t assess the active modes and elevation angles; these vary Using a laser radar (lidar), the intensity of the light ck-scattered markedly with ionospheric conditions and transmitter frequency. by the atmospheric constituents at heights above 50km gives the Equations are available for the raypaths at oblique incidence height distributions of neutral-air density and the temporal and through ionospheric models composed of separate segments of spatial statistics of high-altitude atmospheric aerosols. Minor at- simple analytic form (Appleton and Beynon, 1947; Croft and mospheric constituents may be detected with tunable dye lasers Hoogasian, 1968). Combinations of parabolic, quasiparabolic, li- from their atomic and molecular-resonance scattering. near and quasilinear segments are typically employed (Bradley and Dudeney, 1973; Dyson and Bennett, 1988) with the segment par- ameters determined from numerical prediction maps of the vertical- Propagation prediction procedures incidence ionospheric characteristics, as given by data from the world network of ionosondes (CCIR, 1990e) (see Figure 9.4). Cal- Long-term predictions based on monthly median ionospheric data culations over a fixed path for a range of frequencies indicate the are required for the circuit planning of v.l.f.-h.f. ground-based largest frequency (the basic m.u.f. or maximum usable frequency) systems. Estimates of raypath launch and arrival angles are needed that propagates via a given mode. Assuming some statistical law for for antenna design, and of the relationship between transmitter the day to day variability of the parameters of the model they also power and received field strength at a range of frequencies, SO that give the availability, which is the fraction of days that the mode can the necessary size of transmitter and its frequency coverage can be exist. Received signal strengths are then determined in terms of the determined. Since there are appreciable day to day changes in the transmitter power and a number of transmission loss and gain The 