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INVITED PAPER

PHASED-ARRAY RADARS:PAST, ASTOUNDINGBREAKTHROUGHS ANDFUTURE TRENDS

Phased-anmj radars Juwe seen ever increasingly wider use around the wodd over the last five decades. Inrecent years, they have seen breakthroughs that led to capabilities not possible in the past. This isexemplified by the devehrpment ofGaAs integrated niicroivave circuits called monolithic microwaveintegrated circuits (MMIC) that make it pos.nble to build active electronically scanned arrays (AESA) thathave lighter weight, smaller volume, higher reliability and. lower cost. These developnwnts liave readied thepoint where it is now possible to build a low-cosi 35 GHz phased airayfor a missile seeker costing$30/element (total cost of array including all electronics divided by the number of elements). This is madepos.sible because integration allows the whole T/R module to be put on a single chip. For some applications,it wiU soon be possible to put multiple receivers or transmitters on a single chip. The advances provided byMoore's Law have now made it feasible to do digital beam forming with all its nmnerous advantages. Thisarticle describes these advances and also covers the potential for GaN and SiC chips that have thecapability of higher peak power by a factor often than G

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Fig. 1 Example luhe (T) and salkl'ntate (SS)ssiff plitiscd arrays having large

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SHUTTLERADARTOTOGRAPHMISSION(SRTCISIR

/'Vg. .3 Other phused arraif siisivmHdeveloped or ttnder development.

vd (see Figure 6).2 DARPA alsohindt 'd (lt'\('l(}pin('nl of;! $10 X-l)aii[l, 10 iiiW. siiiule-cliip T/R niocl-ule.^ A 76 CM/, photOL-tclifd Hdtnumlens arra)' costing only a few dollarsWHS de\el()pe(l inr aiitoniotixi' cruise

l'

TABLE IBREAKTHROUGHS

Ffiased iirniy.s i'\

(SBX) nidur35 ( ; M / $19 K active pluiscd anny L()w-C(wl MKMS ])lia.sd

GaN.SiCSiGt', CMOS

Digila! bcaiiil'oriiiingk and iisscinlily

M!M

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45,056 Best-of Breed Tranimit/Receiver modulci

* 128 modules per lubarray(352 tubarrayi total)

Time-delay steering at thesubarray level tupportswideband waveformi

* Nine luper-iubarrays - ctockodto tuppreif grating lobe

'> Xnii (iniiii (iirhUcrtiiri-.

Sure, we sell lots of microwaveand millimeter-wave components.But, let's face it, sometimes you'renot looking for just a component,you're looking to create an entiresystem. Come to us for thecomplete solution. Give usa call and talk to one of ourengineers. Together we'lldesign the system thatexactly meets your needs.

ReceiversTransceivers/TransmittersSwitcii MatricesBlock ConvertersRadar SubsystemsCoherent Converters i iCommunication Systems ;Integrated Amplifiiar Ass

LOW-CO5T MEMS PHASED ARRAYIf only we had a low loss pliiLse sliirter. Then we ooultl go

back to the passive-architecture electronically scannedphased array with one mociule feeding many phase shifters(10, for example). This could potentially reduce the eost ol'iUi electronically scanned pluised array by a factor of aboutten. Micro-electromechanical systems (MEMS) offer thispromise. MEMS s\vitches have improved their reliability- bythree orders of magnitiide over what was reported in Octo-ber 2003'' to a life of 6(X) billion switches.-^ -^ There is stillneed for improvement in the loss. The loss through a fonr-bit phase shifter nsed in a 1-D scanned radonie antenna^ ^ ^ ^ ^ space-fed lens (RADANT) is -1.25

dB. Two lenses are needed for a 2-Dscan so tliat the two-way loss for a 2-Dscanned RADANT array would be ~5clB. but proi^ress is being made.*''

GaN AND SiC CHIPSWide bandgap GaN and SiC

MMIC chips offer the potential ofone to two orders inciease in T/Rmodule power (see Figure 7).2"Table 2 summarizes the major ad-vantages of GaN. Tables 3 and 4compare GaN with Gay\s. This tech-nology would make it possible to up-grade an existing AESA by replacingthe GaAs T/R modules with GaN orSiC T/R modules having ten timesthe power. This provides either a tentimes improvement in search volnme

Raytheon Buirt SI9KSeeker Array; -600 EL-$30/EL, -40mW/EL

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or a 78 percent increase iti trackrange.^ '** CREE provides commercialSiC hybrid devices putting out 10 to60 W'for lip to 4 GHz luul GaN liy-hrid de\ices putting (iiit 15 to 120 Wfrom UHF to 40 GHz.'f^ Their goal isto provide in one package 550 Wpeak and 30 to 40 W average linearpower output using a single-stageFET. CREE supports the design ofMMIC SiC and GaN cliips. For GaNMMIC tliey provide 60 W saturated

from 2.5 to 4 GHz and 25 W saturat-ed from 5 to 6 GHz. See References16-18 for detailed siin'ex'S of state-of-the-art on GaN and SiG.'SiGe CHIPS

SiGe has tlie advantage of using Sias a su})strate. the te'chnolog\' of thelow-cost, commercial integrated-cir-cuit industry and whose exten.sive re-sources can be drawn upon. It offersthe potential of liiglier performance

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Integra's Miniaturized Power Amplifier (MPA) devices arecompletely impedance matched to 50 ohms internal to thepackage. The MPAL series utilizes gold metali2ed siliconLDMOS technology. They are characterized for simple pulsedoperation (lOOus, 10%) with low and constant quiescent current.They are capable of operating under other pulsed operationalmodes and under other classes of bias.

2.7 - 3.1 GHz and 3.0 3.5 GHz frequency bandsFlexible class of operation: AB, B or AFlexible pulsed operational modes

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at low cost. SiGe does not competewith GaAs with respect to microwa\eoutput power or noise figure. It offerslow cost and the ability to integratemany functions on a single chip. Inaddition to microwave power ampli-fiers and a low-noise figure receiveron one chip, it can have A/Ds anddigital circuitry. It can have CMOSon the same chip. Si CMOS. GTRI is

TABLE IIGaN TECHNOLOGY ADVANTAGES

GaN lias the highest [xnu'r dciisih nlaiivexisting tec'hin)!og\' enahlcs retiiEL-cil chip size - IDWI.T cost

tor a given power sni;il!er FET size enables broader

bantlwjdth because of higher impeditnceGaN capable nf higber operating voltage

jn power systemGaN on SiC ha.s icondiicti\'ity eonipared inaiiitain moderate efiiiiiriel ternps ;U high

TABLE IIIADVANTAGES OF MMIC GaN

OVER GaAi MMIC Provides .sy.steiii adMinlages re: weiglil,

coLiliiig. priiTie [lower, enst. sensitiviU', range 2H V vs. 10 Vat ix current (I) provides

5- Wx power witli - same gainiuid elik'ieniy as C.nAs

Iligl) voltage Ga/\s Pl'lEMT al 10 t(} 20 Vlit lower [ pro\ides only ] .5 to 2x power.

CiiiN iu Hit)

- 1

47 390(Z)/490 (SiC)(WViii-K)

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CMOS CHIPSCMOS now operates at microwave

Ireqiiencies. It. too, uses a silicon suh-strate and is the technology widely usedill the coinputtT iuthistrv. It Imlils thfpromise of low cost and low powei' (orthe receiver parts of T/R modules. LikeSiCe. it lias tlie advantage of allowinjjthe integration of many functions on asiujrle chip, even more so than SiGf.One ciiip can have RF, IF, haseljand.microprocessor, memory, tunable filtersand A/Ds a .system on'a chip (SOC). Ittan be c()ini)inetl with CaAs or GaN forthe microwave power amplifier and lownoise flpue receiver. Using CaN has thead\antage oi beintj robn.st euon^h that alimitrr may not be needed. Si to,tietlifrwith CMOS offers the possibility- of theintegration of many receive and/or tran.s-mit channels on a single chip.

DIGITAL BEAM FORMING (DBF)DBF hiLs am\ecl lor microwave .AESA radars (see Figure

8). It provides many sipiificant advantages over anulojf hramfomiing.' For large arrays I used to say DBF Is only being

DBF: Formt J beamipointing at jammerand one dt target

SLCOUTPUT

i) Addplivc-adaplive array (Brookner and Howell, Pro(\

developing a SiCk' single-chip T/R module for use in an.\I*;S.A radar. Its initial design had a peak power of > 50inW using a nvo-stage power amplifier (PA). Work is un-der way to achieve 1 W peak by usingthree stages.'^ The cost per element ofan .AES.A using such a module is expect-('

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done at subarniy. This is no longer tnie.l-llta II;LS doiH' it at thr element level tora 2.'5(X)-eIeinent iirray at S-band, a nia-jor lireakthroitgh. Using DBF elimi-nates the analog coinhinin

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lobe. It is to adjust ndaptively the uin-plitiule aiul pliase weisfhts of the arraySO as to put nulls in the directions oi"the jammers while maintaining themain l)eam poiutin;[;in the reijuirt-d di-rection. The.se amplitude and phaseweight adjustmeuts iire made hitscd onthe jamming sigiKiIs recei\'eil b)' the ai'-ray and tlieir ciJcnlated direction. Thissv'stem is kuouii as a luJly atlaptive ar-ra)' processor". Here again it is useiul ttjview tlie fully adaptive array froni an-

other physical point of \iew. Just as pre-viously we viewed the SLX" as an arrav'that puts nulls in die direction of thejanuners. we can now go the other wayand \ie\v the (ully adaptive arrayprocessor iw a SLC.^'"

Fidiy adaptive arrays have beentoo (lifficiilt to implement for largearrays up to uow. the luirdv\'iu-e andtile processing load being too great,To reduce the complexity, eonsidera-tion lias been given to doing fully

44 Visit http://mwj. hotims.com/16337-53

adaptive arniv proce.ssing at the sub-array level. This reduces the numberoi elements from thousantis to a fewtens. This is what is done on theUK's Multi-fuiictiou p]lectronit;LlIyScainied Adaptive Hatlar (MESAIi)."

Witli ttie advaueti-s of DBF, it is nowpossible to think ol aehieving the per-formance ot fully adaptive proci'ssiugwithout its couiplexity. In fact, tlieequiviJent jammer suppressi(jn of a ful-ly adaptive array without its computa-tion and transient penalties can heachieved. This can be accomplishedwith adaptive-adaptive array pnK*ess-irig.' This involves no more than Itx-at-ing digitally where the jamnu'rs are.then ]K)intiug lieains at tliesc jainiucrs(these beams are effectively eigen-l)eams)^-'"-"* luid using these lu-iims asside lobe cancellers for the uiaiu beam(see Figure 9). For a U)(K)-cleuit'iit ar-ra\' having to cope with 10 jammers, wenow have to invert a 10 x 10 matrix in-stead of a KXX) X KXK) niatiix and thetransient time is reduced iiy a factor ot100. lu a elas.siciJ full)' adaptive arra),one does not make use of" tlie loctitionof the jammers. But we can eiLsily de-termine their hjcation rather than toput on biiudei's. Tliis method i.s e(julva-leut to the method of Principal C o^tnjx)-nents.^ The jammers c;ui easily be lo-cated by doing a Fast Fourier Trans-form aeross [he array This vvil[ notlocate jammers less tliaii a Ix^anivvidthapart, but fbr uiany applications it maybe g(K)d enough. If lietter jamuier VAU-ci'llation is needed, then two stjuintedbeiuns about .'3/4 of a beamvvitlth apartcan be used fbr each located jammer.Tliis is betmise for closely spaci'd jam-mers, less tlian a beamvvidth apart, liteeigen}>eams are sum and differencebeams."' Alternately the Music algo-ritlim can be used.''^ Adaptive-adaptiveanay pnx-essing is in the .same spirit iH.sthe knowledge aided techniquesDAHPA has been recently fundingknown as Knowledge Aided SensorSignal Processor & Expert System(K.\SSPFR).^^ which they have appliedto Space-time Adaptive Proces.siug(STAP) discussed in the next section.

STAPSTAP is adaptive-an ay proee.s.siiip;

of a piil.se Doppler vvavefbrui. It pro-vides adaptive nulling of ground clut-ter and jammers on a moving plat-(oriu. On a ino\ing platfbrui it placesa 2-D Doppler-angle null where the

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cluttfv is.^ STAP is being used on thenew carrier-based E2-D AdvancedHawkeyes AN/APY-9 radar sliown inFignre l.'^ ^ It is used in tlie littoralLTivironnifint.'''^ It can also be nsed tocancel ont ground clutter tor agiound-based pulse Doppler radar.^'

PACKAGING AND ASSEMBLYIt is now possible to package and

assemble active pbascd arrays havinglow cost, li"hf weitiiit and small \-ol-

nnie. The technique involves the useof coinniercial printed wiring boards(PCB) and no packages for indi\idnalT/R modules. An X-band building-block ana\' of" 128 elements and T/Rmodules was built having a size of" 7.4X 10.1 X 0.21 in.2

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to 18 GHz.3>-i'' The Georgia TeclinicalKesearch Institute {GTRI) is devel-oping an urniy luiviniT; 33:1 instantii-nt'Oii.s haiKKvicltli witli potential oi100: l.'^ "^TUBE ADVANCEMENTS

Tubes arc niakijig major ad-\ances. '- ' '-^ 2 Tlu' a\'ailaliilily ofpovv-orful software now allows tlie designt()ber20

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l.T. G.J. Frazer, Y.I. Ahraniii\ilc-li and B.A. 24.Joliiison, "Spatially Wavcrorni DiverseUadar: Pcrspct-tives lor Ilifih Fre(]iient:y 2.5.OTIIH." 0(17 lERR Radar Conferenoe,April 17-20. 2(XI7. Boston. MA. 26.

l(i. Vlilligmi, cl ;J., HnilarC'.on-2007.17. Kopp,/Wi3S. (. :TH1. Hoiiziw Magazine. 2()6.39. Machine Design. Maaii 22. 2007.40. R..^ , Moiiziiigo and T.W. Miller. Inlnxluc-

tion to Adaptive .-\rrr/(/.v. Jolin Wiley &Sons Inc.. I9SII.

41. M.f:. WVlls. "MK.SAH Arlapliv.- Nnlllng,"lEECiilloijuinni. jnm-, 1990.

42. J. (iiifiri and J. Hrrpn. "Military- Hadar,"Jiinf 2.5-27. 2(X)7.

43. K. Tmt and li. Cnnirninjf.s. ct al.. Inti'nia-liorial SynipcKsinni iiri Pila.scd-array Sys-tems anrl Technology, OttobL-r 2003,Boston. MA.

41. S. Brierle); Raj'thcon Co.Eli BrookfMr nv-rtiM't/ ltl\tii:ii: '/-ij/.v fi-fi Til,' citiiC