Abstract: An HF communications system comprising a plurality of radio stations, each radio station including a transceiver and a control means therefor, the control means including a processor and a memory, the memory storing a model of the ionosphere of a designated region in digital form, wherein the order to establish reliable communications channels between the stations, the system undergoes the following procedure: a first station computes from its stored model a propagation frequency window and determines a set of communications channels within the frequency window, the transceiver of the station thereafter monitoring the noise and interference present in the set of channels in order to reject channels having noise and interference levels higher than a predetermined level, and the first station establishing communications with one or more second stations on communication channels of the set in order to actively sound the channels in order to establish which channels are suitable for message transmission.