CHAPTER 1INTRODUCTIONThe upper part of the atmosphere above 50 km is of importance to humanity. It acts as a shield against extreme ultraviolet rays and X-rays harmful to human life. It also plays an important role in communication and navigation. Regions of the atmosphere are defined in terms of ionization, temperature and composition. In terms of ionization, the region extending from 50 to 1000 km above the earth's surface is called the ionosphere. It is a weakly ionized gas containing a large number of neutral molecules and a relatively smaller number of electrons and ions. The main source of ionization is the rays emitted by the sun, which results in the formation of superimposed layers, namely D, E, F1 and F2. In each layer, the ionization density peaks at a certain altitude and decreases above and below. Region D has the lowest ionization density and is only available during the day. The ionization density has a maximum value around 90 km. It is an absorbing region for radio waves. Region E is known as the current carrying region. Its range is approximately 90 to 125 km. The maximum density in this layer is around 120 km. It contains both the normal E layer and the sporadic E layer. The electron density in the E region begins to decrease with sunset and reaches an equilibrium value at night. Region F extends 125 to 600 km. It is divided into two layers, namely F1 and F2 during the day, which merge to form a single layer during the night. The F1 region has a maximum density at around 200 km. It disappears after sunset. The F2 region has a maximum density at around 300 km during the day and the region extends to higher altitudes at night. After sunset, the electron density increases near the peak of the F layer. The density reaches an equilibrium value in the middle of the paper......ns in phase but also in amplitude. In order to develop the basic equations of scintillation theory, the following assumptions were made by Tatarskii [1971] (1) The temporal variations of the irregularities are much slower compared to the wave period and the time taken by the signal to pass through the irregularity layer is negligible compared to the time scale of temporal variation and (2) the operating wavelength is much slower. smaller than the characteristic size of the irregularities. If a radio wave of constant amplitude arrives at the irregularity slab of thickness L (Fig. 2.2) where the layer is assumed to be a phase change screen, the wave changes phase as it propagates through She. The emerging wave is represented by U0(ρ) = A0 exp [ -j φ(ρ)]