Tropospheric scatter

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Tropospheric vs line of sight communications
Boswell Bay, Alaska White Alice Site, Tropospheric scatter antenna and feeder.
Pacific Scatter System

Tropospheric scatter (known as "troposcatter" among practitioners) is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at particular frequencies are randomly scattered as they pass through the upper layers of the troposphere. Radio signals are transmitted in a tight beam aimed at the tropopause, midway between the transmitter and receiver sites; as the signals pass through the troposphere they are scattered, allowing the receiver station to pick up the signal.[1] A related system is meteor burst communications, which uses the ionized trails of meteors to improve the strength of the scattering.

Normally, microwave signals, transmitted at various frequencies, usually around 12 Gigahertz (GHz) or 19 GHz, are only used for ‘line of sight’ applications, where the receiver can be ‘seen’ from the transmitter. However, tropospheric scatter signals use a frequency of around 2 GHz.

Because the troposphere is turbulent and has a high proportion of moisture the tropospheric scatter radio signals are refracted and consequently only a proportion of the radio energy is collected by the receiving antennae. 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.

High gain dish or billboard antennae are 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. Typically, dish antennae with isotropic gains of between 40 decibels(dB) and 60dB are used with transmitter powers of 1 Kilowatt(kW) to 10 kW.

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.

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.

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 antenna (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.

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Tropospheric Scatter Communications Networks[edit]

The Tropospheric scatter phenomenon has been used to build both civilian and military communication links in a number of parts of the world, including:-

ACE High
NATO in Europe.
BT (British Telecom)
United Kingdom - Shetland to Mormond Hill
PTT[disambiguation needed] (a Spanish telecomms company)
Spain - Nogueira[disambiguation needed] to Artzamendi
CNT[disambiguation needed] (Canadian telecomms company)
Tsiigehtchic to Galena
Hay River - Port Radium - Lady Franklin Point
Cuba - Florida
Guanabo to Florida City
AT&T Corporation
Chatham, NC - Buckingham, VA - Charlottesville, VA - Leesburg, VA - Hagerstown, MD
Texas Towers - Air defence radars.
Texas Tower 2
Georges Shoal, in 56-foot (17 m) deep water, 110 miles (180 km) east of Cape Cod 41°44′N 67°47′W / 41.733°N 67.783°W / 41.733; -67.783 , linked to North Truro, MA.
Texas Tower 3
Nantucket Shoals, in 80-foot (24 m) water, 100 miles (160 km) south-east of Rhode Island 40°45′N 69°19′W / 40.75°N 69.317°W / 40.75; -69.317 , linked to Montauk AFB, Long Island, NY.
Texas Tower 4
Un-named Shoal (Unofficially: Old Shaky), in 185-foot (56 m) water, 84 miles (135 km) south-east of New York City 39°48′N 72°40′W (Destroyed, with 28 killed, during a storm on 15 January 1961), linked to Highlands, NJ mainland station.
Texas Tower 1
Cashes Ledge (Lat. 42° 53'N., Long. 68° 57'W., 36-foot depth), 100 miles east of New Hampshire, not built.
Texas Tower 5
Brown's Bank (Lat. 42° 47'N., Long. 65° 37'W., 84foot depth), 75 miles south of Nova Scotia, not built.
Mid Canada Line
A series of five stations (070, 060, 050, 415, 410) in Ontario and Quebec around the lower Hudson Bay.
Pinetree Line, Pole Vault
A series of fourteen stations providing communications for Eastern seaboard radar stations of the US/Canadian Pinetree line, running from N-31 Frobisher Bay, Baffin Island to St. Johns, Newfoundland and Labrador.
White Alice
A Military and Civil Communications network with seventy-one stations stretching up the Western seaboard from Port Hardy, Vancouver island North to Barter Island (BAR) and east to Shemya, Alaska (SYA) in the Aleutian Islands.
DEW Training
A training facility for White Alice and the DEW line tropo-scatter network, between Pecatonica, Illinois to Streator, Illinois.
DEW Line
Several tropo-scatter networks providing communications for the extensive air-defence radar chain in the far north of Canada and the US.
NARS
NATO Air-Defence network stretching from RAF Fylingdales, via Mormond Hill, UK, Sornfelli (Faroe Islands), Höfn, Iceland to Keflavik DYE-5, Rockville.
ET-A, USAREUR
A US Army network from RAF Fylingdales to a network in Germany and a single station in France (Maison Fort).
486L, MEDCOM
A US Navy network covering the European coast of the Mediterranean Sea from San Pablo, Spain in the West to Adana AFB, Turkey in the East, with headquarters at Ringstead in Surrey, England.
Royal Air Force

Communications to British Forces Germany, running from Swingate in Kent to Lammersdorf in Germany.

BARS
A Warsaw Pact tropo-scatter network that stretching from near Rostok in the DDR (Deutsches Demokratisches Republik), Czechoslovakia, Hungary, Poland, Belarussia USSR, Ukraine USSR, Romania and Bulgaria.
SEVER
A Soviet right network stretching right across the USSR.
India - USSR
A single section from Srinigar, Kashmir, India to Dangara, Tajikstan, USSR.
Indian Air Force
An air-defence network covering the Northern borders of India with at least 32 stations.
Peace Ruby, Spellout, Peace Net
An air-defence network set up by the United States in Iran pre-revolution. Spellout built a radar and comms network in the north of iran. Peace Ruby built another air-defence network in the south and Peace net integrated the two networks.
Bahrain - UAE
A tropo-scatter system linking Al Manamah, Bahrain to Dubai, United Arab Emirates.
RAFO
A tropo-scatter communications system providing military comms to the former SOAF - Sultan of Oman's Air Force, (now RAFO - Royal Air Force of Oman), across the Sultanate of Oman.
RSAF
A Royal Saudi Air Force tropo-scatter network linking major airbases and population centres in Saudi Arabia.
Yemen
A single system linking Sana'a with Sa'dah.
BACK PORCH and IWCS
Two networks run by the United States linking military bases in Thailand and South Vietnam.
Phil - Tai - Oki

A system linking the Philippines with Taiwan.

Japanese Troposcatter Networks
Two networks linking Japanese islands from North to South.

Tactical Troposcatter Communication systems[edit]

As well as the permanent networks detailed above, there have been many tactical transportable systems produced by several countries:-[2]

Soviet / Russian Troposcatter Systems
MNIRTI R-423-1 Brig-1/R-423-2A Brig-2A/R-423-1KF
MNIRTI R-444 Eshelon / R-444-7,5 Eshelon D
MNIRTI R-420 Atlet-D
NIRTI R-417 Baget/R-417S Baget S
NPP Radiosvyaz R-412 A/B/F/S TORF
MNIRTI R-410/R-410-5,5/R-410-7,5 Atlet / Albatros
MNIRTI R-408/R-408M Baklan
Peoples Republic of China (PRoC), Peoples Liberation Army (PLA) Troposcatter Systems
CETC TS-504[ Troposcatter Communication System
CETC TS-510/GS-510 Troposcatter Communication System
Western Troposcatter Systems
AN/TRC-97 Troposcatter Communication System
AN/TRC-170 Tropospheric Scatter Microwave Radio Terminal[3]
AN/GRC-201 Troposcatter Communication System


US Army TRC-170 Tropo Scatter Microwave System

The U.S. Army and Air Force use tactical tropospheric scatter systems developed by Raytheon for long haul communications. The systems come in two configurations, the original "heavy tropo", and a newer "light tropo" configuration exist. The systems provide four multiplexed group channels and trunk encryption, and 16 or 32 local analog phone extensions. The U.S. Marine Corps also uses the same device, albeit an older version.

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