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
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
or behind obstacles.
Especially
radioRadio is the transmission of signals 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, like all
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
including light emissions, travel in straight lines.
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
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
or behind obstacles.
Especially
radioRadio is the transmission of signals 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, like all
electromagnetic radiationElectromagnetic radiation is a ubiquitous phenomenon that takes the form of self-propagating waves in a vacuum or in matter. It consists of electric and magnetic field components which oscillate in phase perpendicular to each other and perpendicular to the direction of energy propagation...
including light emissions, travel in straight lines. At low frequencies (below approximately 2
MHzThe hertz is a unit of frequency. It is defined as the number of complete cycles per second. It is the basic unit of frequency in the International System of Units , and is used worldwide in both general-purpose and scientific contexts...
or so) these signals travel as ground waves, which follow the Earth's curvature due to
diffractionDiffraction is normally taken to refer to various phenomena which occur when a wave encounters an obstacle. It is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings...
with the layers of atmosphere. This enables
AMAmplitude 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-
noiseIn common use, the word noise means unwanted sound or noise pollution. In both analog and digital electronics, noise or signal noise is an unwanted random addition to a wanted signal; it is called noise as a generalisation of the audible noise heard when listening to a weak radio transmission...
environments to be received well after the transmitting
antennaAn antenna is a transducer designed to transmit or receive electromagnetic waves. In other words, antennas convert electromagnetic waves into electrical currents and vice versa. Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication, wireless...
has dropped below the
horizonThe horizon is the apparent line that separates earth from sky.It is the line that divides all visible directions into two categories: those that intersect the Earth's surface, and those that do not...
. Additionally, frequencies between approximately 1 and 30 MHz, can be reflected by the
F1/F2 LayerThe ionosphere is the uppermost part of the atmosphere, 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 apply. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the
lightLight is electromagnetic radiation, particularly radiation of a wavelength that is visible to the human eye ....
that the eye may sense. Therefore, as the ability to visual sight a transmitting antenna (with regards to the limitations of the eye's resolution) roughly corresponds with the ability to receive a signal from it, the propagation characteristic of
high-frequencyRadio frequency is a frequency, or rate of oscillation, of electromagnetic radiation within the range of about 3 Hz to 300 GHz. This range corresponds to the frequency of alternating current electrical signals used to produce and detect radio waves...
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
FMIn telecommunications, frequency modulation conveys information over a carrier wave by varying its frequency . In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal...
radio, at comparatively low freqencies of around 100 MHz, easily propagates through buildings and forests.
Line-of-sight propagation as a prerequisite for radio distance measurements
Travel timeTravel time may refer to* the duration of a journey, see travel and travel journal* the best season for traveling, see Wanderlust* in physics the travel time of light or a radio waves from a transmitter to a receiver, due to the signals propagation speed. It can be used for distance measurement and...
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
RADARRadar is an object detection system that uses electromagnetic waves to identify the range, altitude, direction, or speed of both moving and fixed objects such as aircraft, ships, motor vehicles, weather formations, and terrain. The term RADAR was coined in 1941 as an acronym for RAdio Detection And...
, to
Real Time LocatingRTLS may refer to:* Real-time locating - international standard for asset and staff tracking using wireless hardware and real-time software* Real time location system - general techniques for asset and staff tracking using wireless hardware and real-time software* Ravenna Training and Logistics...
and to
LIDARLIDAR is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses...
.
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
microwaveMicrowaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300MHz and 300 GHz. This is an extremely broad definition including both UHF and EHF , and various sources use different boundaries...
transmitters can be foiled by a 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 zoneIn optics and radio communications , a Fresnel zone, named for physicist Augustin-Jean Fresnel, is one of a number of concentric ellipsoids of revolution which define volumes in the radiation pattern of a...
should be kept free of obstructions.
Reflected radiation from the
ground planeIn 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 Phones
Although the frequencies used by 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 cellphone propagation environment highly complex, with multipath effects and extensive
Rayleigh fadingRayleigh 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 cellphone services these problems are tackled using:
- rooftop or hilltop positioning of base stations
- many base station
The term base station can be used in the context of land surveying, wireless computer networking, and wireless communications.-Land surveying:...
s (a phone can typically see six at any given time)
- rapid handoff
In cellular telecommunications, the term handoff refers to the process of transferring an ongoing call or data session from one channel connected to the core network to another. In satellite communications it is the process of transferring satellite control responsibility from one earth station to...
between base stations (roaming)
- extensive error correction and detection
In mathematics, computer science, telecommunication, and information theory, error detection and correction refers to techniques to ensure that data is transmitted without errors, even across unreliable media or networks.-General definitions of terms:...
in the radio link
- sufficient operation of cellphone in tunnels when supported by slit 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 cellphone in buildings of concrete with steel reinforcement
- temporal failure inside metal constructions as elevator cabins, trains, cars, ships
See also
- Non-line-of-sight propagation
Non-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 Fresnel zone....
- Anomalous propagation
Anomalous 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...
- Real Time Locating
- Knife-edge effect
In 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....
- Multilateration
Multilateration, also known as hyperbolic positioning, is the process of locating an object by accurately computing the time difference of arrival of a signal emitted from that object to three or more receivers...
- Unilateration
Unilateration is a special breed of RTLS, i.e. locating or positioning systems. Unilateration is a special simplification applied to Real time locating as a technology to observe a process from any distance...
- Radio propagation
- 2-Ray Ground Reflection Model
In wireless and mobile radio telecommunications finding path differences between Line-of-sight and ground reflected rays is important. It is necessary for finding the phase difference between the electric field components and the time delay of the arriving rays for the purpose of large-scale path...
- Rician fading
Rician 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 two different paths , and at least one of the paths is changing...
, stochastic model of line-of-sight propagation
External links