Troposcatter 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.
Troposcatter basics
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 principle of troposcatter radio communications
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.
Summary
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.
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