mbly, Dusseldorf, Physics of geomagnetic phenomena, ed. Matsushita, S. and ITU, Geneva. Campbell, W.H., Academic Press, London, 793. CCIR (1990f) REPORT 894-2. CCIR HF propagation prediction Mitra, S.N. (1949) A radio method of measuring winds in the method (Third CCIR computer-bused method for estimation of ionosphere, Proc. IEE, 46 Pt.III, 441. MUF, sky-wave field strength, signal-to-noise ratio, LUF and Muldrew, D.B. (1965) F-layer ionisation troughs deduced from basic circuit reliability), Documents of XVIIth Plenary Assem- Alouette data. J. Geophys. Res. 70.2635. bly, Dusseldorf, ITU, Geneva. Munro, G.H. (1950) Travelling disturbances in the ionosphere, Cohen, R. and Bowles, K.L. (1961) On the nature of equatorial Proc. Roy. Soc. 202A. 208. spread-F.J. Geophys. Res. 66. 1081. Nielson, D.L. (1968) The importance of horizontal F-region drifts Coll, D.C. and Storey, J.R. (1964) lonospheric sounding using to transequatorial VHF propagation, Scatter propagation of coded pulse signals, Radio Science. 68D. 1155. radio waves, ed Thrane, E. AGARD Conference Proceedings, Croft, T.A. (1968) Special issue on ray tracing, Radio Science. 3. 1. No.37 NATO, Neuilly-sur-Seine, France. Croft, T.A. (1972) Skywave backscatter: a means for observing our Pierce, E.T. (1965) Nuclear explosion phenomena and their bearing environment at great distances, Rev. Geophys. and Space on radio detection of the explosions, Proc. IEEE, 53. 1944. Physics. 10. 73. Pierce, J.A. (1965) OMEGA, IEEE Trans. Aer. and Elect. Syst. 1. Croft, T.A. and Hoogasian, H. (1968) Exact ray calculations in a quasi-parabolic ionosphere with no magnetic field, Radio Piggott, W.R. and Rawer, K. (1972) U.R.S.I. handbook of ionogram Science. 3. 69. interpretation and reduction, 2nd edn. Rep. UAG-23, Dept. of Davies, K. (1969) Ionospheric radio waves. Blaisdell, Waltham, Commerce, Boulder, USA. Mass. Ratcliffe, J.A. (1956) Some aspects of diffraction theory and their Davies, K. (1990) lonospheric radio. Peter Peregrinus, London. application to the ionosphere, Reports 