lectron density and collision frequency; ion density; composition Rocket of neutral atmosphere Cross-modulation Electron density and collision frequency Ground Partial reflection Electron density and collision frequency Ground Lidar Neutral air density; atmospheric aerosols; minor constituents Ground The swept-frequency ground-based ionosonde consisting of a co- Since reflection takes place from a height where the sounder located transmitter and receiver was developed for the earliest of frequency is equal to the ionospheric plasma frequency, and since ionospheric measurements and is still the most widely used probing the group path can be related to that height provided the electron instrument. The transmitter and receiver frequencies are swept densities at all lower heights are known, the data from a full synchronously over the range from about 0.5-1MHz to perhaps frequency sweep can be used to give the true-height distribution of 20MHz depending on ionospheric conditions, and short pulses electron density in the E- and F-regions up to the height of maxi- typically of duration 100us with a repetition rate of 50/s are trans- mum electron density of the F2-region. The conversion of group mitted. Calibrated film or computer digitised records of the received path to true height requires assumptions regarding missing data echoes give the group path and its frequency dependence. below the lowest height from which echoes are received and over In practice these records require expert interpretation because: regions where the electron density does not increase monotonically with height. The subject of true-height analysis is complex and has Multiple echoes occur, corresponding to more than one traverse been considered by several research groups for many years (Bey- between ionosphere and ground (so-called multiple-hop non, 1967). An internationally agreed procedure is now available modes) or when partially reflecting sporadic-E or F-region (Titheridge, 1985). Commercially manufactured pulse sounders use irregularities a