c. W. Davis III The Airborne Seeker Test Bed
The Airborne Seeker Test Bed is a recently operational instrumentation system containing a closed-loop tracking, semi-active seekerwiththe capabilityto record high fidelity signals pertaining to radar seeker phenomenology, target scattering character istics, electronic countermeasures, and acquisition and tracking performance. The unique capabilities of the test bed will be used to collect data and develop computer models for evaluating and predicting missile performance. Test bed data will be used to evaluate the susceptibility of U.S. aircraft to missile attack, and to explore new directions for future systems. The test bed is also designed to supportthe development of advanced seekers and new electronic counter-countermeasure techniques, and to demonstrate their capabilities in flight.
Major problems in missile seeker design are systems. Specific features have been designed target-detection sensitivity and the effects of andincorporatedinto the testbedto supportthe ground clutter and modem electronic counter development of techniques for electronic measures (ECM). The low radar cross sectionof counter-countermeasures (ECCM), and to weapons such as cruise missiles and future demonstrate those techniques in flight. fighterand bomberaircraft, aswell as the ability The seeker is the system on the missile that ofmodem targets to fly at low altitudes, makes performs the on-board target sensing for flight the missile intercept problems even more diffi gUidance, with the ultimate purpose ofbringing cult. In addition, the effectiveness of certain the intercepting missile's warhead within a le modem countermeasures at degrading missile thal radius ofthe target. Becauseofadvancesin performanceisnotwell understood. Theseprob radar cross-section reduction techniques and lems combine to presentstressing challenges to ECM, future seekers will face increasingly so current missile defenses. phisticated threats. In fact, the advent of low Lincoln Laboratory has undertaken a signifi radar ~ross-section air vehicles has made cant effort to help solve these seeker problems, countermeasures more attractive because the with the initial emphasis on radar-guided mis radiated power necessary to mask a vehicle's siles. The central element in this effort is the radar return has decreased to the point where Airborne SeekerTestBed, a flyinginstrumenta small countermeasure devices are now tion system that carries a closed-loop tracking practical. seeker and also records high-fidelity signals re Future radar seekers will require higher lated to radar phenomenology (clutter, multi sensitivity and more effective clutter rejection. path), target scattering characteristics (bistatic These seekers will probably incorporate dual radar cross section, scintillation statistics, polarizationsensors andmultispectralsensors, angleglint), ECM, and overall seekeracquisition such as a combination of infrared and radar. and tracking performance. The purpose behind Thefuture enhancementswill helpdiscriminate the developmentofthese capabilitiesis to collect and reject false targets, including decoys. As a data and to develop computer models that will consequence, the burden ofdecision making on assist in the design offuture seekers and in the board the seeker will increase, as will the asso prediction of missile performance. Measured ciated compleXity in signal processing. The data from the test bed will be used to evaluate system architecture oftheAirborne SeekerTest the susceptibility of U.S. aircraft to missile at Bed was selected to address this set of seeker tack and to investigate new concepts for future problems.
The Lincoln Laboratory Journal, Volume 3. Number 2 (1990) 203 Davis - The Airborne SeekerTest Bed
Seeker Performance Issues ever, because the accelerations of target maneuvering smear the signal and make The ability ofa rriissile to intercept a target is the integration ineffective beyond a cer often limited by seeker sensitivity. ground-clut tain period. ter rejection, or ECM immunity. The specific Significant performance gains are difficult to waveform used by the seeker, which provides achieve, as indicated in the first three categories varying degrees of resolution in Doppler fre listed above. Even the gains created by a longer quency and range, also has performance impli signal integration period could be insufficient cations. These subjects are reviewed in the for future low cross-section targets. A possible following paragraphs. solution to the sensitivity problem is to launch the missile and gUide it remotely by a powerful Seeker Sensitivity command-guidance system out to a range close enough for the seeker to detect the target. The Low cross-section targets tax the capabilities command-guidance radar would be a larger ofintercepting missiles by requiring them to be system that could solve the sensitivity problem more sensitive to target detection in the pres by increasing transmitter power and/or an ence of natural thermal noise. The following tenna gain. The seeker in these systems would methods canincrease the sensitivity ofa missile need the capability to acquire the targetautono system: mously, since the command gUidance may be 1. Increase transmitter power. For example, unable to put the seeker onto the target directly. doubling the transmitter power would in Whatever missile system architecture is con crease the free-space detection range by a sidered, the seeker performance implications of factor of 1.2 on a given target. the low cross-section threat, and concepts for 2. Increase transmitter and/or receiver an possible improvements, provide a significant tenna gains. Doubling the dimensions of challenge. one of the antennas would increase the free-space detection range by a factor of Ground Clutter 1.4 on a given target. In practice, the narrow diameter of a missile restricts Even ifthe seekerpossesses enough sensitiv antenna size, which also restricts achiev ity. the ground clutter can limit performance by able antenna gain. Typical air-to-air masking the target return. Figure 1 illustrates missile seeker antennas are 5 to 7 in the clutter environment as viewed by the mis in diameter; similar antennas for sur sile. The specific case shown is for a semi-active face-to-air missiles range from 9 to 16 in. surface-to-air missile with continuous-wave 3. Lower the receiver noisejl.oor. Current sys (CW) illumination (see the box titled "Radar tems have noise floors within 4 to 10 dB of Guided Missiles" for a definition of missile theoretical limits. A 3-dB improvement types). For outbound targets (the tail chase (which is a challenge to achieve) would in scenario), ground clutter seen through the an crease the free-space detection range by a tenna sidelobes directly obscures the target factor of 1.2 on a given target. return. For inbound targets, the target return 4. Increase the received signal integration pe competes with the noise sidebands from the riod. A longer signal integration period missile's receiver oscillators (and other system would reduce the effect of thermal noise. specific sources), which spill into what would Faster signal processing could lead to in have been a clutter-free portion of the Doppler tegration periods up to 10 times longer spectrum. The level of these noise sidebands is than current systems. an increase in inte proportional to the strongest signal (usually gration that could extend the free-space main-beam clutter) in the receiver. Ifthe target detection range by a factor of 1.8. The is high enoughin Dopplerfrequency (which cor benefits of integration are limited. how- responds to a high missile-to-target closing ve-
204 The Lincoln Laboratory Journal. Volume 3. Number 2 (l990) Davis - The Airborne Seeker Test Bed locity), the target will appear in a region of the an idiosyncratic Achilles' heel in the threat spectrum where the noise sidebands have system. For example, a missile that relies on reached a floor level. In low-clutter situations three consecutive signal bursts to perform tar this floor level is the thermal noise floor of the get-angle measurement can be confused by a receiver. distorted or amplified signal sent by the target Figure 1 indicates that the missile is more every third burst. The idiosyncratic techniques capable of intercepting targets in the incoming generally exploit some vulnerable characteris target region than in the outgoing target region. tic of the victim's receiver architecture, signal Therefore, air vehicle designers must generally processing technique, or control logic emphasize lowering the nose cross section to (and consequently require a knowledge of the enhance a vehicle's ability to penetrate radar characteristic). These ECM techniques are gen defenses. erally classified, since the enemy can eliminate Signal integration reduces the effective level the specific vulnerability if the weakness being ofthe noise sidebands, with respectto the target exploited is known. signal, and reduces the sidelobe clutter. The Other more fundamental techniques are dif seeker designer further combats the effects of ficult to overcome, even ifthe enemy has knowl clutterbyattemptingto achieve lowsidelobes on edge ofthem. A decoy deployed by the target, for the antenna and high oscillator stability (low example, can be an actual radar target, physi noise sidebands) in the receiver. cally separated from the true target. Since the decoy is a real target, the seeker cannot elimi nate it by a simple change in processing al Electronic Countermeasures gorithm. We must devise a more compli Many categories of ECM currently exist. cated method to discriminate the true target Certain ECM techniques are designed to exploit from the decoy. In addition to expendable or
Missile
Radar Antenna Target Outbound ....:::::Siii~Llnbound /
Inbound Target
Doppler Frequency
Fig. 1-Received clutter spectrum for a semi-active missile. The clutter return spreads out in Doppler frequency; clutter approachedby the missile has a positive Doppler, clutterdirectly beneath the missile has zero Doppler, and clutter behind the missile has a negative Doppler. Incoming targets appear at a higher Doppler frequency than any clutter; outbound targets have a Doppler frequency that appears in the sidelobe clutter region.
The Lincoln Laboratory Joumal. Volume 3. Number 2 (1990) 205 Davis - The Airborne Seeker Test Bed
Radar-Guided Missiles
Radar-guided missilesexistin aperture. the radarona missile is ofthe missile interceptcan be ar four basic categories: command not as powerful as a ground ranged to minimize the effects of guided. active homin<1. semi-ac based or aircraft-mounted radar. clutter. The missile antenna may tive homin<1, and passive homing. To achieve long ranges on low be looking down at the earth. but A missile system canbe designed cross-section tar<1ets. active the earth is shielded from the to use each of these methods in homin<1 must be combined with illuminator by intervening ter combination. Forexample. a sys other means such as command rain. A further advantage of the tem can have commandgUidance gUidance to getthe missilewithin semi-active architecture is that for most of the missile fly-out. homingrange to the tar<1et. Active electronic countermeasures in followed bysemi-active homingin missiles have the attractive fea tended to frustrate themissileare the terminal phase of fli<1ht. tures of fve-and-jorget. which often directed back toward the A command-guided missile can increase the fIre power of a radar source. The missile is not has neither a radar transmitter given fire control system. Adisad radiating. so the tar<1et will not nor a radar receiver on board. A vantage of active missile seekers know the missile's location and separate radar (usually ground is higher cost. since a radar may not be able tojamthe missile based) tracks both the tar<1et and transmitter is reqUired in the seeker. the outgoing missile and com missile. Figure A also indicates the ex putes the trajectory chan<1es A high proportion of the istence ofa rear reference signal. needed to <1uide the missile to its world's radar missile inventories For a semi-active seeker to sup tar<1et. These flight commands use a semi-active architecture. a port coherent si<1nal proceSSing are communicated to the missile scheme in which the missile car (such as Doppler llitering) the by a data link. Theaccuracyofthe ries only a radar receiver. not a missile must either carry a stable missile intercept is limited by the transmitter (see Fi<1. A). The radar frequency re~rence or have a precision with which the radar transmitter that illuminates the receiver dedicated to listening to can determine the target and target is in a separate unit that is the direct si<1naI from the illumi missile locations. This precision either Q'round based or airborne. nator (the rear receiver). degrades as the range to the This architecture has several A passive homing seeker intercept increases. leadin<1 to advantages. Delivering sufficient gUides itself by radio emissions larger miss distances. Command radar energy to a target at long from the tar<1et. These emissions <1uidance may be necessarywhen ran<1es requires high transmitter are from the target's own radar or other modes of missile <1uidance power and hi<1h-gain antennas otheron-board radiatingsensors. are inadequate because of clut (thus requirin<1 a large antenna The advantages of passive hom ter. jamming. or missile receiver aperture). both ofwhich are diffI ing are that it does not require a sensitivity problems. Many for cult to achieve on a missile con separate illuminationradarand it eign missiles employ command strained in wei<1ht and volume. operates qUietly (a powerful illu gUidance modes because a com Stronger illumination is more mination signal recognized by the mand-<1uided missile does not easily provided by a Q'round target is a warning of the immi require a complex on-board based system or. to a lesser ex nent arrival ofa missile). A disad seeker system. tent. by a fIghter aircraft. This vantage isthatthe passive seeker Missile homing gUidance is advantage isespeciallyimportant depends on the presence oftarget needed to achieve a smaller miss when the target has a low radar emissions durin<1 homing. and distance. which is especially return. which requires hi<1her il these emissions are not con important for air-to-air missiles luminator powers to achieve tar trolled by the missile system. that carry small warheads. An get detection. Another disadvantage is that the active missile carries its own Another advantage of the seeker must operate with a vari radar. complete with transmitter semi-active approach illustrated ety ofdifferentwaveforms thatare and receiver (e.g.. the U.S in Fig. A is that for surface-to-air specific to particular targets. and AMRAAM). Because of a small missiles. which use a ground these waveforms are not optimal payload capacity and antenna based illuminator. the geometry for missile homin<1. towed decoys. other fundamental techniques of target angle, and terrain bounce jam include wavefront distortion (known as cross ming, in which a brightly illuminated eye), for corrupting the seeker's measurement spot on the ground is created to draw the
206 The Lincoln Laboratory Joumal. Volume 3. Number 2 (I 990) Davis - The Airborne Seeker Test Bed
Illuminated t Illumination Terrain Clutter Radar Horizon
Fig. A-Asemi-active interceptscenario. The missile homesin on the reflections ofthe illuminatorsignal offthe target. Ifthe intercept occurs beyondthe illuminator's clutterhorizon, the missile will benefitfrom a reduction ofground clutter.
missile away from its target. technique that diminishes both the mainlobe These fundamental, or robust, countermea clutter and sidelobe clutter. The pulsed wave sures are not as implementation-specific as the form design is ultimatelya compromise between idiosyncratic type of ECM described above. target Dopplerfrequency ambigUities and target Because of their fundamental nature, robust range ambigUities. The measured Doppler fre countermeasures can be investigated generi quency and range values are offset from their cally; thatis, experiments can be performed that absolute values by integer multiples of an do not require specific military ECM hardware. ambigUity interval. The size of the ambigUity Aprincipal challengefor future seekerdesigners interval is determined by the pulse-repetition is to devise schemes for dealing with these fun frequency. The larger the unambiguous range damental types of ECM. interval, the smaller the unambiguous Doppler interval, and vice versa. The use of multiple Waveform Selection pulse-repetition frequencies is reqUired to re solve these ambigUities. Most semi-active missiles operate with CWor A pulsed waveform is more difficult for a interrupted-CW illumination. These waveforms semi-active missile to process because the allow the use of Doppler filtering to separate receiver must synchronize itself with the target and clutter. The CW waveforms provide range-gate timing. The U.S. Patriot missile ex the seeker with no range information on the emplifies one method for achieving this syn target; the Doppler frequency uniquely deter chronization by sending the received signal mines the relative velocity of the target. back to the illuminator at the ground (the A pulsed waveform coupled with a range source of the timing) for target processing, gated receiver is used to introduce additional rather than performing the necessary pro separation of target and clutter in the range cessing on board. Despite this complexity, dimension. Figure 2 illustrates the reduction in however, future seekers will probably in clutter that can be achieved by range gating, a corporate pulsed waveforms and range-
The Lincoln Laboratory Joumal. Volume 3. Number 2 (J 990) 207 Davis - The Airborne SeekerTest Bed
(a) Clutter Geometry Signal Spectrum (Plan View)
Sidelobe Frequency Clutter Area at Target Doppler
(b) Clutter Geometry Signal Spectrum (Plan View) for Selected Range Gate
Frequency
Fig. 2-A comparison ofthe clutterresolution ofcontinuous-wave (CW) andpulsedwaveforms. (a) Forthe C W waveform, differentDopplerbins in the clutterspectrum divide the terrain into strips ofconstantDoppler frequency (called isodops). The area ofground in one ofthese strips multipliedby the antenna gains and clutter reflectivity value determines the strength of the clutter signal in that Doppler bin. (b) In a pulsed Doppler waveform, consecutive signal samples represent clutter returns for different ranges from the seeker. When coupled with the resolution provided by Doppler binning, the clutter level in a given range Dopplercell is generally reduced in comparison to CWmethods because the ground area in a cell is less. gated receivers to reduce clutter problems. The test bed instrumentation was de signed with enough sensitivity for the Airborne Seeker Test Bed study of low cross-section targets. It pro vides dynamic in-flight dual-polarized Figure 3 shows the Lincoln Laboratory Air measurements of the target radar cross borne Seeker Test Bed that was designed to section, scintillation (amplitude fluctua support investigations into the problems de tions), and glint (an interference effect that scribed in the previous section. Direct appli induces large angle-measurement errors), cations of the test bed are in the following all measured at transmitter and receiver areas: angles representative of missile intercept 1. Target radar cross-section measurement. geometries.
208 The Lincoln Laboratory Joumal. Volume 3. Number 2 (1990) Davis - The Airborne seeker Test Bed
2. Bistatic clutter database measurement. A generated by selected ECM. These mea dual-polarized database of bistatic (differ surements directly characterize the ECM ent angles to the transmitter and receiver) signals. and the measureddatacanalso be ground cluttercanbe developed for sites of used as input to seeker system simula interest and used in clutter modeling. tions that predict the ECM effect on the Clutter measurements also support live modeled missile system. firing tests byhelpingto selectthe firing ge 5. ECCMalgorithmdevelopment. Anotheruse ometries. for the ECM signal data is to look for dis 3. Measurement oj operational scenarios oj criminants. or signal features that can be targets in clutter. Target and test bed flight used to separate false signalsfrom the true paths are chosen to simulate realistic in target. New ECCM algorithmswill be devel tercept trajectories for evaluating target oped and tested on the measured ECM detectability and trackability. data. These algorithms can then be tested 4. ECM evaluation. The test bed has a high in real time on the test bed dUring a simu fidelity capability to measure the signals lated intercept.
FUR (8-12/lm)
Fig. 3-Principalelements ofthe Airborne Seeker Test Bedon the Falcon 20aircraft. The principalsensor, a large X-band dual-polarized monopulse antenna, is supported by a large number of instrumentation channels and a high-speed recorder. A Forward Looking InfraredSensor (FUR) provides an angle reference on targetposition for use with the radar data. A wing-mounted C-band radar locates the target and directs the other sensors.
The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 209 Davis - The Airborne Seeker Test Bed
The data obtained through the five activities achieved on the test bed by extending the inte listed abovewill be applied to the developmentof gration time; in fact, Doppler resolutions more computer models of seeker performance, from than five times finer than those of a missile are phenomenology models (clutter, target radar practical. Note that the target spectrum is not cross-section dynamics) to missile fly-out mod affected by test bed speed, and the limits on els with six degrees of freedom. From these targetintegration time due to targetacceleration models we will extrapolate measured results to apply without scaling. terrains other than military test ranges and These important improvements in perfor evaluate the performance of new concepts for mance have been implemented so that the mea future missile systems. sured data will be more precise than the data The test bed represents a captive-cany con available to a practical missile system. To model cept (the box titled "Methods for Evaluating existing systems the test bed data will be de Missile Performance" reviews missile perform graded to match the performance ofthe system ance categories). A principal advantage of a being studied. The original unspoiled data are captive-carry experimentis that it allows opera important both to the phenomenology studies tion in the actual real-world environment and (clutter and target signature) and to illustrate offers the possibility of repeatable trajectories achievable seeker performance. and systematic profiling. Aspecific advantage of the Airborne Seeker Test Bed is that using a Overview oJTest Bed Hardware dedicated passengerjet (instead of pod-mount ing the sensor equipment on a military jet) of Figure 3 shows the major elements of the fers room for high-fidelity instrumentation and AirborneSeekerTestBed. The principal payload recording with operator interaction. of the Airborne Seeker Test Bed is the Instru Whenever possible, the elements of the test mentation Head (IH), which is an X-band serni bed instrumentation were designed for better active radar receiver configured as a missile performance than the corresponding elements seeker. The principal sensor of the IH is a large ofan actual missile. This level ofperformance is dual-polarized antenna mounted on a modified possible because of the advantages offered by HAWK seeker gimbal. Behind the antenna are a the test bed platform. The nose-mounted pri large number of instrumented channels. Ray mary antenna allows the use ofa hemispherical theon Missile Systems Division in Tewksbury, radome that minimizes polarization and angle Mass., served as subcontractor on the IH and measurement distortion, and facilitates a large the Forward Looking Infrared System (FUR) antenna aperture to provide increased sensitiv discussed later in the paper. ity. The benign vibration environment provided The IH receiver was designed to accommo by the jet platform supports better reference date a variety ofdifferent waveform types, rang oscillator stability to improve the clutter rejec ing from CW waveforms to experimental range tion performance. Signal integration can occur gated (pulsed) waveforms. This capability over longer time periods, which further in makes the IH compatible with existingillumina creases the sensitivity and clutter rejection of tors such as the HAWK High Power Illuminator the system. (HPI), the AWG-9 (on the F-14 fighter), and the The slower speed of the test bed leads to a APG-63 and APG-70 (on the F-15 fighter), as compressed clutter spectrum because the Dop well as experimental and simulator radars. pler spread ofground clutter is proportional to The Georgia Tech Research Institute (GTRI) the aircraftvelocity. Thevelocityofthe Falcon 20 constructed an experimental radar called is 2.5 times slower than the velocity of a typical the Waveform Simulator for use with the test missile. For the number of Doppler cells across bed, and it can generate every waveform the clutter to be comparable to a missile, the usable by the IH. A separate radio data link Doppler resolution in the test bed must be 2.5 from the illuminator synchronizes the times finer than in the missile. This resolution is IH receiver when the test bed is used with
210 The Lincoln Laboratory Journal. Volume 3. Number 2 () 990) Davis - The Airborne Seeker Test Bed
Methods for Evaluating Missile Performance
Live Firing belly of a piloted aircraft. Close hom antennas in the chamber approximations to real inter repre ent clutter and tar experimental interrupted iUuminators that der the nose is currently empty and available periodically send brief bursts of illumination for another optical sensor. energy. In addition to the IH, the test bed carries a Forward Sensor Antenna C-band Beacon Tracking Radar (BTR) under the wing. The BTR tracks a transponder on The forward sensor antenna, which is the the target aircraft and provides target-angle, principal sensor of the IH, is a large (I6-in range, and range-rate information in real time diameter) X-band dual-polarized monopulse to the other sensors on the test bed, so antenna. Figure 4 shows how the antenna that those othersensors can be tuned to the tar was constructed as a sandwich, with an off-the get signal even if they haven't yet detected shelf slotted waveguide array that senses the the target. Another test bed sensor mounted vertical polarization. An array of microstrip di under the nose is an 8-to-I2-,um-band infrared poles with a microstrip feed layer embedded imaging device (the FUR) that provides preci beneath it senses the horizontal polarization. sion angle data on the target. A second pod un- Figure 5 is a photograph of the forward sensor The Lincoln Laboratory Joumal. Volume 3. Number 2 (1990) 211 Davis - The Airborne Seeker Test Bed ...... •.••....•...... :...... 40 em ...... -.•••...... , •.•...... ...... Mierostrip Mierostrip Slotted H & V Dipoles Feed WavegulC!e Monopulse (HPOL) Network (VPOL) Networks Fig. 4-The construction of the Instrumentation Head (IH) antenna. A standard slotted waveguide array (for vertical polarization) has a microstrip feed network and an array ofmicrostrip dipoles (for horizontalpolarization) layered on top of it. Two monopulse combiners, one for each polarization, are fastened to the back side of the antenna. antenna mounted on the nose ofthe Falcon 20. subtracting signals from the upper and The signals from the vertically and horizon lower halves of the antenna. tally polarized antenna elements are calibrated 4. Diagonaldifference beam. This component in gain and phase and combined to determine is not used in normal monopulse tracking, the unique polarization state of the incoming but it rounds out the complete setoflinear signal. The ability to make polarimetric mea combinations of the four antenna quad surements oftarget, clutter, and ECM signals is rants. a key feature of the IH. The complete set of four signals yields all of Most modem missile seekers employ mono the information available from the antenna (the pulse tracking in which the measurement of set of four signals represents four equations in target angle is made by comparing signal levels four unknowns). In addition to monopulse pro received simultaneously by differently shaped cessing, signal processing schemes related to antenna beams. The forward sensor antenna ECCM can be explored by using the full set of is composed of four separate quadrants that calibrated monopulse signals. The four mono are combined to form the signals for four mono pulse components in each of two polariza pulse components. These monopulse compo tions (vertical and horizontal) result in eight nents are signal channels simultaneously available to 1. Sum beam. This beam pattern is the nor the receiver. mal pencil beam associated with a high gain antenna. It results from adding the Instrumentation Head Receiver signals from all four quadrants of the an tenna. Figure 6 shows a block diagram that illus 2. Azimuth difference beam. This beam pat trates some of the high-level features of the IH tern yields a null signal when the target receiver, and Table 1 summarizes the speci is centered left to right. It results from fications of the IH receiver. The top half of subtracting signals from the left and Fig. 6 illustrates one of the eight forward sen right halves of the antenna. sor antenna channels. This channel splits 3. Elevation difference beam. This beam pat into two paths: one with a wideband filter that tern yields a null signal when the target captures the entire clutter Doppler spec is centered top to bottom. It results from trum, and one with a narrowband filter center- 212 The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) Davis - The Airborne Seeker Test Bed ed on the target. The narrowband filter rejects used. In either configuration the vertical and the clutter signals to improve the fidelity of horizontal polarization signals are recorded in the recorded target signature. With a pulsed the rear receiver. These two rear channels, waveform, three independently controllable along with the eight forward channels, make a range gates are in each of the narrowband total of 10 wideband channels in the IH. channels, and a split gate channel (in the monopulse sumchannels only) provides a range The Forward Looking tracking-error signal. The IH has a total of 26 Infrared Sensor narrowband channels. The bottom half of Fig. 6 illustrates the rear The role of the FUR in the test bed is to receiverthat receives the directpath signalfrom provide a precision angle reference to the target the illuminatorvia the rearantennain the tail of to compare with the radar data from the IH (Fig. the aircraft. The signal passes through fre 7). In particular, the pointing direction ofthe RF quency-locked and phase-locked loops to pro seekercanbe superimposed onthe target image vide the stable frequency reference to mix down to indicate the effects ofECM. The FUR forms a the front channels. The oscillator used for this 1V-compatible image from light in the infrared function is from theAIM-7 Sparrowmissile, and band (thermal radiation) with wavelengths of 8 the rear loop design is similar to the Sparrow to 12 JIm. The particular infrared device we use narrowband rear receiver. was manufactured by Kollmorgen and was in If problems related to missiles that carry tended for security surveillance (for example. in their own on-boardfrequency reference need in prison perimeter security). It was selected as a vestigation. the rear loop can be bypassed low-costinfrared sensorfor angle measurement and a separate fixed local oscillator can be on the test bed. Because it was not designed as Fig. 5-The Falcon 20 nose unit is shown with the radome removed, which reveals the dual polarizedX-bandantenna. The rightpodholds the FURsensor(behind the orangezincsulfide window). The left pod is currently empty and available for a future payload. The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 213 Davis - The Airborne SeekerTest Bed Doppler Voltage-Controlled Oscillator (9 Narrowband Channel (26) 4 & 10 kHz Bandwidth Wideband or Videoband Instrumentation Channel (8) Local 56 & 923-kHz Bandwidth Oscillator Wideband or Videoband Channel (2) 56 & 923-kHz Bandwidth Fig. 6-A diagram of the IH receiver. The upper halfof the diagram shows one of the eight forward sensor channels. The incoming signal is split into a wideband (clutter) channelanda setofnarrowband (target) channels. The lowerhalfofthe diagram shows the rear receiver that locks onto the illumination signal to provide the frequency reference for tuning the front receivers. an instrumentation system, no precise calibra Mountingthe FURin a gyro-stabilized gimbal tion of the infrared signal intensity is provided in the nose of the aircraft provides excellent by the FUR. Under appropriate conditions this image stability; the gimbal allows the FUR to FUR is capable of detecting small aircraft tar point in the same direction as the radar an gets at ranges on the order of 30 km. tenna. A hot-spot tracker has been added to the FUR to keep the gimbal pointed at the target. The FURusuallywill notacquire the targetuntil the latter portion of the intercept, a limitation that is acceptable in the test bed application because the precision angle information is most important during the last phases of the inter cept. Angle errors are less important to a seeker earlier in the intercept because the missile still has time at longer ranges to correct tracking errors. Beacon Tracking Radar Figure 8 shows the Beacon Tracking Radar Fig. 7-The Forward Looking Infrared Sensor. The tele (BTR) that serves as a reference radar for the scope mirroris visible atthe front ofthe sensor; the structure seeker test bed. The BTR, which was designed behind it is the scanner that generates a TV format raster. The FUR field of view is 4.5° by 3.5°. The sensor as and constructed by the Sierra Nevada Corpora mounted has an angular accuracy of 1 mrad (0.06°). tion in Reno. Nev.. is used to direct other on- 214 The Lincoln Laboratory Journal. Volume 3. Number 2 (J 990) Davis - The Airborne Seeker Test Bed Table 1. Specifications for Instrumentation Head Receiver Item Requirement Frequency 9750-to-10,050 MHz inclusive Signal waveforms Continuous wave (CW) and pulsed Pulsewidth (min) 0.78ps PRF 20 kHz to 400 kHz Maximum signals (at antenna port) Operating Front -10 dBm Rear OdBm Survivable Front +60 dBm PK +30 dBm AVG Rear +30 dBm PK +20 dBm AVG Polarization of rear sensor and forward sensor Horizontal (H) & vertical (V) Oscillator stability Noise sidebands <15 kHz Microwave LO dominant 15 kHz to 3 MHz -80 dBclkHz Discrete sidebands <15 kHz -(80 + 20 log f/15) dBc f = frequency separation in kHz 15 kHz to 3 MHz -80 dBc System noise figure Forward sensor $; 8 dB Rear sensor $;15 dB Receiver bandwidth Narrowband 4 kHz & 10 kHz Wideband 56 kHz Videoband 923 kHz Coherent processing interval (Cpr) For data collection 50 ms For auto track 0.5 to 16 ms Channel-to-channel tracking accuracy (after calibration) Gain 0.5 dB (1a) Phase 3.0° rms Absolute amplitude error (including calibration) <±1.0dB IR adjunct sensor Gimbal limits ± 50° pitch; ± 40° yaw Angle accuracy (static positioning) $; 1.0 mrad rms The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 215 Davis - The Airborne SeekerTest Bed Fig. 8-The Beacon Tracking Radar(BTR). This C-bandradartracks a transponderplacedon the target aircraft or at a ground reference point. The BTR records target angle, range, and range rate, which are used to aim the other test bed sensors. Valid data collection therefore begins before the other systems have acquired the target. board sensors to the target location in angle, to acquisition by the other sensors, but only if range, and range rate. It can locate a target prior the target has a beacon, or radio repeater, that MUX/ Avionics Formatter Recorder VME-VME Link Sensor Console Control Chassis Chassis Panel Aircraft Computer Beacon IR/EO Tracker Fig. 9-A blockdiagram oftheprincipaltestbedcomputersystems. The seekercomputer(shown in red) directly controls the IH (Doppler tuning, gain controls, track processing, and gimbal control). The aircraft computer (shown in blue) coordinates and records sensor reports, and represents the primary operator interface for controlling the test bedsystems. 216 The Lincoln Laboratory Journal, Volume 3, Number 2 (J 990) Davis - The Airborne Seeker Test Bed sends back a strong signal in response to the allows high data rates to be supported in BTR interrogations. Because this signal is at a bursts. The data multiplexer can also support different frequency (C-band) from the other test an additional DCRSi recorder. bed sensors, signal interference does not occur. Figure 11 is a photograph of the operator's The BTR is designed to locate the target at a control panel. The operator can see full signal typical initial range of30 km. The BTR is housed spectra in real time and make decisions in a standardAST-4 pod and weighs210lbs; the throughoutthe intercept. Events dUring a target other pod is empty and is used for aerodynamic intercept happen quickly, and not enough time balance. is available for an operatorto type commands on a computer keyboard. Consequently. specific Computers and Data Recording software functions are tied to single button presses on the control panel. For example, in Figure 9 is a block diagram of the primary an ECM mission the operatormight press a but computers of the test bed. The red area in the ton to force a reacquisition in response to figure indicates the digital system associated information shown on the screen. with the IH. This system, called the seeker Mter a data collection mission, a software computer, is responsible for receiver gain system implemented on Sun workstations ac control, Doppler tUning, antenna gimbal con cesses and processes the data from the test trol, and performance of the closed-loop target bed. A single intercept flight pass can generate tracking. The blue area in the figure is the 700 MB of data, which can consist of 10 wide aircraft computer that controls the test bed sys band channels, 26 narrowband channels. and tems. It coordinates and records the sensor a variety of sensor reports from both on board reports (IH, FLIR, BTR, inertial navigation sys and off board the test bed. A sophisticated ar tem. and global positioning system) and chitecture has been developed to allow an an provides the primary operator interface for alyst qUick and convenient access to a desired the test bed. portion of the data. The analyst can then The computers are multitasking multi-CPU define processing operations on the data to gen systems based on Motorola 68020 CPUs in a erate the desired data products. A quick-look VME bus. Most of the system software is pro capability for checking data quality, gener grammed in the C language. The signal proces ating signal spectra. and observing ECM ef sor is fully software programmable; the seeker fects is available in the field. Full data calibra and aircraft computer chassis have room to tion and processing is performed back at the accommodate hardware enhancements and a Laboratory. The analysis team at Lincoln Lab second signal processor. These features are oratory provides continuity of the knowledge included to support future additions and modi base over the life of the project. fications to the test bed. The bulk of the radar data from the IH does Falcon 20 notenterthe computers; it is passed to the high speed recorder by a custom data multiplexer The Dassault Falcon 20 aircraft is a medium developed by TEK Microsystems of Burlington, sized business jet designed to carry nine pas Mass. Figure 10 shows the data flow paths sengers. Two major external modifications were supported by the data multiplexer. The high made to the aircraft: wing hard points were speed data recorder is an Ampex DCRSi rotary added to support pods (a factory kit was avail head cassette recorder that can support data able), and the nose was modified to support the rates up to 13.3 MB/sec. Since the data rate new radome. the two optics pods. and the in from the IH can reach 50 MB/sec in some creased weight. The Falcon 20 airframe is rated modes. the data multiplexer can be pro for speeds up to 0.88 Mach, but the combina grammed to optimize the recording. which tion of engine performance and the increased The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 217 Davis - The Airborne Seeker Test Bed recording and coordinates with the ground 4 Mb/sec crew. 51 Mb/sec 3.3Mb/sec 10 Wide 26 Narrow Test Intercept Mission band band x2 (I,Q) This section describes data from a typical @ 192 kHz target intercept experiment. At the time of this @ 1.28 MHz @ 64 kHz writing we are doing our initial testing and calibration, and have not yet collected data on any military aircraft. Although the experiment described here was designed to verifY system performance, it illustrates many ofthe capabili Buffer Memory Buffer Memory ties of the Airborne Seeker Test Bed. 10 x 128 K Word 26 x4 K Word In the test-range environment, two racetrack I T trajectories are set up-one for the target and I I onefor the testbed-toprovide the desired track Tape Bus crossingangle. (See Figure 13.) Ground control Controller Controller lers assist in getting the aircraft into suitable Data Multiplexer initial positions. As the test bed enters the straight leg of the intercept trajectory, the BTR acquires the target. From this point in the test, the other test bed sensors can be directed by the BTR. The measurements described below were Recorder performed in civilian airspace over New Hamp shire and Massachusetts, rather than at a test Fig. 10-A block diagram of the data multiplexer. The range. No ground controllers were involved multiplexer is the interface between data generated by the (other than the local control towers that the IHandthe Ampextape recorder. Operating modes existfor pilots reported to periodically), and the pilots continuous recording or periodic snapshots of selected channels. used landmarks to set up the trajectories. The experiment was configured as a simu dragofthe external modifications limitsspeed to lated head-on intercept with the test bed 170 m 0.73 Mach dUring level flight. above the altitude ofthe target. The target used Figure 12 illustrates the layout ofequipment in the experiment was a Beechcraft Bonanza, a in the Falcon 20 aircraft. The inertial navigation small single-engine propeller-driven aircraft. system, which is mounted in the nose on a rigid The test bed velocity was kept low (180 kts, or plate, produces aircraft roll, pitch, and heading 91 m/sec) to extend data recording and to sim readings that provide a reference for the IH plifY pilot procedures in these initial exercises. antenna mount. An auxiliary power unit in the The GTRI Waveform Simulator radar built tail provides a dedicated 15 kVA of electrical to support the test bed provided a vertically power to the project. A freon cooling system polarized CW illumination waveform. installed in the tail of the aircraft handles the The signals in the following figures were re increased heat load generated by the project ceived by the forward sensor antenna through equipment. the monopulse sum beams. Since the illumi Threetestbed operators fly in the cabin along nator is vertically polarized, signals re with the two pilots. One operator controls the ceived through the vertical antenna channels overall system through the computer con are called copolarized. Signals from the hori sole, the second operator controls the FUR zontal antenna channels are called cross system, and the third operator monitors signal polarized. 218 The Lincoln Laboralory Journal. Volume 3. Number 2 (1990) Davis - The Airborne SeekerTest Bed Fig. 11-The operator control panel. The upper-left screen displays re ceived Doppler spectra; the upper-right screen lists target track-file infor mation. The centerscreen displays system status and the aim pointofthe gimballedsensors. The buttons on the lowerpanelcontrol the stages ofan intercept (data recording, sensoracquisition andreacquisition), andinitiate mission-specific functions undersoftware control. Figure 14shows a plotofthe received Doppler drop in its Doppler frequency as the test spectra as a function of time. These data are bed flies past is clearly evident in the figure taken from the wideband vertically polarized at approximately 50 sec. monopulse sum channel. Signal intensity is Figure 15 shows a single Doppler spectrum color coded as indicated; yellow represents the taken from the narrowband copolarized (verti strongest signals. The wide bright band in the cal) monopulse sum channel at 33 sec into the center corresponds to the ground clutter seen run. The received signal is integrated for 64 bythe forward sensorantenna. This signal stays msec, which when processed with a Kaiser at a relatively constant Doppler frequency Bessel windowing function yields a Doppler until the end of the trajectory, when the test resolution of40Hz. The Dopplerfrequency ofthe bed flies beyond the strong clutter sources. As central target line is slightly over 10kHz, which the test bed flies over a clutter source, its corresponds to a closing velocity of315 kts (160 Doppler frequency decreases. The narrow Dop m/sec) between test bed and target. We can see pler line to the right of the clutter is the in certain characteristics of the Bonanza target coming Bonanza aircraft. The characteristic surrounding the central Doppler line (the skin The Lincoln Laboratory Journal. Volume 3, Number 2 (J 990) 219 Davis - The Airborne Seeker Test Bed Forward Sensor Fig. 12-Layoutofequipmentin the Falcon 20. The inertial measurementunitin the nose determines the roll, pitch, and heading values ofthe test bed as well as its position in space. Most of the system electronics are in the racks along the left side ofthe aircraft. The auxiliarypower unit in the tail provides the electricalpowerfor allofthe project equipment. return). The broadened region of Doppler side appear on jet aircraft targets if the engine tur bands is due to propeller modulation of the bine blades are visible. radar signals; this broadening is also visible in Figure 16 shows the signal strength of the the spectra of Fig. 14. Similar modulations Bonanzaskin return dUring the first 50 seconds of the fly-by. The signals for the copolarized (V-pol) returns and cross-polarized (H-pol) re " ---, turns were measured simultaneously dUring ././ '\ ./ \ the fly-by. The copolarized signal tends to + ././ \ dominate, but the cross-polarized return is fre t ././ I North ./ I quently seen to be stronger. Figure 17, which shows the instantane ././././ ~/ ous polarization state of the target over a brief . /./ Tar et Illuminator t g interval of time, is another representa ~ ,,-, • 7 -, tion of the radar cross-section data. The ~ / \ '\ coordinates shown are a rectangular pro II '\./' ./ \ \ jection of the Poincare polarization sphere, I I which has left-hand circular (LHC) polari \ I zation at the north pole, right-hand cir '\, Airborne Seeker Test Bed .// "------,, cular (RHC) polarization at the south pole, + and a range of elliptical polarizations in be Ground Tracking Stations tween. The linear polarizations at various rotation angles are represented around the Fig. 13-The flight-path geometryfora simulatedintercept. The testbedflies radiallyoutboundfrom the illuminator. The equator. If the Bonanza did not depolarize the racetrack path of the target is oriented to yield the desired incident vertically polarized signal, we would crossing angle between test bed and target. Ground con see the return signal clustered at V-pol, indi trollers directboth aircraft to cause the intercept to occur at cated by the large dot in the center ofthe figure. a selected location. The test bed is 170 m higher than the target for safety reasons and for providing a look-down The various scattering centers on the aircraft geometry. distort the incident polarization, however, and 220 The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) Davis - The Airborne Seeker Test Bed 102 90 E'-..:120 (() ~ Q; -130 ~ o U 60 Q.. Q) ~ -140 .!E.- > Q) ·w E u i= £. -150 30 -160 8 10 12 14 16 Frequency above Rear Reference (kHz) o Fig. 15-Measured Doppler spectrum in the narrowband monopulse sum channel for the copolarized return signal, o 2 4 6 8 10 12 at a pointcorresponding to 33sec in Fig. 14. The Bonanza Frequency (kHz) return appears at 10kHz. The character of the propeller modulations is clearly evident as the widely spread base region around the central skin line. The narrowband plots were processed to a Doppler resolution of40 Hz. -80 -100 -120 -140 -160 -180 these individual contributions combine with various phase shifts to yield the complicated Amplitude (dBm) behavior shown. Fig. 14-The received Doppler spectra as they evolved in For comparison, Fig. 18 shows the corre time during the intercept. The legendshows signalintensity sponding polarization return with the antenna encodedas color. The brightstripe at5 kHz is groundclutter seen through the sidelobes of the IH antenna. The target main beam centered on clutter (this measure signal is visible as the narrowstripe at 10kHz. The test bed ment was made separately from the Bonanza passes the target at approximately 50 sec, when the target intercept). Note that the clutter depolarizes the Doppler rapidly falls off. The figure also shows a spreading of the target Doppler spectrum because ofsignal modula V-pol illumination signal significantly, but the tions caused by the propeller on the Bonanza aircraft. data are distributed and clustered differently -110 E -120 (() ~ Q; ~ -130 0 Q.. "0 Q) -140 ·w> u Q) a:: -150 Cross-Polarized -160 0 10 20 30 40 50 Time (sec) Fig. 16-A plot ofthe signal strength (in dB below a milliwatt) ofthe target skin return versus time for both the copolarizedandcross-polarizedcomponents. Even though the illuminating signalis vertically polarized, the signalscatteredbythe target is dominatedby the vertical component only halfthe time. The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 221 Davis - The Airbome SeekerTest Bed 90 ;-.:H Vi-- ---,H H V H 90,------.---,1------, LHC I , LHC . :.' OJ ...... - . ,.....- . .. <1l \.:. . :-,- .',...... ~~-..-- . . ~ .- • .. - •• •••••• #•• .. :"" ... -._.1·~.. ..: .~. • .1 .,. , v- 'Je • •• .....~: . ~,. . ~ ~ ••••• .. c ~ 01--:-" •• • ~.I\". ..•... . 0 .-fl't:.. ". ':\ ".': ._..-. . Linear ._ • ..,.. •....(e : ...... , _ Linear ~ .- .., . .,.. .I.'. . .-g •• ~ ....-.. l' . , ' :::l ::::J •• ~ ...~.~ ••• '","' ... ..,.-' .. .-. -'-t... .., .... () -,...~ ~ .. ;.:. .. .a.", ~:. ._.. ': .= ..... ~.... .- I.- _:...... : .... 0 ... .:... _.. . e... o· .: .._ .. .. .: L...-~ •I • -90 ._•....I:..-10·_· -----' RHC -90 L-- ---L.---''--=---__-----' RHC -180 0 180 -180 0 180 Orientation (deg) Orientation (deg) Fig. 17-The true polarization state of the target return is Fig. 18-The polarization state ofan interval ofclutterdata derived from the amplitude and phase of the received plotted on a rectangular representation of the Poincare vertical and horizontal polarized signal components. The sphere. Even though the received polarization is diffusely polarization state isplottedhere on a rectangularrepresen distributed, it clusters around the center dot that indicates tation ofthe Poincare sphere. The equator ofthis sphere is the vertical polarization of the illumination signal. the locus oflinearpolarizations that range from horizontal on the left to vertical in the middle to horizontal on the right. high-fidelity data in a repeatableand systematic Up and down excursions on the plot represent increasing ellipticityin the receivedpolarization, with left-handcircular measurementprogram makes the testbed espe (LHC) polarization at the north pole (the top edge of the cially valuable for investigatingadvanced seeker graph), and right-hand circular (RHC) polarization at the concepts and electronic countermeasures and southpole (the bottom edge ofthe graph). The receivedpo larization, though fairly random, forms two distinctclusters, for developing signal processing schemes to one around verticalpolarization andone aroundhorizontal defeat countermeasures. Though only in the air polarization. The centerdot indicates the vertical polariza for a few weeks as ofthiswriling, the testbed has tion of the illumination signal. already demonstrated the basic functionality required for its mission, from the proper per from those of the target returns of Fig. 17. formance of all sensor systems, operating Figure 19 shows an FUR image of the Bo modes, and data recording to the execution of nanza as it appeared at a range of 0.8 km. For clutter and target intercept measurements. most of an intercept the target aircraft is un In the near future the Airborne Seeker Test resolved; it appears as a single pixel on the Bed will operate atWhite Sands Missile Range in video screen. At this close range, the Bonanza a variety of tests involving clutter, target, and outline is seen as dark (cool) against the warm ECM measurements, with ground-based and er earth background. The bright spot on the airborne illuminators. A database of bistatic nose of the aircraft is the exposed hot engine. desert clutter will be collected and compared to A computer-generated cross hair superim results from other clutter measurements made posed on the FUR video indicates the radar at the same locations. Bistatic target radar aim point obtained from the IH. The motion cross-section measurements will be collected on of the cross hair provides a visual indication a T-38 aircraft both to demonstrate test bed of the dynamic behavior of the radar track and capabilities and to perfect flight procedures. is useful for demonstrating the degree of ECM Both ground-based and airborne illuminators angle deception. In Fig. 19 the cross hair to the will be used in the clutter and target measure left of the Bonanza aircraft shows where the IH ments. Intercepts will be performed on aircraft was positioned at the time, and is shown only eqUipped with angle-deception ECM to investi for illustration. gate the jamming characteristics and identitY possible discriminants. Summary After the tests at White Sands a number of tests are planned with other air vehicles of Air Lincoln Laboratory's Airborne Seeker Test Force interest to investigate their specific vul Bed represents a powerful tool for investigating nerabilities to missile seekers. Long-term plans missile seekerperformance. The ability to collect for the test bed include the development and 222 TIle Lincoln Laboratory Joumal. Volume 3. Number 2 (J 990) Davis - The Airbome SeekerTest Bed Fig. 19-A FUR image of the target Bonanza aircraft, taken near the end of the intercept at a range of 0.8 km. The hot engineparts appearas a positive contrast (brighter) againstthe earth background, while the bodyandwings ofthe aircraft appear as a negative contrast (darker) against the warm earth. A computer-generated cross hairsuperimposed on the FUR video indicates the aim point of the IH radar. demonstration of advanced ECCM algorithms. support is currently being continued into the the addition of other sensors. and the flying of operational phase by Lt. Col. Phil Soucy of the advanced-concept brass-board seekers. Air Force. Acknowledgments A complex project necessarily relies on the hard work and careful attention ofa large group The Airborne SeekerTest Bed was developed of people. Their tireless help has been and under thejoint sponsorship ofthe Air Force and continues to be essential to the success of the Army. I would specifically like to acknowl the project. Many people have been involved edge Dr. David L. Briggs of Lincoln Laboratory in the test bed development. including numer and Lt. Col. James P. Hogarty of the Air Force. ous Raytheon contributors. I will mention only the originators of the seeker test bed concept. the principal members of the Airborne.Seeker Colonel Hogarty and Joseph Durham of the Test Bed subcontractor teams: Raytheon pro Army Missile and Space Intelligence Command gram manager Victor Weisenbloom. Larry provided the sponsorship necessary to make Durfee, and Tom Clougher. all of the Raytheon this project a reality. Program direction and Missile Systems Division; Andy Reddig ofTEK The Lincoln Laboratory Journal. Volume 3. Number 2 (1990) 223 Davis - The Airbome SeekerTest Bed Microsystems; and George Clary and Stan Jim Clarke. Al Shaver. David Bruce. Dan McDonald of the Sierra Nevada Corp. Sparrell. Dave Kohr. Pete Szymansky. Cynthia The Lincoln Laooratory personnel who have Eldridge. and Lucy Smiley; engineers Dick seen the test bed into first flight are Group Simard. Mark Green. and John Parkins; tech Leaders Dr. Lewis Thurman and Dennis nical assistants Al Davis. John Allen. Bob Keane; system engineers Louis Hebert. Paul Cavanaugh. and Dick Thibodeau; pilots Juodawlkis. Dr. Randy Avent. and Dr. Al Charlie Magnarelli and Mike Radoslovich; Hearn; software developers Ken Gregson. and chief mechanic Bob Murray. , CURTIS W. DAVIS III is the Assistant Leader of the Air Defense Techniques Group. He received his B.S.. M.S., and Ph.D. degrees from the Ohio State University. where he worked on impulse radar for underground probing. While at Oillo State he built ultrawideband measurement systems. designed antennas. and mathematically modeled radar performance. Since joining Lincoln Laboratory in 1979 Curt has been involved in the instrumentation and performance analyses of air defense radar systems. His activities have included addingcomputerrecordingcapabil ity to an X-band ground-clutter measurement radar: the design. construction, and operation ofa helicopter-mounted field -strengthmeasurement system for multipath profiling; developmentofmultipath modeHngalgOrithms; and various radar system analysis activities. Curt was responsible for defining the basicarchitectureand specifications oftheNr borne Seeker Test Bed system. and he serves as the test bed's project engineer. • 224 The Lincoln Laboratory Joumal. Volume 3. Number 2 (1990)