2.1. The Ionosphere2.1.1. Historical Overview2.1.2. Formation and Structure2.1.3. The D-, E-, and F-Regions2.2. Spatial and Temporal Variability2.2.1. Radio Sounding at Vertical Incidence2.2.2. Measurements and Models2.2.3. Disturbances and Storms2.3. Oblique Propagation2.3.1. Equivalence Relationships2.3.2. Point-to-Point Links2.3.3. Frequency, Elevation, and Ground Range2.4. Ionospheric Modes2.4.1. Ordinary and Extraordinary Waves2.4.2. Multipath Propagation2.4.3. Amplitude and Phase Fading