A mobile communications system provides communications with mobile users in the interference zones of communications services having priority of use of the frequency band in which the mobile user operates. The frequency band has been previously allocated for use by the other services, such as fixed microwave service (FMS); however, by means of the invention, the mobile user is able to conduct communications operations on a subband or subbands in that frequency band. A position locating system determines the position of the mobile user and if the user is within an interference zone, the mobile user is assigned subbands for operation in the previously allocated frequency band that are outside the subbands of operation of the FMS. A memory accessible by the processor selecting the frequency band of operation of the mobile user includes descriptions of the transmit and receive interference zones of each FMS. The processor selects the frequency subband of operation of the mobile user based on its position relative to the interference zone or zones. The system may have surface and/or space nodes and a network controller that monitors the mobile user's position and controls the nodes and user to subbands. Mobile users are isolated from the fixed microwave sites either by continuous management of their frequency assignments by calculations and commands sent from the network controller to the mobile user or by computations by the mobile user's unit. The mobile system is frequency agile by operating in dynamically selectable frequency subbands.