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The \'elocity of propagation of electromagnetic waves was measured at t he surface of t he ear t h, using a radio-wave interferometer operating at a freq ucncy of 172.8 Mc. The measured phase velocity, converted to velocity in vacuum, or t he " free-s pace" value, was found to be 299795.1 ± 3.1 km/sec . The uncertainty of ± 3.1 km/sec includes a 95-percen t co nfiden ce in terval for t he mean , plus an estimated limi t to t he systematic e rro r of ± O.7 km/sec. Based on a 50-percent confidence interval (probable e rror of the mean ), th e uncertain ty, including t he est imated li mit to the systematic error of ± O.7 km/sec, becomes ± 1.4 km/sec . The accuracy with which t he free-space phase velocity of radio waves could be mcasurcd was limited primarily by t hc accuracy to which t he refractive index of a ir could be obtained from measured values of pressure, temperatu re, and relative humidity.

A measurement of the velocity of propagation of very-high-frequency radio waves at the surface of the earth