Radiolocation
Encyclopedia
Radiolocating is the process of finding the location
of something through the use of radio wave
s. It generally refers to passive uses, particularly radar
—as well as detecting buried cable
s, water
mains
, and other public utilities. It is similar to radionavigation, but radiolocation usually refers to passively finding a distant object rather than actively one's own position. Both are types of radiodetermination
. Radiolocation is also used in Real Time Locating Systems (RTLS
) for tracking valuable assets.
ed waves (as in radar or passive RFID). The received signal strength (RSSI) may be used to estimate distance if the transmitted power and dependence of signal strength on distance is known. The time of arrival
(TOA) can provide a distance estimate if the time of transmission and speed of propagation are known. Combining TOA data from several receivers at different known locations (differential time of arrival, DTOA) can provide an estimate of position even in the absence of knowledge of the time of transmission. The angle of arrival
(AOA) at a receiving station can be determined by the use of a directional antenna, or by differential time of arrival at an array of antennas with known location. AOA information may be combined with distance estimates from the techniques previously described to establish the location of a transmitter or backscatterer. Alternatively, the AOA at two receiving stations of known location establishes the position of the transmitter. The use of multiple receivers to locate a transmitter is known as multilateration
.
Various challenges are encountered in each location technique. Use of RSSI to locate a transmitter from a single receiver requires that both the transmitted (or backscattered) power from the object to be located are known, and that the propagation characteristics of the intervening region are known. In empty space, signal strength decreases as the inverse square of the distance for distances large compared to a wavelength and compared to the object to be located, but in most real environments, a number of impairments can occur: absorption, refraction, shadowing, and reflection. Absorption is negligible for radio propagation in air at frequencies less than about 10 GHz, but becomes important at multi-GHz frequencies where rotational molecular states can be excited. Refraction is important at long ranges (tens to hundreds of kilometers) due to gradients in moisture content and temperature in the atmosphere. In urban, mountainous, or indoor environments, obstruction by intervening obstacles and reflection from nearby surfaces are very common, and contribute to multipath distortion: that is, reflected and delayed replicates of the transmitted signal are combined at the receiver. Signals from different paths can add constructively or destructively: such variations in amplitude are known as fading
. The dependence of signal strength on position of transmitter and receiver becomes complex and often non-monotonic, making single-receiver estimates of position inaccurate and unreliable. Multilateration using many receivers is often combined with calibration measurements ("fingerprinting") to improve accuracy.
TOA and AOA measurements are also subject to multipath errors, particularly when the direct path from the transmitter to receiver is blocked by an obstacle. Time of arrival measurements are also most accurate when the signal has distinct time-dependent features on the scale of interest—for example, when it is composed of short pulses of known duration—but Fourier Transform
theory shows that in order to change amplitude or phase on a short time scale, a signal must use a broad bandwidth. For example, to create a pulse of about 1 nsec duration, roughly sufficient to identify location to within 0.3 m (1 foot), a bandwidth of roughly 1 GHz is required. In many regions of the radio spectrum, emission over such a broad bandwidth is not allowed by the relevant regulatory authorities, in order to avoid interference with other narrowband users of the spectrum. In the United States, unlicensed transmission is allowed in several bands, such as the 902-928 MHz and 2.4-2.483 GHz Industrial, Scientific, and Medical ISM band
s, but high-power transmission cannot extend outside of these bands. However, several jurisdictions now allow ultrawideband transmission over GHz or multi-GHz bandwidths, with constraints on transmitted power to minimize interference with other spectrum users. UWB pulses can be very narrow in time, and often provide accurate estimates of TOA in urban or indoor environments.
Radiolocation is employed in a wide variety of industrial and military activities. Radar systems often use a combination of TOA and AOA to determine a backscattering object's position using a single receiver. In Doppler radar
, the Doppler shift is also taken into account, determining velocity
rather than location (though it helps determine future location). Real Time Location Systems RTLS using calibrated RTLS, and DTOA, are commercially available. The widely-used Global Positioning System (GPS) is based on TOA of signals from satellites at known positions.
A stud finder
can also be an example of radiolocation, if it uses radio waves rather than ultrasound
.
s. Most often, this is done through trilateration
between radio tower
s. The location of the Caller or handset
can be determined several ways:
The first two depend on a line-of-sight
, which can be difficult or impossible in mountain
ous terrain
or around skyscraper
s. Location signatures actually work better in these conditions however. TDMA
and GSM networks such as Cingular and T-Mobile
use TDOA.
CDMA networks such as Verizon Wireless
and Sprint PCS tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies.
Composite solutions, needing both the handset and the network include:
Initially, the purpose of any of these in mobile phones is so that the public safety answering point (PSAP) which answers call
s to an emergency telephone number
can know where the caller is and exactly where to send emergency services. This ability is known within the NANP (North America
) as wireless enhanced 911
. Mobile phone users may have the option to permit the location information gathered to be sent to other phone numbers or data
networks, so that it can help people who are simply lost or want other location-based service
s. By default, this selection is usually turned off, to protect privacy
.
Location (geography)
The terms location and place in geography are used to identify a point or an area on the Earth's surface or elsewhere. The term 'location' generally implies a higher degree of can certainty than "place" which often has an ambiguous boundary relying more on human/social attributes of place identity...
of something through the use of radio wave
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s. It generally refers to passive uses, particularly 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...
—as well as detecting buried cable
Cable
A cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
s, water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
mains
Mains
Mains may refer to:* Mains electricity * Mains power around the world* Electricity transmission* Public utility, "mains services", including electricity, natural gas, water, and sewage disposal...
, and other public utilities. It is similar to radionavigation, but radiolocation usually refers to passively finding a distant object rather than actively one's own position. Both are types of radiodetermination
Radiodetermination
As defined by FS-1037C, radiodetermination is the determination of the position, velocity or other characteristics of an object, or the obtaining of information relating to these parameters, by means of the propagation properties of radio waves....
. Radiolocation is also used in Real Time Locating Systems (RTLS
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....
) for tracking valuable assets.
Basic principles
An object can be located by measuring the characteristics of received radio waves. The radio waves may be transmitted by the object to be located, or they may be backscatterBackscatter
In physics, backscatter is the reflection of waves, particles, or signals back to the direction they came from. It is a diffuse reflection due to scattering, as opposed to specular reflection like a mirror...
ed waves (as in radar or passive RFID). The received signal strength (RSSI) may be used to estimate distance if the transmitted power and dependence of signal strength on distance is known. The time of arrival
Time of arrival
Time of arrival , also called time of flight , is the travel time of a radio signal from a single transmitter to a remote single receiver. By the relation between light speed in vacuum and the carrier frequency of a signal the time is a measure for the distance between transmitter and receiver...
(TOA) can provide a distance estimate if the time of transmission and speed of propagation are known. Combining TOA data from several receivers at different known locations (differential time of arrival, DTOA) can provide an estimate of position even in the absence of knowledge of the time of transmission. The angle of arrival
Angle of arrival
Angle of arrival measurement is a method for determining the direction of propagation of a radio-frequency wave incident on an antenna array...
(AOA) at a receiving station can be determined by the use of a directional antenna, or by differential time of arrival at an array of antennas with known location. AOA information may be combined with distance estimates from the techniques previously described to establish the location of a transmitter or backscatterer. Alternatively, the AOA at two receiving stations of known location establishes the position of the transmitter. The use of multiple receivers to locate a transmitter is known as multilateration
Multilateration
Multilateration 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...
.
Various challenges are encountered in each location technique. Use of RSSI to locate a transmitter from a single receiver requires that both the transmitted (or backscattered) power from the object to be located are known, and that the propagation characteristics of the intervening region are known. In empty space, signal strength decreases as the inverse square of the distance for distances large compared to a wavelength and compared to the object to be located, but in most real environments, a number of impairments can occur: absorption, refraction, shadowing, and reflection. Absorption is negligible for radio propagation in air at frequencies less than about 10 GHz, but becomes important at multi-GHz frequencies where rotational molecular states can be excited. Refraction is important at long ranges (tens to hundreds of kilometers) due to gradients in moisture content and temperature in the atmosphere. In urban, mountainous, or indoor environments, obstruction by intervening obstacles and reflection from nearby surfaces are very common, and contribute to multipath distortion: that is, reflected and delayed replicates of the transmitted signal are combined at the receiver. Signals from different paths can add constructively or destructively: such variations in amplitude are known as fading
Fading
In wireless communications, fading is deviation of the attenuation that a carrier-modulated telecommunication signal experiences over certain propagation media. The fading may vary with time, geographical position and/or radio frequency, and is often modelled as a random process. A fading channel...
. The dependence of signal strength on position of transmitter and receiver becomes complex and often non-monotonic, making single-receiver estimates of position inaccurate and unreliable. Multilateration using many receivers is often combined with calibration measurements ("fingerprinting") to improve accuracy.
TOA and AOA measurements are also subject to multipath errors, particularly when the direct path from the transmitter to receiver is blocked by an obstacle. Time of arrival measurements are also most accurate when the signal has distinct time-dependent features on the scale of interest—for example, when it is composed of short pulses of known duration—but Fourier Transform
Fourier transform
In mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
theory shows that in order to change amplitude or phase on a short time scale, a signal must use a broad bandwidth. For example, to create a pulse of about 1 nsec duration, roughly sufficient to identify location to within 0.3 m (1 foot), a bandwidth of roughly 1 GHz is required. In many regions of the radio spectrum, emission over such a broad bandwidth is not allowed by the relevant regulatory authorities, in order to avoid interference with other narrowband users of the spectrum. In the United States, unlicensed transmission is allowed in several bands, such as the 902-928 MHz and 2.4-2.483 GHz Industrial, Scientific, and Medical ISM band
ISM band
The industrial, scientific and medical radio bands are radio bands reserved internationally for the use of radio frequency energy for industrial, scientific and medical purposes other than communications....
s, but high-power transmission cannot extend outside of these bands. However, several jurisdictions now allow ultrawideband transmission over GHz or multi-GHz bandwidths, with constraints on transmitted power to minimize interference with other spectrum users. UWB pulses can be very narrow in time, and often provide accurate estimates of TOA in urban or indoor environments.
Radiolocation is employed in a wide variety of industrial and military activities. Radar systems often use a combination of TOA and AOA to determine a backscattering object's position using a single receiver. In Doppler radar
Doppler radar
A Doppler radar is a specialized radar that makes use of the Doppler effect to produce velocity data about objects at a distance. It does this by beaming a microwave signal towards a desired target and listening for its reflection, then analyzing how the frequency of the returned signal has been...
, the Doppler shift is also taken into account, determining velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
rather than location (though it helps determine future location). Real Time Location Systems RTLS using calibrated RTLS, and DTOA, are commercially available. The widely-used Global Positioning System (GPS) is based on TOA of signals from satellites at known positions.
A stud finder
Stud finder
A stud finder is a handheld device used to determine the location of wood and metal framing studs used in light-frame construction after the walling surface has been installed....
can also be an example of radiolocation, if it uses radio waves rather than ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
.
Mobile phones
Radiolocation is also used in cellular telephony via base stationBase station
The 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. Most often, this is done through trilateration
Trilateration
In geometry, trilateration is the process of determinating absolute or relative locations of points by measurement of distances, using the geometry of circles, spheres or triangles. In addition to its interest as a geometric problem, trilateration does have practical applications in surveying and...
between radio tower
Radio masts and towers
Radio masts and towers are, typically, tall structures designed to support antennas for telecommunications and broadcasting, including television. They are among the tallest man-made structures...
s. The location of the Caller or handset
Handset
On a telephone, the handset is a device the user holds to the ear to hear the audio sound. Modern-day handsets usually contain the phone's microphone as well, but in early telephones the microphone was mounted directly on the telephone itself, which often was attached to a wall at a convenient...
can be determined several ways:
- angle of arrival (AOA) requires at least two towers, locating the caller at the point where the lines along the angles from each tower intersect
- time difference of arrival (TDOA) resp. time of arrival (TOA) works using 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...
, except that it is the networks that determine the time difference and therefore distance from each tower (as with seismometerSeismometerSeismometers are instruments that measure motions of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources...
s) - location signature uses "fingerprinting" to store and recall patternPatternA pattern, from the French patron, is a type of theme of recurring events or objects, sometimes referred to as elements of a set of objects.These elements repeat in a predictable manner...
s (such as multipath) which mobile phone signals are known to exhibit at different locations in each cell
The first two depend on a line-of-sight
Line-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...
, which can be difficult or impossible in mountain
Mountain
Image:Himalaya_annotated.jpg|thumb|right|The Himalayan mountain range with Mount Everestrect 58 14 160 49 Chomo Lonzorect 200 28 335 52 Makalurect 378 24 566 45 Mount Everestrect 188 581 920 656 Tibetan Plateaurect 250 406 340 427 Rong River...
ous terrain
Terrain
Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...
or around skyscraper
Skyscraper
A skyscraper is a tall, continuously habitable building of many stories, often designed for office and commercial use. There is no official definition or height above which a building may be classified as a skyscraper...
s. Location signatures actually work better in these conditions however. TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
and GSM networks such as Cingular and T-Mobile
T-Mobile
T-Mobile International AG is a German-based holding company for Deutsche Telekom AG's various mobile communications subsidiaries outside Germany. Based in Bonn, Germany, its subsidiaries operate GSM and UMTS-based cellular networks in Europe, the United States, Puerto Rico and the US Virgin Islands...
use TDOA.
CDMA networks such as Verizon Wireless
Verizon Wireless
Cellco Partnership, doing business as Verizon Wireless, is one of the largest mobile network operators in the United States. The network has 107.7 million subscribers as of 2011, making it the largest wireless service provider in America....
and Sprint PCS tend to use handset-based radiolocation technologies, which are technically more similar to radionavigation. GPS is one of those technologies.
Composite solutions, needing both the handset and the network include:
- assisted GPSAssisted GPSAssisted GPS, generally abbreviated as A-GPS or aGPS, is a system which can, under certain conditions, improve the startup performance, or time-to-first-fix of a GPS satellite-based positioning system. It is used extensively with GPS-capable cellular phones as its development was accelerated by...
(wirelessWirelessWireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
or TVTelevisionTelevision is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
) allows use of GPS even indoors - Advanced Forward Link Trilateration (A-FLT)
- Timing Advance/Network Measurement Report (TA/NMR)
- Enhanced Observed Time Difference (E-OTDE-OTDE-OTD is the acronym for the Enhanced Observed Time Difference.Enhanced Observed Time Difference is a standard for the location of mobile telephones. The location method works by multilateration. The standardisation was first carried out for GSM by the GSM standard committees in LCS Release 98...
)
Initially, the purpose of any of these in mobile phones is so that the public safety answering point (PSAP) which answers call
Telephone call
A telephone call is a connection over a telephone network between the calling party and the called party.-Information transmission:A telephone call may carry ordinary voice transmission using a telephone, data transmission when the calling party and called party are using modems, or facsimile...
s to an emergency telephone number
Emergency telephone number
Many countries' public telephone networks have a single emergency telephone number, sometimes known as the universal emergency telephone number or occasionally the emergency services number, that allows a caller to contact local emergency services for assistance. The emergency telephone number may...
can know where the caller is and exactly where to send emergency services. This ability is known within the NANP (North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
) as wireless enhanced 911
Enhanced 911
Enhanced 911, E-911 or E911 in North America is one example of the modern evolution of telecommunications based system meant as an easy way to link people experiencing an emergency with the public resources that can help. The dial-three-digits concept first originated in the United Kingdom in 1937....
. Mobile phone users may have the option to permit the location information gathered to be sent to other phone numbers or data
Data
The term data refers to qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which...
networks, so that it can help people who are simply lost or want other location-based service
Location-based service
A Location-Based Service is an information or entertainment service, accessible with mobile devices through the mobile network and utilizing the ability to make use of the geographical position of the mobile device....
s. By default, this selection is usually turned off, to protect privacy
Privacy
Privacy is the ability of an individual or group to seclude themselves or information about themselves and thereby reveal themselves selectively...
.