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A method of measuring interplanetary plasmastream velocities using bistatic-radar transmissions from earth to a spacecraft is proposed. Nonrelativistic plasma velocities are considered, and the presence of a uniform interplanetary magnetic field is included. The effect of the streaming motion of the plasma medium is shown to introduce a second-order Faraday polarization change on a linearly polarized transmitted wave. Furthermore, this small second-order effect can in principle be conveniently measured using twoway propagation between the two transmission termini. This method represents an improvement over existing means of velocity measurement using plasma probes since the radar measurements establish the average stream velocity over the straight-line propagation path as opposed to the point samples of the plasma probe.