compact range rcs

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August 2005

RCS Measurements in a Compact Rangeby Jeff Fordham, Vice President, and Marion Baggett, Software Engineering Manager, MI Technologies

Introduction

The Radar CrossSection (RCS) of anobject is defined as,

the area intercepting thatamount of power which,when scattered isotropi-

cally, produces a returnat the radar equal to thatfrom the target. In sim-pler terms, RCS is the pro-jected area of a sphere thathas the same radar returnas the target. The unit ofmeasure for an objectsRCS is decibels persquare meter, or dBsm.The power received by aradar for a target indi-

cates how well the radarcan detect or track thattarget. For this reason,much research and efforthas been put into reducingthe signature of variousaircraft, ships and otherobjects. See Figure 1 fora schematic and definitionof RCS.

The task of creatingdesigns based upon theo-retical modeling and sim-

ulation must be provenin the end. With RCSdesigns, this involves tak-ing the object to an RCSmeasurement facility andmeasuring the radar returnof the object. There aremany types of RCS mea-surement techniques andRCS measurement rangepossibilities. Compactranges are one of the most

popular methods for mea-suring the RCS of variousobjects. Compact Ranges(CR) have the advantagesof indoor testing in a con-trolled environment, directwhole-body measurements

of the target and RF sig-nal security if the com-pact range is in a shieldedenclosure. See Figure 2for a picture of a CR usedfor RCS measurements.

In short, a Compact

Range consists of a largeoffset fed reflector anda feed to illuminate thereflector. Once properlyaligned, a CR will producea plane-wave zone withminimal phase taper, mini-

mal amplitude ripple andapproximately 0.5 to 1.0dB amplitude taper overthe designed plane-wavezone. For more informa-tion on the design andcapabilities of compactranges, the reader shouldconsult the literature baseregarding compact rang-es. This article will focusinstead on considerations

involved for making goodRCS measurements in aCR RCS measurementfacility.

RF MeasurementConsiderationsThere are many designchallenges involved inRCS ranges that affect sys-tem performance.

Critical design factorsinclude:

Clutter Rejectionor Reduction Targetresponse and returns fromthe range must be sepa-rated. These returns caninclude: objects in therange, such as walls andtarget mount; interactionof the target with theseobjects and any unwantedsignals in the range itself.See Figure 3 for an illus-

Figure 1: The Definition of RCS.

Figure 2: Compact Range Reflector and Feedlocated at the Instituto Nacional de TecnicaAeroespacial (INTA) in Spain. This CR is capableof testing the RCS of objects up to 5.5 meterswide by 5 meters tall.


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MICROWAVE PRODUCT DIGEST pg 2

tration of clutter in a mea-surement.

Dynamic Range Areceiver with high dynamicrange (on the order of 85 to

90 db) is required to isolatelarge target responses fromclose smaller ones. Thisrequirement is particularlytrue for whole body eval-uations. Some complextargets can exhibit chang-es in RCS on this order

over a few hundredths ofa degree. See Figure 4 fora typical dynamic rangemeasurement.

Frequency Coverage

and Switching Speed AnRCS instrumentation sys-tem usually requires awide operating bandwidth.Many test frequencies maybe required. A fast switch-ing speed will greatlyincrease range throughput

by allowing many frequen-cies to be collected in asingle target rotation.

M e a s u r e m e n tCapabilities A targets

RCS is a function of targetposition, frequency, polar-ization and other factors.Some of the more com-mon measurements are:

RCS vs. Angle(See Figure 5)

RCS vs. CrossRange(See Figure 6)

RCS vs. angle vs.frequency

(See Figure 7) RCS vs. Range Axis

vs. Angle(See Figure 8)

RCS imaging(See Figure 9)

RCS PowerSpectrum (Dopplermeasurements)

Polarimetricmeasurements

RCS vs.

Bi-static angle R i n g d o w n -

Characterized by RFenergy storage withinthe system, ringdown hassignificant impact uponoverall measurement per-formance. The two mostsignificant contributors toringdown are related tothe impulse response ofthe CR feeds and energy

reflections between com-ponent connections dueto impedance mismatch.When an energy pulse istransmitted to an anten-na, the VSWR at the feedreflects some energy backto the transmitted outputof the radar. The VSWRat that point will reflectsome of the energy backto the feed. The cycle con-tinues until all the energy

is radiated or absorbedby the cable. See Figure10 showing measuredreceived power vs. timefor an example of feed

ringdown time. In thefigure, note that betweenthe 30 nsec and 180 nsecpoints, the reflection ofthe CR rises above theringdown and VSWR.

Single vs. DualFeeds The preferred tech-nical approach implementsa dual feed system in orderto minimize RCS perfor-mance risk. Single feedsystems have a somewhat

lower complexity, but havenotable technical draw-backs. With single feedsystems, there are a limitedamount of practical modifi-cations to be made in orderto reduce ringing. Primaryreductions typically involvecharacterization of feedringing performance, poten-tially leading to expensiveand time consuming feed

design changes. Also, feedringing characterization initself, is an involved andhighly technical activ-ity which must be factoredinto program risk. Dualfeed systems, offer a sig-nificant increase in transmitto receive isolation at theexpense of increased systemcomplexity in controllingthe receive gating pathway.

Sensitivity Typical

Range specifications of -80dBsm provide a challengingrequirement and require sig-nificant attention to detail.A detailed RCS link budgetfor a compact range sys-tem utilizing the radar rangeequation is required. Subtledesign implementations suchas minimization of RF cablelengths, selection of appro-priate RF cable type, utiliza-tion of receiver inte gration

Figure 3: Illustration of Clutter and Leakagesources in a CR vs. time.

Figure 4: Dynamic Range Test data collected onthe MI-1797 receiver at 10 GHz:

Input signal set at the .1 dB compression pointfor the MI-1797 (top trace)The other four traces are with the Transmitand Receive cables terminated in a load andaveraging set at 1, 8, 32 and 1024 samples


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gain and prudent maximiz-ing of duty cycle to mini-mize associated loss have tobe considered. Individuallyeach of these design ele-

ments may not seem criti-cal, but when viewed inthe composite they form asignificant contribution tooverall system sensitivity.

Isolation AndMeasurement RepeatabilityTo meet a -80 dBsmrequirement will requirecare in selecting RF com-ponents. Selecting a dualfeed configuration withits better isolation com-

bined with high compo-nent switch isolationprovides for more robustsystem performance. MITechnologies proven gat-ing system cascades mul-tiple elements to achievesignificant improvement inisolation, up to 170 dB.

Attention to detail inthermal packaging andproximity placement of

components is required.Background subtractionof artifacts and noise isonly possible if consistentand repeatable measure-ments can be achieved.Focused design attentionto thermal heat sinks andforced air cooling of activeRF components as well asselection of ultra phasestable cables are signifi-cant keys to improving

background subtraction. Range Throughput& Human Factors Consideration for through-put and human factors arerequired to support bothproduction and engineer-ing development projects.Adequate considerationmust be given to optimizingthroughput of productionantennas and ease of conver-

sion between the Antenna-RCS configurations. Hightechnical performance with-out consideration for thepractical day-to-day use of

the range is not acceptable.Several key aspects of thesystem architecture must beconsidered to ensure effi-cient configuration changes.A heavy-duty quick-changearbor mount design andman lift can be utilized toimprove human factors.

Applicable RCS LinkBudget EquationIn order to make high

quality measurements withrepeatable results, a carefulexamination of the expec-tation on received powermust be made. This care-ful examination is madeby reviewing the expectedlink budget of the RCSmeasurement range.

The link budgets arederived from the RCSrange equation, which is

shown below.

Where:Pr = Receive powerPt = Transmit powerG = Antenna gains = RCSl = WavelengthN = Integration GainktB = Boltzmans

Constant times ReceiverBandwidthLt = Losses betweenTransmitter and FeedLr = Losses betweenFeed and LNALg = Gating LossNF = LNA NoiseFigureR = RangeLength (Equal to CRFocal Length)


Figure 5: RCS vs. Angle

Figure 6: RCS vs. Cross Range

Figure 7: RCS vs. Frequency and Angle


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The radar range equa-tion assumes that thepotential error sourcesassociated with the feedsisolation, ringdown and

VSWR are inconsequen-tial. The range equationalso assumes that thereare no residual reflectionsfrom other items in thechamber present within ameasured pulse window.In a physically realizableRCS range, there are prac-tical limitations to specif-ic devices that can causeperformance to be lessthan indicated by the RCS

range equation.

Typical Equipment andthe MI SolutionMI Technologies approachfor the range architectureis a gated CW systembased upon a set of modu-lar hardware and softwarewhich can be efficientlyconfigured for RCS test-ing. Given that many RCS

ranges are intended forboth antenna and RCStesting, not only will thetraditional system hard-ware be important, butthe subtle, less obvioussystem design elementsmust also be balancedin order to meet variousrange requirements.

Figure 11 illustratesan RCS Measurementarchitecture. The system

includes RF data acquisi-tion, positioning and com-puting subsystems. TheRCS hardware additionsto a basic receiver subsys-tem include a dual feedcluster, pulse generatorand utilization of a mod-ular RCS RF subsystemwhich is located in closeproximity behind the CRfeeds. Software specific to

RCS testing for post-pro-cessing analysis, displayand reporting is added tocomplete the system.

The key RF measure-

ment element of thesystem is the MI-1797Microwave Receiver, pro-viding RF data acquisi-tion from .1- 20 GHz. Afull description of the MI-

1797 receiver can be foundon the MI Technologieswebsite (www.MI-Technologies.com). TheMI-1797 coupled with the

MI-3001 Data Acquisitionand Analysis Workstation,enables the user to fullyautomate the measure-ment process.

At a high speed acquisi-

tion rate of 10,000 CWmeasurements per second(100 usec per sample) andsingle sample dynamicrange of 85-90 dB over

0.1 20 GHz, the MI-1797 provides significantperformance advantag-es. Multi-frequency andmulti-port measurementsare supported with a high


Figure 8: RCS vs. Range Axis vs. Angle. (a) Angular Scan Axis shown as X-Axis. (b) Angular Scan Axis shown along Polar Axis

Figure 9: RCS of three spheres with an RCS of -60 dBsm on top of a foamcolumn. (a) RCS of Spheres and Column. (b) RCS of the Spheres after using

Background Subtraction to remove influence of the foam column.


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speed frequency switching synthe-sizer and two-port multiplexers.

The key elements and subsystemsof the measurement system are com-posed of:

MI-1797 Microwave Receiver Two MI-3100 Series

Synthesizers (LO and TXSource)

RX Low Noise Amplifier LO Extender and Control Data Acquisition Co-

Processor (DAC) TX & RX Mixers RF Couplers Two Port Multiplexers RF Range Cables including

High Performance PhaseStable Cable

RCS RF subsystem includingAuxiliary Control and PowerAnd Gating Control

Pulse Generator Unit RCS Dual Feed Cluster

(See Figure 12 for a photo of

Figure 10: Plot of Received Power vs. Time showing system ring-down. The X-axis is range delay in nsec and the Y-axis is relativereceive power. This plot was created by shifting the receive gate intime and plotting the received power.

Figure 11: Block Diagram of Gated CW RCS Measurement System using an MI-1797 as the receiver.


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a typical feed cluster RF Range

Cables includingHigh PerformancePhase Stable Cable

Typical PerformanceBaseline MI-1797 receiverdynamic range perfor-mance data is presentedin Figure 4. The dataillustrates the significantmeasurement capabilityof the MI-1797 receiverat a variety of measure-ment integration samplesizes. Even at low samplesizes, resulting in the fast-

est test times, the MI-1797receiver shows outstand-ing signal measurementperformance.

Using the RCS linkbudget equation from sec-tion 3.0, Table 1 showsthe typical sensitivity lev-els that can be achievedfor a CR with a 24 ft focallength and a dual feedconfiguration.

ConclusionThe use of compact rang-es to measure the RCSis a well proven methodfor testing a wide vari-

ety of targets. The closeproximity in time of theradar feeds, the CR andtarget present significantdesign and measurementchallenges. Care must betaken to ensure that themany sources of errors inthe measurement are prop-erly designed and account-ed for to ensure a goodmeasurement. Currentgeneration of gated CW

systems, such as the MITechnologies gated CWsystem presented in thiswhite paper are capableof making very accuratemeasurements on welldesigned ranges.

The ability to makesensitive measurementstwenty-four hours a day,seven days a week, in asecure facility using little

real estate makes meetingthe design challenges forthis type of measurementsystem worthwhile. Formore information, please

call (800)854-3660 or visitwww.mi-technologies.com


Figure 12: RCS Feed Cluster using widebandsinuous antennas as feeds. Each feed is duallinearly polarized.

7.3 meter Focal Length

Duty Cycle 17%

10 ft Target, 40 nsec gate

Parameter 2 GHz 10 GHz 18 GHz 26.5 GHz 40 GHz

KTB (10khz) -134 -134 -134 -134 -134 dBm

(4p)3 33 33 33 33 33 dB

(Range)4 34.6 34.6 34.6 34.6 34.6 dB

TX Losses 2 2 2 2 2 dB

RX Losses 2 2 2 2 2 dB

LNA Noise Figure 3 3 3 4 4 dBPower Transmitted 27 27 27 30 24 dBm

(Antenna Gain)2 16 20 28 20 28 dB

(Wavelength)2 -16.5 -30.5 -35.6 -38.92 -42.5 dB

Duty Cycle Loss 15.2 15.2 15.2 15.2 15.2 dB

Integration Gain 12 21 18 27 30 dB

Sensitivity -82.8 -81.7 -81.7 -81.4 -80.8 dBsm

Table 1: Sensitivity Calculation

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