A method of determining relative phase data of a signal received at multiple antennas, the signal being a phase-modulated data signal having a plurality of periods of constant frequency, the method comprising: within each period of constant frequency, sampling the signal received by a first one of the multiple antennas and sampling the signal received by at least one different one of the multiple antennas; determining from the received samples a data symbol value represented by the received signal for each of the periods of constant frequency; and using the determined data symbol value information to eliminate phase variations between the samples received by the different antennas within each constant frequency period arising from the modulation of the data signal to generate relative phase data for said different antennas.