Phase comparison of remote oscillators by tone transmission via communication satellites is considered. By introducing loop-back tones at each station to produce a virtual downlink frequency equal to the uplink frequency, the path phase between two stations is made independent of the direction of transmission. All nondispersive path effects are then canceled by using reverse path transmissions. The effective period for phase comparison is the mean period of the uplink and downlink transmissions. This represents a 2–order of magnitude increase in effective frequency over the previously used two-tone method. The precision of phase comparison via satellites is now limited by uncertainties of ionospheric effects.