1. A method for operating a slave transmitter to provide coherent transmission with a master transmitter, the method comprising: forming a first signal for a data frame for modulation and coherent transmission from the slave transmitter with a transmission of a corresponding signal from the master transmitter; maintaining an estimate of a time-varying phase offset determined from a frequency reference at the slave transmitter and a transmission received at the slave transmitter from the master transmitter; modifying the first signal for the data frame prior to modulation according to a time variation of phase within the data frame determined from the estimate of the time-varying phase offset; and modulating the modified first signal according to the frequency reference at the slave transmitter.