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The Flat Twin ABM Radar: Not as Capable as Previously Believed

4 Technical Intelligence Remaei


W.frln. Nolle*

Source! of Mcthodl Involved (WN INTEL)


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Disclosure Subject to Criminal Sanctions

micrcekbc copy of ihi> eaenl It (tillbit from OIR/ DLBfrU-atl-TITTk printed ciaw. from CPAS/IMCOV;or AIMlo uaerid CTASIMCjcapt of Dl report, on be ar-naaad ihroaili CPAS/IMC.

All material on Ihii pi K

ii UacliuJ.fl

directorate of intelligence

The Flat Twin ABM Radar: Not as Capable as Previously Believed

a Technical Intelligence Report

ai prepaid b;

. JOfftce of Scientific "lTT3 Wr.inoru Kewarch. with owtributinns from

Cctrnmeniianilq OSWR


Tho Flat Twin ABM Radar: Noi as Capable as Previously Believed


analysis of ihe Soviet Flat Twin ballistic missile defense radarit is not as capable as previously believed.

ursevere constraints imposed

Flat Twin by itsiu strengthens our beliefidespread, fast-paced Soviet ABM deployment using the Flat Twin is unlikely because of the number of radars required, as well as tbe extreme difficulty of modifying tbe Flat Twin to make it perform elTeclivcJy.

Our analysis of Ihe Rat Twin's antenna indicates that the Flat Twin is much less capable in off-boresight scanning for track and search than we had previously estimated^

"^indicate lhai the Rat Twin hasscanning capability of aboutIS degrees in azimuthforindicates lhat Ihe Rat Twin can

search less0 degrees. This reassessed search capability isless than the earlier estimateegrees

Because of the Rat Twin's scanningidespread ABM system using the Flat Twin would require an overwrielming number ofystem deployed al Moscow andf the most important areas in the Soviet Union would requireat Twin radars. These numbers arc aboutercent higher than our previous assessment. Although the Soviets would require fewer Rat Twin radars to defendigh-priority deployment sites under the START treaty, the number required is Hill considerable. Under the START treaty limit ofS ballistic missilelevel to be achieved bymodeling indicatesoviei defense would requireat Twin radars.otential future START treaty rsertttitlingS ballistic missile warheads, we calculate that the number of Flat Twin radars required for defense would be reduced to

Given the Rat Twin's liniitalionsidespread ABM system, wc believe that tbe Soviets wouldew type of ABM radar. We wouldew radar toreatly improved scanetter multiptc-larget-tracking capability, and greater detection range.ignificant reduction in Ihe number of radars requiredidespread ABM system





ke Flat Twin Dcaaga

Ihe Hal Twin Be Modified?

of Hal Twin* Umite-

Region UmiuiloM

of Flat Twin Radati Required Tofdeapread ABM Defense

Under Correal USorce Lew_


AABM Radar on the

,, ,


[>eployrneni Analyaii

T*i-- Array

ot tie Fta( Twin In

Sovieti have used tbe Rat Twin engagement radar In divene ways throughout its history. The Rat Twin radar was initially an integral part of an ABM program called thehich we believe was under development for nationwide deployment before the aigrrkng of tbe ABM Treatyespite the demise of that program, the Flat Twin has continuedupport weapons stems't lyyti Kn 11 "Si and Saryshagart

to Mat Twin raoar was deaigrsed' and was almost certainly intended to be trriddy deployed as part ofystem. Before Ihe aicniog of the ABM Treatyhe Soviets had


^repirations for ABM testingit the Sovieu intersdeel lo gun froenfter signing Ibe ABM Treaty is less clear. However, the demise of tbeystem oo cssiTcd by (beith the abandonment of thenterceptor program. We believe that Rat Twin testing conducted2 probably stemmedequirement to continue toapidly depioyable ayttemedgeho treaty

fsccause ibe Rat Twin is transportable and had been associated with an ABM system intended to be widely deployed, we considered tbe possibility of the Soviet* breaking out of ibe ABM Treatyast-paced, nationwide ABM system based on the Rat Twin radar and the Gazelle interceptor. We explored (he limitations of the Ra( Twin radar and the advantages! to the Soviets ofewidespread ballistic missile defense

Flat Twin Radar Characteristics

have boih transmit and

'receive pbased-array antenna systems indicatingtransmitter capability fromf

:BvTiTWot>pe and operationalTBiffer significantly in performance.

Twin Is amultifunction,'n ,wo VC[-'oni., IB re similar'

Und general appeajance.en

system (hatonventional, line-fed. rellecior-typc transmit antennahascd-array receive antenna. Tbe operational Flat Twin radars, f_

Analysis of all-source data indicates that the Flat Twin radar consists of an electronically steered, phascd-array antenna mountededestal Ibat allows mechanical steering in both azimuth andOnce mechanically directed, the Rat Twin

electronically scansimited searchJetree Flat Twin search area represents only J

multiple targets using barsdovcr data from long-range percent of thea (sec figure 3]

i no. electronic scanning feature gives the radarapability topatial sector more quickly than could mechanical scanning Twin-equipped missile battery could engage targets arriving from any direotroti, although not limultancousl;


*inai (Be relocation lime, eadudingTaT&rcbertkout could be reduced to fewer than four monthsr


bore cate


Scan limitation of the Flat Twin Design


(the Flat Twin is much lest capable in off-boresight scanning than our previous estimates indi-


flat Twinaximum scanningegrees from the antenna's boresightand elevation for"J

indicates that the Flat Twin can search leashis reassessed search capability bless than the earlier estimate ofdegrees. which was based on our presumed requirements for an operationally deployed Flat Twin. For example, the

' Ihe differences ia teaa lagse resultrnulreertcatigher tliul-to-mnK ratio for thr anarch prcecra.equired, the radar eaa tiaci rargrii allowt Jlgtul-lo-nrale ratio, pcrmiitini acaaa up

reduced numbei of phase shifters in (hein Ihe de-

crease in ils scanning capabilityheoretical maximum ofdegrees to about(The scan angle of an antenna decreases as the number of phaseeduced..

the Rat Twin antenna's faceubarrays of aboutifferent sizes arrangedseudorandom pattern (see figureore array,robably for beacon tracking of ABM interceptors (seehe lotal area of the antenna wouldaximum0 phase shifters felemenu) if spaced one-half wavelength apart. However, the volume of space available behind tbe face, in addition io one hole per subarray for coaxial wiring, stiggesu only one phase shifter perhb reduced number ofpercent reduction of the thrwretical maximum. The reduced number of phase shifters also minimizes the production cost of the rada

' Our analyn Indicates that the clrmcnu within eachan

allBiBTeeml,iFiat osvly pluses of ibe iadindual

than cactichanted la areaIn the latehen the tUt Twin wu Seniets wen haiie. dtffiesl

lietlw iHftP

Hal Twin Bottom Module

TVffarB4fra-mv, ddOibulrJ itMA

believe ihat the Flat Twin'* scanning capability is limited by the occurrence of secondaryating lobes) when steered otT-bore-sigbt. US researchbasri-nrraywitbpercent reduction in phase shifters shows thatadar wouldcan angle0 degrees with side lobes more thanecibels (dB) below tbe main beamrating lobe more thanB below (he main beam (sec' This research agree* with previously published Soviet papers using similarGrating lobes degrade the antenna pattern and tbe desired overall system performance. By using random-sized subarrays, several grating lobes appear at different angles, reducing tbe effect on the antenna

' la an antenna radsstfo* pattern, ihc letier lobra of prcorrunclv

Occnuinc amclilvdc as (iihr- side of Ihe main lot* are ihe MOV


pattern. The maximum grating lobes calculated for the Flat Twin antenna arc at leastB down wheaL tlie antenna is steeredegrees off-boiesigh>

Can the Hal Twin Be ModiSedT

We believe that the effort involved in realigning the Flat Twin toignificant increase in off-boresighi scanning capability probably would betoew phased-array radar. Because of Flat Twin's tinique antenna aubarray design, any modification in tbc number of phase shifters to increase tbe off-boresight scanningwould bo difficult to achieve because of physical


Figure 5

Tradeoff! of Scan Angle for Fewer Phase Shifters,

ii of Flat Twin's limited Performance Defeedabfc Region Liars! la liners

We calculated the regions that can be defendedlat Twin/Gaietle ABM system using various acanefcndablebe area on tbe ground (hat can be defended by ooe ABM unit. Determinedimple, one-cat-one timeline analysis, these defends We regions represent Ihe most optimistic ex-tent of coverage and do not reflect tsegradatieeis caused bv_multiplc targets, nuclear bursts, or other effects.

As (be Flat Twin's off-borescanningeduced, the dependable regionlat Twia/Ga-aelk system significantly decreases. Our modeling indicatesat Twindegree off-boresight search capability could defend onlyquare kilometers, which isercent of the area defcndablecgree radarS Minuleman III ' Z} Simllaie regiotss also orxut against other US It Vs when lhc scan angle u

Wt believe this scanning limitation of ihe Flat Twin indicate* ihai.Jhe radar cannot enure wtdclv mated

jtlie limned


w SL


can severely constrains the sire of the coverage of Ibe reentry complex- For example, our analysis shows lhat ihe US-degree tracking limitaiimum croasrange distance between twoarriving RVg of betweennddegree limit, however, would allow Use Hal Twin to engage multiple RVs wi(hiakm cross-range. The croasrange limitation would require the Soviets lo deploy additional Flat Twin radar;rge regions Mnaenws Petersburg)!


f Flat Twin Radar* Required Toidespread ABM Defense Requirements Under Current US Siraiegit Forte Letelt. We have examined Ibe effect ihat the reduced orT-boresighl scanning capability of the Flat Twin would have on Soviet deploymentidespread

ABM defense system,elected.set of II)deploymentrequireoMO HalTmrin radars

deployment al Moscow andf the moat important areas,at Twin radars would be required. The number of Flat Twin radars required in this analysis assumes ruradover data from the LPARsr_over data, we calculate that more then twice as many Flat Twinwould be required to defend these alto

The reduced off-borauihl scanning capability of Ihe Flat Twin increases Fill Twin depigment estimates by aboutercent (seereviouse determined thai defenseelected setigh-priority deployment areas would requirelat Twin radars to defend against an attack by US missile forces. The previouso indicated that deployment at Moscow and only

f tbe mosi important deployment areas would require ho0 Flat Twins. These esJciaktioo* auumed thai lhc Flat Twin mdrr -jv'dS-degree OtT-borcatght capability

tt vmiir start. The Sovyeta wouldfewer Flat Twin radar* lo defendigh-priority deployment lite* under tbe START treaty Under the START treaty limit ofS ballistic missile warheads the level to be

TW Deployment Analysis

Studieshave allowed usstimateof components that would be requireda widespread ABU defense system usingTwin and ihe Gazelle, likely deploymentIdentified and roughly

Most of the areas selectedv/HVfrililary or Industrial facilities thai would beto Soviet wat-fightlng capabilities Some areas with low target density were selected If they contained one or more largrti of critical Importance to Soviet offensive or defensive capobllltlei

We postulated rtprttintailve US missile attacks to determine the number and type of RVs that the Soviets might anticipateiven area We then estimated In detail the number and optimumof radars and Interceptors that would bed to handle thr specific threat to each area

probability lhat one or more RVs will not leak through the defenseite has loo few radars. Srnsltivity analysts5 percent level toeasonable echnpromlst between deploymentand expected Irakap

Our analysis Is Intended only lonapshot of an early stage In the Soviei planning processerritorial defense. In planning an actual deployment, ihe Soviets would have lo anticipate any offensive response, most of which would have the effect of Increasing the number of txmponents requiredthe Sovltls would be awareidespread ABM defense system In violation of the ABM Treaty would be viewed as an open-ended commitment to iteadily Increau the number of ABM components as long ai the United States deployedefeat them

The numerical ranges In our estimates of required numbers of radars re/tea variations In themade about the attack scenario and componen performance. In all cases, wcpercent "radar confidenceadar confidence is the





Twinotential future STARTrnhiin-S ballistic missile warheads, we calculate that the number of Flat Twin radars required for defense would be reduced too 4J0 radar

ew ABM Radar on tbe Horizon*

If the Soviets became committed toidespread ABM defense system, we believe thai they have two viable options. Mostew radar would be spedfically designed to replace the Flat

Twin.adar similar lo ibe Horse leg radar, the engagement radar prototype for theystem's Pill Boi battle rnanagefneni radar, could be de.rk.pco

We believeeplacement for the Rat Twin radar would perform significantly better than Ibe Flat Twin because of tbe evolution of tKhrjologv since the Flat Twin's incerXioo la. The new radar wouldreatly improved scanetter multiple-tar get-tr aching capability, and greaterrange.adar could be designed to be


allowing deploymentay, rather than transportable like tbe Flat Twin, which haia sia-weck assembly time. We believe that the Soviet* coo Id develop andew radar at Kamchatka or Saryshagan within three to five year* after theyecision to build ft

A new mobile radar would be more appropriate than

the Rat Twinidespread ABM defensemobility aloneew system would increaseradar tocjuire-

rrtcntt indicatesew radar could reduce the required number of radars by as much asercent (see. This reduction in the required number of radars could increase Iheation-wide ABM defense system to thec believe that ihc Soviets arc not likely to haveadar

ew endoatrnospberic missile available for^ifombination is now inafter ibe0 at the earll-

The Soviets could also accrue tignificanl technical advantage* byorse Leg-type radar atorse Leg radar would provide aa additional ABM lest and uaieittg site for Ibeystem. It would not, however, provide the Sovieti the ability touickly deployed, widespread ABM aytiem. Each Hone Leg and Pill Boi radar will take year* lo construct aod would be detected keg before ihey became operational The Horse leg and the Pill Boa arc systems that the Soviets would be more likely


lawpraclo. RjnUroo SorklorHmI abm Drftiae Sjsten|

toon and progressedore rapid pace than observed In Soviet ABM programs over the pastean, these interceptors woo Id not be ready for deployment until aficr ibcut wc do not know If ibe interceptors under development arefor tbc upgraded Moscow ABM system or arc panrogram toore rapidly deployable option. If ihey are, in part, intended for an optionore rapidly deploysble ABM system, it la highly likely that the Sryriets are alsoradars suitable for rapid deploymeni

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