Frequencies of radar echoes from the planets Mercury and Venus have recently been measured to about 1 part in 10 10 at times when the line-of-sight component of the relative velocity between Earth and target was as large as 10~4c, thus in principle permitting the second-order "longitudinal" term in the Doppler formula to be tested at about the 1% level.Under the assumption that the "fixed" stars and the Newtonian center of mass of the solar system determine an inertial frame, the Doppler shift A/, derived according to special relativity, is given by 1 * 2 Af V&v \i-^eju-ivij f> (1) where/ is the transmitted frequency; jSj, j5 2 , and j8 3 denote the velocities (expressed as fractions of the speed of light c) of the radar antenna at the time of transmission (t^, of the target planet at reflection (t 2 ), and of the antenna at echo reception (t s ), respectively; and e 12 , e 23 are unit vectors pointing from the antenna at t x to the position of the planet at t 2 and from the planet at t 2 to the antenna at t 3

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