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. 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 stations. As a result, licenses for new stations were put on hold in what is known as the "television freeze" of 1948.
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. In 1954 the results from both test series were complete and construction began on the first troposcatter system, the
Pole Vault system that linked
Pinetree Line radar systems along the coast of
Labrador. Using troposcatter reduced the number of stations from 50
microwave relays 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, 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 The
propagation losses are very high; only about one
trillionth () 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 antennas, but these were soon replaced by
billboard antennas which were somewhat more robust, an important quality given that these systems were often found in harsh locales. Paths were established at distances over . They required antennas ranging from and amplifiers ranging from to . These were analogue systems which were capable of transmitting a few voice channels. 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 , though greater distances can be achieved depending on the climate, terrain, and data rate required. Typical antenna sizes range from while typical amplifier sizes range from to . Data rates over 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 ‘information’ channel was carried on the tropo system; generally up to 32 analogue ( bandwidth) channels. Modern military systems are "wideband" as they operate 4-16 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 – to ) 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 ( 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
diversity propagation paths were used to ensure 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
polarised feed horns – 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 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. == Tropospheric scatter communications networks ==