Since 1965 the dual-frequency method has been used for measuring densities of plasmas near planets and in interplanetary space. Signals at two frequencies are sent from an antenna on the earth to a spacecraft, or vice versa. The speed of the higher-frequency signal is less affected by plasma than that of the lower-frequency signal. The difference of delays provides a measure of the integrated effect of plasma along the radio path. In essence, the higher-frequency signal serves as a benchmark against which the lower-frequency effect is measured. Two markedly different sets of equipment have thus far been used to perform dual-frequency observations; their strengths and weaknesses are reviewed, and it shown that many of the weaknesses could be overcome by using separate sites on the earth from which to transmit the low and high frequencies. A survey of the expected error in measurements using separated sites suggests that the critical element is time synchronization between them. The best available device for performing this function is the cesium beam clock; six such clocks were tested under conditions that accurately simulate an operational system. The test results are related to errors that would arise in plasma densities derived by this method. While the feasibility of site separation is dependent upon the specific mission to be accomplished, it appears practicable at the present state of the art for many planetary, solar wind, and comet applications.