e earth's atmosphere and their properties also affect radio wave propagation. The troposphere extends to a height of about 10 km, and in this area, the bending of radio waves by refraction (tropospheric refraction) causes the distance of the radio horizon to exceed the optical horizon. The stratosphere extends to a distance of approximately 10 60 km above the earth's surface, and the temperature in this region remains relatively constant. The ionosphere then extends from 60 to 1,000 km above the surface of the earth and contains several different layers in which ionization (the splitting of molecules into positive and negative ions) occurs. The presence of free electrons in the ionosphere has a significant effect on the propagation of radio waves. Ultraviolet rays from the sun cause this ionization.
Intense ionizing agents exist in the upper most levels of the ionosphere, but there is very little atmosphere so that the ionization density (the number of electrons per unit volume) is low. At lower heights, atmospheric pressure and ionization increase until electron density reaches its maximum level. Below this height, atmospheric pressure increases even as ionization decreases from a loss of energy (www.gordon.army.mil/acd/tcs/hf/propag8n.htm).
The ACTS Program was developed to help the U.S. industry maintain a preeminent position in the field of satellite communications. The program develops advanced technologies and tests them in actual user trials. In its first year, the ACTS Program proved that it had fulfilled its mission by helping U.S. industry to develop new communications technologies, among them on board switching and baseband processing, on demand channel assignments, Ka band transmission, wide bandwidth (900 MHz) channels, and techniques to study and compensate for rain fade attenuation at these frequencies. Also, the ACTS satellite now makes it possible for many companies, government agencies, and universi...