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Non-line-of-sight propagation
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This article is about radio transmissions. For non-line-of-sight artillery fire, see Indirect fire.
Non-line-of-sight (NLOS) and near-line-of-sight are radio transmissions across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone.
Many types of radio transmissions depend, to varying degrees, on line of sight (LOS) between the transmitter and receiver. Obstacles that commonly cause NLOS conditions 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 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 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 is 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 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 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
Either as a straight line free of any form of visual obstruction, even if it is actually too distant to see with the unaided human eye
As a virtual LOS i.e., as a straight line through visually obstructing material, thus leaving sufficient transmission for radio waves to be detected
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 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 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 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 a few miles to a few thousand miles.
Contents
[hide]
1 Radio waves as plane electromagnetic waves
2 What is line-of-sight?
3 How are plane waves affected by the size and electrical properties of the obstruction?
3.1 Obstruction Size
3.2 Electrical properties of obstructions that may cause NLOS
3.2.1 Good conductors (poor insulators)
3.2.2 Good insulators (poor conductors)
3.2.2.1 Plane waves and intrinsic impedance
3.2.2.2 Reflection losses at dielectric boundaries
3.2.3 Intermediate materials with finite conductivity
4 Means of achieving non-line-of-sight transmission
4.1 Passive random reflections
4.2 Passive repeaters
4.3 Active repeaters
4.4 Groundwave propagation
4.5 Tropospheric scatter links
4.6 Refraction through the Earth's atmosphere
4.7 Anomalous propagation
4.8 Ionospheric propagation
4.9 Finite absorption
4.10 Other methods
5 How is positioning accuracy affected by NLOS conditions?
6 References
7 Further reading
8 External links
Radio waves as plane electromagnetic waves[edit]
From Maxwell's equations[1] we find that radio waves, as they exist in free space in the far field or Fraunhofer region behave as plane waves.[2][3] In plane waves the electric field, magnetic field and direction of propagation are mutually perpendicular.[4] To understand the various mechanisms that allow successful radio communications over NLOS paths we must consider how such plane waves are affected by 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.
What is line-of-sight?[edit]
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 millimetre (mm) at 300 GHz to 30 kilometres (km) at 10 kHz. Even 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 more like 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 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.[5] Please remember that propagation conditions on different frequencies are different, so traditional duplex with different uplink and downlink frequencies is not necessarily reciprocal.
How are plane waves affected by the size and electrical properties of the obstruction?[edit]
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 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.
Obstruction Size[edit]
Broadly, there are three approximate sizes of obstruction in relationship to a wavelength to consider in a possible NLOS path—those that are:
Much smaller than a wavelength
The same order as a wavelength
Much larger than a wavelength
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 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 the obstruction.
Electrical properties of obstructions that may cause NLOS[edit]
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 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
tan  δ = σ ω ϵ 0 ϵ r {\displaystyle \tan \delta ={\frac {\sigma }{\omega \epsilon _{0}\epsilon _{r}}}}
where
σ {\displaystyle \sigma } is the conductivity of the material in siemens per metre (S/m)
ω = 2 π f {\displaystyle \omega =2\pi f} is the angular frequency of the RF plane wave in radians per second (rad/s) and f {\displaystyle f} is its frequency in hertz (Hz).
ϵ 0 {\displaystyle \epsilon _{0}} is the absolute permittivity of free space in farads per metre (F/m)
and
ϵ r {\displaystyle \epsilon _{r}} is the relative permittivity of the material (also known as dielectric constant) and has no units.
Good conductors (poor insulators)[edit]
If σ ≫ ω ϵ 0 ϵ r {\displaystyle \sigma \gg \omega \epsilon _{0}\epsilon _{r}} 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.[6] 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 the urban environment, for example bridges, metal clad buildings, storage warehouses, aircraft and electrical power transmission towers or pylons.
Good insulators (poor conductors)[edit]
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 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 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 gases but plasmas.
Plane waves and intrinsic impedance[edit]
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 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:[7]
η = μ 0 μ r ϵ 0 ϵ r {\displaystyle \eta ={\sqrt {\frac {\mu _{0}\mu _{r}}{\epsilon _{0}\epsilon _{r}}}}}
where
μ 0 {\displaystyle \mu _{0}} is the absolute permeability in henries per metre (H/m) and is a constant fixed at 4 π ⋅ 10 − 7 {\displaystyle 4\pi \cdot 10^{-7}} H/m
μ r {\displaystyle \mu _{r}} is the relative permeability (unitless)
ϵ 0 {\displaystyle \epsilon _{0}} is the absolute permittivity in farads per metre (F/m) and is a constant fixed at 8.85 ⋅ 10 − 12 {\displaystyle 8.85\cdot 10^{-12}} F/m
ϵ r {\displaystyle \epsilon _{r}} is the relative permittivity or dielectric constant (unitless)
For free space μ r = 1 {\displaystyle \mu _{r}=1} and ϵ r = 1 {\displaystyle \epsilon _{r}=1} , therefore the intrinsic impedance of free space η 0 {\displaystyle \eta _{0}} is given by
η 0 = μ 0 ϵ 0 {\displaystyle \eta _{0}={\sqrt {\frac {\mu _{0}}{\epsilon _{0}}}}}
which evaluates to approximately 377 Ω {\displaystyle \Omega } .
Reflection losses at dielectric boundaries[edit]
In an analogy of plane wave theory and transmission line theory, the definition of reflection coefficient Γ {\displaystyle \Gamma } is 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}} , is given by:
Γ 21 = η 2 − η 1 η 2 + η 1 {\displaystyle \Gamma _{21}={\frac {\eta _{2}-\eta _{1}}{\eta _{2}+\eta _{1}}}}
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}
Intermediate materials with finite conductivity[edit]
Most materials of 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 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 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.
Means of achieving non-line-of-sight transmission[edit]
Passive random reflections[edit]
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 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 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[edit]
Passive repeaters may be used to achieve NLOS links by deliberately installing a precisely designed reflector at a critical position to provide 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 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.
Active repeaters[edit]
An active repeater is a powered piece of equipment essentially comprising 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 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 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.
Groundwave propagation[edit]
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 a component of the field is propagated a few metres into the surface of the Earth. The propagation is very low loss and communications over thousands of miles 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.
Tropospheric scatter links[edit]
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 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.
Refraction through the Earth's atmosphere[edit]
The obstruction that creates an NLOS link may be 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.[8] To model the change in refractive index with altitude, 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 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 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 the VHF and UHF bands, including FM and TV terrestrial broadcast services.
Anomalous propagation[edit]
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 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 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.[9] 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 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).
Ionospheric propagation[edit]
The mechanism of ionospheric propagation in supporting NLOS links 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.[10] 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 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 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.
In the latter half of the twentieth century, alternative means of communicating over large NLOS distances were developed such as satellite communications and submarine optical fiber, both of which potentially carry much larger bandwidths than HF and are much more reliable. Despite their limitations, HF communications only need relatively cheap, crude equipment and antennas so they are mostly used as backups to main communications systems and in sparsely populated remote areas where other methods of communication are not cost effective.
Finite absorption[edit]
If an object that changes a LOS link to NLOS is not a good conductor but an intermediate material, it absorbs some of the RF power incident upon it. However, if it has finite thickness the absorption is also finite and the resulting attenuation of the radio waves may be tolerable and an NLOS link may be set up using radio waves that actually pass through the material. As an example, wireless local area networks (WLANs) often use finite absorption NLOS links to communicate between a WLAN access point and WLAN client(s) in the typical office environment. The radio frequencies used, typically a few gigahertz (GHz) normally passes through a few thin office walls and partitions with tolerable attenuation. After many such walls though or after a few thick concrete or similar (non-metallic) walls the NLOS link becomes unworkable.
Other methods[edit]
Earth–Moon–Earth communication, Meteor burst communications, and Sporadic E propagation are also other methods of achieving communications past the radio horizon.
How is positioning accuracy affected by NLOS conditions?[edit]
In most of the recent localization systems, it is assumed that the received signals propagate through a LOS path. However, infringement of this assumption can result in inaccurate positioning data.[11] For Time of Arrival based localization system, the emitted signal can only arrive at the receiver through its NLOS paths. The NLOS error is defined as the extra distance travelled by the received signal with respect to the LOS path. The NLOS error is always positively biased with the magnitude dependent on the propagation environment.
References[edit]
Jump up ^ Pozar, David M. (2005); Microwave Engineering, Third Edition (Intl. Ed.); John Wiley & Sons, Inc.; pp 5-9. ISBN 0-471-44878-8.
Jump up ^ Ramo, Whinnery and Van Duzer;"Fields and Waves in Communication Electronics"; John Wiley & Sons, Inc; pp 322-324. ISBN 0-471-58551-3
Jump up ^ Morton, A H; "Advanced Electrical Engineering"; Pitman Publishing Ltd.; pp 387-389. ISBN 0-273-40172-6.
Jump up ^ A. J. Baden Fuller;"Microwaves, Second Edition"; Pergammon Press; p 47. ISBN 0-08-024228-6.
Jump up ^ Ramo, Whinnery and Van Duzer (op. cit); pp 717-719.
Jump up ^ A. J. Baden Fuller (op. cit.); p152
Jump up ^ A. J. Baden Fuller (op. cit.); pp45-47
Jump up ^ Tennent, R. M. (Ed.); "Science Data Book; The Open University; p66
Jump up ^ Hutchinson, Chuck K8CH; "The ARRL Handbook for Radio Amateurs 2001 78th Ed.";The American Radio Relay League, Inc.ISBN 0-87259-186-7
Jump up ^ Kennedy, George; "Electronic Communication Systems"; MacMillan/McGraw-Hill. ISBN 0-07-112672-4
Jump up ^ Wang Wei; Xiong Jin-Yu; Zhu Zhong-Liang (2005). "A new NLOS mitigation algorithm in location estimation". IEEE Transactions on Vehicular Technology. IEEE Vehicular Technology Society. 54 (6): 2048–2053. doi:10.1109/TVT.2005.858177. ISSN 0018-9545.
Further reading[edit]
Bullington, K.; "Radio Propagation Fundamentals"; Bell System Technical Journal Vol. 36 (May 1957); pp 593–625.
"Technical Planning Parameters and Methods for Terrestrial Broadcasting" (April 2004); Australian Broadcasting Authority. ISBN 0-642-27063-5
External links[edit]
Research on "Non-line-of-sight (NLOS) Localisation for Indoor Environments" by CMR at UNSW
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British anti-invasion preparations of 1803–05
From Wikipedia, the free encyclopedia
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James Gillray, Buonaparte, 48 hours after landing!--Vide John Bulls home-stroke, armed en masse (1803).
British anti-invasion preparations of 1803–05 were the military and civilian responses in the United Kingdom to Napoleon's planned invasion of the United Kingdom. They included mobilization of the population on a scale not previously attempted in Britain, with a combined military force of over 615,000 in December 1803.[1] Much of the southern English coast was fortified to repel a French landing. However Napoleon never attempted his planned invasion and so the preparations were never put to the test.
Contents
[hide]
1 Background
2 British armed forces
2.1 Regular Army
2.2 Army of Reserve
2.3 Militia
2.4 Volunteers
2.5 Royal Navy
3 Fortifications
3.1 Martello towers
3.2 Royal Military Canal
3.3 Western Heights
4 Telegraphs
5 Contingency plans
6 Legacy
7 See also
8 Notes
9 References
10 External links
Background[edit]
In the aftermath of the French Revolution of 1789 Britain and France were at war almost continuously from 1793 to 1802, and then from 1803 to 1815, interrupted by the brief Peace of Amiens of 1802–3 and Napoleon's first exile at Elba in 1814–15. In late 1797, Napoleon said to the French Directory that:
[France] must destroy the English monarchy, or expect itself to be destroyed by these intriguing and enterprising islanders...Let us concentrate all our efforts on the navy and annihilate England. That done, Europe is at our feet.[2]
Napoleon inspecting the army at Boulogne, 15 August 1804.
However Napoleon decided against invading for the time being and instead unsuccessfully attacked British interests in Egypt. In March 1802 the two countries signed the Treaty of Amiens, which brought to an end nearly nine years of war. However both the British Prime Minister Henry Addington and Napoleon viewed the peace as temporary, and so it was, with Britain declaring war on France on 18 May 1803.[3] William Pitt replaced Addington as Prime Minister on 10 May 1804.
In 1803 Napoleon turned his attention to invading England once more, saying: "All my thoughts are directed towards England. I want only for a favourable wind to plant the Imperial Eagle on the Tower of London".[4] Napoleon now planned an invasion on a bigger scale than 1798 and 1801, and built a new armada for the effort.[5] He assembled the Grande Armée of over 100,000 at Boulogne.
British armed forces[edit]
Commander-in-Chief of the Forces The Prince Frederick, Duke of York and Albany.
Regular Army[edit]
Addington's government had kept the Regular army at 132,000 men during the Amiens interlude, with 18,000 in Ireland and 50,000 in Great Britain (the rest serving abroad).[6]
Army of Reserve[edit]
In 1803 50 of the 93 regiments of the Army gained a second battalion, which became known as the Army of Reserve. These 50,000 men would be raised by ballot within one year. However each recruit was liable to serve in Great Britain only. These Reserve soldiers could volunteer as Regular soldiers and consequently receive money.[7] After 9 months recruiting fewer than 3,000 of the 42,000 men were balloted men.[8] Within one month of recruiting it had enlisted 22,500 men but by the end of 1803 it was short of the required 50,000 by 15,000 so the government stopped recruiting for it.[9]
Militia[edit]
The Militia was a territorial-based infantry to be used only for home defence and was not a standing army. It was to be raised by ballot. The government in December 1802, fearing war, held a Militia ballot. The ballot was run by churchwardens and overseers of the poor in each parish. A list of men aged between eighteen and forty-five, with many exceptions (such as seamen and Thames watermen), was posted on the front of the church door. However, if a man found himself on the list he could get out of serving by paying a fine or getting another man to fill his place.[10] Four months after the ballot, and a week into the war, the Militia was filled to 80% of the 51,000 required.[9]
Volunteers[edit]
Main articles: Volunteer corps (1794–1814) and Yeomanry
The British government had no choice—in view of Napoleon's avowed determination to invade the United Kingdom—but to rely on the patriotism of the people through a volunteer movement.[11] The volunteers' function, as laid down in July 1803 by the Commander-in-Chief of the Forces, Prince Frederick, Duke of York, was to conduct guerrilla warfare against the French occupying forces. They would operate in small bodies to harass, instill panic and wear out the French army. They were never to get deeply engaged with French troops and to retire when pressed. Local knowledge was to be used as much as possible and they were also expected to cut off French pillaging detachments.[12] In anticipation to criticisms that arming the people was dangerous, the Secretary at War in Addington's government (Charles Yorke) said on 18 July 1803 in introducing a Bill for amending the Defence of the Realm Act:
I say, that in these times, it is better to run the hazard even of the people making a bad use of their arms, than that they should be actually left in a state of entire ignorance of the use of them. For my own part, I can safely aver, that I cannot see any real danger which is likely to accrue to the internal peace of the Country, when I consider the present dispositions and feelings of the people.[13]
William Pitt, in response, agreed:
William Pitt.
I am sure there is not an heart that palpitates in a British bosom that will not rouse for the common cause, and cordially join for the defence of the country. ... That there was a time, Sir, when it would have been dangerous to entrust arms with a great portion of the people of this country, I have strong reasons to know, because it must be in the recollection of every man that incendiaries were at work amongst them; and so successful in the promulgation of revolutionary doctrines, as to have disposed them to exert any means, however desperate, which they thought could be successful, in subverting the Government and Constitution. But that time is now past; and, I trust, those who have been so grosly deluded have seen their error. At least I am convinced, that if any such there still remain, the portion is so small, that if armed and dispersed in the same ranks with their loyal fellow subjects, they would be converted by their example; and, like them, rejoice in the blessings of our happy constitution; like them glory to live under its auspices, or die in its defence.[14]
Even the prominent anti-war MP Charles James Fox supported the Bill:
This is the first measure which I could...come down to support, being a measure for the defence of the country...the mass of the country; acting, not in single regiments, but as a great mass of armed citizens, fighting for the preservation of their country, their families, and every thing that is dear to them in life...an armed mass of the country, who are bound by every feeling and by every tie to defend that country to the last drop of their blood, before they will give way to him and his invading forces.[14]
During 1803 the government's call for volunteers to resist an invasion was met with a massive response. However the government was unprepared for the numbers of volunteers, as within a few weeks 280,000 men had volunteered. On 18 August Addington issued a circular discouraging new volunteers "in any county where the effective members of those corps, including the yeomanry, shall exceed the amount of six times the militia".[15] This had little effect since by the first week of September there were 350,000 volunteers.[16] At least half of the volunteers of the summer and autumn of 1803 were not equipped with their own weapon. When the government tried to issue them with pikes this was met with contempt and attacked by the Opposition leader William Windham.[17]
The second half of 1803 marked the height of the invasion scare.[18][19][20][21][22] When the King reviewed 27,000 volunteers in Hyde Park, London on 26 and 28 October 1803, 500,000 people were estimated to have turned out on each of the two days to witness the event. The Chief Constable of Bramfield (John Carrington) travelled from Hertfordshire to see it, saying: "I never saw such a sight all my days".[19] These were the best-attended of reviews of volunteers which between 1797 and 1805 "were often of daily occurrence".[23]
In 1804 returns to Parliament recorded a total of 480,000 volunteers in uniform. In addition there were the regular forces and the militia, which meant that nearly one-in-five able-bodied men were in uniform.[24] The Speaker, addressing the King at the prorogation of Parliament on 13 August 1803, said "the whole Nation has risen up in Arms".[25] Addington called the volunteer movement on 4 September 1803 "an insurrection of loyalty".[26] The response to the call to arms to resist invasion in these years has impressed some historians: "If some dissentient voices were heard in 1797–8 when the aftermath of the Revolution still lingered in the land, there was increased enthusiasm in the patriotism of 1801, and burning ardour coupled with absolute unanimity in that of 1803–5".[27]
Royal Navy[edit]
First Lord of the Admiralty John Jervis, 1st Earl of St. Vincent.
The Royal Navy kept a constant blockade of French harbours from Toulon to the Texel, just outside artillery range, waiting for a French ship to sail close enough to be attacked.[28] Rear-Admiral of the United Kingdom Admiral Cornwallis had a fleet off Brest and Commander-in-Chief of the North Sea Admiral Keith possessed a fleet between the Downs and Selsey Bill.[20] A further line of British ships lay close to the English coast to intercept any French ships that broke through the blockade.[20] The French were unwilling to venture out of their ports and so in the two years only nine flotilla ships had been captured or sunk by the Royal Navy.[29] During the end of December 1803 a violent storm blew Cornwallis's fleet from Brest and it had to stay in Torbay, giving the French fleet two days opportunity to invade. Upon hearing this Addington gave orders to prepare for an imminent invasion, but the French never used this opportunity.[30]
First Lord of the Admiralty Lord St. Vincent is said to have told the House of Lords: "I do not say the French cannot come, I only say they cannot come by sea".[31][32]
A volunteer force called the Sea Fencibles had been formed in 1793; they manned small armed boats, watch and signal towers, and fixed and floating batteries along the coasts.[33]
Fortifications[edit]
Brigadier General William Twiss.
In July 1803 the Duke of York argued for the construction of field fortifications as soon as possible because "the Erection of such Works must be immediate with a view to their probable utility", placing them at "Points where a Landing threatens the most important interests of the Country".[34] In August he requested from Addington further funds for such fortifications, who eventually consented. The Duke of York's priorities were the building of considerable fortifications at the Western Heights looking over the port of Dover and then the construction of Martello towers along the Kent and Sussex coasts.[35]
Martello towers[edit]
Main article: Martello towers
Across the coast of Kent and Sussex the government constructed a series of well-fortified towers, known as Martello towers, between 1805 and 1808. In September 1804 Pitt's government ordered General Sir William Twiss to scout the south-east coast of England for possible sites for Martello towers, to be used as artillery emplacements. Twiss earmarked eighty-eight appropriate sites between Seaford and Eastwear Bay. Twiss also headed the team that designed the Martello towers.[36] Twiss's plan was adopted in October at a defence conference in Rochester attended by Pitt, Lord Camden, the Duke of York, Lord Chatham, Major General Sir Robert Brownrigg, Lieutenant Colonel John Brown and Twiss.[37] Eventually seventy-four towers were built, with two (Eastbourne and Dymchurch) being considerably larger with eleven guns and housing 350 soldiers. These became known as Grand Redoubts.[37] A second line of twenty-nine towers, from Clacton-on-Sea to Slaughden near Aldeburgh, was constructed by 1812, including a redoubt at Harwich. Forty were built in Ireland.[38]
Martello tower.
Royal Military Canal[edit]
Main article: Royal Military Canal
During the summer of 1804, Lieutenant Colonel Brown was sent to examine the coast to discover if flooding of Romney Marsh would be viable in case of invasion. Brown thought it would not be and believed security would be improved by the excavation of "a cut from Shorncliffe battery, passing in front of Hythe under Lympne heights to West Hythe...being everywhere within musket shot of the heights".[39] Brown then advocated an extension of the water barrier: "...cutting off Romney Marsh from the county, opening a short and easy communication between Kent and Sussex but, above all, rendering unnecessary the doubtful and destructive measure of laying so large a portion of the country waste by inundation".[39] This became known as the Royal Military Canal, with construction starting in 1805 and completion in 1810.
Western Heights[edit]
The Drop Redoubt at the Western Heights.
Main article: Dover Western Heights
General Twiss recommended a fortress at Dover, with work beginning in 1804. The Western Heights at Dover consisted of three parts: the Drop Redoubt, the Citadel and the Grand Shaft.[40] The Drop Redoubt was a detached fort close to the steep cliffs, surrounded by ditches and intended for soldiers to go out and attack French infantry, built between 1804 and 1808. The Citadel was a larger fort surrounded by ditches and was still unfinished when war with France ended in 1815. The Grand Shaft was a barracks containing sixty officers and 1300 NCOs and soldiers, begun in 1806 and completed in 1809.[40]
Telegraphs[edit]
In order for the government to better communicate with the coast in case of invasion, a system of telegraphs was constructed. In January 1796, a telegraph line from the Admiralty in London to Deal, Kent had been built. In December 1796 another telegraph line was constructed between the Admiralty and Portsmouth. In May 1806 a telegraph line from Beacon Hill on the Portsmouth line to Plymouth was built. A subsequent telegraph line was built between the Admiralty and Great Yarmouth, thus telegraphs covered the south-east, south-west and East Anglia.[41] Before, messages between Portsmouth and London had taken several hours (two days for Plymouth) to reach their destination but now communications between London and Portsmouth took only fifteen minutes. Between London and Hythe it was eleven minutes.[41]
Contingency plans[edit]
King George III.
The King made contingency plans in the event of a French landing, with a courtier writing on 13 November 1803: "The King is really prepared to take the field in case of attack, his beds are ready and he can move at half an hour's warning".[42] Another courtier wrote:
The king certainly has his camp equipage and accoutrements quite ready for joining the army if the enemy should land, and is quite keen on the subject and angry if any suggests that the attempt may not be made. ... God forbid he should have the fate of Harold.[43]
The King wrote to his friend Bishop Hurd from Windsor on 30 November 1803:
We are here in daily expectation that Bonaparte will attempt his threatened invasion; the chances against his success seem so many that it is wonderful he persists in it. I own I place that thorough dependence on Divine Providence that I cannot help thinking the usurper is encouraged to make the trial that the ill-success may put an end to his wicked purposes. Should his troops effect a landing, I shall certainly put myself at the head of my troops and my other armed subjects to repel them. But as it is impossible to foresee the events of such a conflict, should the enemy approach too near to Windsor, I shall think it right the Queen and my daughters should cross the Severn, and send them to your Episcopal Palace at Worcester; by this hint I do not the least mean they shall be any inconvenience to you, and shall send a proper servant and furniture for their accommodation. Should this event arise, I certainly would rather have what I value most in life remain, during the conflict, in your diocese and under your roof than in any other place in the island.[23]
The government's contingency plans laid down that the King would go to Chelmsford if the French landed in Essex, or to Dartford if they landed in Kent, along with the Prime Minister and the Home Secretary. Lord Cornwallis would be in command of the reserve army. The Royal Arsenal artillery and stores and the Ordnance Board's powder magazines in Purfleet, would be put on the Grand Junction canal to the new ordnance depot at Weedon, Northamptonshire. Soldiers would be paid in gold instead of paper money. The Bank of England books would be sent to the Tower of London and its treasure would be entrusted to Sir Brook Watson, the Commissary General, who would transport it in thirty wagons (guarded by a relay of twelve Volunteer escorts) across the Midlands to join the King at Worcester Cathedral. The Stock Exchange would close and the Privy Council would take charge in London. The press would be forbidden from printing troop movements and official government communiqués would be distributed. If London fell to the French, the King and his ministers would retreat to the Midlands and "use the final mainstays of sovereignty – treasure and arms – to keep up the final struggle".[43][44]
Legacy[edit]
By the 1 September 1805 Napoleon's invasion camps were empty due to the Grande Armée marching against the Austrians. The Battle of Trafalgar on 21 October relieved British fears of invasion as Admiral Nelson destroyed the combined French-Spanish fleet.[45] However the threat of invasion remained so long as Britain was at war with France. At Tilsit in July 1807 Napoleon and the Russian Tsar Alexander I agreed to combine the navies of Europe against Britain. The British responded with a pre-emptive attack on the Danish fleet at Battle of Copenhagen and they also made sure the French never secured the Portuguese fleet.[46] Captain Edward Pelham Brenton in his Naval History of Great Britain noted that after Trafalgar "another French navy, as if by magic sprang forth from the forests to the seashore, manned by a maritime conscription exactly similar in principle to that edict by which the trees were appropriated to the building of ships".[47] The First Lord of the Admiralty, Lord Melville, noted after the war that given time Napoleon would "have sent forth such powerful fleets that our navy must eventually have been destroyed, since we could never have kept pace with him in building ships nor equipped numbers sufficient to cope with the tremendous power he could have brought against us".[47]
See also[edit]
British anti-invasion preparations of World War II
Notes[edit]
Jump up ^ Charles John Fedorak, Henry Addington, Prime Minister, 1801-1804. Peace, War and Parliamentary Politics (Akron, Ohio: The University of Akron Press, 2002), p. 165.
Jump up ^ H. F. B Wheeler and A. M. Broadley, Napoleon and the Invasion of England. The Story of the Great Terror (Nonsuch, 2007), p. 7.
Jump up ^ Peter A. Lloyd, The French Are Coming! The Invasion Scare 1803–05 (Spellmount Publishers Ltd, 1992), p. 8.
Jump up ^ Wheeler and Broadley, p. 9.
Jump up ^ Frank McLynn, Invasion. From the Armada to Hitler. 1588–1945 (London: Routledge, 1987), p. 98.
Jump up ^ Lloyd, pp. 119-20.
Jump up ^ Lloyd, p. 121.
Jump up ^ Lloyd, p. 123.
^ Jump up to: a b Lloyd, p. 126.
Jump up ^ Lloyd, pp. 121-22.
Jump up ^ Linda Colley, Britons. Forging the Nation, 1707–1837 (Yale University Press, 2005), p. 284.
Jump up ^ Admiral Sir Herbert W. Richmond, The Invasion of Britain. An Account of Plans, Attempts & Counter-Measures from 1586 to 1918 (Methuen, 1941), p. 54.
Jump up ^ The Times (19 July 1803), p. 1.
^ Jump up to: a b The Times (19 July 1803), p. 3.
Jump up ^ Norman Longmate, Island Fortress. The Defence of Great Britain 1603–1945 (Pimlico, 2001), pp. 285-6.
Jump up ^ Longmate, p. 286.
Jump up ^ Longmate, pp. 286-7.
Jump up ^ Wheeler and Broadley, p. 10.
^ Jump up to: a b Colley, p. 225.
^ Jump up to: a b c McLynn, p. 100.
Jump up ^ Longmate, p. 284.
Jump up ^ Wendy Hinde, George Canning (Purnell Book Services, 1973), pp. 118-9.
^ Jump up to: a b Wheeler and Broadley, p. 14.
Jump up ^ Alexandra Franklin and Mark Philp, Napoleon and the Invasion of Britain (The Bodleian Library, 2003), p. 13.
Jump up ^ The Times (13 August 1803), p. 2.
Jump up ^ Philip Ziegler, Addington. A Life of Henry Addington, First Viscount Sidmouth (Collins, 1965), p. 200.
Jump up ^ Wheeler and Broadley, p. 13.
Jump up ^ Lloyd, pp. 55-6.
Jump up ^ Lloyd, p. 66.
Jump up ^ Fedorak, p. 167.
Jump up ^ Longmate, p. 267.
Jump up ^ Lloyd, p. 197.
Jump up ^ http://www.historyofwar.org/articles/weapons_sea_fencibles.html
Jump up ^ Fedorak, pp. 167-68.
Jump up ^ Fedorak, p. 168.
Jump up ^ Lloyd, p. 166.
^ Jump up to: a b Lloyd, p. 167.
Jump up ^ Longmate, p. 278.
^ Jump up to: a b Lloyd, p. 159.
^ Jump up to: a b Longmate, p. 279.
^ Jump up to: a b Longmate, p. 269.
Jump up ^ John Brooke, King George III (Panther, 1974), p. 597.
^ Jump up to: a b Lloyd, p. 93.
Jump up ^ Brooke, p. 598.
Jump up ^ Correlli Barnett, Britain and Her Army. A Military, Political and Social History of the British Army 1509–1970 (Cassell, 2000), p. 252.
Jump up ^ Richard Glover, Britain at Bay. Defence against Bonaparte, 1803-14 (London: George Allen & Unwin Ltd, 1973), pp. 17-18.
^ Jump up to: a b Glover, p. 19.
References[edit]
Correlli Barnett, Britain and Her Army. A Military, Political and Social History of the British Army 1509–1970 (Cassell, 2000).
John Brooke, King George III (Panther, 1974).
Linda Colley, Britons. Forging the Nation, 1707–1837 (Yale University Press, 2005).
J. E. Cookson, The British Armed Nation, 1793–1815 (Clarendon Press, 1997).
Charles John Fedorak, Henry Addington, Prime Minister, 1801-1804. Peace, War and Parliamentary Politics (Akron, Ohio: The University of Akron Press, 2002).
Alexandra Franklin and Mark Philp, Napoleon and the Invasion of Britain (The Bodleian Library, 2003).
Austin Gee, The British Volunteer Movement, 1794–1815 (Clarendon Press, 2003).
Richard Glover, Britain at Bay. Defence against Bonaparte, 1803-14 (London: George Allen & Unwin Ltd, 1973).
Wendy Hinde, George Canning (Purnell Book Services, 1973).
Peter A. Lloyd, The French Are Coming! The Invasion Scare 1803–05 (Spellmount Publishers Ltd, 1992)
Norman Longmate, Island Fortress. The Defence of Great Britain 1603–1945 (Pimlico, 2001).
Frank McLynn, Invasion. From the Armada to Hitler. 1588–1945 (London: Routledge, 1987).
Mark Philp, Resisting Napoleon: The British Response to the Threat of Invasion, 1797–1815 (Ashgate, 2006).
Admiral Sir Herbert W. Richmond, The Invasion of Britain. An Account of Plans, Attempts & Counter-Measures from 1586 to 1918 (Methuen, 1941).
H. F. B Wheeler and A. M. Broadley, Napoleon and the Invasion of England. The Story of the Great Terror [1908] (Nonsuch, 2007).
Philip Ziegler, Addington. A Life of Henry Addington, First Viscount Sidmouth (Collins, 1965).
External links[edit]
BBC History - Dan Cruickshank - Napoleon, Nelson and the French Threat
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value: Préparatifs anti-invasion britanniques de 1803–1805 – French

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