re present. transmitter powers of about 1kW. Sounders with powers of around 2. The ordinary and extraordinary waves are sometimes reflected 100kW and a lower frequency limit of a few kHz have been from appreciably different heights. operated successfully in areas free from m.f. broadcast interference, 3. Oblique reflections occur when the ionosphere is tilted. to study the night time E-region. This has a maximum plasma frequency of around 0.5MHz. Internationally agreed procedures for scaling ionosonde records Pulse-compression systems (Coll and Storey, 1964) and C.W. chirp (ionograms) have been produced (Piggott and Rawer, 1972). Auto- sounders (Fenwick and Barry, 1966) offer the possibility of im- matic scaling software has been developed for use with digital proved signal/noise ratios and echo resolution. In the chirp sounder ionograms (Reinisch and Huang, 1983) and comparisons with ma- system, originally developed for use at oblique incidence, the trans- nual scaling lead to periodic refinements. mitter and receiver frequencies are swept synchronously so that the lonospheric probing techniques 9/9 finite echo transit time leads to a frequency modulation of the dispersions may be observed either at the ground or in satellites receiver i.f. signals. These signals are then spectrum-analysed to (Helliwell, 1965). produce conventional ionograms. Receiver bandwidths of only a Continuous wave and pulsed signals, transmitted and received at few tens of hertz are needed so that transmitter powers of a few ground-based terminals, may be used in a variety of ways to study watts are adequate. Other ionosondes have been produced and used irregularities or fluctuations in ionisation. Cross-correlation ana- operationally which record data digitally on magnetic tape (Piggott lyses of the amplitudes on three spaced receivers, of pulsed signals and Rawer, 1972). An ionosonde has been successfully flown in an of fixed frequency reflected from the ionosphere at near vertical aircraft to investigate geographical changes in electro