Frequency synthesizers at first and second stations that communicate with each other via a transmission link having a constant propagation delay time are started and synchronized by transmitting a tone from the first station to the second station via the link. The output frequencies of the two synthesizers correspond with each other and occur for the same length of time. At the second station, the tone is detected to cause the derivation of a tone burst having a predetermined frequency and duration. At the second station, a predetermined time interval after the derivation of the trailing edge of the tone burst, activation of the synthesizer at that station is initiated. The tone burst and frequencies derived from the synthesizer at the second station are transmitted in sequence from the second station to the first station via the link. At the first station, the tone burst is detected; detection of the trailing edge of the tone burst initiates operation of the synthesizer at the first station. The frequencies derived from the synthesizer at the second station are received at the first station and are combined with the frequencies derived from the synthesizer at the first station to derive a constant beat frequency as the two synthesizers are stepped.