A software-defined radio system has a plurality of fixed radio receivers each operable to receive radio signals in a receiving band, to sample a received radio signal to produce a sample stream, and to send the sample stream over a network. The radio system includes at least one fixed sync signal transmitter operable to transmit predetermined sync signals in said receiving band to receivers of the aforementioned plurality. The radio system further comprises a data processing system which is connected to the network for receiving sample streams from the receivers. The data processing system is operable to align samples of a data signal contained in sample streams from different receivers by: detecting a sync signal in those sample streams; determining a timing offset between samples of the sync signal in those sample streams in dependence on predetermined locations of the different receivers and the transmitter of that sync signal; and aligning the samples of the data signal in dependence on the timing offset.