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VOLUME 12, NUMBER 2, 2000 LINCOLN LABORATORY JOURNAL 355 Radars for the Detection and Tracking of Cruise Missiles Lee O. Upton and Lewis A. Thurman The advent of the modern cruise missile, with reduced radar observables and the capability to fly at low altitudes with accurate navigation, placed an enormous burden on all defense weapon systems. Every element of the engagement process, referred to as the kill chain, from detection to target kill assessment, was affected. While the United States held the low-observable- technology advantage in the late 1970s, that early lead was quickly challenged by advancements in foreign technology and proliferation of cruise missiles to unfriendly nations. Lincoln Laboratory’s response to the various offense/defense trade-offs has taken the form of two programs, the Air Vehicle Survivability Evaluation program and the Radar Surveillance Technology program. The radar developments produced by these two programs, which became national assets with many notable firsts, is the subject of this article. I , Defense Advance Research Projects Agency (DARPA) requested that Lincoln Labo- ratory develop and lead a new program in air de- fense against cruise missiles. The initial focus of the work at Lincoln Laboratory was to quantitatively as- sess and verify the capability of U.S. cruise missiles to penetrate Soviet air defenses. Two principal areas of technological concentration emerged from this study: (1) understanding and modeling the environmental factors, such as propagation and clutter, that directly affect a defensive system’s capability to detect and en- gage a low-altitude, low-observable air vehicle; and (2) measuring, developing, and demonstrating the ra- dar and infrared detection technologies required to address this difficult threat. Between 1982 and 1986 the program sponsorship was transferred from DARPA to the U.S. Air Force, and in 1983 this pro- gram was renamed the Air Vehicle Survivability Evaluation program (AVSE), which continues to this day. This article gives a short history of the AVSE pro- gram and several of the radar developments that re- sulted from the program, including the Airborne Seeker Test Bed. In 1983 the U.S. Navy (particularly the Naval Sea Systems Command and the Office of Naval Research) began sponsorship of a Lincoln Laboratory program, complementary to the AVSE program, which was originally focused on the U.S. ship-based defense against foreign antiship cruise missiles. The major de- velopment of this program, called Radar Surveillance Technology, was the Radar Surveillance Technology Experimental Radar (RSTER). Recently, the RSTER mission was modified to address issues related to the operation of airborne radars, including applications to the Air-Directed Surface-to-Air Missile (ADSAM) concept. A later section of this article gives a short his- tory of the Radar Surveillance Technology program and describes the development of the RSTER system. Radar Development: Air Vehicle Survivability Evaluation The purpose of the AVSE program is to understand and predict the survivability of U.S. air vehicles against existing or new enemy air defenses. A process, illustrated in Figure 1, was developed early in the pro- gram to provide these predictions of air-vehicle sur- vivability. Close ties with the intelligence community helped to define the enemy air-defense-system pa-

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VOLUME 12, NUMBER 2, 2000 LINCOLN LABORATORY JOURNAL 355

Radars for the Detection andTracking of Cruise MissilesLee O. Upton and Lewis A. Thurman

■ The advent of the modern cruise missile, with reduced radar observables andthe capability to fly at low altitudes with accurate navigation, placed anenormous burden on all defense weapon systems. Every element of theengagement process, referred to as the kill chain, from detection to target killassessment, was affected. While the United States held the low-observable-technology advantage in the late 1970s, that early lead was quickly challengedby advancements in foreign technology and proliferation of cruise missiles tounfriendly nations. Lincoln Laboratory’s response to the various offense/defensetrade-offs has taken the form of two programs, the Air Vehicle SurvivabilityEvaluation program and the Radar Surveillance Technology program. The radardevelopments produced by these two programs, which became national assetswith many notable firsts, is the subject of this article.

I , Defense Advance Research ProjectsAgency (DARPA) requested that Lincoln Labo-ratory develop and lead a new program in air de-

fense against cruise missiles. The initial focus of thework at Lincoln Laboratory was to quantitatively as-sess and verify the capability of U.S. cruise missiles topenetrate Soviet air defenses. Two principal areas oftechnological concentration emerged from this study:(1) understanding and modeling the environmentalfactors, such as propagation and clutter, that directlyaffect a defensive system’s capability to detect and en-gage a low-altitude, low-observable air vehicle; and(2) measuring, developing, and demonstrating the ra-dar and infrared detection technologies required toaddress this difficult threat. Between 1982 and 1986the program sponsorship was transferred fromDARPA to the U.S. Air Force, and in 1983 this pro-gram was renamed the Air Vehicle SurvivabilityEvaluation program (AVSE), which continues to thisday. This article gives a short history of the AVSE pro-gram and several of the radar developments that re-sulted from the program, including the AirborneSeeker Test Bed.

In 1983 the U.S. Navy (particularly the Naval Sea

Systems Command and the Office of Naval Research)began sponsorship of a Lincoln Laboratory program,complementary to the AVSE program, which wasoriginally focused on the U.S. ship-based defenseagainst foreign antiship cruise missiles. The major de-velopment of this program, called Radar SurveillanceTechnology, was the Radar Surveillance TechnologyExperimental Radar (RSTER). Recently, the RSTERmission was modified to address issues related to theoperation of airborne radars, including applicationsto the Air-Directed Surface-to-Air Missile (ADSAM)concept. A later section of this article gives a short his-tory of the Radar Surveillance Technology programand describes the development of the RSTER system.

Radar Development: Air VehicleSurvivability Evaluation

The purpose of the AVSE program is to understandand predict the survivability of U.S. air vehiclesagainst existing or new enemy air defenses. A process,illustrated in Figure 1, was developed early in the pro-gram to provide these predictions of air-vehicle sur-vivability. Close ties with the intelligence communityhelped to define the enemy air-defense-system pa-

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rameters; vehicle radar cross-section measurementsand models were generally provided by the U.S. in-dustrial developers via their government sponsors.Lincoln Laboratory’s role was to build phenomeno-logical models and predictive survivability models, asneeded. Over the years this role has necessitated theneed to develop instrumentation systems and to usethese systems and sensors in air-vehicle measure-ments. These measurements are key to a confidentprediction process since they are subsequently com-pared with system-analysis predictions. Over the lasttwenty years the system-analysis models and method-ology that have been developed have greatly benefitedfrom the corrective process afforded by these mea-surements. Much of the effort of the AVSE programis classified, however, and that portion is not dis-cussed in this article.

The effectiveness of defense against cruise missilesis highly dependent on the radar cross section of anair vehicle versus frequency. Figure 2 presents a no-tional representation of the variation in radar crosssection of an air vehicle versus frequency. Note thatthe radar cross section of an air vehicle is lower at S-band and X-band (the track/kill portion of the killchain) than at HF, VHF, and UHF (the surveillanceportion of the kill chain). Modern methods, such asairframe shaping and the use of absorbing material,have been used to considerably reduce the cross sec-tions of air vehicles. These techniques are particularlyeffective at higher frequencies. The effect of an air

vehicle’s reduced radar cross section is a reduction inthe air defense’s effective battle space. Reduced radarcross section also lends itself to the employment ofvarious electronic countermeasures.

The early years of the AVSE program tended to fo-cus on phenomenology and analytic modeling of So-viet defenses; the middle years saw growing efforts infield instrumentation and field testing. Most of therecent work has emphasized missile seekers, counter-measure and counter-countermeasure issues, and in-frared systems. The Lincoln Laboratory infrared sys-

FIGURE 2. Variation with frequency of the radar cross sec-tion of a typical air vehicle. The dashed extensions suggestthe cross-section behavior for very low and very high fre-quencies, where radar wavelength becomes much longer ormuch shorter, respectively, than the physical length of theair vehicle. The figure also shows the frequency domains oc-cupied by most surveillance and fire-control radars.

FIGURE 1. Air-vehicle-survivability prediction process. The engagement-analysis computer pro-gram combines the measurement data and defense-system models to predict the survivability ofthe air vehicle in the defense-system engagement scenario. The accuracy of the prediction is veri-fied by using airborne experimental captive-carry tests.

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FIGURE 4. Phase One radar equipment at Lethbridge,Alberta, Canada. Clutter surveys at five different frequen-cies were performed at forty-two different sites.

tems associated with the AVSE program, namely, theground-based infrared measurement sensor and thepod-mounted airborne infrared imager, are men-tioned here for completeness but are not discussedextensively in this article (since the focus here is onradar). Chronologically, the AVSE program first em-phasized the surveillance aspect of air defense, thenthe fire control (target tracking) aspect, and currentlythe target intercept (kill) aspect.

Monostatic Clutter Measurement(Phase Zero and Phase One Radars)

An initial AVSE program objective was to accuratelypredict the performance of surface-sited radarsagainst low-altitude targets. This capability required agreatly improved understanding of clutter phenom-enology, which led to plans for a major new programof ground-clutter measurements.

This new program occurred in two phases: PhaseZero, a pilot phase that involved a small noncoherentX-band radar; followed by Phase One, the full-scalecoherent-radar data-collection program at five fre-quencies (VHF, UHF, L-band, S-band, and X-band).

Figure 3 shows a photograph of the Phase Zero mea-surement instrumentation, and Figure 4 shows a pho-tograph of the Phase One measurement instrumenta-tion. The Phase One radar was a computer-controlledinstrumentation radar specifically designed forground-clutter measurements. It had high data-raterecording capability and could maintain coherenceand stability sufficient for 60-dB two-pulse-cancelerclutter attenuation in post-processing.

Because Soviet-type terrains were of principal in-terest, and the prairie provinces of Canada provided agood analog of this terrain, measurements were pri-marily made in Canada with the assistance of the Ca-nadian government. In addition, measurements weremade at selected sites in the United States for a totalof forty-two Phase One sites. These measurements re-sulted in a large land-clutter measurement database.This calibrated clutter database was used to developan empirically based clutter-modeling capability. Anew site-specific approach was adopted in model de-velopment, based on the use of digitized terrain eleva-tion data to distinguish between visible and maskedregions to the radar. Extensive analysis of the newclutter-measurement database led to a progression ofincreasingly accurate statistical clutter models for lay-ing down the clutter strengths in visible regions ofclutter occurrence [1, 2].

Figure 5 shows mean and median clutter reflec-tivity as a function of depression angle at the radar

FIGURE 3. Phase Zero radar equipment in Dundurn, Sas-katchewan, Canada. X-band clutter surveys were performedat more than one hundred sites.

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antenna for typical rural terrain, observed at X-bandfrequency with horizontal transmit and receive polar-izations. Each plotted point is the result of a combi-nation of many similar measurements (e.g., on theorder of hundreds) in general rural terrain. It is onlyby means of such extensive averaging that theground-clutter dependencies with angle emerge em-pirically to provide a general predictive capability.Both mean and median are observed to rise mono-tonically with increasing depression angle, and thespread in low-angle clutter-amplitude statistics is de-fined by the mean-to-median ratio that decreases rap-idly with increasing depression angle. The curves inFigure 5 illustrate that at low depression angles neargrazing incidence, clutter is a widespread spikey pro-cess dominated by discrete sources, but with increas-ing angle, spread diminishes and the process gradu-ally begins to transition to one of homogeneousRayleigh statistics, which are more typical of clutterobserved from airborne regimes. (Airborne cluttermeasurements were made by Lincoln Laboratory in1980 by utilizing an airborne X-band and L-bandsynthetic-aperture radar system from the Environ-mental Research Institute of Michigan. These mea-surements are not detailed in this article.)

Low-Angle Propagation Measurements

Propagation of radar signals can affect radar targetand clutter returns, especially at low frequencies (e.g.VHF). In 1982, to understand this phenomenon bet-ter and to improve prediction models, Lincoln Labo-

ratory built a propagation measurement instrumenta-tion module that could be conveniently carried on ahelicopter. In a typical experiment, the helicopter-borne instruments recorded received signal strengthversus height from the radar of interest, at variousranges and azimuths around the radar. Measured ter-rain profiles were used in conjunction with reflectionand diffraction theory to deduce the relative impor-tance of each effect. The principal insights of thepropagation work are incorporated in the LincolnLaboratory Spherical Earth with Knife Edge (SEKE)model [3], which is used along with some more exactmodels in AVSE system analyses.

Fire-Control Experiments

In the early 1980s many defense planners were inter-ested in the performance of fire-control radars, espe-cially the Soviet SA-10 Flap Lid radar, against low-fly-ing cruise missiles. This interest led to LincolnLaboratory’s involvement in two X-band tracking sys-tems: the L-X radar and the FLEXAR radar. The AN/TPN-19 aircraft-approach radar, derived from theRaytheon prototype Hostile Weapons Location Sys-tem, was modified for Lincoln Laboratory and deliv-ered in 1982 as the L-X radar. It was a dual-frequencyinstrumentation system that collected signature andmetric data on a variety of targets, and participated intwelve air-launched and ground-launched cruise-mis-sile tests at the Dugway, Utah, test range in 1983 and1984. Of special note was the vertically polarized X-band system that employed a reflector, a small phasedarray, and a monopulse feed, and formed a pencilbeam (2° azimuth, 1.5° elevation angle).

There was much debate on how well the SA-10Flap Lid’s receiver performed in canceling groundclutter, and this capability was a significant factor inthe system’s ability to track low-altitude, low-observ-able targets. Since the United States had no direct ac-cess to the SA-10, the next best thing was to look forexisting U.S. systems to evaluate the technology lim-its. The Hughes Aircraft Company had developed anexperimental X-band, phased-array, fire-control radarfor Navy shipboard applications. This radar, calledFLEXAR, had state-of-the-art clutter-rejection capa-bility. The Laboratory conducted field experimentswith the FLEXAR system from 1983 to 1986.

FIGURE 5. General variation of ground-clutter strength withdepression angle.

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FLEXAR was first used to characterize the clutter-re-jection capability of high- and medium-pulse-repeti-tion-frequency ground radars, and it added muchneeded real-world data to the Flap Lid clutter-cancel-lation debate. It was later used at Eglin Air ForceBase, Florida, and at the China Lake, California, testrange to evaluate U.S. electronic countermeasuresagainst Soviet radars.

Another asset used to investigate the Flap Lid typeof radar was the Waveform Simulator (WFS), whichwas originally developed by the Georgia Tech Re-search Institute for the Army Missile System Intelli-gence Command’s CROSSBOW office, now calledthe Threat Systems Office. The WFS was then trans-ferred to Lincoln Laboratory in 1990, where it hasbeen used extensively as an illuminator for the Air-borne Seeker Test Bed (ASTB) and in conjunctionwith the ASTB to evaluate the SA-10’s potential sys-tem performance in clutter.

VHF Instrumentation Radar

Even before the development of the modern cruisemissile, the Soviets had deployed thousands of VHFground radars for aircraft surveillance and early warn-ing. In 1983 the Laboratory initiated a competitive

procurement for a VHF test-range instrument, in or-der to have an instrumentation-quality VHF radar toinvestigate the issues associated with low-frequencysurveillance. General Dynamics of Fort Worth, Texas,delivered this VHF radar in 1985. It is a substantialbut transportable radar featuring a 150-ft-wide an-tenna, as shown in Figure 6, and it can emulate Rus-sian VHF radars such as Tall King and Spoon Rest(although it has superior electronic performance).What is particularly interesting, especially for clutterand electronic-countermeasure measurements, is thatthe VHF instrumentation radar can selectively trans-mit in horizontal and vertical polarizations and re-ceive in both polarizations simultaneously. The radarhas undergone a number of modifications and up-grades, including extensive waveform changes and theaddition of a sidelobe canceler, to enhance its useful-ness to the test community.

The principal contribution of the VHF instru-mentation radar has been the development of realisticappraisals of VHF radar capability against low-ob-servable air vehicles. VHF-radar performance predic-tions are rich in phenomenological questions relatingto low-elevation-angle propagation and ground-clut-ter effects, and this radar was a national test bed to ex-plore and define these effects.

Airborne Seeker Test Bed

The Airborne Seeker Test Bed (ASTB) is an aircraft-mounted instrumentation system used for developingand evaluating missile-seeker technology. Since theinitial flight in March 1990, the ASTB has been usedon 550 flights to collect radar and infrared data criti-cal for understanding air-defense issues.

The impetus for the ASTB came from a contro-versy within the defense community, including Lin-coln Laboratory and Raytheon, over the performanceof the improved HAWK surface-to-air missiles andSparrow air-to-air missiles in live firings against U.S.cruise missiles in the early 1980s. The expense of mis-sile live firings does not permit the collection of a suf-ficiently large database to completely assess the effec-tiveness of missile seekers in all scenarios of interest.Furthermore, it is challenging to represent the perfor-mance of a seeker in a realistic electromagnetic envi-ronment through computer modeling or hardware-

FIGURE 6. The VHF instrumentation-quality radar used as atest bed to investigate problems in low-frequency surveil-lance, including target detection, clutter, and electronic-countermeasure performance. The person standing to theleft of the pedestal indicates the very large size of this radar.

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in-the-loop hybrid simulations. A key challenge is tocapture the effects of propagation and clutter, and theinteraction of electronic countermeasures with thesephenomena. Therefore, Lincoln Laboratory specifiedan airborne missile-seeker instrumentation platformto directly capture the performance of missile seekersin complex environments, and to record instrumenta-tion-quality data to support the development of morerealistic computer models of seeker performance andthe environment.

Construction of this instrumentation platform be-gan in 1986 with the award of a contract to RaytheonMissile Systems Division in Bedford, Massachusetts,to build the primary sensor—a calibrated, dual-polar-ization, eight-channel, X-band, semiactive instru-mentation system. The Laboratory developed the air-borne data-recording and processing system, andadded additional measurement support systems, in-cluding an 8-to-12-µm infrared camera, a Global Po-sitioning System (GPS) receiver, and a pod-mountedC-band beacon tracker. By providing information onthe angular position of the X-band seeker antenna,the position of the ASTB aircraft, and the position ofa C-band beacon-equipped target, these auxiliary sys-tems bring an element of scientific control to airbornemissile-seeker measurements. By March 1990 the sys-tem was fully integrated into a Dassault Falcon-20twin-engine jet aircraft, shown in Figure 7.

The purpose of the ASTB was to produce high-fidelity test data related to semiactive radar-seekerphenomenology, target scattering characteristics,electronic countermeasures, electronic-counter-coun-termeasure technique development, and missile-seeker acquisition and tracking performance [4]. TheASTB radar receiver operates with ground-based ra-

dars such as the HAWK missile illuminator, the spe-cial-purpose WFS radar, or modern airborne radars,including those on the F-15 and F-16 aircraft. Theseradars track the target aircraft and provide the illumi-nating radar signal received by the ASTB. Typically,the ASTB climbs or dives toward a target aircraft on aproportional navigation collision course. The effectsof target cross section, ground clutter, and electroniccountermeasures have been evaluated in intercept sce-narios for a variety of air vehicles at national testranges, including White Sands Missile Range, NewMexico; Eglin Air Force Base, Florida; the Utah Testand Training Range, Nellis Air Force Base, Nevada;Edwards Air Force Base, California; and the Naval AirWarfare Center’s Weapons Division Test Ranges atChina Lake and Point Mugu, California.

In the study of cruise-missile offensive and defen-sive interactions, such as semiactive missile interceptsand towed-decoy and terrain-bounce electronic coun-termeasures, it is desirable to understand not only themonostatic clutter but also the bistatic clutter charac-teristics and their effects on radars and radar seekers.In the development of the bistatic clutter models,however, neither the existing bistatic clutter data northe theoretical bistatic clutter models were found tobe adequate. The main reason is that the bistatic clut-ter is more difficult to investigate than the monostaticclutter because of additional complexities such astransmitter/receiver clutter-cell geometry and in-creased number of bistatic angular variables, resultingin increased efforts and costs for measurements.

In 1990, as interest increased on the developmentof the bistatic clutter models in order to understandthe operation of missile seekers against low-flyingmissiles, Lincoln Laboratory, with DARPA sponsor-ship, carried out extensive bistatic clutter measure-ments by using advanced test assets and supportingequipment. The ASTB was used in conjunction withthe WFS or with a terrain-bounce antenna mountedon a Lear Jet as a transmitter to gather bistatic clutterover a wide range of bistatic angles and in a variety ofterrain types.

The ASTB was instrumented to properly guide theaircraft position and antenna pointing during themeasurements with a GPS satellite and a C-band bea-con-tracking radar in range and angle. Both the GPS

FIGURE 7. Dassault Falcon-20 twin-engine jet aircraft. Thisplatform was the original Airborne Seeker Test Bed (ASTB).

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and beacon-tracking radar data were used in post-mission analyses to reconstruct target position as wellas antenna position and pointing. With these test as-sets and a systematic test plan to cover the bistaticangles of interest, it was possible to gather bistaticmeasurement data for the development of the bistaticclutter models.

From these measurements the X-band bistatic clut-ter models were developed. As examples, Figure 8shows a land-clutter model of a rough desert terrainfrom White Sands Missile Range and a sea-cluttermodel (sea state 3) from the Point Mugu Naval AirWarfare Center test range. These models may be usedto predict the clutter effects on the operation of mis-sile seekers against low-flying cruise missiles.

Electronic countermeasures, known as endgamecountermeasures (EGCM), can be used against mis-sile seekers during the last few seconds before targetintercept. Techniques known as counter-endgamecountermeasures (CEGCM) have been postulated toallow a missile to continue to guide to the target. TheASTB has been used to collect bistatic radar targetand clutter data, as discussed in previous sections, andto support the development of CEGCM concepts. InDecember 1995, a high-speed digital signal processorwas added to the ASTB to process radar returns andcontrol the pointing of the instrumentation seeker.This effort culminated in the first real-time demon-stration of a class of radio-frequency (RF) CEGCMtechniques in June 1996 at White Sands.

The success of the ASTB led the U.S. Air Forcesponsor to request the expansion of system capability.In late 1993, the ASTB performed its last mission onthe Falcon-20 aircraft, and the system was installedon a Gulfstream II twin-engine jet in fall 1994. TheGulfstream II is a more capable aircraft in terms ofpayload and endurance. Up to five external sensorpods can be carried on the aircraft, and it has roomfor additional sensor operators. Figure 9 shows theconfiguration of the ASTB on the Gulfstream II air-craft. Modifications have been recently made to ex-tend the frequency of operation of the ASTB, andseveral additional RF and infrared seekers are beingprepared for future tests.

In May 1995, the ASTB mission was augmentedto collect data on infrared-seeker phenomenology

FIGURE 8. (a) Bistatic clutter-measurement geometry, (b)mean normalized clutter reflectivity versus β, and (c) meannormalized clutter reflectivity versus δ, a measure of angulardistance from the specular direction. The bistatic cluttermodels specify measurements of mean clutter reflectivity atX-band, with vertical polarization.

(infrared clutter, atmospheric propagation), target in-frared signatures, and infrared-seeker acquisition andtracking performance. Adding a pod-mounted air-borne infrared-imager system accomplished this task.From a missile-seeker-technology point of view, therole of the airborne infrared-imager system is analo-gous to the role of the X-band instrumentation head.The dual-band, radiometrically calibrated infraredcamera is used to evaluate current and proposed infra-

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red seekers. A pod-mounted infrared missile seeker(Sidewinder or AIM-9M) is carried by the aircraft toevaluate the performance of current U.S. infraredmissiles, especially in clutter background.

Radar Development: Radar SurveillanceTechnology

The initial focus of the Radar Surveillance Technol-ogy program, as discussed earlier, was the U.S. ship-based defense against foreign antiship cruise missiles.A later focus was on the radar detection and track of alow-flying cruise missile by the naval fleet-surveil-lance airborne-radar system, the E-2C. The conceptpresented in Figure 1 for the air-vehicle-survivabilityprediction process can also be applied to the RadarSurveillance Technology program. Here the air ve-hicle is a foreign cruise missile, the defense system isthe U.S. Navy’s Aegis air-defense system or its E-2Cairborne surveillance radar systems, and the phenom-enological measurements and the air-vehicle measure-

ments are accomplished by the program’s Radar Sur-veillance Technology Experimental Radar (RSTER).The same concept of prediction using available mea-surements and models and closing the predictionloop by using measurements on air vehicles is, ofcourse, valid in these scenarios.

Radar Surveillance Technology Experimental Radar

The RSTER is a UHF, phased-array, moving-target-indicator system capable of detecting targets in thepresence of heavy clutter and jamming interference.The interference is mitigated through the use of adap-tive-nulling capability in elevation angle and ultralowsidelobes in azimuth. Development of the RSTERsystem began in 1983. During the first two years ofthe program, the antiship-missile threat characteris-tics were established, and engagement analyses wereperformed. Radar design studies were conducted thattook into account the target platform’s capabilitiesand the threat environment. These studies of clutter

FIGURE 9. Current version of the ASTB. The ASTB is a fully instrumented, highly calibrated, airborne data-collection system.The Gulfstream II platform carries an assortment of sensor pods (e.g., an RF seeker, the airborne infrared imager, and an AIM-9M seeker), a C-band beacon-tracker subsystem that is used to point the sensor pods at beacon-carrying targets, and nose-mounted advanced array antennas. Data from the sensors and support equipment are recorded on a wideband digital recorderfor post-mission data analysis and interpretation.

Monopulse antenna4 port

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X-band antennas(16-in diameter)

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FIGURE 10. Radar Surveillance Technology ExperimentalRadar (RSTER), as originally deployed at Lincoln Labora-tory. The 5-m-high × 10-m-wide, 14-channel antenna is con-nected to the transmitter and receiver subsystems via a 33-channel rotary coupler. Signals from the individual channelsare digitized and processed to achieve adaptive digitalbeamforming (in elevation), pulse compression, Doppler fil-tering, constant false-alarm-rate detection, multitargettracking, synthetic displays, data recording, and targettrack-file generation. The last capability is required to directother sensors and weapon systems. All the radar sub-systems are contained in the two forty-foot trailers shown atthe base of the tower.

levels, radar performance, and radar equipment con-figurations led to a RSTER system design in 1986.

After the initial design, technology developmentbegan on the antenna, transmitter, and digital adap-tive beamformer. A contract was awarded to Westing-house to develop an ultralow-sidelobe vertically po-larized phased-array antenna that was five by tenmeters and consisted of fourteen channels of antennaelements. During 1991 and 1992, the spatially adap-tive digital signal processor was designed and built atLincoln Laboratory. Following the completion of thesignal processor, the assembly of the RSTER systemwas concluded, and system-level testing was con-ducted during the first half of 1992. Figure 10 showsRSTER as first installed at the Laboratory. Eachchannel had twenty-four elements connected to aprecision corporate feed that applied a fixed Cheby-

FIGURE 11. RSTER ultralow-sidelobe array azimuth princi-pal plane. The array beam in the azimuth (horizontal) planeis formed within the array by using precision analog tech-niques. The gain of the antenna is the ratio of the peak ofthe beam to the isotropic level, and is within a very small per-centage of achieving the theoretical maximum. Sidelobesaway from the main beam are governed in amplitude by arrayfundamentals, weighting tapers, and electronic phase andamplitude errors. The RSTER array’s sidelobe levels sug-gest performance levels rarely achieved in a laboratory, butwhich are demonstrated here in a fielded experimental radar.

shev amplitude taper. The corporate feed design pro-duced azimuth sidelobes that were more than 60 dBbelow the main lobe, as shown in Figure 11. The an-tenna was steered mechanically in azimuth and elec-tronically in elevation angle and was designed to beused over the 400-to-500-MHz band. Westinghousewas also commissioned to build the very stable four-teen-channel solid state transmitter. The transmitterhad a pulse-repetition-frequency range of 300 to1500 Hz and produced 10 kW of peak power in eachof the fourteen channels, for a total peak power of140 kW [5].

Following initial tests, RSTER was shipped toWallops Island, Virginia, where—acting as an Aegisadjunct radar—it participated in exercises involvingjamming and clutter, including heavy chaff. RSTERsuccessfully detected and tracked high-flying, low-ra-dar-cross-section targets in real time and designatedthe targets to the AN/SPY-1B radar. The RSTER sys-tem met or exceeded all its design goals with regard toazimuth sidelobes, moving-target-indicator (MTI)performance, and adaptive-null depth in these testsand demonstrations. Figure 12 shows an adaptive-nulling result involving detection of a Lear Jet in the

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presence of a strong jammer. RSTER’s adaptive nullprovides about a 45-dB reduction in the jammingnoise level.

As part of the Mountaintop program, RSTER wasshipped to the Pacific Missile Range Facility atMakaha Ridge on Kauai, Hawaii, in July 1994. Fig-ure 13 shows an aerial view of the Makaha Ridge site.In 1995, the radar was moved to Kokee Park onKauai to act as a simulated airborne radar in the U.S.Navy’s Cruise-Missile-Defense Advanced ConceptTechnology Demonstration (ACTD). The goal of the

FIGURE 14. U.S. Navy Cruise-Missile-Defense Advanced Concept Technology Demonstration (ACTD) on theKokee Park mountaintop on Kauai, Hawaii. Multiple radar sensors were netted via the Cooperative Engagement Ca-pability (CEC) communication system to detect, track, launch against, and destroy a low-flying surrogate cruisemissile (the BQM drone) that was flying beyond the line of sight of the surface-based Aegis missile system. Thetest demonstrated the air-directed surface-to-air missile (ADSAM) system concept in the Mountaintop test venues.

FIGURE 13. Aerial view of the Makaha Ridge site at the Pa-cific Missile Range Facility, Kauai, Hawaii.

ACTD was to provide over-the-horizon detectionand engagement of low-flying targets by using a sen-sor suite at the 3800-ft Mountaintop site, whichserved as a surrogate airborne radar; this scenario, il-lustrated in Figure 14, demonstrated the air-directedsurface-to-air missile (ADSAM) system concept inthe Mountaintop test venues. The elevated site con-sisted of RSTER providing surveillance and acquisi-tion and an MK-74 fire-control system providingprecision tracking and target illumination. TheMountaintop sensors were interconnected to each

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FIGURE 15. ACTD scenarios with RSTER as the surveillance sensor. A single high-altitude calibra-tion scenario and three missile-firing low-altitude intercept scenarios were performed as the testcomplement for the Mountaintop ACTD. Success was achieved for all the scenarios, which broughtthe U.S. Navy closer to the ADSAM capability needed for future littoral warfare.

other and to an Aegis Cruiser (CG 70-USS Lake Erie)by using the U.S. Navy’s Cooperative EngagementCapability (CEC). BQM-74E drones flying at closeto Mach 1 were used as surrogate low-altitude cruisemissiles. The drones were engaged by the modifiedAegis SM-2 surface-to-air missile.

After several months of integration testing, highlysuccessful live-fire concept demonstrations were ac-complished in January 1996. Figure 15 illustrates thefour different flight scenarios that were flown. In sce-nario 1 the drone was inbound at an altitude of15,000 ft. In the other three scenarios the drones wereflown inbound at an altitude of fifty feet. The princi-pal measure of success for the ACTD was intercept(to within a lethal radius) in all three low-flying sce-narios beyond the 18-nm radar horizon of the ship’sAN/SPY-1B radar to the BQM-74E at an altitude offifty feet. The tests were all completed successfully.

The Future

Cruise missiles will continue to be improved in all as-pects of their performance, and they will continue toproliferate to many nations of the world. LincolnLaboratory will continue to work on behalf of theDepartment of Defense to pursue the system con-

structs, technology developments, and proof-of-con-cept demonstrations that are required to maintain thepreeminent position the United States holds in low-observable air defense and low-observable air vehicles.Radar sensors such as those developed for the ASTBand RSTER programs will provide the legacy for fu-ture detection and tracking of cruise missiles.

R E F E R E N C E S1. J.B. Billingsley, “Ground Clutter Measurements for Surface-

Sited Radar,” Technical Report 786 Rev. 1, Lincoln Laboratory(1 Feb. 1993), DTIC #AD-A262472.

2. J.B. Billingsley, Low-Angle Radar Land Clutter: Measurementsand Empirical Models (William Andrew/SciTech Publishing,Norwich, N.Y., to be published in fall of 2001).

3. S. Ayasli, “SEKE: A Computer Model for Low AltitudePropagation over Irregular Terrain,” IEEE Trans. AntennasPropag. 34 (8), 1986, pp. 1013–1023.

4. C.W. Davis III, “The Airborne Seeker Test Bed,” Linc. Lab. J.3 (2), 1990, pp. 203–224.

5. B.D. Carlson, L.M. Goodham, J. Austin, M.W. Ganz, andL.O. Upton, “An Ultralow-Sidelobe Adaptive Array An-tenna,” Linc. Lab. J. 3 (2), 1990, pp. 291–310.

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. is the head of the TacticalSystems Technology division.The focus of the division isresearch and development oftechniques for target detec-tion, tracking, and identifica-tion of airborne and surfacevehicles. Prior to his currentposition, he was the associatehead of the Tactical SystemsTechnology division, and priorto that he was the associatehead of the Air Defense Tech-nology division. In his threeyears at the Kiernan ReentryMeasurements Site (KREMS)at Kwajalein he was a systemengineer at the ALCOR radarand the Lincoln Laboratorysection leader of the ALTAIRradar. He has been deeplyinvolved in the evolution ofthe Air Vehicle SurvivabilityEvaluation Project from itsinitial emphasis on radarphenomenology through thedevelopment of radar andinfrared assets such as theVHF radar and the InfraredMeasurement System, leadingto today’s emphasis on fire-control radars and seekerscentered around the waveformsimulator and the AirborneSeeker Test Bed. He receivedB.S., M.S., and Ph.D. degreesin electrical engineering fromPurdue University. He is amember of the IEEE.

. is the assistant director ofLincoln Laboratory. Prior tohis current position, he wasthe head of the Tactical Sys-tems Technology division, andprior to that he was the headof the Air Defense Technologydivision. He was on assign-ment with the Defense Ad-vanced Research ProjectsAgency (DARPA) from 1989to 1992 as the DARPA pro-gram manager of an airborne-sensor development program.The program featured manyscientific, algorithmic, andimplementation advances inairborne-sensor technology.For his work on this assign-ment, he received the Secretaryof Defense award for technicalexcellence. He also was theMillimeter Wave (MMW)Radar section head inKwajalein, Marshall Islands,from 1982 to 1984, duringwhich time the MMW radarbecame operational. He joinedLincoln Laboratory in 1978after a period of employmentwith the RCA Corporation inMoorestown, New Jersey. Hegraduated from Tufts Collegeof Engineering with a B.S.E.E.degree and from the Universityof Pennsylvania with anM.S.E.E. degree. Heis a member of the IEEE.