Troposcatter communications & propagation
- troposcatter or tropospheric scatter is a form of radio signal propagation for radio communications links up to distances up to about 1000 km using the troposphere
One useful form of radio communications technology for applications where path lengths of around 800 km are needed is known as tropospheric scatter or troposcatter. It is a reliable form of radio communications link that can be used regardless of the prevailing tropospheric conditions. Although reliable, when using troposcatter, the signal strengths are normally very low. Accordingly troposcatter radio communications links require high powers, high antenna gains and sensitive receivers.
Troposcatter is often used for commercial radio communications applications, normally on frequencies above 500 MHz for over the horizon links. It is ideal for remote telemetry, or other links where low to medium rate data needs to be carried. Where viable, troposcatter provides a means of communication that is much cheaper than using satellites.
As the name implies, troposcatter uses the troposphere as the region that affects the radio signals being transmitted, returning them to Earth so that they can be received by the distant receiver. Troposcatter relies on the fact that there are areas of slightly different dielectric constant in the atmosphere at an altitude of between 2 and 5 kilometres. Even dust in the atmosphere at these heights adds to the reflection of the signal. A transmitter launches a high power signal, most of which passes through the atmosphere into outer space. However a small amount is scattered when is passes through this area of the troposphere, and passes back to earth at a distant point. As might be expected, little of the signal is "scattered" back to Earth and as a result, path losses are very high. Additionally the angles through which signals can be reflected are normally small.
The area within which the scattering takes place is called the scatter volume, and its size is dependent upon the gain of the antennas used at either end. In view of the fact that scattering takes place over a large volume, the received signal will have travelled over a vast number of individual paths, each with a slightly different path length. As they all take a slightly different time to reach the receiver, this has the effect of "blurring" the overall received signal and this makes high speed data transmissions difficult.
It is also found that there are large short term variations in the signal as a result of turbulence and changes in the scatter volume. As a result commercial troposcatter propagation systems use multiple diversity systems. This is achieved by using vertical and horizontally polarised antennas as well as different scatter volumes (angle diversity) and different frequencies (frequency diversity). Control of these systems is normally undertaken by computers. In this way troposcatter radio communications systems can run automatically giving high degrees of reliability.
Although troposcatter requires high power transmitters, sensitive receivers and high gain antennas, it provides a very convenient data transmission system for many radio communications applications. Although there are limitations, it nevertheless provides a cost effective data communications system, cheaper than using satellites, for many medium distance applications. For example it was used by offshore oil rigs in the North Sea off the UK to provide a medium speed data link back to the mainland.
By Ian Poole
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