Radar Clutter And Chaff - Massachusetts Institute Of .

Introduction to Radar SystemsRadar Clutter and ChaffRadar Course 1.pptODonnell 10-26-01MIT Lincoln Laboratory

Disclaimer of Endorsement and Liability The video courseware and accompanying viewgraphs presented onthis server were prepared as an account of work sponsored by anagency of the United States Government. Neither the United StatesGovernment nor any agency thereof, nor any of their employees, northe Massachusetts Institute of Technology and its Lincoln Laboratory,nor any of their contractors, subcontractors, or their employees,makes any warranty, express or implied, or assumes any legal liabilityor responsibility for the accuracy, completeness, or usefulness of anyinformation, apparatus, products, or process disclosed, or representsthat its use would not infringe privately owned rights. Reference hereinto any specific commercial product, process, or service by trade name,trademark, manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring by the UnitedStates Government, any agency thereof, or any of their contractors orsubcontractors or the Massachusetts Institute of Technology and itsLincoln Laboratory. MIT LincolnLaboratoryThe views and opinions expressed herein do not necessarilystateorreflect those of the United States Government or any agency thereof orany of their contractors or subcontractorsRadar Course 2.pptODonnell (1) 6-19-02

Radar terWaveformGeneratorSignal gs(Radar Clutter)DopplerProcessingMain ComputerDetectionRadar Course 3.pptODonnell (1) 6-19-02Tracking &ParameterEstimationConsole /DisplayRecordingMIT Lincoln Laboratory

Why Understand Radar Clutter?Naval Air Defense ScenarioRadar echo is composed of: Backscatter from target ofinterest Receiver noiseAtmospheric noiseInterferenceFrom other radarsJammers Backscatter from unwantedobjectsGroundSeaRainChaffBirdsGround trafficRadar Course 4.pptODonnell (1) 6-19-02342636 10.PPTRMO 6-22-2000MIT Lincoln Laboratory

Outline Radar Course 5.pptODonnell (1) 6-19-02MotivationGround ClutterSea ClutterRainChaffBirds and InsectsMIT Lincoln Laboratory

Attributes of Ground Clutter Mean value of backscatter from ground clutter– Very large size relative to aircraft–Varies statistically Frequency, spatial resolution, geometry, terrain type Doppler characteristics of ground clutter return– Innate Doppler spread small (few knots) Mechanical scanning antennas add spread to clutter– Relative motion of radar platform affects Doppler ofground clutter Ship AircraftRadar Course 6.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Photographs of Ground Based Radar’s PPI(Different Levels of Attenuation)Mountainous Region of Lakehead, Ontario, CanadaPPI Set for 30 nmi.0 dB60 dBCourtesy of IEEE. Used with permission.Source: Shrader, W. "Radar Technology Applied to Air Traffic Control," IEEE Transactions on Communications, Vol COM-21, No. 5, May 1973. IEEE.Radar Course 7.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Photographs of Ground Based Radar’s PPI(Different Levels of Attenuation)0 dB30 dBRadar Course 8.pptODonnell (1) 6-19-0210 dB40 dB20 dB50 dBMIT Lincoln Laboratory

Geometry of Radar ClutterRadarElevation ViewhcT / 2φ½ cT sec φPlan ViewClutterθBRadarRθ Bσ0 Radar Course 9.pptODonnell (1) 6-19-02σAA RθB [½ cT sec φ]MIT Lincoln Laboratory

Calculation of Ground Clutter0.01 m2 Typical Value of σo -20 dB m2cT σ Clutter σo A σoR θB2– For ASR-9 (Airport Surveillance Radar)cT 100mθB 1.5o 0.026 radiansR 60 km20.01 m2x 100 m x 60,000 m x 0.026 radians 1500 m2 σ Clutter 2mFor σ Target 1Smallsingle-engineaircraftRadar Course 10.pptODonnell (1) 6-19-02m2σ Targetσ Clutter 11500σ Targetσ Clutter M ust suppress clutter by a factor of1500 x 20 30,000 45 dB 20For gooddetectionMIT Lincoln Laboratory

Joint U.S./Canada Measurement Program Phase One radar– VHF, UHF, L-, S-, X-bands Measurements conducted1982 – 1984 Archival data at LincolnLaboratoryRadar Course 11.pptODonnell (1) 6-19-02 42 sitesData shared with Canadaand the United KingdomMIT Lincoln Laboratory

Clutter ngClutter Coefficient σ oImage from Billingsley, J. B. Ground Clutter Measurements for Surface Sited Radars. Tech Report 786, Rev. 1.Lexington, MA: Lincoln Laboratory, February 1, 1993. Courtesy of Lincoln LaboratoryRadar Course 12.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Clutter PhysicsLobingFree shadowingPropagation Factor FClutter Coefficient σ oClutter Strength σ oF 4Image from Billingsley, J. B. Ground Clutter Measurements for Surface Sited Radars. Tech Report 786, Rev. 1.Lexington, MA: Lincoln Laboratory, February 1, 1993. Courtesy of Lincoln Laboratory1) Radar Parameters Frequency, f Spatial resolution, ARadar Course 13.pptODonnell (1) 6-19-022) Geometry Depression angle(Range R, Height H)3) Terrain Type Landform Land coverMIT Lincoln Laboratory

Mean Ground Clutter Strengthvs. Frequency0–10Mean of σ F4 (dB)–20General Rural (36 /36PolarizationHVHVFrequency (MHz)Radar Course 14.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Outline Radar Course 15.pptODonnell (1) 6-19-02MotivationGround ClutterSea ClutterRainChaffBirds and InsectsMIT Lincoln Laboratory

Attributes of Sea Clutter Mean cross section of sea clutter depends on many variables– Wind and weatherSea State– Radar frequency– Radar Polarization– Range resolution– Cross range resolution– Grazing angle– Too many variablesMean sea backscatter is about 100 timesless than ground backscatterRadar Course 16.pptODonnell (1) 6-19-02Figure by MIT OCW.MIT Lincoln Laboratory

World Meteorological OrganizationSea StateSea StateWave Height (m)Wind Velocity (knots)Descriptive Term0 to 10 to 0.10 to 6Calm, Rippled20.1 to 0.57 to 10Smooth, Wavelets30.6 to 1.211 to 16Slight to Moderate41.2 to 2.417 to 21Moderate to Rough52.4 to 422 to 27Very Rough64 to 628 to 47HighRadar Course 17.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Sea SpikesFigure by MIT OCW. Grazing angle 1.5 deg. Horizontal polarization At low grazing angles, sharp sea clutter peaks, knownas “sea spikes”, begin to appear These sea spikes can cause excessive false detectionsFrom Lewis and Olin, NRLRadar Course 18.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Outline Radar Course 19.pptODonnell (1) 6-19-02MotivationGround ClutterSea ClutterRainChaffBirds and InsectsMIT Lincoln Laboratory

Attributes of Rain Clutter Rain both attenuates and reflects radar signals Problems caused by rain lessen dramatically with longerwavelengths (lower frequencies)– Much less of a issue at L-Band than X-Band Rain is diffuse clutter (wide geographic extent)– Travels horizontally with the wind– Has mean Doppler velocity and spreadReflected energyRain dropTransmitted energyRadar Course 20.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

PPI Display Radar Normal VideoClear Day (No Rain)Day of Heavy Rain10 nmi Range Rings on PPIDisplayAugust 1975, FAA TestCenterAtlantic City, New JerseyAirport Surveillance RadarS BandDetection Range - 60 nmi ona 1 m2 targetRadar Course 21.pptODonnell (1) 6-19-0210 nmi Range Rings on PPIDisplayAugust 1975, FAA TestCenterAtlantic City, New JerseyMIT Lincoln Laboratory

Reflectivity of Uniform Rain(σ in dBm2/m3)Figure by MIT OCW. Rain reflectivity increases as f 4 (or 1 / λ4)– Rain clutter is an issue at S-Band and a significant one atX-Band or higher frequenciesRadar Course 22.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Measured S-Band Doppler Spectra of Rain0– 10– 20– 30– 40– 50– 60– 70– 80– 90– 60 Kt0 60 Kt0– 10– 20– 30– 40– 50– 60– 70– 80– 90Azimuth 320 0– 10– 20– 30– 40– 50– 60– 70– 80– 90– 60 KtDoppler Velocity0 60 Kt– 60 KtRain is not Gaussian In these examples the rainfall ratewas approximately 20 mm/hr Winds 30 kts on ground, 50 kts at6000 ftMean velocity varies as stormmoves by radar0 60 KtDoppler VelocityDoppler Velocity Radar Course 23.pptODonnell (1) 6-19-02Azimuth 330 dBdBAzimuth 90 0– 10– 20– 30– 40– 50– 60– 70– 80– 90Velocity Spread6 kts– 60 Kt0 60 KtDoppler VelocityMIT Lincoln Laboratory

Outline Radar Course 24.pptODonnell (1) 6-19-02MotivationGround ClutterSea ClutterRainChaffBirds and InsectsMIT Lincoln Laboratory

Attributes of Chaff Large number of dipoles (metallic or metallic coated)– High reflectivity per pound– Optimum length 1/2 of radar wavelength– Moves with the wind Uses of chaff– MaskingLarge cloud can shield aircraft or missiles in or near the cloud– DeceptionChaff “puff” can emulate a missile / aircraft and cause falsedetectionsPackets of chaff can divert radar tracker from targetRadar Course 25.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Chaff Reflectivity and Density Resonant Metallic Dipoles–σ .18 λ2 (in m2) Average Cross Section per Dipole– Bandwidth 10-15% of center frequency– Fall rates 0.5 to 3 m/s Aluminum foil dipoles (.001 in. x .01 in. x λ/2 long)–σ 3000 W / f (in m2)– W weight in lb, f frequency in GHz– At S-Band, 400 lb yields 265,000 m2 or 54.3 dBsmRadar Course 26.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

AN/ALE-38 Chaff-Dispensing SystemMotorGearChaff Roll (6 each)Cartridge (6 each)Chaff RollChaffRoll HubAir CylinderWater SeparatorRam Air InletTake-upRollers(12 each)Air Ducts (6 each)0.02 in Polyester Film0.10 inTake-upRollerChaffTake-upRollerAir Ductn12.5 i13 inChaff(Aluminized Glassor Aluminum Foil)ChaffDipole (6 rows)Radar Course 27.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Movie of ChaffRadar Course 28.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Outline Radar Course 29.pptODonnell (1) 6-19-02MotivationGround ClutterSea ClutterRainChaffBirds and InsectsMIT Lincoln Laboratory

Bird Breeding Areas and Migration RoutesGadwallNorthern FlickerVirginia RailPhotos courtesy of vsmithuk, sbmontana, and khosla.Figure by MIT OCW.During the breeding season along the Gulf Coast, sea and wading bird colonies existthat have up to 60,000 birds. 10,000 birds are common. These birds are large;weighing up to 1 kg and having wingspreads from 0.75 to several meters.MIT Lincoln LaboratoryRadar Course 30.pptODonnell (1) 6-19-02

Bird Breeding Areas and Migration RoutesSpotted TowheeBlack TernNorthern HarrierPhotos courtesy amkhosla, Changhua Coast Conservation Action, and amkhosla.Figure by MIT OCW.Within the lower Mississippi Valley, 63 blackbird roosts have been identified with over1 million birds each. Many smaller roosts also exits. These birds disperse 30 miles fordaily feeding.Radar Course 31.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Radar Properties of Birds Even though the radar echo of birds isrelatively small, bird densities are so great thatbirds can often overload a radar with falsetargets Since birds move at relatively low velocities,their speed, if measured, can be used topreferentially threshold out the low velocitybirds.Figure by MIT OCW.Radar Course 32.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Bird Example from Dallas-Fort WorthRadar&BeaconBeacon-OnlyRadar UncorrelatedRadar CorrelatedRadar Course 33.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Attributes of Birds Birds are actually moving point targets–Velocity usually less than 60 knots Mean radar cross section is small, but a fraction of birdreturns fluctuate up to a high level (aircraft like)– Cross section is resonant at S-Band and L-Band Lots of birds per square mile– 10 to 1000 bird / square mile Birds cause a false target problem in many radars– Significant issue for when detecting targets with lowcross sectionsRadar Course 34.pptODonnell (1) 6-19-02MIT Lincoln Laboratory

Insects Insects can clutter the displayand prevent detection ofdesired targets Density of insects can bemany orders of magnitudegreater than that of birds Insect flight path generallyfollows that of the wind Cross section can berepresented as a sphericaldrop of water of the samemass Insect echoes broad side are10 to 1,000 times than whenviewed end onFigure by MIT OCW.Radar Course 35.pptODonnell (1) 6-19-02MIT Lincoln Laboratory