Editing Tropospheric scatter (section)

==Overview==
===Discovery===
Prior to [[World War II]], prevailing radio physics theory predicted a relationship between frequency and diffraction that suggested radio signals would follow the curvature of the Earth, but that the strength of the effect would fall off rapidly and especially at higher frequencies. In spite of this widespread belief, during the war there were numerous incidents in which high-frequency radar signals were able to detect targets at ranges far beyond the theoretical calculations. In spite of these repeated instances of anomalous range, the matter was never seriously studied.{{sfn|Stecker|1960}}

In the immediate post-war era, the limitation on [[television]] construction was lifted in the United States and millions of sets were sold. This drove an equally rapid expansion of new television stations. Based on the same calculations used during the war, the [[Federal Communications Commission]] (FCC) arranged frequency allocations for the new VHF and UHF channels to avoid interference between stations. To everyone's surprise, interference was common, even between widely separated<!--in distance, or frequency?--> stations. As a result, licenses for new stations were put on hold in what is known as the "television freeze" of 1948.{{sfn|Stecker|1960}}

[[Bell Labs]] was among the many organizations that began studying this effect, and concluded it was a previously unknown type of reflection off the [[tropopause]]. This was limited to higher frequencies, in the UHF and microwave bands, which is why it had not been seen prior to the war when these frequencies were beyond the ability of existing electronics. Although the vast majority of the signal went through the troposphere and on to space, the tiny amount that was reflected was useful if combined with powerful transmitters and very sensitive receivers. In 1952, Bell began experiments with [[Lincoln Labs]], the MIT-affiliated [[radar]] research lab. Using Lincoln's powerful microwave transmitters and Bell's sensitive receivers, they built several experimental systems to test a variety of frequencies and weather effects. When [[Bell Canada]] heard of the system they felt it might be useful for a new communications network in [[Labrador]] and took one of the systems there for cold weather testing.{{sfn|Stecker|1960}}

In 1954 the results from both test series were complete and construction began on the first troposcatter system, the [[Pole Vault (communications system)|Pole Vault]] system that linked [[Pinetree Line]] radar systems along the coast of [[Labrador]]. Using troposcatter reduced the number of stations from 50 [[microwave relay]]s scattered through the wilderness to only 10, all located at the radar stations. In spite of their higher unit costs, the new network cost half as much to build as a relay system. Pole Vault was quickly followed by similar systems like [[White Alice Communications System|White Alice]], relays on the [[Mid-Canada Line]] and the [[DEW Line]], and during the 1960s, across the Atlantic Ocean and Europe as part of [[NATO]]'s [[ACE High]] system.

===Use===
[[File:Tropo Scatter communications.jpg|thumb|Pole Vault used circular parabolic antennas; later systems generally used squared-off versions sometimes known as "billboards".]] 
The [[Path loss|propagation loss]]es are very high; only about one [[trillionth]] ({{scinote|fn=eng|0.00000000001}}) of the transmit power is available at the receiver. This demands the use of antennas with extremely large [[antenna gain]]. The original Pole Vault system used large [[parabolic reflector]] [[Dish antenna|dish]] antennas, but these were soon replaced by [[billboard antenna]]s which were somewhat more robust, an important quality given that these systems were often found in harsh locales. Paths were established at distances over {{convert|1000|km|mi}}. They required antennas ranging from {{convert|9|to|36|m}} and amplifiers ranging from {{val|1|u=kW}} to {{val|50|u=kW}}. These were analogue systems which were capable of transmitting a few voice channels.

Troposcatter systems have evolved over the years. With [[communication satellite]]s 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 {{convert|50|and|250|km}}, though greater distances can be achieved depending on the climate, terrain, and data rate required. Typical antenna sizes range from {{convert|1.2|to|12|m}} while typical amplifier sizes range from {{val|10|u=W}} to {{val|2|u=kW}}. Data rates over {{val|20|u=Mbit/s}} can be achieved with today's technology.

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 (signal processing)|bandwidth]] ‘information’ channel was carried on the tropo system; generally up to 32 analogue ({{frequency|4|kHz}} bandwidth) channels. Modern military systems are "wideband" as they operate 4-16&nbsp;Mbit/s digital data channels.

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 – {{frequency|3|MHz}} to {{frequency|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, North Yorkshire|Scarborough]], were capable of transmitting and receiving 156 analogue ({{frequency|4|kHz}} bandwidth) channels of data and telephony to / from North Sea oil production platforms, using [[frequency-division multiplexing]] (FDMX) to combine the channels.
  
Because of the nature of the turbulence in the troposphere, quadruple [[Antenna diversity|diversity propagation]] paths were used to ensure {{percentage|9998|10000|2}} 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 antennas (spaced several metres apart) and two differently [[Polarization (waves)|polarised]] [[feed horn]]s – one using vertical polarisation, the other using horizontal polarisation. 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 difference]]s 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.

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