Communications system

A frequency hopping radio communications system has a plurality of radios each incorporating a transmitter and a receiver. When a radio is not transmitting or receiving, a switch switches it into a mode in which it monitors the signal strength in each of the available hop channels so as to assess the level of interference, and selects a subset of the total number of channels, those having the lowest interference level. The identities of this subset of channels are stored in a store. When the radio is to transmit a message, it first transmits the identities of the subset of channels stored in the store-using the full number of hop channels (the information being transmitted with sufficient redundancy to overcome likely interference). The receiving radios receive the identities of the subset of channels. The transmitting radio then transmits, pseudo-randomly, on this subset only and each of the receivers receives on this subset only. In this way, transmission and reception takes place only on the channels where the interference is low.