Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the

location In geography, location or place is used to denote a region (point, line, or area) on Earth's surface. The term ''location'' generally implies a higher degree of certainty than ''place'', the latter often indicating an entity with an ambiguous bou ...

of something through the use of

radio wave Radio waves (formerly called Hertzian waves) are a type of electromagnetic radiation with the lowest frequencies and the longest wavelengths in the electromagnetic spectrum, typically with frequencies below 300 gigahertz (GHz) and wavelengths g ...

s. It generally refers to passive, particularly

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

—as well as detecting buried cables, water mains, and other public utilities. It is similar to '' radionavigation'' in which one actively seeks its own position; both are types of ''

radiodetermination As defined by FS-1037C and ITU Radio Regulations, 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 propagati ...

''. Radiolocation is also used in real-time locating systems (RTLS) 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 backscattered waves (as in radar or passive

RFID Radio-frequency identification (RFID) uses electromagnetic fields to automatically identify and track tags attached to objects. An RFID system consists of a tiny radio transponder called a tag, a radio receiver, and a transmitter. When tri ...

). A stud finder uses radiolocation when it uses radio waves rather than

ultrasound Ultrasound is sound with frequency, frequencies greater than 20 Hertz, kilohertz. This frequency is the approximate upper audible hearing range, limit of human hearing in healthy young adults. The physical principles of acoustic waves apply ...

. One technique measures a distance by using the difference in the power of the received signal strength (RSSI) as compared to the originating signal strength. Another technique uses the time of arrival (TOA), when the time of transmission and speed of propagation are known. Combining TOA data from several receivers at different known locations (time difference of arrival, TDOA) 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 Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth ( geopositioning). When more than three distances are involved, it may be called multilateration, f ...

. Estimates are improved when the transmission characteristics of the medium is factored into the calculations. For RSSI this means electromagnetic permeability; for TOA it may mean non-line-of-sight receptions. 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 In science, an inverse-square law is any scientific law stating that the observed "intensity" of a specified physical quantity is inversely proportional to the square of the distance from the source of that physical quantity. The fundamental ca ...

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 In wireless communications, fading is the variation of signal attenuation over variables like time, geographical position, and radio frequency. Fading is often modeled as a random process. In wireless systems, fading may either be due to mul ...

. 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 In mathematics, the Fourier transform (FT) is an integral transform that takes a function as input then outputs another function that describes the extent to which various frequencies are present in the original function. The output of the tr ...

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 ns 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 The ISM radio bands are portions of the radio spectrum reserved internationally for ''industrial, scientific, and medical'' (ISM) purposes, excluding applications in telecommunications. Examples of applications for the use of radio frequency (RF ...

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 The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. The ''Doppler effect'' is named after the physicist Christian Doppler, who described t ...

is also taken into account, determining

velocity Velocity is a measurement of speed in a certain direction of motion. It is a fundamental concept in kinematics, the branch of classical mechanics that describes the motion of physical objects. Velocity is a vector (geometry), vector Physical q ...

rather than location (though it helps determine future location). Real Time Location Systems RTLS using calibrated RTLS, and TDOA, are commercially available. The widely used Global Positioning System ( GPS) is based on TOA of signals from satellites at known positions.

Mobile phones

Radiolocation is also used in

cellular telephony Mobile telephony is the provision of wireless telephone services to mobile phones, distinguishing it from fixed-location telephony provided via landline phones. Traditionally, telephony specifically refers to voice communication, though t ...

via

base station Base station (or base radio station, BS) is – according to the International Telecommunication Union's (ITU) Radio Regulations (RR) – a " land station in the land mobile service." A base station is called '' node B'' in 3G, '' eNB'' in L ...

s. Most often, this is done through

trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth ( geopositioning). When more than three distances are involved, it may be called multilateration, f ...

between radio towers. The location of the Caller or handset 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

, except that it is the networks that determine the time difference and therefore distance from each tower (as with

seismometer A seismometer is an instrument that responds to ground displacement and shaking such as caused by quakes, volcanic eruptions, and explosions. They are usually combined with a timing device and a recording device to form a seismograph. The out ...

s) * location signature uses "

fingerprint A fingerprint is an impression left by the friction ridges of a human finger. The recovery of partial fingerprints from a crime scene is an important method of forensic science. Moisture and grease on a finger result in fingerprints on surfa ...

ing" to store and recall

pattern A pattern is a regularity in the world, in human-made design, or in abstract ideas. As such, the elements of a pattern repeat in a predictable manner. A geometric pattern is a kind of pattern formed of geometric shapes and typically repeated l ...

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, which can be difficult or impossible in

mountain A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. Although definitions vary, a mountain may differ from a plateau in having a limited summit area, and is usually higher t ...

ous

terrain Terrain (), alternatively relief or topographical relief, is the dimension and shape of a given surface of land. In physical geography, terrain is the lay of the land. This is usually expressed in terms of the elevation, slope, and orientati ...

or around

skyscraper A skyscraper is a tall continuously habitable building having multiple floors. Most modern sources define skyscrapers as being at least or in height, though there is no universally accepted definition, other than being very tall high-rise bui ...

s. Location signatures actually work ''better'' in these conditions however. TDMA and

GSM The Global System for Mobile Communications (GSM) is a family of standards to describe the protocols for second-generation (2G) digital cellular networks, as used by mobile devices such as mobile phones and Mobile broadband modem, mobile broadba ...

networks such as Cingular and

T-Mobile T-Mobile is the brand of telecommunications by Deutsche Telekom Deutsche Telekom AG (, ; often just Telekom, DTAG or DT; stylised as ·T·) is a partially state-owned German telecommunications company headquartered in Bonn and the largest telec ...

use TDOA.

CDMA Code-division multiple access (CDMA) is a channel access method used by various radio communication technologies. CDMA is an example of multiple access, where several transmitters can send information simultaneously over a single communicatio ...

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: * assisted GPS (

wireless Wireless communication (or just wireless, when the context allows) is the transfer of information (''telecommunication'') between two or more points without the use of an electrical conductor, optical fiber or other continuous guided transm ...

or TV) allows use of GPS even indoors * Advanced Forward Link Trilateration ( A-FLT) * Timing Advance/Network Measurement Report ( TA/NMR) * Enhanced Observed Time Difference ( E-OTD) Initially, the purpose of any of these in mobile phones is so that the public safety answering point (PSAP) which answers

call Call or Calls may refer to: Arts, entertainment, and media Games * Call (poker), a bet matching an opponent's * Call, in the game of contract bridge, a bid, pass, double, or redouble in the bidding stage Music and dance * Call (band), from L ...

s to an

emergency telephone number An emergency telephone number is a number that allows a caller to contact local emergency services for assistance. The emergency number differs from country to country; it is typically a three-digit number so that it can be easily remembered and ...

can know where the caller is and exactly where to send

emergency services Emergency services and rescue services are organizations that ensure public safety, security, and health by addressing and resolving different emergencies. Some of these agencies exist solely for addressing certain types of emergencies, while ot ...

. This ability is known within the NANP (

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

) as wireless

enhanced 911 Enhanced 911 (E-911 or E911) is a system used in North America to automatically provide the caller's location to 911 dispatchers. 911 is the universal emergency telephone number in the region. In the European Union, a similar system exists known ...

. Mobile phone users may have the option to permit the location information gathered to be sent to other phone numbers or

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

networks, so that it can help people who are simply lost or want other

location-based service Location-based service (LBS) is a general term denoting software service (economics), services which use geographic data and information to provide services or information to users. LBS can be used in a variety of contexts, such as health, indoor ...

s. By default, this selection is usually turned off, to protect

privacy Privacy (, ) is the ability of an individual or group to seclude themselves or information about themselves, and thereby express themselves selectively. The domain of privacy partially overlaps with security, which can include the concepts of a ...

International regulation

Radiolocation service (short: RLS) is – according to ''Article 1.48'' of the International Telecommunication Union's (ITU)

Radio Regulations Radio regulation refers to the regulation and licensing of radio in international law, by individual governments, and by municipalities. International regulation The International Telecommunication Union (ITU) is a specialized agency of the Unit ...

(RR) – defined as "A

radiodetermination service As defined by FS-1037C and ITU Radio Regulations, radiodetermination is: the determination of the Real-time locating system, position, velocity or other characteristics of an object, or the obtaining of information relating to these parameters, b ...

for the purpose of radiolocation", where radiolocation is defined as: "radiodetermination used for purposes other than those of radionavigation."

Classification

This '' radiocommunication service'' is classified in accordance with ''ITU Radio Regulations'' (article 1) as follows:

Radiodetermination service As defined by FS-1037C and ITU Radio Regulations, radiodetermination is: the determination of the Real-time locating system, position, velocity or other characteristics of an object, or the obtaining of information relating to these parameters, b ...

(article 1.40) *Radiolocation service (article 1.48) ** Radiolocation-satellite service (article 1.49) The ''radiolocation service'' distinguishes basically * Radiolocation mobile station land-mobile, air-mobile, sea-mobile (article 1.89) * Radiolocation land station (article 1.90)

Examples

Radar antenna.jpg, Bundesarchiv Bild 101I-356-1845-08, Frankreich, Radar an der Kanalküste.jpg, Cavalierairforcestationparcs.jpg, APAR.jpg, File:Boeing E-3 Sentry 090512-F-7550B-902.jpg, File:Radar TRML-3D.jpg,

Satellites

Radiolocation-satellite service (short: RLSS) is – according to ''Article 1.49'' of the International Telecommunication Union's (ITU)

(RR) – defined as ''«A radiodetermination-satellite service used for the purpose of radiolocation. This (radiocommunication) service may also include the feeder links necessary for its operation''.» The ''radiolocation-satellite service'' distinguishes basically *Earth radio stations *Feeder links and *Space radio stations For example military radar sensors in earth satellites operate in the ''radiolocation-satellite service'' n this service. ;Examples of

radio station Radio broadcasting is the broadcasting of audio (sound), sometimes with related metadata, by radio waves to radio receivers belonging to a public audience. In terrestrial radio broadcasting the radio waves are broadcast by a land-based rad ...

s in the ''radiolocation-satellite service'' : ORS-2.jpg, NRO Lacrosse.jpg, Космос-954.jpg,

Frequency allocation

The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012). In order to improve harmonisation in spectrum utilisation, the majority of service-allocations stipulated in this document were incorporated in national Tables of Frequency Allocations and Utilisations which is within the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared. ; Example of

frequency allocation Frequency allocation (or spectrum allocation) is the part of spectrum management dealing with the designation and regulation of the electromagnetic spectrum into frequency bands, normally done by governments in most countries. Because radio pr ...

Stations

Land station

A radiolocation land station is – according to ''article 1.90'' of the International Telecommunication Union's (ITU)

ITU Radio Regulations The ITU Radio Regulations (RR) is a basic document of the International Telecommunication Union (ITU) that regulates on law of nations scale radiocommunication services and the utilisation of radio frequencies. It is the supplementation to th ...

(RR) – defined as "a

radiolocation service Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the geolocalization, location of something through the use of radio waves. It generally refers to passive, particularly radar—as well as detecting bur ...

not intended to be used while in motion." Each ''radiolocation station'' shall be classified by the '' radiocommunication service'' in which it operates permanently or temporarily. In accordance with ''ITU Radio Regulations'' (article 1) this type of ''radio station'' might be classified as follows:
Radiodetermination station (article 1.86) of the

(article 1.40 ) * Radionavigation mobile station (article 1.87) of the radionavigation service (article 1.42) * Radionavigation land station (article 1.88) of the radionavigation service * Radiolocation mobile station (article 1.89) of the radiolocation service (article 1.48) *Radiolocation land station ;Selection ''radiolocation land stations'': 2012-08 FuMG 65 Funkmessgeraet Wuerzburg Riese anagoria 01.JPG, German Radar Wurzburg Riese (FuMG 65) Radar antenna.jpg,

ALTAIR Altair is the brightest star in the constellation of Aquila (constellation), Aquila and the list of brightest stars, twelfth-brightest star in the night sky. It has the Bayer designation Alpha Aquilae, which is Latinisation of name ...

(ARPA Long-Range Tracking and Instrumentation Radar) 14a-WFF Radar.png, NASA Wallops Flight Facility Radar Antenna radar L-band TAR Finland.JPG, Antenna radar L band TAR Finland C-band Radar-dish Antenna.jpg, 50 Feet dish Antenna of a 3 kW C band Radar FPS-108 Cobra Dane.jpg, Intelligence-gathering phased array radar FPS-108 COBRA DANE Cavalierairforcestationparcs.jpg, Phased array radar AN/FPQ-16 PARCS Lägern - Hochwacht - Skyguide 20100524 17-29-30 ShiftN.jpg, Skyguide radar, Hochwacht in Boppelsen on Lägern (Switzerland)

Mobile station

Radiolocation mobile station is – according to ''article 1.89'' of the International Telecommunication Union's (ITU)

(RR)ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.89, definition: ''radiolocation mobile station'' – defined as "A

intended to be used while in motion or during halts at unspecified points." Each ''radiolocation station'' shall be classified by the '' radiocommunication service'' in which it operates permanently or temporarily. In accordance with ''ITU Radio Regulations'' (article 1) this type of ''radio station'' might be classified as follows:
Radiodetermination station (article 1.86) of the

(article 1.40 ) * Radionavigation mobile station (article 1.87) of the radionavigation service (article 1.42) * Radionavigation land station (article 1.88) of the radionavigation service *Radiolocation mobile station ;Selection ''radiolocation mobile stations'': Radar TRML-3D.jpg, Radar PRV-17 2009 G2.jpg, RAAF radar.jpg, Luer1.jpg, MIM-104 Patriot Radar unit (JASDF).jpg, Nike Hercules IFC radars.JPG, Nike Hercules IFC radars LOPAR and the tracking radars (MTR, TTR, TRR) f.l.t.r. APAR.jpg, Mark 68 director containing SPG-53.jpg, Weapon control radar Sbx underway.jpg, ME-110G-2 at RAF Hendon.jpg,