changes in the incident cosmic noise at techniques. the ground caused by ionospheric absorption. D-region electron densities and collision frequencies may be 9.6.1 Long-term predictions inferred from oblique or vertical-path measurements of signal am- plitude, phase, group-path delay and polarisation at frequencies of The first requirement of any long-term prediction is a model of the 10Hz to 100kHz, with atmospherics as the signal sources at the ionosphere. At v.l.f. waves propagate between the Earth and the lower frequencies. Vertically radiated signals in the frequency range lower boundary of the ionosphere at heights of 70km by day and 1.5-6MHz suffer weak partial reflections from heights of 75-90km. 90km by night as if in a two-surface waveguide. Very low frequency Measurements of the reflection coefficients of both the ordinary and extraordinary waves, which can be of the order of 10-5 enable field-strength predictions are based on a full-wave theory that in- cludes diffraction and surface-wave propagation. For paths beyond electron density and collision-frequency data to be deduced (Bel- 1000km range only three or fewer waveguide modes need to be rose and Burke, 1964). Pulsed signals with high transmitter antenna considered. A general equation gives field strength as a function of systems and very sensitive receivers are needed. As well as in-situ range, frequency, ground-electrical properties and the ionospheric probes in rockets, there are a wide range of other schemes for reflection height and reflection coefficients (CCIR, 1990d). Unfor- determining electron density and collision frequency, involving the tunately the reflection coefficients vary in a complex way with study of wave-fields radiated between the ground and a rocket. electron density and collision frequency, the direction and strength These use combinations of frequencies in the v.l.f.-v.h.f. range and of the Earth's magnetic field, wave frequency and angle of in- include the measurement of differential-Doppler frequency, absorp- cidence, S