 | This article includes a list ofgeneral references, butit lacks sufficient correspondinginline citations. Please help toimprove this article byintroducing more precise citations.(February 2019) (Learn how and when to remove this message) |
Radiolocation, also known asradiolocating orradiopositioning, is the process of finding thelocation of something through the use ofradio waves. It generally refers to passive, particularlyradar—as well as detecting buried cables,water mains, and otherpublic utilities. It is similar toradionavigation in which one actively seeks its own position; both are types ofradiodetermination. Radiolocation is also used inreal-time locating systems (RTLS) for tracking valuable assets.
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 bebackscattered waves (as in radar or passiveRFID). Astud finder uses radiolocation when it uses radio waves rather thanultrasound.
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 thetime 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. Theangle 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 asmultilateration.
Estimates are improved when the transmission characteristics of the medium is factored into the calculations. For RSSI this meanselectromagnetic permeability; for TOA it may meannon-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 theinverse 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 tomultipath 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 asfading. 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—butFourier 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 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 MedicalISM bands, but high-power transmission cannot extend outside of these bands. However, several jurisdictions now allowultrawideband 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. InDoppler radar, theDoppler shift is also taken into account, determiningvelocity 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.
Radiolocation is also used incellular telephony viabase stations. Most often, this is done throughtrilateration betweenradio towers. The location of theCaller orhandset can be determined several ways:
The first two depend on aline-of-sight, which can be difficult or impossible inmountainousterrain or aroundskyscrapers. Location signatures actually workbetter in these conditions however.TDMA andGSM networks such asCingular andT-Mobile use TDOA.
CDMA networks such asVerizon Wireless andSprint 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 thepublic safety answering point (PSAP) which answerscalls to anemergency telephone number can know where the caller is and exactly where to sendemergency services. This ability is known within theNANP (North America) as wirelessenhanced 911. Mobile phone users may have the option to permit the location information gathered to be sent to otherphone numbers ordata networks, so that it can help people who are simply lost or want otherlocation-based services. By default, this selection is usually turned off, to protectprivacy.
Radiolocation service (short:RLS) is – according toArticle 1.48 of theInternational Telecommunication Union's (ITU)Radio Regulations (RR)[1] – defined as "Aradiodetermination service for the purpose of radiolocation", where radiolocation is defined as: "radiodetermination used for purposes other than those of radionavigation."
Thisradiocommunication service is classified in accordance withITU Radio Regulations (article 1) as follows:
Radiodetermination service (article 1.40)
Theradiolocation service distinguishes basically
Radiolocation-satellite service (short:RLSS) is – according toArticle 1.49 of theInternational Telecommunication Union's (ITU)Radio Regulations (RR)[2] – defined as«Aradiodetermination-satellite service used for the purpose of radiolocation. This (radiocommunication) service may also include thefeeder links necessary for its operation.»
Theradiolocation-satellite service distinguishes basically
For example military radar sensors in earth satellites operate in theradiolocation-satellite service n this service.
| Name | Country | Sensoric |
|---|---|---|
| Lacrosse | USA | military radar (imaging) reconnaissance satellite |
| SAR-Lupe | Germany | military radar (imaging) reconnaissance satellite |
| IGS | Japan | radar reconnaissance and optoelectronic reconnaissance |
| RORSAT | Russian Federation | Radar Ocean Reconnaissance SATellite |
The allocation of radio frequencies is provided according toArticle 5 of the ITU Radio Regulations (edition 2012).[3]
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.
| Allocation to services | ||
| Region 1 | Region 2 | Region 3 |
24.65-24.75GHz
| 24.65-24.75
| 24.65-24.75
|
Aradiolocation land station is – according toarticle 1.90 of theInternational Telecommunication Union's (ITU)ITU Radio Regulations (RR)[4] – defined as "aradio station inradiolocation service not intended to be used while in motion."Eachradiolocation station shall be classified by theradiocommunication service in which it operates permanently or temporarily.
In accordance withITU Radio Regulations (article 1) this type ofradio station might be classified as follows:
Radiodetermination station (article 1.86) of theradiodetermination service (article 1.40 )
Radiolocation mobile station is – according toarticle 1.89 of theInternational Telecommunication Union's (ITU)ITU Radio Regulations (RR)[5] – defined as "Aradio station inradiolocation service intended to be used while in motion or during halts at unspecified points."Eachradiolocation station shall be classified by theradiocommunication service in which it operates permanently or temporarily.
In accordance withITU Radio Regulations (article 1) this type ofradio station might be classified as follows:
Radiodetermination station (article 1.86) of theradiodetermination service (article 1.40 )
.jpg)
