1. Tropospheric scatter

2. Tropospheric scatter
A tropospheric scatter system can bridge large distances while a microwave relay system (below) requires multiple relay stations due to its line of sight limitation.

3. Boswell Bay, Alaska White Alice Site, Tropospheric scatter antenna and feeder.

4. Pacific Scatter System
Tropospheric scatter (also known as troposcatter) is a method of communicating with microwave radio signals over considerable distances – often up to 300 kilometres (190 mi), and further depending on terrain and climate factors.

5. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at UHF and SHF frequencies are randomly scattered as they pass through the upper layers of the troposphere.
Radio signals are transmitted in a narrow beam aimed just above the horizon in the direction of the receiver station. As the signals pass through the troposphere, some of the energy is scattered back toward the Earth, allowing the receiver station to pick up the signal.
Normally, signals in the microwave frequency range travel in straight lines, and so are limited to line of sight applications, in which the receiver can be 'seen' by the transmitter.

6. Communication distances are limited by the visual horizon to around 30–40 miles (48–64 km). Troposcatter allows microwave communication beyond the horizon.
Because the troposphere is turbulent and has a high proportion of moisture the tropospheric scatter radio signals are refracted and consequently only a tiny proportion of the radio energy is collected by the receiving antennas.
Frequencies of transmission around 2 GHz are best suited for tropospheric scatter systems as at this frequency the wavelength of the signal interacts well with the moist, turbulent areas of the troposphere, improving signal to noise ratios.

7. Overview
Historically, high gain dish or billboard antennas were required for tropospheric scatter systems as the propagation losses are very high; only about one billion-billionth (1 x 10−12) of the transmit power is available at the receiver.
Paths were established at distances over 1,000 km. They required antennas ranging from 9 meters to 36 meters and amplifiers ranging from 1 kW to 50 kW.
These were analogue systems which were capable of transmitting a few voice channels.

8. Troposcatter systems have evolved over the years
Troposcatter systems have evolved over the years. With communication satellites used for long-distance communication links, current troposcatter systems are employed over shorter distances than previous systems, use smaller antennas and amplifiers, and have much higher bandwidth capabilities.
Typical distances are between 50 km and 250 km, though greater distances can be achieved depending on the climate, terrain, and data rate required.
Typical antenna sizes range from 1.2 meters to 12 meters while typical amplifier sizes range from 10W to 2 kW. Data rates over 20Mbit/s can be achieved with today's technology.

9. Tropospheric scatter is a fairly secure method of propagation as dish alignment is critical, making it extremely difficult to intercept the signals, especially if transmitted across open water, making them highly attractive to military users.
Military systems have tended to be ‘thin-line’ tropo – so called because only a narrow bandwidth ‘information’ channel was carried on the tropo system; generally up to 32 analogue (4 kHz bandwidth) channels. Modern military systems are "Wideband" as they operate 4-16 Mbit/s digital data channels.

10. Civilian troposcatter systems, such as the British Telecom (BT) North Sea oil communications network, required higher capacity ‘information’ channels than were available using HF (high frequency – 3 to 30 MHz) radio signals, before satellite technology was available.
The BT systems, based at Scousburgh in the Shetland Islands, Mormond Hill in Aberdeenshire and Row Brow near Scarborough, were capable of transmitting and receiving 156 analogue (4 kHz bandwidth) channels of data and telephony to / from North Sea oil production platforms, using frequency-division multiplexing (FDMX) to combine the channels.

11. Because of the nature of the turbulence in the troposphere, quadruple diversity propagation paths were used to ensure 99.98% reliability of the service, equating to about 3 minutes of downtime due to propagation drop out per month.
The quadruple space and polarisation diversity systems needed two separate dish antennae (spaced several metres apart) and two differently polarised feed horns – one using vertical polarisation, the other using horizontal polarisation.

12. This ensured that at least one signal path was open at any one time
This ensured that at least one signal path was open at any one time. The signals from the four different paths were recombined in the receiver where a phase corrector removed the phase differences of each signal.
Phase differences were caused by the different path lengths of each signal from transmitter to receiver. Once phase corrected, the four signals could be combined additively.

13. Troposheric Propagation:
The lowest part of the earth’s atmosphere is called the troposphere. Typically, the troposphere extends from the surface of the earth to an altitude of approximately 9 km at the poles and 17 km at the equator.
This upper boundary is referred to as the tropopause and is defined as the point at which the temperature in the atmosphere begins to increase with height. Within the troposphere, the temperature is found to decrease with altitude at a rate of approximately 7o C per km .
The earth’s weather system is confined to the troposphere and the fluctuations in weather parameters like temperature, pressure and humidity cause the refractive index of the air in this layer to vary from one point to another.

14. It is in this context that the troposphere assumes a vital role in the propagation of radio waves at VHF ( MHz) and UHF ( MHz) frequencies.
The meteorological conditions therefore influence the manner in which radio wave propagation occurs in the troposphere both on a spatial and temporal scale.

15. Refractive Index, Refractivity and Modified Refractivity:
In general, the refractive index, n, of the troposphere decreases with altitude to simplify the mathematics involved variations in the horizontal are neglected and horizontal homogeneity of the refractive index of the troposphere is assumed in most discussions on this topic.
A typical value for n at sea level is A few s above sea level, this might decrease to a value such as
For all practical purposes, at this scale, this change in the refractive index is negligibly small, with hardly any visible deviation.
However, immediately above the surface of the sea, it is often this small (but rapid) change in the refractive index profile that facilitates the formation of meteorological phenomena called evaporation ducts.

16. A convenient way of expressing these unwieldy numbers is to use the concept of refractivity instead. Refractivity, N, is defined as follows:
where P = total atmospheric pressure (in mb);
T = atmospheric temperature (in K);
e = water vapour pressure (in mb).

17. All three terms, P, T and e fall with height in an exponential manner, resulting in a corresponding decrease in N with height.
A standard atmosphere, therefore is one in which the refractivity varies with altitude according to equation.
Using Snell’s law, a radio ray projected into the atmosphere will have to travel from a denser to rarer medium and will refract downwards towards the surface of the earth.
The curvature of the ray, however, will still be less than the earth’s curvature. The gradient of refractivity in this case generally varies from 0 to –79 N-units per kilo.

18. When the refractivity gradient varies from –79 to –157 N-units per kilo, a super refractive condition is said to prevail in the troposphere and the ray will refract downwards at a rate greater than standard but less than the curvature of the earth .
A refractivity gradient that is even less than –157 N-units per kilo will result in a ray that refracts towards the earth’s surface with a curvature that exceeds the curvature of the earth.
This situation is referred to as trapping and is of particular importance in the context of evaporation ducts.

19. Finally, if the refractivity gradient is greater than 0 N units per kilo, a sub refractive condition exists and a radio ray will now refract upwards, away from the surface of the earth.
Depending on the existing conditions in the troposphere, a radio wave will undergo any of the types of refraction:
sub refraction, standard refraction, super refraction or trapping. Figure1 illustrates the four refractive conditions discussed above.

21. While dealing with radio propagation profiles, the curved radio rays are replaced with linear rays for the purpose of geometric simplicity. To account for drawing radio rays as straight lines, the earth radius has to be increased.
The radius of this virtual sphere is known as the effective earth radius and it is approximately equal to four-thirds the true radius of the earth (i.e. roughly 6400 km). A more classical form of representing n is that of modified refractivity, M.

22. In this case, the surface of the earth is represented by a flat plane and the radio rays are constituted by curves that are determined by Snell’s law and the corresponding value of M at each point along the radio link.
The following is the expression for M
N h,
where N = refractivity (in N-units),
h = height above sea level (in s),
a = radius of the earth (in s).

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