Line-of-sight propagation
Encyclopedia
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line. The rays or waves may be diffracted, refracted, reflected, or absorbed by atmosphere and obstructions with material and generally cannot travel over the horizon
or behind obstacles.
At low frequencies (below approximately 2 MHz
or so) radio
signals travel as ground waves, which follow the Earth's curvature due to diffraction
with the layers of atmosphere. This enables AM
radio signals in low-noise
environments to be received well after the transmitting antenna
has dropped below the horizon
. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2 Layer
, thus giving radio transmissions in this range a potentially global reach (see shortwave radio), again along multiply deflected straight lines. The effects of multiple diffraction or reflection lead to macroscopically "quasi-curved paths".
However, at higher frequencies and in lower levels of the atmosphere, neither of these effects are significant. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light
that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic of high-frequency
radio is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon".
In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM
radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere and there were no atmosphere, the radio
horizon would be a circle.
The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Antenna heights above 1000000 ft (189 mi; 305 km) will cover the entire hemisphere and not increase the radio horizon.
Radio wave propagation
is affected by atmospheric conditions, ionospheric absorption
, and the presence of obstructions, for example mountains or trees.
Simple formulas that include the effect of the atmosphere give the range as:
The simple formulas give a best-case approximation of the maximum propagation distance but are not sufficient to estimate the quality of service at any location.
s. It refers to the circular segment of earth profile which blocks off long distance communications. Since the geometric line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. The usual effect of the declining pressure of the atmosphere with height is to bend radio waves down toward the surface of the Earth, effectively increasing the Earth's radius, and the distance to the radio horizon, by a factor around 4/3. This k-factor can change from its average value depending on weather.
(i.e., line of sight) can readily be calculated.
Let R be the radius of Earth and h be the altitude of a telecommunication station. Line of sight distance d of this station is given by the Pythagorean theorem
;
Since the altitude of the station is much less than the radius of the Earth,
If the height is given in metres, and distance in kilometres,
If the height is given in feet, and the distance in miles,
(geometric) distance. Usually a factor k is used in the equation above
k > 1 means geometrically reduced bulge and a longer service range. On the other hand, k < 1 means a shorter service range.
Under normal weather conditions k is usually chosen to be 4/3. That means that, the maximum service range increases by % 15
for h in m. and d in km.
for h in ft. and d in miles ;
But in stormy weather, k may decrease to cause fading in transmission. (In extreme cases k can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge.
of radio waves between transmitters and receivers can be measured disregarding the type of propagation. But, generally, travel time only then represents the distance between transmitter and receiver, when line of sight propagation is the basis for the measurement. This applies as well to RADAR
, to Real Time Locating
and to LIDAR
.
This rules: Travel time measurements for determining the distance between pairs of transmitters and receivers generally require line of sight propagation for proper results. Whereas the desire to have just any type of propagation to enable communication may suffice, this does never coincide with the requirement to have strictly line of sight at least temporarily as the means to obtain properly measured distances. However, the travel time measurement may be always biased by multi-path propagation including line of sight propagation as well as non line of sight propagation in any random share. A qualified system for measuring the distance between transmitters and receivers must take this phenomenon into account. Thus filtering signals traveling along various paths makes the approach either operationally sound or just tediously irritating.
transmitters can be foiled by tree branches, or even heavy rain or snow.
If a direct visual fix cannot be taken, it is important to take into account the curvature of the Earth when calculating line-of-sight from maps.
The presence of objects not in the direct visual line of sight can interfere with radio transmission. This is caused by diffraction effects: for the best propagation, a volume known as the first Fresnel zone
should be kept free of obstructions.
Reflected radiation from the ground plane
also acts to cancel out the direct signal. This effect, combined with the free-space r−2 propagation loss to a r−4 propagation loss. This effect can be reduced by raising either or both antennas further from the ground: the reduction in loss achieved is known as height gain.
s (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects:
The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive Rayleigh fading
. For mobile phone services these problems are tackled using:
Other conditions may physically disrupt the connection surprisingly without prior notice:
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
or behind obstacles.
At low frequencies (below approximately 2 MHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
or so) radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
signals travel as ground waves, which follow the Earth's curvature due to diffraction
Diffraction
Diffraction refers to various phenomena which occur when a wave encounters an obstacle. Italian scientist Francesco Maria Grimaldi coined the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665...
with the layers of atmosphere. This enables AM
Amplitude modulation
Amplitude modulation is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent...
radio signals in low-noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
environments to be received well after the transmitting antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
has dropped below the horizon
Horizon
The horizon is the apparent line that separates earth from sky, the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not. At many locations, the true horizon is obscured by trees, buildings, mountains, etc., and the resulting...
. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the F1/F2 Layer
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
, thus giving radio transmissions in this range a potentially global reach (see shortwave radio), again along multiply deflected straight lines. The effects of multiple diffraction or reflection lead to macroscopically "quasi-curved paths".
However, at higher frequencies and in lower levels of the atmosphere, neither of these effects are significant. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the propagation characteristic of high-frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
radio is called "line-of-sight". The farthest possible point of propagation is referred to as the "radio horizon".
In practice, the propagation characteristics of these radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (a function of both the transmitter and the antenna characteristics). Broadcast FM
Frequency modulation
In telecommunications and signal processing, frequency modulation conveys information over a carrier wave by varying its instantaneous frequency. This contrasts with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant...
radio, at comparatively low frequencies of around 100 MHz, are less affected by the presence of buildings and forests.
Radio horizon
The radio horizon is the locusLocus (mathematics)
In geometry, a locus is a collection of points which share a property. For example a circle may be defined as the locus of points in a plane at a fixed distance from a given point....
of points at which direct rays from an antenna are tangential to the surface of the Earth. If the Earth were a perfect sphere and there were no atmosphere, the radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
horizon would be a circle.
The radio horizon of the transmitting and receiving antennas can be added together to increase the effective communication range. Antenna heights above 1000000 ft (189 mi; 305 km) will cover the entire hemisphere and not increase the radio horizon.
Radio wave propagation
Radio propagation
Radio propagation is the behavior of radio waves when they are transmitted, or propagated from one point on the Earth to another, or into various parts of the atmosphere...
is affected by atmospheric conditions, ionospheric absorption
Ionospheric absorption
Ionospheric absorption is the scientific name for absorption occurring as a result of the interaction between various types of electromagnetic waves and the free electrons in the ionosphere, which can interfere with radio transmissions.-Description:...
, and the presence of obstructions, for example mountains or trees.
Simple formulas that include the effect of the atmosphere give the range as:
The simple formulas give a best-case approximation of the maximum propagation distance but are not sufficient to estimate the quality of service at any location.
Earth bulge and atmosphere effect
Earth bulge is a term used in telecommunicationTelecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s. It refers to the circular segment of earth profile which blocks off long distance communications. Since the geometric line of sight passes at varying heights over the Earth, the propagating radio wave encounters slightly different propagation conditions over the path. The usual effect of the declining pressure of the atmosphere with height is to bend radio waves down toward the surface of the Earth, effectively increasing the Earth's radius, and the distance to the radio horizon, by a factor around 4/3. This k-factor can change from its average value depending on weather.
Geometric distance to horizon
Assuming a perfect sphere with no terrain irregularity, the distance to horizon from a high altitude transmitterTransmitter station
Transmitter station refers to terrestrial infrastructure for transmitting radio frequency signals. The station maybe used for, wireless communication, broadcasting, microwave link, mobile telephone etc.-Choice of location:...
(i.e., line of sight) can readily be calculated.
Let R be the radius of Earth and h be the altitude of a telecommunication station. Line of sight distance d of this station is given by the Pythagorean theorem
Pythagorean theorem
In mathematics, the Pythagorean theorem or Pythagoras' theorem is a relation in Euclidean geometry among the three sides of a right triangle...
;
Since the altitude of the station is much less than the radius of the Earth,
If the height is given in metres, and distance in kilometres,
If the height is given in feet, and the distance in miles,
The actual service range
The above analysis doesn’t take the effect of atmosphere on the propagation path of the RF signals into consideration. In fact, the RF signals don’t propagate in straight lines. Because of the refractive effects of atmospheric layers, the propagation paths are somewhat curved. Thus, the maximum service range of the station, is not equal to the line of sightLine-of-sight propagation
Line-of-sight propagation refers to electro-magnetic radiation or acoustic wave propagation. Electromagnetic transmission includes light emissions traveling in a straight line...
(geometric) distance. Usually a factor k is used in the equation above
k > 1 means geometrically reduced bulge and a longer service range. On the other hand, k < 1 means a shorter service range.
Under normal weather conditions k is usually chosen to be 4/3. That means that, the maximum service range increases by % 15
for h in m. and d in km.
for h in ft. and d in miles ;
But in stormy weather, k may decrease to cause fading in transmission. (In extreme cases k can be less than 1.) That is equivalent to a hypothetical decrease in Earth radius and an increase of Earth bulge.
Example
In normal weather conditions, the service range of a station at an altitude of 1500 m. with respect to receivers at sea level can be found as,
Line-of-sight propagation as a prerequisite for radio distance measurements
Travel timeTravel time
Travel time may refer to* Travel, movement of people between locations* Travel journal, record made by a voyager* Propagation speed, term in physics to measure things such as the speed of light or radio waves...
of radio waves between transmitters and receivers can be measured disregarding the type of propagation. But, generally, travel time only then represents the distance between transmitter and receiver, when line of sight propagation is the basis for the measurement. This applies as well to RADAR
Radar
Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...
, to Real Time Locating
RTLS
RTLS may refer to:* Real-time locating system - general techniques for asset and staff tracking using wireless hardware and real-time software* Ravenna Training and Logistics Site - an Ohio Army National Guard base....
and to LIDAR
LIDAR
LIDAR is an optical remote sensing technology that can measure the distance to, or other properties of a target by illuminating the target with light, often using pulses from a laser...
.
This rules: Travel time measurements for determining the distance between pairs of transmitters and receivers generally require line of sight propagation for proper results. Whereas the desire to have just any type of propagation to enable communication may suffice, this does never coincide with the requirement to have strictly line of sight at least temporarily as the means to obtain properly measured distances. However, the travel time measurement may be always biased by multi-path propagation including line of sight propagation as well as non line of sight propagation in any random share. A qualified system for measuring the distance between transmitters and receivers must take this phenomenon into account. Thus filtering signals traveling along various paths makes the approach either operationally sound or just tediously irritating.
Impairments to line-of-sight propagation
Low-powered microwaveMicrowave
Microwaves, a subset of radio waves, have wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz and 300 GHz. This broad definition includes both UHF and EHF , and various sources use different boundaries...
transmitters can be foiled by tree branches, or even heavy rain or snow.
If a direct visual fix cannot be taken, it is important to take into account the curvature of the Earth when calculating line-of-sight from maps.
The presence of objects not in the direct visual line of sight can interfere with radio transmission. This is caused by diffraction effects: for the best propagation, a volume known as the first Fresnel zone
Fresnel zone
In optics and radio communications , a Fresnel zone , named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids which define volumes in the radiation pattern of a circular aperture...
should be kept free of obstructions.
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...
also acts to cancel out the direct signal. This effect, combined with the free-space r−2 propagation loss to a r−4 propagation loss. This effect can be reduced by raising either or both antennas further from the ground: the reduction in loss achieved is known as height gain.
Mobile telephones
Although the frequencies used by mobile phoneMobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s (cell phones) are in the line-of-sight range, they still function in cities. This is made possible by a combination of the following effects:
- r−4 propagation over the rooftop landscape
- diffraction into the "street canyon" below
- multipath reflection along the street
- diffraction through windows, and attenuated passage through walls, into the building
- reflection, diffraction, and attenuated passage through internal walls, floors and ceilings within the building
The combination of all these effects makes the mobile phone propagation environment highly complex, with multipath effects and extensive Rayleigh fading
Rayleigh fading
Rayleigh fading is a statistical model for the effect of a propagation environment on a radio signal, such as that used by wireless devices.Rayleigh fading models assume that the magnitude of a signal that has passed through such a transmission medium will vary randomly, or fade, according to a...
. For mobile phone services these problems are tackled using:
- rooftop or hilltop positioning of base stations
- many base stationBase stationThe term base station can be used in the context of land surveying and wireless communications.- Land surveying :In the context of external land surveying, a base station is a GPS receiver at an accurately-known fixed location which is used to derive correction information for nearby portable GPS...
s (a phone can typically see six at any given time) - rapid handoff between base stations (roaming)
- extensive error correction and detectionError detection and correctionIn information theory and coding theory with applications in computer science and telecommunication, error detection and correction or error control are techniques that enable reliable delivery of digital data over unreliable communication channels...
in the radio link - sufficient operation of mobile phone in tunnels when supported by split cable antennas
- local repeaters inside complex vehicles or buildings
Other conditions may physically disrupt the connection surprisingly without prior notice:
- local failure when using the mobile phone in buildings of concrete with steel reinforcement
- temporal failure inside metal constructions as elevator cabins, trains, cars, ships
See also
- Anomalous propagationAnomalous propagationAnomalous propagation includes different forms of electromagnetic wave propagation that are not encountered in a standard atmosphere. While technically the term includes propagation with larger losses than in standard atmosphere, in practical applications it is most often meant to refer to cases...
- Field strength in free spaceField strength in free spaceField strength in free space is a term in telecommunications. It is the field strength caused by a half wave dipole under ideal conditions. The actual field strength in terrestrial environments is calculated by empirical formulas based on this field strength....
- Fresnel zoneFresnel zoneIn optics and radio communications , a Fresnel zone , named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids which define volumes in the radiation pattern of a circular aperture...
- Knife-edge effectKnife-edge effectIn electromagnetic wave propagation, the knife-edge effect or edge diffraction is a redirection by diffraction of a portion of the incident radiation that strikes a well-defined obstacle such as a mountain range or the edge of a building....
- MultilaterationMultilaterationMultilateration is a navigation technique based on the measurement of the difference in distance to two or more stations at known locations that broadcast signals at known times. Unlike measurements of absolute distance or angle, measuring the difference in distance results in an infinite number of...
- Non-line-of-sight propagationNon-line-of-sight propagationNon-line-of-sight or near-line-of-sight is a term used to describe radio transmission across a path that is partially obstructed, usually by a physical object in the innermost Fresnel zone....
- Over-the-horizon radarOver-the-horizon radarOver-the-horizon radar, or OTH , is a design concept for radar systems to allow them to detect targets at very long ranges, typically up to thousands of kilometers...
- Radio propagation
- Radial (radio)Radial (radio)In RF engineering, radial has two different meanings, both referring to lines which radiate from a radio antenna.-Planning:...
- Real Time Locating
- Rician fadingRician fadingRicean fading is a stochastic model for radio propagation anomaly caused by partial cancellation of a radio signal by itself — the signal arrives at the receiver by several different paths , and at least one of the paths is changing...
, stochastic model of line-of-sight propagation
External links
- http://www.wireless-center.net/Cisco-Wireless-Networking/728.html
- http://web.telia.com/~u85920178/data/pathlos.htm#bulges
- Article on the importance of Line Of Sight for UHF reception
- Approximating 2-Ray Model by using Binomial series by Matthew Bazajian