Tropospheric scatter

Tropospheric scatter, also known as troposcatter, is a method of communicating with microwave radio signals over considerable distances – often up to 500 kilometres (310 mi) and further depending on frequency of operation, equipment type, terrain, and climate factors. 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.[1]

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.
Boswell Bay, Alaska White Alice Site, Tropospheric scatter antenna and feeder.
Pacific Scatter System

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. Communication distances are limited by the visual horizon to around 48–64 kilometres (30–40 mi). Troposcatter allows microwave communication beyond the horizon. It was developed in the 1950s and used for military communications until communications satellites largely replaced it in the 1970s.

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 transmitted 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.

Overview

Discovery

Previous 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. However, 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.[2]

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.[2]

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.[2]

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 scatted 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
Pole Vault used circular parabolic antennas, later systems generally used squared-off versions sometimes known as "billboards".

The propagation losses are very high; only about one trillionth (10×10^−12) 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 1,000 kilometres (620 mi). They required antennas ranging from 9 to 36 metres (30 to 118 ft) and amplifiers ranging from 1 kW to 50 kW. 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 50 and 250 kilometres (31 and 155 mi), though greater distances can be achieved depending on the climate, terrain, and data rate required. Typical antenna sizes range from 1.2 to 12 metres (3 ft 11 in to 39 ft 4 in) while typical amplifier sizes range from 10 W to 2 kW. Data rates over 20 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 ‘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 MHz 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 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

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

Allied Command Europe Highband (ACE High),  NATO
NATO military radiocommunication and early warning system throughout Europe from the Norwegian-Soviet border to the Turkish-Soviet border.
BT (British Telecom),  UK
United Kingdom - Shetland to Mormond Hill
Fernmeldeturm Berlin,  West Germany
Torfhaus-Berlin, Clenze-Berlin at Cold War times
Portugal Telecom,  Portugal
Serra de Nogueira (northeastern Portugal) to Artzamendi (southwestern France)
CNCP Telecommunications,  Canada
Tsiigehtchic to Galena Hill, Keno City
Hay River - Port Radium - Lady Franklin Point
 Cuba -  Florida
Guanabo to Florida City
Project Offices - AT&T Corporation,  United States
Main article: Project Offices
Project Offices is the name sometimes used to refer to several structurally dependable facilities maintained by the AT&T Corporation in the Mid-Atlantic states since the mid-20th century to house an ongoing, non-public, company project. AT&T began constructing Project Offices in the 1960s. Since the inception of the Project Offices program, the company has chosen not to disclose the exact nature of business conducted at Project Offices. However, it has described them as central facilities.[3][4][5]
Texas Towers - Air defence radars,  United States Air Force
Main article: Texas Towers
The Texas Towers were a set of three radar facilities off the eastern seaboard of the United States which were used for surveillance by the United States Air Force during the Cold War. Modeled on the offshore oil drilling platforms first employed off the Texas coast, they were in operation from 1958 to 1963.
Tower ID Location Staffing unit Mainland station Notes
TT-1 Cashes Ledge off New Hampshire coast
42°53′N 68°57′W		 Not built
TT-2 Georges Bank off Cape Cod
41°45′0.00″N 67°46′0.00″W		 762d Radar Squadron North Truro Air Force Station decommissioned 1963
TT-3 Nantucket Shoals
40°45′00.00″N 69°19′0.00″W		 773d Radar Squadron Montauk AFS decommissioned 1963
TT-4 off Long Beach Island, New Jersey
39°48′N 72°40′W		 646th Radar Squadron Highlands Air Force Station collapsed (1961)
TT-5 Browns Bank south of Nova Scotia
42°47′N 65°37′W		 Not built
Mid Canada Line,  Canada
A series of five stations (070, 060, 050, 415, 410) in Ontario and Quebec around the lower Hudson Bay. A series of six stations were built in Labrador and Quebec between Goose Bay and Sept-Îles between 1957 and 1958.[8]
Pinetree Line, Pole Vault,  Canada
Pole Vault was 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 N-22 St. John's, Newfoundland.
White Alice/DEW Line/DEW Training (Cold War era),  United States/ Canada
A former military and civil communications network with eighty stations stretching up the western seaboard from Port Hardy, Vancouver Island north to Barter Island (BAR), west to Shemya, Alaska (SYA) in the Aleutian Islands (just a few hundred miles from the Soviet Union) and east across arctic Canada to Greenland. Note that not all station were troposcatter, but many were. It also included a training facility for White Alice/DEW line tropo-scatter network located between Pecatonica, Illinois, and Streator, Illinois.
DEW Line (Post Cold War era),  United States/ Canada
Several tropo-scatter networks providing communications for the extensive air-defence radar chain in the far north of Canada and the US.
North Atlantic Radio System (NARS),  NATO
NATO air-defence network stretching from RAF Fylingdales, via Mormond Hill, UK, Sornfelli (Faroe Islands), Höfn, Iceland to Keflavik DYE-5, Rockville.
European Tropospheric Scatter - Army (ET-A),  United States Army
A US Army network from RAF Fylingdales to a network in Germany and a single station in France (Maison Fort). The network became active on 1966.[9]
486L Mediterranean Communications System (MEDCOM),  United States Air Force
A network covering the European coast of the Mediterranean Sea from San Pablo, Spain in the west to Incirlik Air Base, Turkey in the East, with headquarters at Ringstead in Dorset, England. Commissioned by the US Air Force in 1966.[10]: Spanish Communications Region 
Royal Air Force,  UK
Communications to British Forces Germany, running from Swingate in Kent to Lammersdorf in Germany.
Troposphären-Nachrichtensystem Bars, Warsaw Pact
BARS tropo-scatter network map
A Warsaw Pact tropo-scatter network stretching from near Rostok in the DDR (Deutsches Demokratisches Republik), Czechoslovakia, Hungary, Poland, Byelorussia USSR, Ukraine USSR and Bulgaria.
TRRL SEVER,  Soviet Union
A Soviet network stretching across the USSR.[11][12]
 India -  Soviet Union
India-USSR troposcatter UHF link on a 1982 stamp of India
A single section from Srinigar, Kashmir, India to Dangara, Tajikistan, USSR.
Indian Air Force,  India
Part of an Air Defence Network covering major air bases, radar installations and missile sites in Northern and central India. The network is being phased out to be replaced with more modern fiber-optic based communication systems.[13]
Peace Ruby, Spellout, Peace Net,  Imperial State of Iran
An air-defence network set up by the United States prior to the 1979 Islamic Revolution. Spellout built a radar and communications network in the north of Iran. Peace Ruby built another air-defence network in the south and Peace Net integrated the two networks.[14][15]
 Bahrain -  UAE
A tropo-scatter system linking Al Manamah, Bahrain to Dubai, United Arab Emirates.
Royal Air Force of Oman,  Oman
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.
Royal Saudi Air Force,  Saudi Arabia
A Royal Saudi Air Force tropo-scatter network linking major airbases and population centres in Saudi Arabia.
Yemen,  Yemen
A single system linking Sana'a with Sa'dah.
BACK PORCH and Integrated Wideband Communications System (IWCS),  United States
Two networks run by the United States linking military bases in Thailand and South Vietnam. Stations were located at Bangkok, Ubon Royal Thai Air Force Base, Pleiku, Nha Trang, Vung Chua mountain (13.742°N 109.196°E / 13.742; 109.196) Quy Nhon, Monkey Mountain Facility Da Nang, Phu Bai Combat Base, Pr Line (11.868°N 108.547°E / 11.868; 108.547) near Da Lat, Hon Cong mountain An Khê, Phu Lam (10.752°N 106.627°E / 10.752; 106.627) Saigon, VC Hill (10.36°N 107.068°E / 10.36; 107.068) Vũng Tàu and Cần Thơ.[16]
Phil-Tai-Oki,  Taiwan
A system linking the Taiwan with the Philippines and Okinawa.[17]
Cable & Wireless Caribbean network
A troposcatter link was established by Cable & Wireless in 1960, linking Barbados with Port of Spain, Trinidad. The network was extended further south to Georgetown, Guyana in 1965.[18][19]
Japanese Troposcatter Networks,  Japan
Two networks linking Japanese islands from North to South.

Tactical Troposcatter Communication systems
Belarusian "Horizon" mobile tropospheric scatter communication system

As well as the permanent networks detailed above, there have been many tactical transportable systems produced by several countries:[20]

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
People's Republic of China (PRoC), People's 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[21]
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.

See also

References

Citations
  1. Telecommunications: Glossary of Telecommunication Terms - tropospheric scatter (pdf) (Technical report). National Telecommunications and Information Administration. 23 August 2000. p. T-21. FED-STD-1037C. Archived (PDF) from the original on 25 December 2016. Retrieved 13 July 2021.
  2. Stecker 1960.
  3. Price, Jay (10 August 2008). "Mysterious Cold War bunker closes". Home & Garden. Charlotte Observer. The McClatchy Company. ISSN 2331-7221. OCLC 9554626. Archived from the original on 6 October 2020. Retrieved 15 July 2021. Although AT&T had dozens of similar communications bunkers across the country, the one in Chatham was part of a heavily armored and heavily guarded group of just five that went by the deceptively bland name of "Project Offices," said Albert LaFrance, who runs two Web sites dedicated to Cold War infrastructure. Unlike the more common AT&T communications bunkers, the Project Offices were apparently designed to shelter high-level government and military officials as part of a plan to preserve at least a skeletal national government in the event of a nuclear attack, LaFrance said. These "Continuance of Government" facilities would need communications capability, but communications wasn't their main mission, he said.
  4. Elliston, Jon (December 13, 2000). "Big Hole, Deep Secret". Indy Week. Retrieved March 8, 2017.
  5. coldwar-c4i.net
  6. indyweek.com (Archived copy)
  7. coldwar-c4i.net
  8. "Camping Canadian: 1958 Bell Quebec-Labrador Troposcatter Network". 27 September 2014.
  9. Elkins, Walter. "European Tropospheric Scatter - Army". USARMYGERMANY.com. Retrieved 15 July 2021. A major segment of the Department of Defense communications network in Europe was activated July 19 (1966). The new system went into operation as part of the ET-A ( European Tropo-Army ) network that spans a number of nations in Western Europe. The system ties in communications from Leghorn, Italy, through the Italian Alps to Bremerhaven, Germany, and from Heidelberg to within a few miles of Paris, adding more than 1,200 channel miles to the US Army Strategic Communications Command's world-wide communications complex.
  10. Elkins, Walter. "Air Force Communications in Europe". USARMYGERMANY.com. Retrieved 15 July 2021.
  11. "Soviet System of Tropospheric Communication TRRL Sever". Retrieved 6 May 2023.
  12. "A dead Dragon, or the remains of Sever TRRL". Russian Urban Exploration. n.d. Archived from the original on 4 March 2021. Sever tropospheric-scatter radio relay line (TRRL Sever) is a former Soviet communications line system designed for establishing communication with the remote regions of the country. The line was 13200 km (8200 miles) long and consisted of 46 tropospheric radio relay stations (TRRS) located mostly along the coasts of the Arctic and the Pacific oceans and major Siberian rivers: the Ob, the Enisey and the Lena.
  13. AF Net
  14. "Air Defense Command And Radars". Imperial Iranian Air Force. Archived from the original on 15 March 2012. Retrieved 15 May 2021.
  15. Clark, Jay; Salute, Joe. "The Philco-Ford Peace Ruby Project". exreps.com. Archived from the original on 15 March 2012. Retrieved 15 May 2021. The Peace Ruby program added AC&W radar sites and communications to the south of Iran, it supplemented the Spellout system to the north which provided radar coverage along the Russian boarder from Mashad to Tabriz. A later project, Peace Net, fully integreted these two systems into a state of the art air control and defense system.
  16. Rienzi, Thomas (1972). Vietnam Studies Communications-Electronics 1962-1970. Department of the Army. p. 14.
  17. Farrow, J. E.; Hause, L. G. (1 January 1981). "3.3 Aspects of Equipment Covered". Recommendations for Digital Radio Common Tactical/Long-Haul Standards (Technical report). Vol. 60. United States Department of Commerce. p. 29. ASIN B003HGFY0K. OCLC 605998563. OL 14852853M. NTIA Report 81-74. Retrieved 15 July 2021 – via Google Books. Failures of the antenna, antenna mounts, or transmission lines have been observed to contribute a major portion of system unavailability. Such outages have been observed on the Scope Com system in Germany, the ETA system in Germany, the Phil-Tai-Oki system in Taiwan and most recently, on the DEB Phase I in Italy.
  18. Burton, William (19 September 2016). "Cable & Wireless in Barbados". BajanThings. Archived from the original on 18 April 2021. Retrieved 15 July 2021. 1960 – Tropospheric scatter radio link established between Barbados and Trinidad. 1965 – Tropospheric scatter system extended south to Guyana via Trinidad and North to Tortola via St. Lucia and Antigua.
  19. Stamp of Guyana (1968)
  20. Kopp, Carlo (1 August 2010). Tropospheric Scatter Communications Systems (html) (Technical report). Air Power Australia. APA-TR-2010-0801. Archived from the original on 6 May 2021. Retrieved 13 July 2021.
  21. Williamson, John, ed. (27 January 2011). "Terrestrial microwave and tropospheric scatter - AN/TRC-170(V) tactical digital tropospheric scatter radio (United States)". Jane's Military Communications (32nd ed.). Janes Information Group. ISBN 978-0710629487. OCLC 751723522. Archived from the original on 5 February 2011. Retrieved 13 July 2021.

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