and control during missions, emergencies, general broadcasts, direction
finding and ATC support.
Authorized users request service from Global HF System stations by using the general net air-ground call sign "MAINSAIL". Any Global station hearing
the call "MAINSAIL" will respond and provide a service. Depending on the service required, the station may request the user to change frequency for
some other frequencies use are 6712, 6739, 8992, 9025, all USB.
Been listening to these transmissions for years, they are definitely not numbers stations. You will hear many US bases using them, like Andrews, Guam,
Ascension Islands, Hickam, Offat.
Numbers stations are much different transmissions. Numbers stations always transmit the same message, over and over. Very rarely do they change, if
they do it is normally just one or two letters difference. GHFS transmissions are different every time, unless it is an important message that needs
to be transmitted but propagation problems mean it has to be repeated.