Radiowave Propagation in Satellite Communications
However this is not the case and effects are introduced primarily by two areas of the atmosphere. One is the troposphere and the other is the ionosphere.
Accordingly satellite propagation or the radio propagation characteristics of the path between the ground and the satellite of great importance. The atmosphere can be divided into several areas. It is found that the temperature varies according to the height. Initially the temperature falls until altitudes of around 10 km are reached. At this point the temperature is around or Celsius.
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It is around this point that the temperature starts to rise again. The region below this inflexion point is known as the troposphere.
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The second area that affects radio signals is known as the ionosphere. This is a region of the atmosphere that starts at altitudes of around 50 km, and extends to more than km. In this region radiation from the Sun mainly in the form of ultraviolet light strikes the gas molecules and atoms causing them to ionise producing a positive ion and a negative electron.
These electrons then affect radio signals, effectively reflecting those in the short wave bands and often returning them to Earth. However this region also affects the signals passing to and from satellites. Beyond the ionosphere the signals can be considered to be in free space, and the region between the upper reaches of the troposphere and the ionosphere is often temred "inner free space. There are a number of different of radio signal propagation effects that are introduced into satellite radio signals by the troposphere and ionosphere.
Transmission in free space has unity refractive index and is loss-less apart from the spreading effect that reduces the signal power over a fixed area with distance away from the source, but no power is actually lost. The troposphere and ionosphere have refractive indices that differ from unity. The troposphere is greater than unity and the ionosphere is less than unity and as a result refraction and absorption occur. The inner free space region also has little effect.
Faraday rotation is an effect that affects satellite propagation. Faraday rotation results from the fact that the ionosphere is a magneto-ionic region. The Faraday rotation of a signal causes different elements of a signal to travel in different ways, particularly rotating the plane of polarisation. This can create some problems with reception.
Radiowave Propagation in Satellite Communications
A linearly polarised signal can be considered as two contra-rotating circularly polarised signals. The phase velocities of these two signals vary in a magnetic medium such as the ionosphere and as a result the polarisation of the signal changes.
The degree of change is dependent upon the state of the ionosphere and it follows the same pattern as that experienced for HF ionospheric communications changing over the course of the day, with the seasons and over the sunspot cycle. Another satellite propagation effect introduced by the ionosphere is termed "ionospheric scintillations. They can also change the angle of arrival of the signals.
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These variations change over a period of between one to fifteen seconds, and they can affect signals well into the microwave region. The variations are caused primarily by the variations in electron density arising in the E region, often as a result of sporadic E but also in the F layer where a spreading effect is the cause.
The level of scintillation is dependent upon a number of factors including the location of the earth station and the state of the ionosphere, as a result of the location, the sunspot cycle, the level of geomagnetic activity, latitude, and local time of day. The scintillations are more intense in equatorial regions, falling with increasing latitude away from the equator but then rising at high latitudes, i.
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The effects are also found to decrease with increasing frequency, and generally not noticeable above frequencies of 1 - 2 GHz. The first edition of Satellite Communications Systems Engineering Wiley was written for those concerned with the design and performance of satellite communications systems employed in fixed point to point, broadcasting, mobile, radio naviga Propagation of Radiowaves Les Barclay Inbunden. Skickas inom vardagar. Radiowave Propagation in Communications was written with two basic objec- tives: Earth-orbiting satellites are employed extensively for the relay of information in a vast array of telecommunications, meteorological, government, and sci- entific applications.
Satellite systems rely on the transmission of radiowaves to and from the satellite and are dependent on the propagation characteristics of the transmission path, primarily the earth's atmosphere. Radiowave propagation thus plays a very important part in the design and ultimate performance of space communications systems.