New constants are derived for the equation relating the radio refractive index, n, at frequencies below 20 GHz to the pressure, temperature, and humidity of the air. The new equation is (n-1) X 106 = N = 77.6(Pa/T)Za -1 -Jr-64.8(e/T)Z,o -1 -Jr-3.776 X 105(e/T2)Z,o -• where Pa is the partial pressure of dry air (mb), T .is the absolute temperature (K), e is the partial pressure of water vapor (mb), and Z•-X and Z•o -x are the inverse compressibility factors for dry air and water vapor, respectively. Improved values of the constants are obtained by considering the relationship between the radio and optical portions of the electromagnetic spectrum. With the use of the full three-term equation including non-ideal gas law effects, the accuracy of the new radio refractive index equation ranges from 0.018%, or about 0.05N, for dry air to 0.048%, or 0.21N, for extremely moist air. sions No. AD-707-919 from the National Technical Information Service Operations Division, Springfield, Va. 22151. Edl•n, B. (1966), The refractive index of air, Metrologia, 2, 71-80. Erickson, K. E. (1962), Investigation of the invariance of atmospheric dispersion with a long-path interferometer, J. Opt. Soc. Amer., 52, 777-780. Essen, L., and K. D. Froome (1951), Dielectric constant and refractive index of air and its principal constituents at at 24,000 Mc/s, Nature, 167, 512-513. Owens, J. S. (1967), Optical refractive index of air: dependence on pressure, temperature, and composition, A ppl. Opt., 6, 51-58. Smith, E. K., and S. Weintraub (1953), The constants in the equation for atmospheric refractive index at radio frequencies, Proc. IRE, 41, 1035-1037. Strickland, A. C. (1942), The dielectric constant of water vapour and its effect upon the propagation of very short waves, RRB 1594,
