Introduction To Radar Systems 2004

Medium PRF ModeHigh PRF ModePowerPowerMedium PRF ModeTrue Doppler FrequencyPRFTrue Doppler FrequencyPRF 1PRF 2PRF 5PRF 4True TargetDopplerPRF 1 In the Doppler domain, the target and clutter alias (fold down)into the range 0 to PRF1, PRF2, etc.– Because of the aliasing of sidelobe clutter, medium PRF radarsshould have very low sidelobes to mitigate this problem In the range domain similar aliasing occurs– Sensitivity Time Control (STC) cannot be used to reduce cluttereffects (noted in earlier lectures) Range and Doppler ambiguity resolution techniquesdescribed in previous lectureRadar Systems CourseAirborne PD 1/1/201029IEEE New Hampshire SectionIEEE AES Society

Medium PRF Pulse Doppler Radar Both range and Doppler ambiguities exist––– Seven or eight different PRFs must be usedInsures target seen at enough Doppler frequencies to resolve rangeambiguitiesTransmitter larger because of redundant waveforms used to resolveambiguitiesThere is no clutter free region––Fewer range ambiguities implies less of a problem with sidelobeclutterAntenna must have low sidelobes to reduce sidelobe clutter Often best single waveform for airborne fighter / interceptor More range gates than high PRF, but fewer Doppler filters for eachrange gate Better range accuracy and Doppler resolution than high PRFsystemsRadar Systems CourseAirborne PD 1/1/201030IEEE New Hampshire SectionIEEE AES Society

Outline Introduction– The airborne radar environment Different airborne radar missions– Pulse Doppler radar in small fighter / interceptor aircraftF-14, F-15, F-16, F-35– Airborne, surveillance, early warning radarsE-2C (Hawkeye), E-3 (AWACS), E-8A (JOINT STARS)– Airborne synthetic aperture radarMilitary and civilian remote sensing missionsTo be covered in lecture 19, later in the course Radar Systems CourseAirborne PD 1/1/2010Summary31IEEE New Hampshire SectionIEEE AES Society

Airborne Surveillance & TrackingRadars Missions and Functions– Surveillance, Tracking, Fire Control– Reconnaissance– Intelligence Examples– Airborne early warning– Ground target detection and tracking Radar modes––––Pulse Doppler radarSynthetic Aperture radarDisplaced Phase Center Antenna (DPCA)Other modes/techniquesElevated radar platforms provide long range andover the horizon coverage of airborne and ground based targetsRadar Systems CourseAirborne PD 1/1/201032IEEE New Hampshire SectionIEEE AES Society

Examples of Airborne RadarsCourtesy of US NavyBoeing 737 AEW&CCourtesy of milintelTRGlobal HawkE-2CAPS-125Courtesy of US Air ForceJOINT STARS E-8AAPY-3AWACSE-3AAPY-1Courtesy of US Air ForceCourtesy of US Air ForceRadar Systems CourseAirborne PD 1/1/201033IEEE New Hampshire SectionIEEE AES Society

AEW Radar CoverageGround Based Surveillance Radar CoverageAirborne Surveillance Radar Coverage Elevating the radar can extend radar coverage well out over thehorizon Range Coverage -400 km to 800 kmGround based radars 400 kmAirborne radar 800 km–– IssuesHigh acquisition and operating costsLimited Antenna sizeRadar Weight and prime powerMore challenging clutter environment––––Radar Systems CourseAirborne PD 1/1/201034IEEE New Hampshire SectionIEEE AES Society

Characteristics of Ground Clutter(from Airborne Platform)vPGround Clutter Doppler Frequency2 vP2 vPfC cos α cos θ sin φλλMainBeamClutterΔf SL MLDoppler Frequency Width(Sidelobe Main Beam Clutter)Δf SL MLSidelobeClutterΔf ML4 vP λDoppler Frequency Width of MainBeam Clutter (Null to Null)2 vP λRadar Systems CourseAirborne PD 1/1/2010DopplerFrequency (Hz)352 vPλΔf MB 4 vP λ 4 vP λ LLIEEE New Hampshire SectionIEEE AES Society

Spread of Main Beam ClutterBeamCenterAircraft Velocity and TrajectoryIndividualClutterScattererα Doppler frequency ofclutter return depends onangle of clutter withvelocity vector of aircraft Doppler frequency ofclutter return at center ofbeam2 VPfC cos θλ Doppler spread of mainbeam clutter can be foundby differentiating thisequationθθBVPRadarSpread of Main Beam ClutterMaximum at θ 90 Adapted fromSkolnik Reference 1Radar Systems CourseAirborne PD 1/1/201036Depression angle of beam neglected2 VPΔf C θ B sin θλIEEE New Hampshire SectionIEEE AES Society

Clutter Spread with a UHF Airborne RadarClutter PowerSpeed of aircraft 400 knotsAntenna beamwidth 7 degreesθ 90 θ Angle between radar beamand the platform velocity vectorθ 60 60Hz35Hz0100200300θ 0 θ 30 30Hz400500600Doppler Frequency (Hz) Both the width of the clutter spectra and its center frequencydepend on the angle θ When the antenna points in the direction of the platform velocityvector, the Doppler shift of the clutter echo is maximum, but thewidth of the spectrum is theoretically zero When the antenna is directed in the direction perpendicular to thedirection of the platform velocity, the clutter center frequency iszero, but the spread is maximumAdapted from Skolnik Reference 1Radar Systems CourseAirborne PD 1/1/201037IEEE New Hampshire SectionIEEE AES Society

Aliasing of Clutter in Low PRF UHFAirborne RadarClutter PowerPRF 360 Hzθ 90 0θ 90 100200300360400Doppler Frequency (Hz) PRF 360 Hz corresponds to a maximum unambiguousrange of 225 nmiA relatively large portion of the frequency domain (Dopplerspace) is occupied by the clutter spectrum because ofplatform motionThe widening of the clutter needs to be reduced in order forstandard clutter suppression techniques to be effectiveRadar Systems CourseAirborne PD 1/1/201038IEEE New Hampshire SectionIEEE AES Society

AEW Airborne Radar Clutter Rejection There are 2 effects that can seriously degrade theperformance of a radar on a moving platform– A non-zero Doppler clutter shift– A widening of the clutter spectrum These may be compensated for by two different techniques– TACCAR (Time Averaged Clutter Coherent Airborne Radar)The change in center frequency of the clutter spectrum– DPCA (Displaced Phase Center Antenna)The widening of the clutter spectrum Radars which have used these techniques, over the years,to compensate for platform motion are Airborne EarlyWarning radarsRadar Systems CourseAirborne PD 1/1/201039IEEE New Hampshire SectionIEEE AES Society

Compensation for Clutter Doppler Shift TACCAR (Time Averaged Clutter Coherent Airborne Radar)– Also called “Clutter Lock MTI” The Doppler frequency shift from ground clutter can becompensated by using the clutter echo signal itself to setthe frequency of the reference oscillator (or coho)– This process centers the ground clutter to zero Dopplerfrequency– The standard MTI filter (notch at zero Doppler) attenuates theground clutter This technique has been used in ground based radars tomitigate the effect of moving clutter– Not used after the advent of Doppler filter processingRadar Systems CourseAirborne PD 1/1/201040IEEE New Hampshire SectionIEEE AES Society

AEW Advances - E-2D and MP-RTIPE-2D E-2D––Courtesy of US NavyMP-RTIP mounted on Proteus Aircraft Mechanically RotatingActive ElectronicallyScanned Antenna (AESA)Space Time AdaptiveProcessing (STAP)MP-RTIP––“Multi-Platform RadarTechnology InsertionProgram”Originally Joint StarsUpgrade ProgramGlobal Hawk and then awide area surveillanceaircraft–Advanced ground targetsurveillance capabilityCourtesy of US Air ForceRadar Systems CourseAirborne PD 1/1/201041IEEE New Hampshire SectionIEEE AES Society

E-3A Sentry - AWACSRadar APY-2E-3A Sentry AircraftS-Band (10 cm wavelength)Range 250 milesHigh PRF waveform to rejectclutter in look down modeLong range beyond thehorizon surveillance modeCourtesy of USAF AWACS Radar (S-Band)––Maritime surveillance modeMission –Long range Surveillance, Command and Control forair tactical environmentRadar System Improvement Program (RSIP)Advanced pulse Doppler waveformsPulse compression addedDetection range doubled (over original radar)Radar Systems CourseAirborne PD 1/1/201042See reference 1IEEE New Hampshire SectionIEEE AES Society

AWACS Radar AntennaRadomeRadar AntennaCourtesy of martin juliaCourtesy of Northrop GrummanUsed with Permission Radome Diameter 30 ftAWACS (APY-1/2) Antenna– Phased array – 26 ft by 4.5 ft ultralow sidelobe arrayElliptically shaped28 slotted waveguides with a total of over 4000 slotsAntenna is mechanically scanned 360 in azimuthUses 28 ferrite reciprocal phase shifters to scan in elevation10 sec rotation (data) rate––––See Skolnik reference 1Radar Systems CourseAirborne PD 1/1/201043IEEE New Hampshire SectionIEEE AES Society

Displaced Phase Center Antenna (DPCA)ConceptT1T2If the aircraft motion is exactly compensated by the movement of thephase center of the antenna beam, then there will be no clutterspread due to aircraft motion, and the clutter can be cancelled with atwo pulse canceller344334 2.pptRMO 9-01-00Viewgraph Courtesy of MIT Lincoln Laboratory Used with permissionRadar Systems CourseAirborne PD 1/1/201044IEEE New Hampshire SectionIEEE AES Society

DPCA for Mechanically Scanned AEW RadarIndividualClutterScattererAngle α offbeamcenterBeam 1Beam 2 A mechanically rotating antenna on a movingplatform that generates two overlapping(squinted) beams can act as a DCPA whenthe outputs of the two squinted beams areproperly combinedThe sum and difference of the two squinted beams aretaken––The sum is used for transmitThe sum and difference are used on receive A phase advance is added to the first pulse and a phase lag is added tothe second pulse beams are taken The added (or subtracted) phase shift depends on aircraft velocity, thePRF, and the scan angle of the radar relative to the aircraft direction The two signals are then subtracted, resulting in the cancellation of theDoppler spread of the clutterRadar Systems CourseAirborne PD 1/1/201045IEEE New Hampshire SectionIEEE AES Society

DPCA – The Math- AbbreviatedIndividualClutterScattererAngle α off Beam 1beamcenterΣ R Sum (2 pulses) of receive signalΔ R Difference (2 pulses) of receive signalBeam 2E22ηThe sum and difference of the two squinted beamsare takenThe sum is used for transmitThe sum and difference are used onreceiveAfter MUCH manipulation, the correctedreceived pulses become:Phasorrepresentation ofclutter echoes from2 successive pulsesE1E2Correctionsapplied to pulsesallowingcancellationηηRadar Systems CourseAirborne PD 1/1/2010e 2 j E 2 tan ηE146e1 j E1 tan ηPulse 1Σ R (α ) j k (v sin θ ) Δ R (α )Pulse 2Σ R (α ) j k (v sin θ ) Δ R (α )ΣConstant k accounts for differences inand Δ patterns, as well as a factor 4 TP/DFor more detail see Skolnik, Reference 1, pp 166-168IEEE New Hampshire SectionIEEE AES Society

Multiple Antenna Surveillance Radar(MASR)CP130-569DPCA OffDPCA OnViewgraph Courtesy of MIT Lincoln LaboratoryUsed with permissionRadar Systems CourseAirborne PD 1/1/201047IEEE New Hampshire SectionIEEE AES Society

Joint Surveillance Target Attack RadarSystem (Joint STARS)Courtesy of US Air Force Employs Interferometric SAR for airborne detection of groundvehicles and imaging of ground and surface targets– Employs APY-3, X Band radar Mission in wide area surveillance mode:– Coverage 50,000 km2– Detect, locate, identify, classify, and track trucks, tanks, and othervehiclesCan differentiate tracked and wheeled vehiclesCan see vehicles at ranges 200 km , moving at walking speedsRadar Systems CourseAirborne PD 1/1/201048IEEE New Hampshire SectionIEEE AES Society

Joint Stars RadarJSTARSAntennaCourtesy of Northrop GrummanUsed with Permission Radar employs a slotted array antenna 24 ft by 2 ft456 x 28 horizontally polarized elementsBeam scans 60 in azimuth; mechanically rotated in elevation–– Aperture can be used as a whole for SAR mappingWhen total aperture is divided into 3 independent apertures in theinterferometric mode, it is used for moving target detectionMoving targets are separated from clutter by different time of arrivals oftarget and clutter in the 3 aperturesDPCA techniques are used to cancel main beam clutter––Radar Systems CourseAirborne PD 1/1/201049IEEE New Hampshire SectionIEEE AES Society

Joint Stars Moving Target DetectionsOperation DesertStorm(Feb 1991)Courtesy of Northrop GrummanUsed with PermissionRadar Systems CourseAirborne PD 1/1/201050IEEE New Hampshire SectionIEEE AES Society

Outline Introduction– The airborne radar environment Different airborne radar missions– Pulse Doppler radar in small fighter / interceptor aircraftF-14, F-15, F-16, F-35– Airborne, surveillance, early warning radarsE-2C (Hawkeye), E-3 (AWACS), E-8A (JOINT STARS)– Airborne synthetic aperture radarSAR basics to be covered in lecture 19Military and civilian remote sensing missionsTo be covered in lecture 19, later in the course Radar Systems CourseAirborne PD 1/1/2010Summary51IEEE New Hampshire SectionIEEE AES Society

Detection of Ground Moving Targets Ground Moving Target Indication (GMTI)– Low or medium PRF pulse Doppler radar used– PRF chosen so that Doppler region of interest isunambiguous in range and Doppler– Ku (16 GHz) or Kα (35 GHz) Band often used, since fixedminimum detectable Doppler frequency will allow detection oflower velocities than X band– APG-67 (X-Band) in F-20 fighter has GMTI mode usingmedium PRF– AWACS has low PRF ship detection mode Side-Looking Airborne Radar (SLAR)– Standard airborne radar subtracts sequential conventionalimages of terrain ( Non-coherent MTI) to detect moving targetsRadar Systems CourseAirborne PD 1/1/201052IEEE New Hampshire SectionIEEE AES Society

Detection of Ground Moving Targets Synthetic Aperture Radar (SAR) with MTI– SARs (discussed in lecture 19) produce excellent images offixed targets on the groundGood cross range resolution obtain by processing sequentialtarget echoes as aircraft moves a significant distance LCross range resolution inversely proportional to L not antenna size D– Moving targets distorted and smeared in SAR image– Can be detected if target Doppler is greater than bandwidthof clutter echo– Requires high PRF to avoid aliasing issues Joint Stars– Uses interferometer for clutter suppression processingRadar Systems CourseAirborne PD 1/1/201053IEEE New Hampshire SectionIEEE AES Society

Summary Difficult ground clutter environment is chief radar design driver forairborne radarsElevated radar platform implies ground clutter at long rangeBoth Doppler frequency of clutter and its spread depend on radarplatform motion and scan angle–– Clutter challenges with Airborne radarsAntenna aperture size often limits frequencies, so that ambiguousrange and Doppler velocity issues arise–Low, Medium and High PRF Modes each have unique clutter issuesDoppler spreading of ground clutter, particularly at broadside, viewingcan degrade performance– Sophisticated clutter suppression techniques can alleviate some ofthese issuesDPCA techniquesMedium and High PRF modes often imply higher power–– Active Electronically Scanned arrays and advanced signalprocessing techniques (STAP) offer significant new capabilities forairborne radarsRadar Systems CourseAirborne PD 1/1/201054IEEE New Hampshire SectionIEEE AES Society

Homework Problems From Skolnik (Reference 1)Problems 3-19, 3-20, 3-21, 3-22, 3-23, and 3-24Show that the maximum Doppler frequency of ground clutter as seenby an airborne radar is––2V h2 1 2 fD λ R Where:V velocity of airborne radarλ radar wavelengthh height of radar above groundR slant rangeShow that, for an airborne radar flying at a constant height above theground, the lines of constant clutter velocity are a set of hyperbolae–The last problem is from Roger Sullivan’s previously referenced textRadar Systems CourseAirborne PD 1/1/201055IEEE New Hampshire SectionIEEE AES Society

References1. Skolnik, M., Introduction to Radar Systems, McGraw-Hill, NewYork, 3rd Ed., 20012. Barton, D. K., Modern Radar System Analysis, Norwood,Mass., Artech House, 19883. Skolnik, M., Editor in Chief, Radar Handbook, New York,McGraw-Hill, 3rd Ed., 20084. Skolnik, M., Editor in Chief, Radar Handbook, New York,McGraw-Hill, 2nd Ed., 19905. Nathanson, F. E., Radar Design Principles, New York,McGraw-Hill, 1st Ed., 19696. Richards, M., Fundamentals of Radar Signal Processing,McGraw-Hill, New York, 20057. Schleher, D. C., MTI and Pulsed Doppler Radar, Artech,Boston, 19918. Long, W. H., et. al, “Medium PRF for the AN/APG-66 Radar”,Proceedings of the IEEE, Vol. 73, No 2, pp 301-311, February1985Radar Systems CourseAirborne PD 1/1/201056IEEE New Hampshire SectionIEEE AES Society

Acknowledgements Niall J. DuffyDr. Allen HearnMark A. WeinerRadar Systems CourseAirborne PD 1/1/201057IEEE New Hampshire SectionIEEE AES Society

Radar Systems Course 3 Airborne PD 1/1/2010 IEEE New Hampshire Section IEEE AES Society Outline Introduction – The airborne radar mission and environment Clutter is the main issue Different airborne radar missions – Pulse Doppler radar in small fighter / interceptor aircraft F-14, F-15, F-16,