f a microwave using partial reflections, J. Geophys, Res. 69. 2799. cavity for measuring changes in a resonance frequency, which in Beynon, W.J.G. (1967) Special issue on analysis of ionograms for turn is related to the refractive index of the enclosed air. Such electron density profiles, ed. URSI Working Group, Radio refractometers are robust, rapid-response instruments which have Science 2. 1119. been widely used as research tools, though they have yet to be Beynon, W.J.G. and Rangaswamy, S. (1969) Model electron density developed in a form suitable for widespread, routine use. profiles for the lower ionosphere, J. Atmosph. Terr. Phys. 31. High-power, centimetric radar is also a valuable technique. By its use it is possible to detect layers or other regions of strong scatter in Blair, J.C., Melanson, L.L. and Tveten, L.H. (1969) HF ionospheric the troposphere, and to study their location and structure. Joint radar ground scatter map showing land-sea boundaries by a radar-refractometer soundings have proved of special interest in spectral separation technique, Electronics Letters, 5. 75. confirming that the radar does indeed detect irregularities in clear- Bradley, P.A. and Dudeney, J.R. (1973) A simple model of the air structure. The application of radar in precipitation studies is, of vertical distribution of electron concentration in the ionosphere, course, a well-known and widely used technique in meteorology; J. Atmosph. Terr. Phys. 35. 2131. although to obtain the detail and precision necessary for radio Boithias, L. (1988) Radio-wave propagation, North Oxford Aca- applications requires careful refinements in technique. demic Publishers, London. 9/18 The lonosphere and the Troposphere Buchau, J. (1972) Instantaneous versus averaged ionosphere. Air Garriott, G.K. and Nichol, A.W. (1965) Ionospheric information Force Surveys in Geophysics No. 241 (Air Force Systems Com- deduced from the Doppler shifts of harmonic frequencies from mand, United States Air Force), 1. earth satellites, J. Atmosph. Terr. Phys. 22