Tropospheric scatter

Lore & Background

Prior to World War II, prevailing radio physics theory predicted that radio signals would follow the curvature of the Earth but that the strength of the effect would fall off rapidly, especially at higher frequencies. During the war, however, high-frequency radar signals repeatedly detected targets at ranges far beyond theoretical calculations, though the matter was never seriously studied. In the immediate post-war era, the lifting of television construction restrictions in the United States led to millions of sets being sold and a rapid expansion of television stations. The Federal Communications Commission arranged frequency allocations based on the same calculations used during the war, but interference was common even between widely separated stations, leading to the 'television freeze' of 1948.

Bell Labs studied this effect and concluded it was a previously unknown type of reflection off the tropopause, limited to higher frequencies in the UHF and microwave bands. In 1952, Bell began experiments with Lincoln Labs, using Lincoln's powerful microwave transmitters and Bell's sensitive receivers. When Bell Canada heard of the system, they took one to Labrador for cold weather testing. In 1954, construction began on the first troposcatter system, Pole Vault, which linked Pinetree Line radar systems along the coast of Labrador. Using troposcatter reduced the number of stations from 50 microwave relays to only 10, all located at the radar stations, and cost half as much to build. 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.

Reader's Guide

Tropospheric scatter systems have evolved significantly over the years. With communication satellites used for long-distance 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 and 250 km, though greater distances can be achieved depending on climate, terrain, and data rate. Antenna sizes range from 1.2 to 12 m, and amplifier sizes range from 1 W to 10 kW. Data rates over 4–16 Mbit/s can be achieved with today's technology.

Tropospheric scatter is a fairly secure method of propagation because dish alignment is critical, making it extremely difficult to intercept the signals, especially if transmitted across open water. This has made them highly attractive to military users. Military systems have tended to be 'thin-line' tropo, carrying up to 32 analogue voice channels, while modern military systems are 'wideband', operating 4–16 Mbit/s digital data channels. Civilian troposcatter systems, such as the British Telecom North Sea oil communications network, required higher capacity channels than were available using HF radio 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 voice channels of data and telephony to and from North Sea oil production platforms, using frequency-division multiplexing.

Because of the nature of turbulence in the troposphere, quadruple diversity propagation paths were used to ensure 99.999% reliability of the service, equating to about 3 minutes of downtime due to propagation dropout 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, allowing the four signals to be combined additively.

Did You Know?

• Only about one trillionth of the transmit power is available at the receiver in a troposcatter system.
• Quadruple diversity propagation paths were used to ensure 99.999% reliability, with about 3 minutes of downtime per month.
• Troposcatter was developed in the 1950s and used for military communications until communications satellites largely replaced it in the 1970s.