ns +15 radians f-superscript(1) are considered as follows. (peak-to-peak) 9.3.1 Refraction Frequency +0.04Hz +0.004Hz f-superscript(1) stability (r.m.s.) The change in direction of propagation resulting from the traverse Absorption 0.1dB 0.01dB of a thin slab of constant ionisation is given approximately by Refraction Negligible Bouger's law in terms of the refractive index and the angle of incidence. A more exact specification including the effects of the Earth's magnetic field is given by the Haselgrove equation solution 9.3.4 Polarisation (Haselgrove, 1954). The refractive index is determined from the Appleton-Hartree equations of the magnetoionic theory (Ratcliffe, Radio waves that propagate in the ionosphere are called charac- 1959; Budden, 1961), as a function of the electron density and teristic waves. There are always two characteristic waves known as electron-collision frequency, together with the strength and direc- the ordinary wave and the extraordinary wave; under certain re- tion of the Earth's magnetic field, the wave direction and the wave stricted conditions a third wave known as the Z-wave can also exist frequency. The dependence on frequency leads to wave dispersion (Ratcliffe, 1959). In general the ordinary and extraordinary waves of modulated signals. Since the ionosphere is a doubly refracting are elliptically polarised. The polarisation ellipses have the same medium it can transmit two waves with different polarisations (see axial ratio, orientations in space that are related such that under below). The refractive indices appropriate to the two waves differ. many conditions they are approximately orthogonal, and electric Refraction is reduced at the greater wave frequencies, and at v.h.f. vectors which rotate in opposite directions (Ratcliffe, 1959). The and higher frequencies it is given approximately as a function of the polarisation ellipses are less elongated the greater the wave fre- ratio of the wave and plasma frequencies, where the plasma fre- quency. Any wave launched into the iono