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• BRYANT, MORSE, NOVAK, AND HENRYTactical Radars for Ground Surveillance

VOLUME 12, NUMBER 2, 2000 LINCOLN LABORATORY JOURNAL 341

L ’ in thearena of tactical battlefield surveillance beganin 1967 with a program to develop a radar sys-

tem that would penetrate jungle foliage and detectmoving hostile intruders. This effort arose during thewar in Vietnam, when major national laboratorieswere called upon to contribute solutions to tacticalbattlefield surveillance involving ground-based andairborne-based sensors.

Ground-based sensors can be loosely grouped intotwo categories. Special ground-penetrating radar sen-sors are used to detect mines and other explosives aswell as hidden tunnels and buried stores. Otherground-based radar systems are used to survey largeregions of terrain within the sensors’ fields of view inorder to detect and identify fixed ground targets andto detect, identify, and track moving ground targets.Airborne sensors designed for tactical battlefield sur-veillance require the ability to survey large areas onthe ground in a timely manner in order to detect andidentify both fixed and moving surface targets thatmay be hidden in ground clutter or protected bycountermeasures. Lincoln Laboratory has developed a

Tactical Radars forGround SurveillanceThomas G. Bryant, Gerald B. Morse, Leslie M. Novak, and John C. Henry

■ Battlefield awareness is the key to battlefield dominance. The fieldcommander who knows the enemy’s location and the types of forces beingdeployed enjoys a great tactical advantage. The problem of detecting andclassifying ground targets presents substantial technical challenges, whichLincoln Laboratory has addressed for nearly three decades in its TacticalTechnology program. Substantial progress has been made in many aspects ofground surveillance since the mid-1960s, but many challenges remain. Thesechallenges include sensor development, signal processing, and target-recognitiontechnology. Among its successes, the Laboratory has provided the foundationfor operational national assets such as the Joint Surveillance Target Attack RadarSystem (Joint STARS) airborne surveillance system. This article describes inchronological order several important Laboratory tactical-radar programs andthe technologies that were developed for both airborne and ground-basedsurface surveillance.

broad understanding of the technology and the phe-nomenology of target detection; the Laboratory hasalso developed a variety of remote sensors, communi-cation strategies, digital processors, signal-processingalgorithms, and data-processing techniques to addressthe concerns of different surface-surveillance systems.This article describes some of the significant Labora-tory radar programs and the technologies that weredeveloped for these ground-based and airborne sys-tems. For more information on these and other pro-grams at the Laboratory, see the articles entitled“Displaced-Phase-Center Antenna Technique,” byCharles Edward Muehe and Melvin Labitt, and “De-velopment of Coherent Laser Radar at Lincoln Labo-ratory,” by Alfred B. Gschwendtner and William E.Keicher, both in this issue.

Foliage-Penetrating Radar (1967–1972)

Field reports from American troops in Vietnam re-vealed that foliage played a major role in concealingthe enemy in most tactical engagements. As a result,Lincoln Laboratory began an investigation of a sur-veillance system that offered a foliage-penetration ca-


342 LINCOLN LABORATORY JOURNAL VOLUME 12, NUMBER 2, 2000

pability. A preliminary Laboratory study in 1966concluded that radar would be able to detect peopleand vehicles moving through dense foliage.

The Camp Sentinel Radar

The foliage-penetration radar program, supportedoriginally by the Defense Advanced Research ProjectsAgency (DARPA) and subsequently by the U.S. AirForce, began in January 1967 [1]. The objective wasthe development of a ground-based radar that coulddetect intruders moving into a small encampment.This ground-based system was named the Camp Sen-tinel Radar. Deployed in Vietnam after an eighteen-month crash development program, it protectedAmerican troops throughout the rest of the war.

At the inception of the Camp Sentinel Radar pro-gram, there was little information on which to basethe radar design. The technical literature providedminimal data on electromagnetic attenuation intropical foliage, but it did suggest that the best operat-ing frequencies were between 20 and 500 MHz. Afrequency (435 MHz) near the upper end was chosenin order to localize any detections to a small azi-muthal region with an antenna small enough for tac-tical deployment.

Three critical questions had to be answered aboutthe propagation of radar signals within relativelydense foliage: (1) How much does the moving foliagespread the frequency spectrum of a signal reflectedfrom a target? (2) What is the frequency spectrum ofclutter signals reflected from windblown foliage? (3)What is the effect of multipath propagation on rangeand azimuth resolution and on subclutter visibility?

Lincoln Laboratory built two radar systems to an-swer these questions. The initial system, called theCamp Sentinel Radar-I, took measurements at a sitelocal to Lincoln Laboratory. This system worked well,and it was used in demonstrations to military observ-ers in the fall of 1967. A second version was mountedin a van and sent to Bisley, Puerto Rico, where foliageclosely simulated the conditions in Vietnam. By Janu-ary 1968, enough measurement data had been accu-mulated to go ahead with a more advanced radar,called the Camp Sentinel Radar-II.

The design of the Camp Sentinel Radar-II incor-porated unique and innovative concepts. The an-

tenna was mounted high above the ground on a rap-idly deployable tower so the electromagnetic wavescould reach a target by propagating over the tops ofthe trees and then be diffracted to the ground, ratherthan by propagating directly through the foliage. Anelectronically scanned cylindrical array sequentiallystepped the antenna beam through thirty-two posi-tions in azimuth to cover 360°. The transmit/receivebeams were stepped so rapidly in azimuth that thesignal processing functioned as if there had beenthirty-two individual radars, each with stationary azi-muth coverage and all operating simultaneously. Theprocessors were able to execute the critical algorithmsneeded to detect small, slow-moving human infiltra-tors from the high-level clutter background createdby the windblown tropical foliage.

The radar control was designed to allow an opera-tor to construct two intrusion fences. These fencescould be made irregular in shape, to match them tothe desired defense perimeter. The operator did notneed to monitor the radar unless an alarm sounded. Ifa detection occurred, the operator simply checked thedisplay to see which range/azimuth sector containedthe intruder and whether the target was incoming oroutgoing. After a short local testing period, the firstLaboratory-built Camp Sentinel Radar-II was

FIGURE 1. The Camp Sentinel Radar-II was first installedand operated in Lai Khe, Vietnam, in 1968. The system wassituated on the perimeter of the U.S. Army camp, with theradar antenna mounted on top of the tower. The remainderof the radar system was located in a bunker behind the hill inthis picture, where the machine-gun tower was located.



shipped to Vietnam in August 1968. Laboratory em-ployees Leonard Bowles and David Rogers spent twomonths in Vietnam, introducing the radar to theThird Brigade, 1st Infantry Division, and instructingArmy personnel in its operation and maintenance.The radar, shown in Figure 1, received immediate ac-ceptance, and the Army used it until the end of thewar.

The U.S. Army’s Harry Diamond Laboratory car-ried out additional development work. The improvedversion, called the Camp Sentinel Radar-III, includeda more powerful transmitter to increase the detectionrange and to provide an additional number of displayoptions. Six of these radars were manufactured andsent to Vietnam, where they remained until U.S.combat troops were withdrawn.

Geodar: Ground-Penetrating Radar (1966–1967)

American forces operating in Vietnam needed a sen-sor system that could detect tunnels. In 1966, afterdiscussions with other laboratories working on thisproblem, Robert Lerner of Lincoln Laboratory con-cluded that a ground-penetrating radar offered possi-bilities, and the Geodar (ground echo detection andranging) program was initiated to investigate the con-cept under DARPA sponsorship. The distinguishingfeature of the Geodar concept was that electromag-netic energy was radiated directly into the ground.

Because the soil-penetration properties of radarwere not known with any precision, it was decided touse a wide band of frequencies, from 50 to 150 MHz,within the generally applicable frequency range. A flatrectangular-shaped antenna of transmission-line de-sign, operating close to the ground surface, radiatedelectromagnetic energy in compact packets of 3-to-5-nsec duration. The antenna, shown in Figure 2, withan effective area of about 3/4 of a square meter, wasdrawn over the ground on a Teflon sled structure.

A first experimental system was quickly assembledin 1966, and proof-of-concept tests were made at asimple tunnel test range at Lincoln Laboratory. Thetests proved that tunnel-like voids in the groundcould be detected. A formal program was then estab-lished to develop a demonstration system for a fieldtest. The first system, Geodar Mark I, was completedin March 1967, and an improved version, GeodarMark II, was completed a few months later [2].

The Geodar systems were tested on tunnels at FortBelvoir, Virginia, and at Raleigh, North Carolina, aswell as on voids implanted in a second Laboratory testrange constructed at Millstone Hill in Westford, Mas-sachusetts. The test results indicated that the Geodarsystems could locate tunnels of two to three feet in di-ameter at depths of up to approximately twenty feetin most alluvial, glacial, and loessial soils.

Several sets of Geodar Mark II were fabricated bySylvania West, and Lerner went with them to Viet-nam. Demonstrations there corroborated the earliertest data and predictions. For a time, the Geodar sys-tem was deployed for perimeter tunnel surveillancearound a U.S. Army Headquarters installation.

Hostile Weapons Location System (1974–1981)

The joint DARPA–U.S. Army Hostile Weapons Lo-cation System (HOWLS) program, which began in1974, focused on the development of techniques tolocate and classify stationary indirect-fire weapons inbackground clutter. In December 1974, the GeneralElectric Company was selected to build the HOWLSairborne radar. The HOWLS system consisted of aKu-band (16 GHz) array radar mounted in a twin-en-gine aircraft, a ground-station recording system, andan off-line processing system in a van [3]. The azi-muth resolution (real-beam) was 0.5° and the range

FIGURE 2. Field operation of the Geodar ground-penetrat-ing radar system. Initial testing of this system consisted ofdragging the antenna over a known test area to detect tun-nel-like voids under the surface. The antenna was con-nected to the radar equipment in the jeep by a cable.



resolution was ten meters, which was a high-resolu-tion system at that time. The program addressed twomajor issues: solve the automatic-target-detectionproblem, and implement this solution on a miniatureunmanned air vehicle (UAV) radar.

Figure 3 shows the airborne and ground-stationcomponents of the HOWLS experimental radar sys-tem. Figure 3(a) shows the HOWLS radar antennamounted under the fuselage of a twin-engine PiperNavajo aircraft. The radar was a Ku-band system witha 500-MHz radio-frequency (RF) bandwidth. Themaximum coherent-pulse bandwidth was 18 MHz(corresponding to a resolution of ten meters), and thecenter frequency could be stepped across the RFbandwidth in sixty-four steps. The system was used todevelop stationary-ground-target detection tech-niques with a spatial resolution of 10 × 10 m and asingle polarization. Figure 3(b) shows the interior ofthe ground-station van with the signal-processing and recording system. The HOWLS system provided the

information needed to design a lightweight and low-cost radar appropriate for mini-UAV applications.

Figure 4 shows a HOWLS ground-radar map ofStockbridge, New York, at 10 × 10-m resolution withtarget detections overlaid. The white squares form acalibration array, while the red squares are detectionsof targets such as eight-inch guns and armored ve-hicles. Speckle in the image was reduced by non-coherently averaging over sixteen independent fre-quencies. Targets in relatively open areas could bedetected, but the false-alarm rate was high from natu-ral and man-made clutter. Resolution was still inad-equate for acceptable classification of stationary tacti-cal targets.

One major accomplishment of HOWLS was thedevelopment of a new theory of target detection thatmore accurately predicted the detection performanceof moderate-resolution radar systems [4]. Earliertheories that used noise models to represent groundclutter gave very optimistic detection results, whichwere in error by one to two orders of magnitude. Thenew theory correctly took clutter inhomogeneity intoaccount, leading to more accurate predictions of per-formance. Figure 5 plots the probability of detectionof armored targets against the number of false alarmsexpected per square kilometer. The dashed black lineis the fit to the experimental data; the average target-

FIGURE 4. A HOWLS radar map of the ground atStockbridge, New York. The white squares are returns froma calibration array, while the red squares are detections oftargets such as eight-inch guns and armored vehicles.

FIGURE 3. (a) The airborne component of the DARPA–U.S.Army Hostile Weapons Location System (HOWLS) in-stalled in a twin-engine Piper Navajo aircraft with the arrayantenna mounted below the fuselage. (b) Data from thissensor were linked to a ground-station van for signal pro-cessing, recording, and display.

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with good resolution like the HOWLS radar wouldbe inadequate for target detection because the sensorwould be swamped with false alarms if it were set toachieve realistic target-detection probabilities. Toachieve the higher detection probabilities with a sig-nificantly lower false-alarm density required muchbetter resolution, such as that achievable with a syn-thetic-aperture radar (SAR) and a polarimetric-mea-surement capability to further distinguish targetsfrom natural and cultural clutter. The performancethat can be realized with such a system is discussedlater in this article in the section entitled “Stationary-Target Detection Employing High Resolution andPolarization.”

Netted Radar Program (1976–1981)

The Netted Radar Program began in 1976 to addressthe moving-target detection problem and to demon-strate that an operational system of distributed radarsconnected by a multiple-user network would providefor the first time a comprehensive view of the battle-field. In addition, the program showed that the U.S.Army’s AN/PPS-5 ground radars could be vastly im-proved by adding modern digital signal-processingtechniques originally developed by Lincoln Labora-tory for use in air-traffic control.

Figure 6 illustrates the Netted Radar System asdemonstrated at Fort Sill, Oklahoma. The networkutilized an airborne radar (a modified version of theHOWLS radar called the Advanced Airborne Radarwith moving-target detection and tracking capability[5]), two modified AN/PPS-5 ground-based radars,and a U.S. Army AN/TPQ-36 ground-based artil-lery-locating radar. The AN/PPS-5 radars wereequipped with modern signal-processing capabilitiesimplemented by Lincoln Laboratory, and were re-named AN/TPS-5X radars [6]. Figure 7(a) shows anAN/TPS-5X antenna looking over Fort Sill, Okla-homa, from nearby Mount Scott. The AN/TPS-5Xradar could detect and track tank-sized targets out toa range of about twenty kilometers. In addition to de-tecting moving ground vehicles, the AN/TPS-5X ra-dar also detected low-flying aircraft, rotating anten-nas, moving personnel, and artillery shell bursts. Itcould also estimate the azimuth positions of noisejammers.

FIGURE 5. Detection performance of HOWLS against sta-tionary targets in clutter. The assumption of a homogeneousground-clutter model predicts very optimistic results, asshown in the upper black curve for a target-to-clutter ratio(T/C) of 6 dB. The HOWLS radar data from experiments inStockbridge, New York, shown as red points, show the prob-ability of detection of armored targets versus the number offalse alarms per square kilometer. The curve-fit HOWLSdata, shown as a black dashed line, indicate a much less op-timistic performance for this medium-resolution radar thanthe homogeneous-clutter model, an error of about two or-ders of magnitude. A new theory developed by Leslie M.Novak at Lincoln Laboratory assumes a nonhomogeneous-clutter model, and predictions from this theory bracket theexperimental results quite well, as shown by the blue dashedlines for T/C = 5 dB and T/C = 8 dB [4].

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to-clutter ratio (T/C ) was 6 dB. The theoreticalcurves for T/C = 5 dB and T/C = 8 dB nicely bracketthe curve for the experimental data. The homoge-neous-clutter model (the top curve in Figure 5) pre-dicts 2 false alarms/km2 for a detection probability of0.8 and a T/C = 6 dB, while the experimental-datacurve shows 200 false alarms/km2 for the same condi-tions—an error of two orders of magnitude. TheHOWLS program demonstrated the potential ofboth automatic-target-detection techniques for sta-tionary targets and the utility of mini-UAV radar sys-tems with their excellent terrain visibility.

The HOWLS program was a pioneering wide-areabattlefield-surveillance effort. It proved that a sensor



All radars were netted together through a TargetIntegration Center, which in turn was connected tothe Army’s Tactical Artillery Fire Direction System(TACFIRE) computer center. The system providedexceptional coverage of the battlefield and greatly re-duced the delay between target detection and artilleryfire on the target. This demonstration clearly showedthe Army the advantages of tracking and reportingmoving targets with an automatic real-time system,compared to the Army’s established “man-in-the-loop” methods.

Figure 7(b) illustrates the operator display for theNetted Radar Demonstration. Each tracked target isautomatically assigned a target number that can beaccessed to give the target’s location and velocity. Forinstance, target 17 is a ground object moving at 12m/sec, while target 16 is a helicopter in the air. Thesystem also provided jammer location and artillery-fire registration by using range and azimuth triangu-lation techniques.

The Fort Sill demonstration clearly showed the ad-vantages of an airborne radar for terrain visibility.

Even in the moderately level terrain of Oklahoma,terrain masking was a serious problem for ground-based sensors, while virtually all targets were visible tothe airborne radar. The airborne radar component ofthe Fort Sill demonstration pointed to the value ofUAV radar with moving-target detection and track-ing capability.

In addition to rapid target acquisition, the advan-tages of combining target information with rapid dis-semination to artillery units were demonstrated inreal time. As an example of the speed and accuracy ofthe networked system, a tank, fortified to withstandhits with inert artillery rounds, was tracked by the ra-dar network as it moved along a roadway in the EastRange target area at Fort Sill. A firing time was com-puted at the Target Integration Center on the basis of(1) the predicted arrival time of the tank at the prede-termined aim point on the road, and (2) the esti-mated flight time of the projectile from gun to aimpoint. Because the artillery had been zeroed in on theaim point, and because the predicted projectile timeof arrival derived from the real-time measurements

FIGURE 6. Elements of the Netted Radar Program demonstration of moving-target detection.Data from the single airborne radar and three ground-based radars were sent over narrowbandVHF links to the Target Integration Center, where they were integrated with the Army’s TacticalArtillery Fire Direction System (TACFIRE) computer center. Fire missions then brought artil-lery fire against designated moving targets on roads in real time.

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was very accurate, the inert round hit the tank anddamaged one of the tank treads. This convincing re-sult demonstrated the improved performance of theAN/TPS-5X radars, the automatic target-acquisitionand tracking functions, the feasibility of radar net-ting, and the potential to provide for more accurateartillery-fire adjustment.

The basic objective of the Netted Radar Programwas to develop the technology for the netting of

battlefield radars. The AN/TPS-5X radars had severallimitations: they could not serve multiple users in adynamic situation, they were easily detected by theirscan motion, and the VHF radio system was not jam-resistant enough for a tactical configuration.

For these reasons, Lincoln Laboratory developedthe Advanced Ground Surveillance Radar (AGSR),which was incorporated as a parallel element of theNetted Radar Program. The AGSR was a moving-tar-get-indicator (MTI) radar. It featured a C-band cylin-drical array antenna, which was electronicallysteerable in azimuth over 360° and capable of simul-taneous multimode radar operation together with anintegral data link. [7] The fully coherent MTI AGSRwas demonstrated at Fort Sill from November 1980through January 1981, where it automatically trackedground vehicles, walking troops, and helicopters. Italso detected and accurately located artillery shellbursts.

The achievement of the Netted Radar Programwas the completely automated fusion of a number ofsurveillance radars in U.S. Army exercises under vari-ous field conditions. The system developed under thisprogram was conceptually similar to the Laboratory’sSAGE (Semi-Automatic Ground Environment) air-defense system developed in the 1950s [8]. The suc-cess of this program set the stage for the netting ofother operational military surveillance radars.

Unmanned-Air-Vehicle RadarProgram (1982–1991)

In 1982, under DARPA and U.S. Army sponsorship,Lincoln Laboratory began a UAV-radar developmentprogram to detect and classify moving targets with alow-power, lightweight radar designed for a UAVplatform. The system provided either a specific angu-lar sector or full 360° surveillance of moving groundvehicles and low-flying helicopters, as illustrated inFigure 8. MTI data were first processed onboard theUAV. Low-bandwidth data were then linked to aground station where precision track, location, andclassification of moving targets were performed andthe results were displayed [9].

Lincoln Laboratory researchers designed the UAVradar, using commercially available components forthe RF parts of the radar such as the transmitter, re-

FIGURE 7. (a) The radio-frequency (RF) portion of a U.S.Army AN/TPS-5X radar looking over Fort Sill, Oklahoma,from Mount Scott. A nearby radar support van housed thesignal-processing and data-processing equipment. (b) Anillustration of the operator display used in the Netted RadarDemonstration. The Fort Sill demonstration clearly showedthe advantages of an airborne radar for significantly reduc-ing terrain masking.

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ceiver, and antenna. One important issue was to pro-vide onboard MTI processing and to provide a low-bandwidth communication link to send the target re-ports to a ground station for classification andreal-time display. Since a suitable commercial proces-sor was not available at that time, the Laboratory de-veloped a state-of-the-art processor together with thenecessary signal-processing and data-processing algo-rithms. Figure 9 shows the equipment as mounted inan Amber UAV fuselage. Figure 10 shows how theentire system was form-fitted in the UAV fuselageand captive-carried on a Twin Otter aircraft for test-ing and evaluation. The radar could be operated fromthe ground station or independently by an operator

in the aircraft. The complex radar data were recordedon a 13-MB/sec recorder, so missions could be flownbeyond the reach of the ground station.

In the spring of 1990, a field demonstration wasconducted at Fort Sill and evaluated by the U.S.Army’s Intelligence School. As an example of real-time wide-area surveillance, Figure 11 shows the de-tection of virtually all of the moving targets found inless than three minutes within a 900-km2 area sur-rounding Fort Sill (above center in the figure) and thetown of Lawton, Oklahoma (just below center). Thedensity of moving targets is clearly visible from thenumber of MTI detections overlaid on the digitizedroad map. Notice the absence of detections or false

FIGURE 8. The unmanned-air-vehicle (UAV) surveillance and tracking radar concept. (a) The remotely con-trolled UAV carries a small coherent moving-target detection radar that provides surveillance of a large areaof the surrounding terrain and sends moving-target reports, generated by an onboard processor, via data linkto a ground station for display and target tracking. UAV position estimates are provided by a Global Position-ing System (GPS) receiver and are used to update an inertial-navigation system that provides filtered UAV po-sition (two dimensions), altitude, and aircraft attitude information. (b) With high look-down angles, the UAVradar has an excellent view of the surrounding terrain for detection and tracking. With its small visible signa-ture and low radar cross section, the UAV is difficult to locate and destroy with surface-fired weapons.

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FIGURE 11. UAV-radar wide-area MTI surveillance at FortSill, Oklahoma. An activity map of moving vehicles showsthe vehicular traffic near Fort Sill and the town of Lawton,Oklahoma. This map represents moving-target detectionsobtained from several 360° scans of the antenna.

FIGURE 9. The moving-target-indicator (MTI) radar mounted in the UAV fuselage. The open panels show thelocation of the radar components within the fuselage. The UAV, with a twenty-foot wing span, was developedby Leading Systems, Irvine, California, under DARPA sponsorship, and was identified by the name “Amber.”

FIGURE 10. The UAV-radar captive-carry flight-test configu-ration. The UAV fuselage was attached, without wings, tail,or engine, to a Twin Otter aircraft for testing. All the UAV ra-dar equipment was contained within the UAV’s fuselage.The Twin Otter aircraft flies at a speed comparable to that ofthe UAV.

alarms in the (restricted access) artillery ranges to thenortheast and northwest. The accuracy of targetplacement can be qualitatively measured by howclosely detections match the underlying road grid.

The radar was also required to supply moving-tar-get reports and tracks against a background map ofthe Fort Sill area. During a blind test, the radar opera-tor was asked to determine the number of vehicles,the speed, and the mix of wheeled and tracked ve-hicles in the convoys sent out by the evaluation team.The radar successfully reported location, speed, andcomposition of the convoys out to ranges of sixteen

kilometers. The field demonstrations were conductedjointly with the Army Research Laboratory. The com-bined efforts and the demonstration results con-vinced Army users of the value of such an MTI radaron a UAV. The Joint UAV Program Office is nowconsidering such applications.

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Lincoln Laboratory has made many significantcontributions to the improvement of airborne MTIsystems. Much of the pioneering work in the area ofsurface surveillance has contributed directly to the de-velopment of national assets such as the U.S. AirForce Joint Surveillance Target Attack Radar System(Joint STARS) airborne surveillance system. Thiswork included the development of improvements toMTI target-detection techniques, SAR imaging, andfixed-target detection, as discussed in this article. Amajor contribution in the area of airborne MTI clut-ter cancellation and target angle estimation was madewith the concept of the displaced-phase-center an-tenna (DPCA), which introduced the multiple-phase-center array concept. The details of DPCA arecovered in the article entitled “Displaced-Phase-Cen-ter Antenna Technique,” by Charles Edward Mueheand Melvin Labitt, in this issue.

Joint STARS is a long-range surface-surveillancemultiple-aperture array radar carried by U.S. AirForce E-8C aircraft. The wide-area surveillance andmoving-target indicator (WAS/MTI) are the radar’sfundamental operating modes. WAS/MTI is de-signed to detect, locate, and identify slow-moving tar-gets. Synthetic-aperture radar/fixed-target indicator(SAR/FTI) provides high-resolution SAR imaging forstationary-target detection and identification.

Stationary-Target Detection EmployingHigh Resolution and Polarization (1982–1996)

In parallel with the UAV MTI work, some of theLaboratory’s effort in tactical technology shifted todeveloping more capable techniques for stationary-target detection and understanding the performancelimits of these techniques. The Laboratory began theAdvanced Detection Technology (ADT) program in1982 under DARPA sponsorship in response to aDepartment of Defense need to examine the poten-tial of MTI radar for use in “smart weapons.” A verycapable instrumentation radar, called the AdvancedDetection Technology Sensor (ADTS), was built forLincoln Laboratory by Goodyear Aerospace. TheLaboratory used this dual-polarized sensor to capturethe full dimensionality of the radar signal. The radar,mounted in a Gulfstream II aircraft as shown in Fig-ure 12, provided well-calibrated, high-resolution,

FIGURE 12. The Lincoln Laboratory airborne Advanced De-tection Technology Sensor (ADTS) had X-band, Ku-band,and Ka-band (monostatic and bistatic) synthetic-apertureradar (SAR) modes. Either (a) the dual-band (X and Ku) an-tenna (reflector type) or (b) the Ka-band horn antenna ismounted in (c) the radome below the Gulfstream II aircraftfuselage, depending on the data-collection requirements.

fully polarimetric, real or synthetic-aperture data.Originally built for Ka-band (33 GHz) operation[10], the radar was later modified to include X andKu-bands together with a bistatic capability as well.

This radar and the associated research programshave played a vital role in the national effort to de-velop automatic target recognition (ATR) of station-ary ground targets. The airborne data acquired by theradar in Figure 12 have been a principal source for de-velopment in the ATR research community. For ex-ample, the high-resolution SAR map of the site inStockbridge, New York, displayed in Figure 13(a)shows targets in typical background clutter. Figure13(b) shows the declared detections that result fromprocessing the data represented in Figure 13(a). Fig-ure 13(c) shows the results of an end-to-end perfor-mance study using a baseline SAR ATR algorithmsuite. These results have formed the fundamental un-derstanding of the value of SAR resolution and mul-tiple polarizations in ATR.

Figure 13(c) summarizes the fundamental trade-offs between false-alarm density versus probability of

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FIGURE 13. (a) The ADTS high-resolution SAR map of theStockbridge, New York, site covered approximately the samegeographical area of natural and cultural clutter as shown inthe low-resolution map in Figure 4. In this high-resolutionSAR map, light pixels indicate strong radar reflections. Lo-cated on the right side of the image is a region containingvarious tactical targets. On the left side of the image is alarge maintenance area containing buildings, vehicles, air-craft, and other man-made objects. (b) Declared detectionsare indicated as white markers on the ADTS SAR map ofthe Stockbridge site. Note the relative absence of false de-tections in the natural-clutter areas. (c) SAR automatic-tar-get-recognition (ATR) detection performance results versuspolarization and resolution for the Stockbridge site. HH rep-resents horizontal polarization (on both transmit and re-ceive), and PWF represents the polarimetric whitening filter.

detection for single and multiple polarizations (opti-mally combined) and for various resolutions [11, 12].Algorithms developed under these programs wereused in the DARPA Semi-Automated IMINT (ImageIntelligence) Program, or SAIP.

Over four hundred data-collection missions were

flown with the ADTS aircraft, satisfying a variety ofmission objectives relating to (1) detection and classi-fication of stationary tactical targets, (2) internal sea-wave detection for antisubmarine warfare, (3) bistaticphenomenology for missile-seeker applications, and(4) SAR for moving-target imaging. Following an ac-tive and productive lifetime, operations with theADTS sensor ended in late 1998.

Synthetic-Aperture Foliage-PenetrationRadars (1987–1996)

Synthetic-aperture foliage-penetration (FOPEN) ra-dars have been developed because of the need to findstationary tactical targets located in deep foliage andnatural camouflage, which hide these targets fromconventional microwave surveillance sensors. Duringthe Vietnam era, as noted earlier, military personnelsuccessfully used the Camp Sentinel ground-based ra-dar to detect enemy soldiers and vehicles moving infoliage. To locate stationary targets, researchers alsoneeded to develop a low-frequency foliage-penetra-tion SAR imaging capability with sufficient resolu-tion to differentiate targets from clutter. Unfortu-nately, no reliable target-recognition algorithms wereavailable at that time to reduce false alarms. Many ofthe currently successful ATR and cueing techniquesfor detecting moving and stationary targets have beendeveloped to be used only for targets in the open.

Since the late 1980s, Lincoln Laboratory, underDARPA and U.S. Air Force sponsorship, has plannedand conducted a number of experiments and data-collection programs utilizing a variety of industry-built sensors to evaluate the use of low-frequency ra-dar to detect and identify tactical targets hidden byfoliage. The results from these efforts have been usedto develop the current automatic target-detection andcueing algorithms that operate on UHF and VHFSAR data. When we use a SAR system to detect ob-jects hidden or obscured by foliage, detection is de-graded in three ways for the higher microwave fre-quencies (>100 MHz). First, the foliage contributesto the clutter return. Second, the foliage attenuatessignal propagation through it. And third, the movingfoliage induces fluctuations in the amplitude andphase of the radar signal, which distort the SAR im-age of the target. These fluctuations affect the image-

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focusing quality and the detection performance ofSAR for targets hidden by foliage.

To obtain a better understanding of the foliage ef-fects, researchers needed an accurate quantitative as-sessment of these issues. In 1990, Lincoln Laboratoryconducted a definitive experiment to measure foliageattenuation and backscatter of heavily forested areas.This experiment was conducted with the NASA/JetPropulsion Laboratory AIRSAR aircraft, which hasUHF, L-band, and C-band SAR radars. Figure 14shows this aircraft equipped with these systems.

The Laboratory conducted tests by overflying aforest site in northern Maine. The site had been care-fully mapped and implemented with a variety of cali-bration reflectors and devices. Phase and amplitudedata from the experiment were coherently integratedto create synthetic-aperture azimuthal patterns thatwould result when imaging a point target obscured bythe foliage. The effects of synthetic-aperture length,frequency, and polarization on the attenuation andazimuthal synthetic beam pattern were investigated.Measurements also showed that less than a one-meterresolution for foliage penetration could be achieved atUHF frequencies. The results, shown in Figure 15,demonstrate the well-defined synthetic-aperture azi-muthal patterns that can be generated even down toresolutions as small as 0.6 meters. This result was cer-tainly a surprise to the conventional wisdom of thetime, and it launched a major Department of Defenseeffort in FOPEN radar technology.

In the past decade, this FOPEN work has been ex-tended to develop a phenomenological understand-ing of foliage penetration and to develop advanced

FIGURE 15. Foliage-penetration (FOPEN) results demon-strate that SAR azimuthal patterns can be generated with in-creasing resolution as small as 0.6 m, which indicates theability of FOPEN systems to form accurate SAR beamsthrough foliage.

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automatic-target-detection and recognition tech-niques. Additional foliage-penetration measurementswere made in 1993 in tropical rain forest and north-ern U.S. forest environments by using the Swedish3-m-resolution Coherent All Radio Band Sensor(CARABAS) and the Stanford Research Instituteultrawideband (UWB) SAR sensor collecting hori-zontal-polarization VHF and UHF data. The datacollected in these experiments were processed intocalibrated SAR imagery, and foliage-induced attenua-tion for all frequency bands was calculated by com-parison of echoes from test reflectors in foliage andthose in the open [13–15].

In support of the current DARPA-sponsored foli-age-penetration SAR project, Lincoln Laboratory iscurrently developing automatic-target-detection andcueing algorithms for VHF and UHF radar. With thedetection of targets now possible with our existing fo-liage-penetration capability, the remaining issue be-comes the identification of threatening targets fromall of the many detections that are reported. Twofalse-alarm-mitigation techniques—change detectionand group detection—have been used to reduce thesefalse alarms substantially.

Wideband FOPEN systems are needed to detecttargets reliably in the presence of foliage, radio inter-ference, and cultural clutter. This requirement has

FIGURE 14. The NASA/Jet Propulsion Laboratory AIRSARaircraft containing the UHF, L-band, and C-band SAR ra-dars. This aircraft was used to measure and characterize ra-dar attenuation and backscatter caused by foliage in heavilyforested areas.



motivated the development of a UWB SAR operatingover 215 to 730 MHz. Its resolution is 0.3 m in rangeand 0.6 m in cross-range; and it has a full range ofpolarizations. The radar was built by the Environ-mental Research Institute of Michigan and installedon a U.S. Navy P-3 aircraft controlled by the NavalAir Warfare Center. This test-bed sensor, funded bythe Air Force Wright Laboratory, was flown in 1995to collect data on tactical targets in geographically di-verse foliage over some 1500 km2 of foliated terrain.Lincoln Laboratory has analyzed these data to deter-mine the system requirements for an operational sys-tem and to identify suitable concepts to be employedwith a tactical platform. Preliminary results from theUWB data suggest that a resolution finer than onemeter is required to reduce false alarms from trees andto provide a sufficient number of independent pixelsfor recognition of potential targets in a radar image.At UHF frequencies, this requirement implies weneed SAR azimuth integration angles greater than 35°with correspondingly long integration times. LincolnLaboratory has also designed a new coherent classifierto use the complex polarimetric UHF data. The clas-sifier performs well on targets in the open, and it cor-rectly classifies a significant portion of targets hiddenunder foliage.

Recently, many researchers have been interested inthe use of an airborne SAR for detection of under-ground targets, large and small, such as mines andtrucks hidden in underground bunkers. In support ofthis interest, a ground-penetration experiment wasconducted in 1993 near Yuma, Arizona. Again, a vari-ety of radars were used, covering the RF band from20 to 500 MHz. Data from this test helped research-ers to develop a phenomenological understanding ofsoil-penetration losses and clutter backscatter, and toinvestigate the signatures of buried targets [16].

Summary

Lincoln Laboratory has had a major influence overthe past thirty years on the science and technology ofbattlefield surveillance for stationary and moving tar-gets. Major developments and tests of airborne andground sensors have been accomplished.

In all these efforts the goals have been to (1) under-stand the fundamental radar and processing issues in-

volved, (2) determine the theoretical performancelimits based on scientific principles and state-of-the-art technology, and (3) confirm those limits with fielddemonstrations. Many problems still remain inbattlefield surveillance, and the ongoing program atthe Laboratory continues to address these challenges.

R E F E R E N C E S1. E.C. Freeman, ed., MIT Lincoln Laboratory: Technology in the

National Interest (Lincoln Laboratory, Lexington, Mass.,1995), pp. 153–155.

2. MIT Lincoln Laboratory: Technology in the National Interest,p. 156.

3. V.L. Lynn, “HOWLS Radar Development,” in MillimeterWave Radars, S.L. Johnston, ed. (Artech House, Dedham,Mass., 1980), pp. 621–634.

4. L.M. Novak and F.W. Vote, “Millimeter Airborne Radar Tar-get Detection and Selection Techniques,” IEEE NAECON-79Conf. Record 2, Dayton, Ohio, 15–17 May 1979, pp. 807–817.

5. MIT Lincoln Laboratory: Technology in the National Interest,p. 160.

6. M.I. Mirkin, C.E. Schwartz, and S. Spoerri, “AutomatedTracking with Netted Ground Surveillance Radars,” IEEE1980 Int. Radar Conf., Arlington, Va., 28–30 Apr. 1980, pp.371–379.

7. J.-C. Sureau, “Applications of Cylindrical Arrays to Surveil-lance Radars,” Int. Conf. on Radar, Paris, 4–8 Dec. 1978, pp.337–343.

8. MIT Lincoln Laboratory: Technology in the National Interest,pp. 15–33.

9. MIT Lincoln Laboratory: Technology in the National Interest,pp. 158–159.

10. J.C. Henry, “The Lincoln Laboratory 33 GHz Airborne Pola-rimetric SAR Imaging System,” IEEE National TelesystemsConf. Proc., Atlanta, 26–27 Mar. 1991, pp. 353–358.

11. L.M. Novak, M.C. Burl, and W.W. Irving, “Optimal Polari-metric Processing for Enhanced Target Detection,” IEEETrans. Aerosp. Electron. Syst. 29 (1), 1993, pp. 234–244.

12. L.M. Novak, S.D. Halversen, G.J. Owirka, and M. Hiett, “Ef-fects of Polarization and Resolution on the Performance of aSAR Automatic Target Recognition System,” Linc. Lab. J. 8(1), 1995, pp. 49–68.

13. J.G. Fleischman, S. Ayasli, E.M. Adams, and D.R. Gosslen,“The July 1990 Foliage Penetration Experiment, Part I: FoliageAttenuation and Backscatter Analysis of SAR Imagery,” IEEETrans. Aerosp. Electron. Syst. 32 (1), 1996, pp. 135–144.

14. M.F. Toups, S. Ayasli, and J.G. Fleischman, “The July 1990Foliage Penetration Experiment, Part II: Analysis of Foliage-Induced Synthetic Pattern Distortions,” IEEE Trans. Aerosp.Electron. Syst. 32 (1), 1996, pp. 145–155.

15. B.T. Binder, M.F. Toups, S. Ayasli, and E.M. Adams, “SARFoliage Penetration Phenomenology of Tropical Rain Forestand Northern U.S. Forest,” IEEE Int. Radar Conf., Alexandria,Va., 8–11 May 1995, pp. 158–163.

16. M.I. Mirkin, T.O. Grosch, T.J. Murphy, S. Ayasli, H. Hellsten,R. Vickers, and J.M. Ralston, “Results of the June 1993 YumaGround Penetration Experiment,” SPIE 2217, 1994, pp. 4–15.



. was leader of the SurveillanceSystems group until he retiredfrom Lincoln Laboratory in1996. He is now working forthe Laboratory as a consultant.His research interests includeradars and signal processing.Before joining the Laboratoryin 1966, he worked for VarianAssociates. He received a B.S.degree in physics from North-eastern University and anM.S. degree in physics fromRensselaer PolytechnicInstitute.

. is a staff member in the Sur-veillance Systems group. Hejoined Lincoln Laboratory in1968. His background is inradar systems and signal pro-cessing. His current interest isin airborne synthetic-apertureradar for surface surveillanceand target classification.He received a B.S.E.E. degreein 1966 and an M.S.E.E.degree in 1968 from theUniversity of Maine. He is asenior member of the IEEE,Eta Kappa Nu, Tau Beta Phi,Phi Kappa Phi, and Sigma Xi.

. is a senior staff member in theSurveillance Systems group.He received a B.S.E.E. degreefrom Fairleigh DickinsonUniversity in 1961, anM.S.E.E. degree from theUniversity of Southern Cali-fornia in 1963, and a Ph.D.degree in electrical engineeringfrom the University of Califor-nia, Los Angeles, in 1971.Since 1977 he has been amember of the technical staffat Lincoln Laboratory, wherehe has studied the detection,discrimination, and classifica-tion of radar targets. He hascontributed chapters on sto-chastic observer theory tovolumes 9 and 12 of the seriesAdvances in Control Theory,and in 2000 he was elected aFellow of the IEEE.

. was an associate leader of theSurveillance Systems groupuntil he retired from LincolnLaboratory in 1999. Hisbackground is in radar-systemdesign and signal processing.His current interest is in theidentification of movingtargets by using high-resolu-tion range profiles and inverse-synthetic-aperture radar im-ages. He received his B.S.E.E.degree from the University ofRhode Island and his S.M.E.E.degree from MIT. He has beenat Lincoln Laboratory since1971.

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