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71-713: The Defence High Frequency Communications Service or the DHFCS is a British military beyond line-of-sight communication system operated by the Ministry of Defence (MOD) and used predominately by the UK Armed Forces, as well as other authorised users. The system operates from six transmitting and receiving sites across the United Kingdom and is controlled from a network control centre located at Forest Moor in North Yorkshire and

142-716: A backup site at Kinloss Barracks in Moray. Overseas sites are located in Ascension Island , Cyprus and Falkland Islands . In 2003 VT Merlin Communications (now Babcock International Group ) were awarded the contract to operate the system for a period of fifteen years on behalf of the Ministry of Defence. The system is to be replaced by the Defence Strategic Radio Service (DSRS) also operated by Babcock Prior to

213-510: A component of the field is propagated a few meters into the surface of the Earth. The propagation is very low loss and communications over thousands of kilometers over NLOS links is possible. However, such low frequencies by definition ( Nyquist–Shannon sampling theorem ) are very low bandwidth, so this type of communication is not widely used. Radio waves in the VHF and UHF bands can travel somewhat beyond

284-469: A few kilometers to a few thousand kilometers. From Maxwell's equations we find that radio waves, as they exist in free space in the far field or Fraunhofer region behave as plane waves . In plane waves the electric field , magnetic field and direction of propagation are mutually perpendicular . To understand the various mechanisms that allow successful radio communications over NLOS paths we must consider how such plane waves are affected by

355-437: A fraction of a second. The RF noise burst from the lightning makes the initial part of the open channel unusable and the ionization disappears quickly because of recombination at low altitude and high atmospheric pressure. Although the hot lightning channel is briefly observable with microwave radar, no practical use for this mode has been found in communications. The mechanism of ionospheric propagation in supporting NLOS links

426-502: A measure of the level of reflection normally at the boundary when a plane wave passes from one dielectric medium to another. For example, if the intrinsic impedance of the first and second media were η 1 {\displaystyle \eta _{1}} and η 2 {\displaystyle \eta _{2}} respectively, the reflection coefficient of medium 2 relative to 1, Γ 21 {\displaystyle \Gamma _{21}} ,

497-441: A path around the obstruction. However they are unacceptable in most urban environments due to the bulky reflector requiring critical positioning at perhaps an inaccessible location or at one not acceptable to the planning authorities or the owner of the building. Passive reflector NLOS links also incur substantial loss due to the received signal being a 'double inverse-square law ' function of the transmit signal, one for each hop from

568-419: A possible NLOS path—those that are: If the obstruction dimensions are much smaller than the wavelength of the incident plane wave, the wave is essentially unaffected. For example, low frequency (LF) broadcasts, also known as long waves , at about 200 kHz has a wavelength of 1500 m and is not significantly affected by most average size buildings, which are much smaller. If the obstruction dimensions are of

639-431: A radio channel or link where there is no visual line of sight (LOS) between the transmitting antenna and the receiving antenna . In this context LOS is taken There are many electrical characteristics of the transmission media that affect the radio wave propagation and therefore the quality of operation of a radio channel, if it is possible at all, over an NLOS path. The acronym NLOS has become more popular in

710-536: A receiving antenna, a receiver, a transmitter and a transmitting antenna. If the ends of the NLOS link are at positions A and C, the repeater is located at position B where links A-B and B-C are in fact LOS. The active repeater may simply amplify the received signal and re-transmit it un-altered at either the same frequency or a different frequency. The former case is simpler and cheaper but requires good isolation between two antennas to avoid feedback , however it does mean that

781-407: A steel bridge or an airplane ) or relatively good conductors to plane waves such as large expanses of concrete building sides, walls etc. Sometimes this is considered a brute force method because, on each reflection the plane wave undergoes a transmission loss that must be compensated for by a higher output power from the transmit antenna compared to if the link had been LOS. However, the technique

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852-399: A visual obstruction on a NLOS link may be anything from negligible to complete suppression. An example might apply to a LOS path between a television broadcast antenna and a roof mounted receiving antenna. If a cloud passed between the antennas the link could actually become NLOS but the quality of the radio channel could be virtually unaffected. If, instead, a large building was constructed in

923-415: A warm layer of air settles above a cool layer. At the interface between them exists a relatively abrupt change in refractive index from a smaller value in the cool layer to a larger value in the warm layer. By analogy with the optical Snell's Law , this can cause significant reflections of radio waves back towards the Earth's surface where they are further reflected, thus causing a ducting effect . The result

994-495: Is absorbed but some can be reflected at its boundaries depending on its relative permittivity compared to that of free space, which is unity. This uses the concept of intrinsic impedance, which is described below. There are few large physical objects that are also good insulators, with the interesting exception of fresh water icebergs but these do not usually feature in most urban environments. However large volumes of gas generally behave as dielectrics. Examples of these are regions of

1065-427: Is cheap and easy to employ and passive random reflections are widely exploited in urban areas to achieve NLOS. Communication services that use passive reflections include WiFi , WiMax , WiMAX MIMO , mobile (cellular) communications and terrestrial broadcast to urban areas. Passive repeaters may be used to achieve NLOS links by deliberately installing a precisely designed reflector at a critical position to provide

1136-418: Is constructed from fabric and full of hot air, both of which are good insulators. Conversely, a metal obstruction of dimensions comparable to a wavelength would cause significant reflections. When considering obstruction size, we assume its electrical properties are the most common intermediate or lossy type. Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in

1207-574: Is directed into the troposphere just above the horizon with sufficient power flux density that gas and water vapour molecules cause scattering in a region in the beam path known as the scatter volume. Some components of the scattered energy travel in the direction of the receiver antennas and form the receive signal. Since there are very many particles to cause scattering in this region, the Rayleigh fading statistical model may usefully predict behaviour and performance in this kind of system. Rain scattering

1278-495: Is given by: The logarithmic measure in decibels ( T r {\displaystyle T_{r}} ) of how the transmitted RF signal over the NLOS link is affected by such a reflection is given by: T r e f = 10 log 10 ⁡ ( 1 − | Γ 21 | 2 ) d B {\displaystyle T_{ref}=10\log _{10}(1-\left|\Gamma _{21}\right|^{2})dB} Most materials of

1349-541: Is in one direction only), duplex (transmission is in both directions simultaneously) or half-duplex (transmission is possible in both directions but not simultaneously). Under normal conditions, all radio links, including NLOSl are reciprocal —which means that the effects of the propagation conditions on the radio channel are identical whether it operates in simplex, duplex, or half-duplex. However, propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies

1420-669: Is less effective. The most common application for this phenomenon is microwave rain radar, but rain scatter propagation can be a nuisance causing unwanted signals to intermittently propagate where they are not anticipated or desired. Similar reflections may also occur from insects though at lower altitudes and shorter range. Rain also causes attenuation of point-to-point and satellite microwave links. Attenuation values up to 30 dB have been observed on 30 GHz during heavy tropical rain. Lightning scattering has sometimes been observed on VHF and UHF over distances of about 500 km (300 miles). The hot lightning channel scatters radio-waves for

1491-423: Is no requirement for land-based receiver sites as communications are one way. The DHFCS is known to have the following capabilities. Non-line-of-sight propagation Obstacles that commonly cause NLOS propagation include buildings, trees, hills, mountains, and, in some cases, high voltage electric power lines. Some of these obstructions reflect certain radio frequencies, while some simply absorb or garble

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1562-439: Is not necessarily reciprocal. In general, the way a plane wave is affected by an obstruction depends on the size of the obstruction relative to its wavelength and the electrical properties of the obstruction. For example, a hot air balloon with multi-wavelength dimensions passing between the transmit and receive antennas could be a significant visual obstruction but is unlikely to affect the NLOS radio propagation much assuming it

1633-522: Is purely a microwave propagation mode and is best observed around 10 GHz, but extends down to a few gigahertz —the limit being the size of the scattering particle size vs. wavelength . This mode scatters signals mostly forwards and backwards when using horizontal polarization and side-scattering with vertical polarization . Forward-scattering typically yields propagation ranges of 800 km (500 miles). Scattering from snowflakes and ice pellets also occurs, but scattering from ice without watery surface

1704-535: Is quite a good insulator when dry and steel is a good conductor. Alternatively the material may be a homogeneous lossy material. The parameter that describes to what degree a material is a conductor or insulator is known as tan ⁡ δ {\displaystyle \tan \delta } , or the loss tangent , given by where and If σ ≫ ω ϵ 0 ϵ r {\displaystyle \sigma \gg \omega \epsilon _{0}\epsilon _{r}}

1775-501: Is similar to that for atmospheric refraction but, in this case, the radio wave refraction occurs not in the atmosphere but in the ionosphere at much greater altitudes. Like its tropospheric counterpart, ionospheric propagation can sometimes be statistically modelled using Rayleigh fading . The ionosphere extends from altitudes of approximately 50 km to 400 km and is divided into distinct plasma layers denoted D, E, F1, and F2 in increasing altitude. Refraction of radio waves by

1846-404: Is simply to circumvent the NLOS condition and place relays at additional locations, sending the content of the radio transmission around the obstructions. Some more advanced NLOS transmission schemes now use multipath signal propagation, bouncing the radio signal off other nearby objects to get to the receiver. Non-Line-of-Sight (NLOS) is a term often used in radio communications to describe

1917-534: Is that radio waves can propagate well beyond their intended service area with less than normal attenuation. This effect is only apparent in the VHF and UHF spectra and is often exploited by amateur radio enthusiasts to achieve communications over abnormally long distances for the frequencies involved. For commercial communication services it cannot be exploited because it is unreliable (the conditions can form and disperse in minutes) and it can cause interference well outside of

1988-487: The Earth itself, such as would exist if the other end of the link was beyond the optical horizon. A very useful property of the Earth's atmosphere is that, on average, the density of air gas molecules reduces as the altitude increases up to approximately 30 km. Its relative permittivity or dielectric constant reduces steadily from about 1.00536 at the Earth's surface. To model the change in refractive index with altitude,

2059-543: The Falkland Islands . The system was originally operated from a network control station (NCS) located at Forest Moor, with an alternate network control station (ANCS) at Kinloss which could be used in the event that Forest Moor station was unavailable. Split site working was introduced in 2011 which allows both the Forest Moor NCS and Kinloss ANCS to control 50% of the network, increasing resilience and recovery time in

2130-581: The Ministry of Defence (MOD) decided to merge the existing high frequency communications systems into one enhanced system and thereafter operate and manage it as the Defence High Frequency Communications Service (DHFCS). The Defence Communication Services Agency (DCSA) (now Information Systems and Services (ISS) under Joint Forces Command ) awarded a £228m public private partnership contract to VT Merlin Communications (now Babcock International Group ) to upgrade and operate

2201-569: The VHF and UHF bands, including FM and TV terrestrial broadcast services. The phenomenon described above that the atmospheric refractive index, relative permittivity or dielectric constant gradually reduces with increasing height is on account of the reduction of the atmospheric air density with increasing height. Air density is also a function of temperature, which ordinarily also reduces with increasing height. However, these are only average conditions; local meteorological conditions can create phenomena such as temperature inversion layers where

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2272-495: The White Alice Communications System covering all of Alaska before the 1960s, when communication satellites largely replaced them. A tropospheric scatter NLOS link typically operates at a few gigahertz using potentially very high transmit powers (typically 3 kW to 30 kW, depending on conditions), very sensitive receivers and very high gain, usually fixed, large reflector antennas. The transmit beam

2343-483: The urban environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons . If σ ≪ ω ϵ 0 ϵ r {\displaystyle \sigma \ll \omega \epsilon _{0}\epsilon _{r}} the material is a good insulator (or dielectric) or a poor conductor and substantially transmit waves that are incident upon it. Virtually no RF power

2414-858: The DHFCS and the US Air Force's (USAF) High Frequency Global Communications System (HFGCS) through a link to the HFGCS at the USAF facility at RAF Croughton in Northamptonshire. A link with the Australian Defence Force's High Frequency Communications System (HFCS) based in Canberra also exists. The service allows real-time strategic communications between users which comprise ground stations, submarines, surface vessels, fixed wing aircraft and helicopters. The DHFCS

2485-455: The Earth's radius been 4/3 of its actual value . The '4/3 Earth's radius' is a useful rule of thumb to the radio communication engineers when designing such a NLOS link. The 4/3 Earth radius rule of thumb is an average for the Earth's atmosphere assuming it is reasonably homogenised , absent of temperature inversion layers or unusual meteorological conditions. NLOS links that exploit atmospheric refraction typically operate at frequencies in

2556-659: The Earths atmosphere , which gradually reduce in density at increasing altitudes up to 10 to 20 km. At greater altitudes from about 50 km to 200 km various ionospheric layers also behave like dielectrics and are heavily dependent on the influence of the Sun . Ionospheric layers are not neutral gases but plasmas . Even if an obstruction is a perfect insulator, it may have some reflective properties on account of its relative permittivity ϵ r {\displaystyle \epsilon _{r}} differing from that of

2627-575: The LOS to the NLOS zone. A communications satellite is an example of an active repeater that does change frequency. Communications satellites, in most cases, are in geosynchronous orbit at an altitude of 22,300 miles (35,000 km) above the Equator . Application of the Poynting Vector to vertically polarized plane waves at LF (30 kHz to 300 kHz) and VLF (3 kHz to 30 kHz) indicates that

2698-575: The UK. The base was manned by a small group of Royal Naval radio operators and electricians (approximately six to a shift). This centre provided HF comms receiver links to bases in the Indian Ocean, Mediterranean, Canada and a RATT ship/shore receiver link to warships at sea. Transmitters were located at HMS Inskip in Lancashire. It also had a small ships company (including an RPO, a PO Caterer and chefs) for

2769-483: The absorption, these are often called lossy materials, although the degree of loss is usually extremely variable and often very dependent on the level of moisture present. They are often heterogeneous and comprise a mixture of materials with various degrees of conductor and insulator properties. Such examples are hills, valley sides, mountains (with substantial vegetation) and buildings constructed from stone, brick or concrete but without reinforced steel. The thicker they are

2840-438: The atmosphere may be approximated to many thin air layers, each of which has a slightly smaller refractive index than the one below. The trajectory of radio waves progressing through such an atmosphere model at each interface, is analogous to optical beams passing from one optical medium to another as predicted by Snell's Law . When the beam passes from a higher to lower refractive index it tends to get bent or refracted away from

2911-585: The atmosphere. Electrical materials through which plane waves may propagate have a property called intrinsic impedance ( η {\displaystyle \eta } ) or electromagnetic impedance, which is analogous to the characteristic impedance of a cable in transmission line theory . The intrinsic impedance of a homogeneous material is given by: where For free space μ r = 1 {\displaystyle \mu _{r}=1} and ϵ r = 1 {\displaystyle \epsilon _{r}=1} , therefore

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2982-404: The context of wireless local area networks (WLANs) and wireless metropolitan area networks such as WiMAX because the capability of such links to provide a reasonable level of NLOS coverage greatly improves their marketability and versatility in the typical urban environments where they are most frequently used. However NLOS contains many other subsets of radio communications. The influence of

3053-661: The creation of the DHFCS, the Royal Air Force (RAF) and Royal Navy (RN) operated their own independent high frequency (HF) communications systems. The RAF's Strike Command Integrated Communications System (STCICS), later known as Terrestrial Air Sea Communications (TASCOMM), operated from six sites within the UK whilst the RN system had twelve sites. The systems overlapped in their capabilities whilst simultaneously having gaps in overall capability, high running costs, lack of flexibility and neither were used to their full capacity. In 2003

3124-475: The day-to-day running of the establishment, including three civilian drivers. HMS Forest Moor was also the setting of a short-lived YTV comedy Thundercloud starring James Cosmo on a stone frigate which the Royal Navy confuse with an actual ship. In 2003 it was transferred to VT Communications (now Babcock International ) under a PPP contract to provide HF communications to the Ministry of Defence via

3195-410: The end of the NLOS link at A or C does not require to change the receive frequency from that used for a LOS link. A typical application might be to repeat or re-broadcast signals for vehicles using car radios in tunnels. A repeater that changes frequency would avoid any feedback problems but would be more difficult to design and expensive and it would require a receiver to change frequency when moving from

3266-426: The event of a loss of service and minimising potential network unavailability. Sites at Anthorn (Formerly HMS Nuthatch) and Skelton in northern England provide a Very Low Frequency (VLF) transmitting capability which is used to send communications to submerged submarines . Due to the low frequencies involved submarines can only receive VLF communications and do not carry VLF transmission aerials. Therefore, there

3337-429: The events are typically from several hours up to several days. Higher frequencies experience the most dramatic increase of signal strengths, while on low-VHF and HF the effect is negligible. Propagation path attenuation may be below free-space loss. Some of the lesser inversion types related to warm ground and cooler air moisture content occur regularly at certain times of the year and time of day. A typical example could be

3408-436: The greater the loss. For example, a wall absorbs much less RF power from a normally incident wave than a building constructed from the same material. Passive random reflections are achieved when plane waves are subject to one or more reflective paths around an object that makes an otherwise LOS radio path into NLOS. The reflective paths might be caused by various objects that could either be metallic (very good conductors such as

3479-434: The ground. In tropospheric scatter (troposcatter) communication systems a powerful beam of microwaves is aimed above the horizon, and a high gain antenna over the horizon aimed at the section of the troposphere though which the beam passes receives the tiny scattered signal. Troposcatter systems can achieve over-the-horizon communication between stations 500 miles (800 km) apart, and the military developed networks such as

3550-516: The horizon by Marconi in the early 20th century prompted extensive studies of ionospheric propagation for the next 50 years or so, which have yielded various HF link channel prediction tables and charts. Frequencies that are affected by ionospheric propagation range from approximately 500 kHz to 50 MHz but the majority of such NLOS links operate in the 'short wave' or high frequency (HF) frequency bands between 3 MHz and 30 MHz. HMS Forest Moor HMS Forest Moor

3621-399: The intrinsic impedance of free space η 0 {\displaystyle \eta _{0}} is given by which evaluates to approximately 377 Ω {\displaystyle \Omega } . In an analogy of plane wave theory and transmission line theory, the definition of reflection coefficient Γ {\displaystyle \Gamma } is

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3692-604: The ionosphere rather than the atmosphere can therefore allow NLOS links of much greater distance for just one refraction path or 'hop' via one of the layers. Under certain conditions radio waves that have undergone one hop may reflect off the Earth's surface and experience more hops, so increasing the range. The positions of these and their ion densities are significantly controlled by the Sun's incident radiation and therefore change diurnally , seasonally and during Sun spot activity. The initial discovery that radio waves could travel beyond

3763-429: The late summer, early morning tropospheric enhancements that bring in signals from distances up to few hundred kilometers (miles) for a couple of hours, until undone by the Sun's warming effect. At VHF and higher frequencies, small variations (turbulence) in the density of the atmosphere at a height of around 6 miles (9.7 km) can scatter some of the normally line-of-sight beam of radio frequency energy back toward

3834-406: The material is a good conductor or a poor insulator and substantially reflects the radio waves that are incident upon it with almost the same power. Therefore, virtually no RF power is absorbed by the material itself and virtually none is transmitted, even if it is very thin. All metals are good conductors and there are of course many examples that cause significant reflections of radio waves in

3905-423: The normal at the boundary according to Snell's Law. When the curvature of the Earth is taken into account it is found that, on average, radio waves whose initial trajectory is towards the optical horizon follows a path that does not return to the Earth's surface at the horizon, but slightly beyond it. The distance from the transmit antenna to where it does return is approximately equivalent to the optical horizon, had

3976-402: The normal radio-horizon. The inversion layer is mostly observed over high pressure regions, but there are several tropospheric weather conditions which create these randomly occurring propagation modes. Inversion layer's altitude for non-ducting is typically found between 100 and 1,000 meters (330 and 3,280 feet) and for ducting about 500 to 3,000 meters (1,600 to 9,800 feet), and the duration of

4047-528: The normal service area. Temperature inversion and anomalous propagation can occur at most latitudes but they are more common in tropical climates than temperate climates, usually associated with high pressure areas (anticyclones). Sudden changes in the atmosphere's vertical moisture content and temperature profiles can on random occasions make UHF , VHF and microwave signals propagate hundreds of kilometers (miles) up to about 2,000 kilometers (1,200 miles)—and for ducting mode even farther—beyond

4118-643: The object or objects that visually obstruct the otherwise LOS path between the antennas. It is understood that the terms radio far field waves and radio plane waves are interchangeable. By definition, line of sight is the visual line of sight, that is determined by the ability of the average human eye to resolve a distant object. Our eyes are sensitive to light but optical wavelengths are very short compared to radio wavelengths. Optical wavelengths range from about 400 nanometer (nm) to 700 nm but radio wavelengths range from approximately 1 millimeter (mm) at 300 GHz to 30 kilometers (km) at 10 kHz. Even

4189-427: The obstruction. The electrical properties of the material forming an obstruction to radio waves could range from a perfect conductor at one extreme to a perfect insulator at the other. Most materials have both conductor and insulator properties. They may be mixed: for example, many NLOS paths result from the LOS path being obstructed by reinforced concrete buildings constructed from concrete and steel . Concrete

4260-502: The only sites which closed entirely. Approximately seventy-two masts were removed in December 2003 and the final two removed in 2015. The service provides HF communications for the Ministry of Defence (Royal Air Force, Royal Navy, British Army and Joint Forces Command), other British government departments and NATO (including its Partnership for Peace organisation). A memorandum of understanding exists to allow cooperation between

4331-455: The path making it NLOS, the channel may be impossible to receive. Beyond line-of-sight ( BLOS ) is a related term often used in the military to describe radio communications capabilities that link personnel or systems too distant or too fully obscured by terrain for LOS communications. These radios utilize active repeaters , groundwave propagation , tropospheric scatter links , and ionospheric propagation to extend communication ranges from

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4402-409: The same order as a wavelength, there is a degree of diffraction around the obstruction and possibly some transmission through it. The incident radio wave could be slightly attenuated and there might be some interaction between the diffracted wavefronts. If the obstruction has dimensions of many wavelengths, the incident plane waves depend heavily on the electrical properties of the material that forms

4473-435: The shortest radio wavelength is therefore about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz, the difference is on the order of 60,000 times so it is not always reliable to compare visual obstructions, such as might suggest a NLOS path, with the same obstructions as they might affect a radio propagation path. NLOS links may either be simplex (transmission

4544-476: The signals; but, in either case, they limit the use of many types of radio transmissions, especially when low on power budget. Lower power levels at a receiver reduce the chance of successfully receiving a transmission. Low levels can be caused by at least three basic reasons: low transmit level, for example Wi-Fi power levels; far-away transmitter, such as 3G more than 5 miles (8.0 km) away or TV more than 31 miles (50 km) away; and obstruction between

4615-507: The system. The contract lasts for fifteen years (ending in 2018) and is funded by a reduction in the number of transmitting, receiving and control sites and a reduction of 266 military personnel. Through the rationalisation of sites, twelve sites (four RAF, seven RN and a joint site in Gibraltar ) were closed between 2003 and 2006. Many sites were used for other military purposes with the sites at Bampton Castle , Chelveston and Milltown being

4686-424: The transmit antenna to the receive antenna. However, they have been successfully used in rural mountainous areas to extend the range of LOS microwave links around mountains, thus creating NLOS links. In such cases the installation of the more usual active repeater was usually not possible due to problems in obtaining a suitable power supply. An active repeater is a powered piece of equipment essentially comprising

4757-469: The transmitter and the receiver, leaving no clear path. NLOS lowers the effective received power. Near Line Of Sight can usually be dealt with using better antennas, but Non Line Of Sight usually requires alternative paths or multipath propagation methods. How to achieve effective NLOS networking has become one of the major questions of modern computer networking. Currently, the most common method for dealing with NLOS conditions on wireless computer networks

4828-413: The type affecting radio wave transmission over NLOS links are intermediate: they are neither good insulators nor good conductors. Radio waves incident upon an obstruction comprising a thin intermediate material are partly reflected at both the incident and exit boundaries and partly absorbed, depending on the thickness. If the obstruction is thick enough the radio wave might be completely absorbed. Because of

4899-532: The visual horizon due to refraction in the troposphere , the bottom layer of the atmosphere below 20 km (12 miles). This is due to changes in the refractive index of air with temperature and pressure. Tropospheric delay is a source of error in radio ranging techniques, such as the Global Positioning System (GPS). In addition, unusual conditions can sometimes allow propagation at greater distances: The obstruction that creates an NLOS link may be

4970-521: Was a Royal Navy land base located in Nidderdale in the borough of Harrogate , North Yorkshire , England . The establishment was first used as one of the Second World War intelligence units, or Y-stations . In 1960 it became the Royal Navy's major high frequency (HF) receiver station, its primary function was to route HF signals from locations abroad to military bases and command centres in

5041-549: Was declared operational in March 2008. Within the UK, the system is divided into three stations of paired transmitter and receiver sites. 'UK North' comprises sites at Kinloss Barracks and Crimond in north east Scotland, 'UK Middle' at Forest Moor and Inskip in northern England and 'UK South' at St. Eval and Penhale Sands in south west England. Three overseas stations, with two sites each, are located in Cyprus , Ascension Island and

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