Warsaw Pact / Russian Emitter Locating Systems / ELINT Systems

WarsawPact/Russian/PLAEmitter Locating Systems / ELINT SystemsTechnical Report APA-TR-2008-0503byDrCarloKopp, AFAIAA,SMIEEE, PEngMay,2008Updated December, 2008Updated September, 2009Updated July, 2010Updated April, 2012©2008 - 2012 Carlo Kopp(Images Czech MoD, RuMoD, Topaz, ERA,MiroslavGyűrösi, Vitaliy Kuzmin, Other)

The Ukrainian Topaz Kolchuga ESMsystemhas received considerable press over the last decade, mostly related toalleged illegal sales to Saddam's regime preceding Operation IraqiFreedom (Image ©Miroslav Gyűrösi).IntroductionReferencesFormer WarPac Systems Topaz Kolchuga/Kolchuga MEmitter Locating SystemTesla-PardubiceKRTP-81/81M Ramona / SoftBall EmitterLocating SystemTesla-PardubiceKRTP-86/91 Tamara / TrashCan Emitter Locating SystemsERA KRTP-96M4 -BORAP EmitterLocating SystemERA Vera EEmitter Locating SystemsRAMET / VTÚOSDD StaniceDalekého Dosahu (Long Range ESMStation)Russian Systems 85V6Vega / OrionELINTSystemKvant 1L222AvtobazaELINTSystemLantan/Almaz-Antey ValeriaE ELINT andEmitter Locating SystemPLA Systems CETC DWL002Passive Detection SystemCETC YLC-20Emitter Locating SystemDifferentialTime Of Arrival Emitter Locating SystemsIntroductionA topic which appears to cropup with monotonous regularity in the overseas press is that of allegedsales or smuggling to nations hostile to the US of former Warsaw Pactequipment "capable of detecting stealth aircraft". These claimsinvariably involve either the Czech designed and built Tesla-PardubiceKRTP-86 Tamara or ERA Vera Emitter Locating Systems, or the Ukrainiandesigned and built Topaz Kolchuga series of Emitter Locating Systems.More than often this equipment is described as 'anti-stealth radar','radar' or 'passive radar', all of which are completely incorrect.The purpose of this analysis is to provide some technical discussion ofthese equipment types and their basic capabilities.Both the Tamara/Vera series, their predecessor the Ramona, and theKolchuga are passive Electronic Support Measures (ESM) systems built toprovide an Emitter Locating System (ELS) capability against airbornetargets emitting radio frequency signals. In this sense they arefunctional analogues of US, French, Israeli and other types ofequipment designed to collect, identify, track and locate RF signalsemitted by airborne targets.These systems were developed during the last two decades of the ColdWar to bolster Warsaw Pact air defence capabilities in the high densityEuropean Theatre, where it was expected that the US would heavily jamall surveillance, acquisition and engagement radars used in theIntegrated Air Defence System (IADS). The intent behind these passivesensors was to provide a capability to passively detect, locate andtrack US and NATO aircraft using their RF emissions, to cue other IADSelements to an engagement.The Czechs made the most progress in this area, developing the Ramonaand Tamara systems using the quite sophisticated DTOA (Time DifferenceOf Arrival) technique, one which did not become widely used in WesternELS equipment until much later.The Kolchuga, Vega/Orion and Avtobaza are more conventional DirectionFinding(DF) systems, with two or more stations they  use multiplebearing measurements to fix the target emitter.The widely propagated public claims that DTOAEmitter Locating Systems are 'passive anti-stealth radars' isdifficult to fathom. All DTOA ELS systems are most effective atdetecting and tracking omnidirectional emitters. For the DTOA ELS tofunction, at least three of the widely spaced antenna/receiver systemsmust detect the very same emission from the target. This is why theWarsaw Pact's Ramona/Tamara family of DTOA systems was used primarilyto track IFF, SSR, VOR/DME, Tacan, JTIDS/Link-16 and other omniemissionsources fromNATO aircraft. A narrow and low sidelobe pencil beam emission from anX/Ku-band radar is even under the most favourable geometricalconditions not going to concurrently illuminate three or more DTOA ELSstations,spaced tens of miles apart,  so the DTOA system cannotperform its geolocating function. With low gain antennas needed toproperly cover the required angular extent, the notion that DTOAsystems can lock on to and track sidelobes from X/Ku-band AESAs issimply not supportable from a basic radio physics perspective. The onlypossible scenario in which such a DTOA ELS could track a VLO aircraftis where the aircraft is transmitting via an omni antenna JTIDS/Link-16terminal while penetrating hostile airspace. This is so unlikely thatit cannot be considered seriously.The only other possible scenario which might be contemplated by thosearguing 'anti-stealth' capabilities for DTOA or DF ELS equipment istheir use as the receiver component in a multi-static radar system,which assumes the volume of airspace in which the VLO aircraft isoperating is also being floodlit by a very high power pulsed emitter inthe UHF/VHF/L-bands. The difficulty then confronted, especially by aDTOA ELS network, is the power-aperture problem. As the angularcoverage of the DTOA ELS stations must be large, this is at the expenseof antenna gain. To achieve a given power-aperture product in themulti-static system, the gain and emitted power at the floodlightingemitter end of the system must be exceptionally large, to compensatefor thelow gain of the receiver components.Claims that conventional DF systems like the Kolchuga can readilydetect and track VLO aircraft also defy analysis. While they havehigher gain antennas compared to the DTOA ELS designs, they areconfronting the probability of intercept problem against a very lowsidelobe AESA, which is power managed, and highly frequency agile. Theycan only detect and track the emitter if the station is sitting insidethe mainlobe of the AESA, and pointing at it when it is emitting. Theonly scenario where this is feasible is if three or more such DFsystems are closely clustered around the target to be attacked, and allare pointed along the threat axis. Were this true, the DF systems thenconfront a geometrical dilution of precision (GDOP) problem, which willseverely impair range accuracy. The claimed use of DTOA techniques inthe Kolchuga is unlikely to correct this problem due to the very shortDTOA baseline.The claim that DTOA or conventional DF Emitter Locating Systems providea useful capability against VLO aircraft is simply not credible. Itscontinuing popularity appears to fit in the same category as claimsthat the B-2A's stealth paint washes off in the rain.

USDoDBandAllocationChartReferencesUkraine's Kolchuga Radar -Embassy of Ukraine in thePeople’s Republic of China - URL:http://www.mfa.gov.ua/china/en/publication/content/12653.htmManfred Bischoff - KRTP-81 RAMONA - URL:http://www.manfred-bischoff.de/RAMONA.htmManfred Bischoff - KRTP-86 TAMARA - URL:http://www.manfred-bischoff.de/TAMARA.htmЧехословацкие станции пассивной электронной разведки - URL:http://pvo.guns.ru/other/czech/tamara/Tamara / Kolchuga - Peter's ADA - URL:http://www.peters-ada.de/tamara.htm531st Passive Surveillance Systems Battalion,  53.Brigade of Passive Surveiilance Systems and Electronic Warfare, Planá /Českých Budějovice, Czech Army - URL:http://www.pasivnisystemy.army.cz/htm/index_en.htmlRamona KTRP-81 Emitter Locating System - Disposal Offer -URL:http://www.armypoint.cz/nabidka-patrace-ramona-krtp-81/d-90513/Igor Peretyagin - Military Parade, 1998, 85V6-AVEGA 3-D ELINT COMPLEX 58, URL:http://milparade.udm.ru/28/058.htmFormer WarPac Systems

Topaz Kolchuga/Kolchuga MEmitter Locating System

Kolchuga ESM antenna array (Image ©Miroslav Gyűrösi). Additional imagesvia Topaz [1], [2].The Topaz Kolchuga is a longrange direction finding Electronic Support Measures receiver system,which if networked can provide the functions of an Emitter LocatingSystem using triangulation and DTOA techniques. The design is claimedto have beennominated for a State Science and Engineering Prize. It was developedduring the 1990s by a consortium including the Special Radio DeviceDesign Bureau public holding company, the Topaz holding company,the Donetsk National Technical University, the Ukrspetsexport statecompany, and the Investment and Technologies Company.Claimed band coverage extents from 130 MHz (VHF) up to the X/Ku-bands.Claimed sensitivity is -110dBW to - 155 dBW. Track capability isclaimed to be 32 concurrent targets.The Kolchuga is also claimed to combine DF techniques with DTOAtechniques. The latter will be limited in angular extent to targetswhich fall into the mainlobes of the respective antenna components forthe band in question.The sale of four systems to the PRC has been reported. There is ongoingspeculation that the system has been supplied to Iran but no validationto date.

Kolchuga ESMconsole(Image ©MiroslavGyűrösi).

Kolchugaondisplayat MAKS 2009. Below, note the revised operator station (©2009VitaliyV.Kuzmin).

Tesla-PardubiceKRTP-81/81M Ramona / SoftBall EmitterLocating System

SemimobileRamona/SoftBallELS variant of the DDR NVAat Gatow AFB (Images ©Miroslav Gyűrösi). Additional image [1].

The Ramona was deployed firstin 1979, as a replacement for the PRP-1 Kopac DTOA ELS which wasdeveloped duringthe 1960s, and retired in the late 1990s. It was superceded inproduction by the mobile KRTP-86 Tamara. The Ramona system was built ina semimobile configuration, either on a ground based platform or 25metre tethered lattice mast. The mast mounted variant weighed in total160 tonnes, and was carried by no less than thirteen Tatra 138/148 10tonne  6x6 trucks. The spherical radome housed the receivers anddatalink transceivers required to operate three or more stations.Deployment of the system on site takes 12 hours.Band coverage was 1 to 8 GHz, with the primary application in locatingand tracking airborne IFF/SSR transponders and TACAN installations.Twenty targets could be tracked concurrently.The Ramona was regarded to be complicated and troublesome to deploy,factors which strongly influenced the design of the subsequent Tamara.Seventeen baseline KRTP-81 systems were built, 14 exported to theSoviet Union, 1 to the DDR, 1 to Syria, and 1 deployed by the CSLA.Fifteen improved KRTP-81M systems were built, the Soviets buying 10,Syria 3 and the CSLA deploying 2 systems.

MastmountedRamonaELS variant of the CSLA during the Cold War.Tesla-PardubiceKRTP-86/91 Tamara / TrashCan Emitter Locating Systems

Early KRTP-86 Tamara ELS of the PVO-Sdeployed with partially elevated mast (Image ©Miroslav Gyűrösi).Additional image [1].The KRTP-84 Tamara was anevolution of the Ramona, designed with high mobility and rapiddeployment as a priority. Testing of prototypes began in 1983, followedby state trials and certification in 1987. A single system is carriedby eight Tatra 815 8x8 trucks (Equivalent to the MAZ-543), comprisingthree RS-AJ/M receiver systems with telescoping masts, and a mix ofRS-KB hardware containers, RS-KM signal processing equipment containerand a ZZP-5 command van. The mast mounted RS-AJ/M can elevate to 8.5,12.5 or 25 metres AGL and can operate at wind strengths below 60 knots,with a structural limit of 100 knots. The cylindrical antenna radomehouses the receiver equipment and datalink transceivers for networkingthe stations. In a typical deployment the receivers are stationed atdistances of 5 to 20 NMI.Cited band coverage is 820 MHz to 18 GHz. Design objectives includedthe tracking of the F-15 at 200 NMI and F-16 at 215 NMI, with the citedrange limit being 240 NMI and limited primarily by the curvature of theearth. Russian sources claim that 72 targets can be tracked within a100° angular sector, these including emitting JTIDS/Link-16 terminals.In 1991 the baseline KRTP-86 was superceded in production by improvedthe KRTP-91 Tamara-M.Russian sources claim that 23 Tamara and Tamara M systems were builtbefore production switched to the Vera series. Of these, theUSSR/Russia acquired 15 Tamara systems and 4 Tamara-M systems, the CSLA4 Tamara M systems, the GDR NVA one Tamara system, with claims that theUS acquired two systems via Oman.

KRTP-91 Tamara antennapackage(Image ©Miroslav Gyűrösi).

KRTP-91Tamarawithantenna fully deployed (Image ©MiroslavGyűrösi).

Tamara ELS of the DDRNVAdeployed with mast fully elevated.

KRTP-91 Tamara console (Image ©MiroslavGyűrösi).

Tamara Concept ofOperations.ERA KRTP-96M4 -BORAP EmitterLocating System

KRTP-96BORAPELSdeployedwith partially elevated mast(Image ©MiroslavGyűrösi).The KRTP-96 series are a follow-on design to the KRTP-91 Tamara series,using a much larger antenna package, and achieving IOC with Czech Armyunits in 1999. The system has a DF capability and is typically deployedin pairs, providing coverage over a 120 degree angular sector.Frequency coverage is L-Band throught to Ku-band. The system can bedeployed in ~1 hr.

KRTP-96 BORAP ELS stowed (Image ©Miroslav Gyűrösi).

KRTP-96 BORAP ELSantenna stowed (Image ©Miroslav Gyűrösi).

Deployed ERA  KRTP-96 BORAP ELS of the 531st PassiveSurveillance Systems Battalion of the Czech Army (Czech Army image).

Deployed ERA  KRTP-96 BORAP ELS of the 531st Passive Surveillance Systems Battalion of the Czech Army (CzechArmy image).ERA Vera EEmitter Locating Systems

ERA Vera E EmitterLocatingSystem. The Vera E equipment displayed to date has been installed ontrailers rather than the robust truck mounted mast system of the Tamaraseries (Image ©Miroslav Gyűrösi).The post Cold War Vera systems are improved derivatives of the Tamara,and have not proven particularly successful in the market, in a largepart due to the fact that the clients most interested in the productare not part of the Western alliance. China was granted export licencesin 2004 for six Vera-E systems, which were revoked after pressure wasbrought to bear by the US State Department. There are reports thatMalaysia, Vietnam, Pakistan and Egypt were interested in acquiring thesystem. The Czech Army has acquired one system, the US DoD one system,and the Estonians one system.

ERA Vera E EmitterLocatingSystem antenna package (Image ©MiroslavGyűrösi).

ERA Vera E Emitter Locating System console(Image ©Miroslav Gyűrösi).

ERA Vera SM of the 531stPassive Surveillance Systems Battalion of the Czech Army(CzechArmy image).

ERA Vera SM of the 531stPassive Surveillance Systems Battalion of the Czech Army (CzechArmy image).RAMET / VTÚOSDD StaniceDalekého Dosahu (Long Range ESMStation)

SDD long range ESM of the 531stPassive Surveillance Systems Battalion of the Czech Army (CzechArmy image).The SDD long range ESM systemwas introduced in 2001 as a replacement for the obsolescent SovietSDR-2 system. Deployed in pairs, this monopulse system is credited witha range of up to 330 NMI with coverage between 0.8 and 8 GHz, effectiveagainst pulse and CW emitters. The antennas use steerable feeds. Thesystem is deployed on TATRA 4x4 ARMAX truck with a KSSK container andtowed antenna suite.

DeployedSDD long range ESM of the 531stPassive Surveillance Systems Battalion of the Czech Army(CzechArmy image).

StowedSDDlongrange ESM of the 531stPassive Surveillance Systems Battalion of the Czech Army (CzechArmy image).Russian Systems

85V6Vega / OrionELINTSystem

Deployed 85V6Vega / OrionELINTSystem (Image ©Miroslav Gyűrösi).

Stowed 85V6Vega / OrionELINTSystem (Image ©Miroslav Gyűrösi).

Antenna package on the85V6Vega / OrionELINTSystem (Image ©Miroslav Gyűrösi).

Kvant 1L222AvtobazaELINTSystem

RosoboronexportDescription(Cite):The Avtobaza ELINT system isdesigned to detect airborne side-looking radars, air-togroundfire-control radars and low-altitude flight control radars, as well asto provide intelligence data for the 1L125M APUR.Compositionequipment vehicle based on the Ural-43203 chassis with theK1.4320 vanED2x16-T230P-1VAS electric power generator in the K1.4320van on the Ural 4310 chassisThe ELINT system displays on the TV screen acquired targets with dataon their direction finding, angular coordinates (azimuth andelevation), radiation signal parameters (carrier frequency, duration,pulse repetition frequency) and radar type classification (sidelooking,fire control, low-altitude flight control radar). The APUR automatedjamming control system is fed with target data (frequency band numberaccording to frequency assignment of jamming systems, type of emittingradars and their angular coordinates) via cable at a range of up to 100metres.

Lantan/Almaz-Antey Valeria E ELINT andEmitter Locating SystemRussian sources have reported thedevelopment of the Valeria E whichappears to be Russian designed and built replacement for theCzechoslovak KRTP-86/91 Tamara series of emitter locating systems. Todate no imagery of the antenna system and vehicles has been published.The system is intended to detect, track and identify airborne emitters,including radars and support jamming aircraft, from VHF through to theKu/Ka bands. A cylindral wideband phased array antenna is employed.Cited Capabilities (Rusarmy.com):Automatic detection and measurement of emitter coordinates(range, azimuth, angle of elevation, altitude and velocity vector);Determining emitter and platform kinematic parameters(air-space, ground and surface);Recognition of the type and class of targets from theiremissions, classification of objectives in terms of threats andpriorities for their destruction;Locating opposing ground based emitters;MASINT/ELINT collection tasks;Threat warning and raid warning;Enemy Electronic OrBat analysis;Threat detection, tracking and cueing for fighter GCI andmedium and long range SAM systems under  conditions of jamming,and attack by anti-radiation missiles, cruise missiles, and lowobservable PGMs.The Valeria E comprises at least four networked subsystems, one centralprocessing and C3 cabin, and three antenna/receiver units. While eachsystem provides a circular coverage footprint within which range,azimuth and elevation can be measured, it is intended that multiplesystems be deployed with overlapping coverage. Elevated terrain isrecommended to improve coverage. The networked components are typicallysited at 10 to 35 km apart.Основныехарактеристики:Principal characteristics:Диапазон, ГГцFrequency coverage [GHz]0,15-18; (0,15-48)0.15 to 18 / 0.15 to 48Диапазон прослушиванияУКВ связи, МГцVHF band coverage [MHz]100-500Зона обзора(относительно ЦП):Coverage footprint:по дальности, кмin range [km]0-500по азимуту, град.in azimuth [deg]круговая 0-360по высоте, кмin altitude [km]30-40Рубежи обнаруженияцелей по собственным излучениям (БРЛС) иПАП, км:Emitter coverage [km]:на высоте, м:at emitter altitude of [m]10 3050 50100 60-7010000 450-500по целям AN/APY-1AN/APY-1 detection range800-850 (до 1000 при наличии прямой видимости)800-850 (up to 1000 LOS permitting)Точность определенияместоположения по сигналам БРЛС:Emitter locating accuracy:по дальности, кмin range [km]0,5-0,7по азимуту, угл. мин.in azimuth [min of arc]1-4по высоте (усредненная по трассе) научастке, м:in altitude (averaged over track) [m]0- 250 км 500-8000- 500 км 2000-2500Количество одновременносопровождаемых целейNumber of tracked targetsДо 200Выдаваемая информацияgenerated outputsтрассы, параметры траекторий движения и типыцелей и ПАПtracks, kinematic parameters, type of emitter and modeПотребляемая мощность,кВт:Power consumption [kW]ЦП, кВт не более 16ВП, кВт не более 15Среднее время наработкина отказ, чMTBF [hr]2000Дежурная смена длямобильного варианта, чел.Crew complement of mobile variant5Количество транспортныхединиц(мобильный/перевозимый вариант)Number of vehicles (mobile/transportable variant)6/5

PLA Systems

CETC DWL002Passive Detection System

The recently disclosed DWL002 is a modernand technically sophisticated digital Emitter Locating System,which builds on ideas employed in the earlier YLC-20 ELS. While designemploys much the same DTOA and Angle Of Arrival [AOA] techniques as themuch older Russian, Ukrainian and Czech ELS systems, the DWL002introduces an important innovation, which is the use of paired primarywideband apertures, displaced in elevation. The resulting phase andtime differences between the upper and lower antennas permitheightfinding, otherwise problematic in earlier single aperture designs.The primary apertures are housed under cylindrical radomes, in anarrangement similar to the KRTP-91 Tamara and ERA Vera systems.Thelower primary aperture is on a telescoping mast, the upper primaryaperture on the articulated folding main mast, which employs hydraulicactuators.Below the upper primary aperture is a package of steerableparabolic antennas, likely operating in the upper X-band or Ku-band.These are employed to provide high data rate links between the three orfour networked DWL002 systems when deployed.The aft of the equipment container also mounts three Yagi antennas, thepurpose of which has not been disclosed. It is most likely that theseare employed for datalinking target track data from the networkedDWL002 systems to other air defence assets.The system is carried on a North Benz ND1260 (Mercedes-Benz NG 80) 6 x6 military truck, common to recent radardesigns such as the YLC-2V, JY-11B, JYL-1, YLC-18, Type 120, Type 305Aand Type 305B. This will result in similar onroad and offroadperformance.The strategic significance of the DWL002is that it is the first DTOA technology ELS which has been designedfrom the outset with the intention of providing robust heightfindingcapability when passively tracking an emitting target. The ability togenerate near-realtime or soft realtime 3D target tracks would beespecially valuable in supporting SAM systems like the S-300PMU2 orHQ-9, as this could be employed to cue the SAM engagement radar veryprecisely to the inbound target. Should the accuracy of the ELS besufficiently high, it could be employed to generate post-launchmidcourse tracking corrections for outbound SAMs.The CETC brochure describes the system thus:“DWL002 Passive Detection System, also calledaspassiveradar, ismainly used in air-defense or seashore monitoring to perform thedetection to perform the detection and location to airborne, shipborneor landbased emitters in complex electromagnetic environment anddisplay the target flight path in real time. The system can alsooperate together with active detection system to form a mutualsupplementary surveillance network.Typical configuration of DVL002 Passive Detection 'system is composedof three reconnaissance stations. One of them serves as master stationand the other two as slave stations. The system can be expandable tofour station configuration with perfect performance of full spatialcoverage and altitude information of air target. Each station iscarried by an individual vehicle.Main Functions:* Realtime & Accurate Location and Tracking* Signal Analysis and Identification* Long Range Detection and Early WarningMain Features:* Passive* Real Time* Very Good MobilityDWL002 Passive Detection System is a three station configuration(expandable to four station configuration). Each station. includingantenna and power generator. is housed and carried by one vehicle.which ensures the good mobility of the system* Remote Control* Advanced techniquesLong base line time difference of arrival (TDOA) location techniquecombined with AOA: Wideband digitized receiver technique; Multilevelcorrelation processing technique with good flight track processingresult: Automatic set up. Chassis leveling techniques and automaticnorth calibration technique to ensure fast deployment and flexibleoperation.”

DLW002 antenna arrangement.

DLW002 CONOPS diagram.

DLW002 console display of target track.CETC YLC-20Emitter Locating System

The CETC YLC-20 isa DTOA/DF system modelled on the Tamara M (via IASC)The Chinese YLC-20 isconceptuallybased on the KRTP-91 Tamara, but incorporates both precision DF andDTOA capabilities to locate airborne and surface based emitters. The only open source material at this time states the YLC-20 isintended to detect, locate and identify:aerial emitting targets using active radar, includingfighters, AEW&C aircraft and UAVs.surface targets including early warning radars, acquisitionradars and fire control radars.emitting communications equipment.Stated band coverage is 380 MHzto 12GHz. Deployment time is claimed to be 1hr, with all system components on 8x8 or 6x6 trucks. At the time ofwriting no good quality imagery of production equipment was availablethrough opensources. This limits current assessments of the system's capabilities.It is likely that DTOA techniques are used for target acquisition andcoarse tracking, and DF techniques used for precision tracking, usingDTOA derived coordinates to cue an interferometric DF antenna.Avaliable material does not state whether a heightfinding capability isprovided, if so this would likely be performed using interferometrictechniques with the DF subsystem. Once better quality imagery of theantenna arrangement becomes available, a more precise definition ofcapabilities and limitations will be possible.It is likely that much of the YLC-20 design is based ondocumentation acquired during the abortive attempt to procure six CzechVera E DTOA ELS systems. The YLC-20 was first disclosed in 2006.

We have yet to see hard evidence that thePLA is integrating the YLC-20 or Kolchuga M with its S-300PMU/PMU1/PMU2SAM batteries. That is however not a technically difficult task toperform and given recent Chinese writings on the use of VHF radar toprovide midcourse guidance for SAMs, something we can be certain thePLA is planning. The principal risk which arises is that emissions fromany network antennas on combat aircraft which can be detected by morethan two DTOA or DTOA/DF ELS would permit passive tracking and providecoordinate data of sufficient accuracy to effect a SAM shot - or vectora fighter (Author).

DifferentialTime Of Arrival Emitter Locating SystemsDTOA systems make use of threeor four widely displaced receiver stations which employ a synchronisedhigh precision clock. All received signals, such as radar pulse trains,IFF emissions, network or datalink packets, etc are identified, sorted,and timestamped, and the collected data relayed to a central processingsite, such as a van. What a TDOA system exploits is the fact that thegeographical location of any emitter which produces a specificdifference in time of arrival to a pair of receivers will fall along ahyperbolic curve termed an "iso-chrone" (curve of like time).With two receivers the observer knows only that the location of theemitter falls somewhere along a curve. With three or more receivers,the observer knows the emitter falls somewhere along several curves.The points where these intersect is where the emitter can be found.

Technical Report APA-TR-2008-0503