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Radar 2009 a 10 radar clutter1

The document discusses radar clutter from unwanted objects like ground, sea, rain, and birds/insects. It provides examples of military radars for which clutter is an issue and outlines factors that affect ground clutter backscatter like terrain type, frequency, and depression angle. Median ground clutter strength values are shown for various terrain types and frequencies.

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IEEE New Hampshire Section Radar Systems Course 1 Clutter 11/1/2009 IEEE AES Society Radar Systems Engineering Lecture 10 Part 1 Radar Clutter Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer
Radar Systems Course 2 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Block Diagram of Radar System Transmitter Waveform Generation Power Amplifier T / R Switch Antenna Propagation Medium Target Radar Cross Section Photo Image Courtesy of US Air Force Used with permission. Pulse Compression Receiver Clutter Rejection (Doppler Filtering) A / D Converter General Purpose Computer Tracking Data Recording Parameter Estimation Detection Signal Processor Computer Thresholding User Displays and Radar Control Buildings (Radar Clutter)
Radar Systems Course 3 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
Radar Systems Course 4 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Why Study Radar Clutter? Bird Flock Ground Urban Buildings Chaff Targets Target Rain Ground Hills Sea Courtesy MIT Lincoln Laboratory Used with permission Naval Air Defense Scenario
Radar Systems Course 5 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Radars for Which Clutter is a Issue SPS-48 OTH Radar AEGIS SPY 1 AWACS E-3A SPS-49 HAWKEYE E-2C Courtesy of US Navy Courtesy of US Navy Courtesy of US Air Force Courtesy of ITT Gillfillan Used with permission Courtesy of US Navy Courtesy of Raytheon Used with permission
Radar Systems Course 6 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Radars for Which Clutter is a Issue F-16 APG-68 FAA ARSR-4 TPS-79 JOINT STARS E-8 AEROSTAT RADAR APG-63 V(2) Courtesy of Alphapapa Courtesy of Northrop Grumman Used with permission Courtesy of US Air Force Courtesy of Boeing Used with permission WEDGETAIL Courtesy of Wings777Courtesy of Lockheed Martin Used with permission
Radar Systems Course 7 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society How to Handle Noise and Clutter
Radar Systems Course 8 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society How to Handle Noise and Clutter If he doesn’t take his arm off my shoulder I’m going to hide his stash of Hershey Bars !! Why does Steve always talk me into doing ridiculous stunts like this ?
Radar Systems Course 9 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Typical Air Surveillance Radar (Used for Sample Calculations) Frequency S-band (2700–2900 MHz) Instrumented range 60 nautical miles Peak power 1.4 mw Average power 875 W Pulse repetition (700–1200 Hz) frequency 1040 Hz average Antenna rotation rate 12.8 rpm Antenna size 4.8 m × 2.7 m Antenna gain 33 dB Radar Parameters FAA - Airport Surveillance Radar Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 10 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
Radar Systems Course 11 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline - Ground Clutter • Introduction • Mean backscatter – Frequency – Terrain type – Polarization • Temporal statistics • Doppler spectra
Radar Systems Course 12 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society 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 of ground clutter Ship Aircraft
Radar Systems Course 13 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Ground Based Radar Displays 0 dB Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Plan Position Indicator (PPI) Display Map-like Display Radial distance to center Range Angle of radius vector Azimuth Threshold crossings Detections
Radar Systems Course 14 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Photographs of Ground Based Radar’s PPI (Different Levels of Attenuation) Attenuation Level 0 dB Attenuation Level 60 dB Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission.
Radar Systems Course 15 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Photographs of Ground Based Radar’s PPI 0 dB 0 dB 10 dB 20 dB 30 dB 40 dB 50 dB 60 dB 70 dB Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Different Levels of Attenuation
Radar Systems Course 16 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Geometry of Radar Clutter Elevation View Radar h φ cT / 2 ½ cT sec φ Plan View Radar Clutter BRθ 0 A σ σ = Bθ A = RθB [½ cT sec φ] Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 17 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Calculation of Ground Clutter • σ Clutter = σo AA = σo c T 2 R θB • Typical Value of σo = -20 dB = 0.01 m2 m2 – For ASR-9 (Airport Surveillance Radar) R = 60 km c T 2 = 100m θB = 1.5o = 0.026 radians • σ Clutter = x 100 m x 60,000 m x 0.026 radians = 1500 m2 ∴ Must suppress clutter by a factor of 1500 x 20 = 30,000 = 45 dB 0.01 m2 m2 For σ Target = 1 m2 For good detection σ Clutter σ Target = 1 1500 σ Clutter σ Target = 20 Small single-engine aircraft INPUT OUTPUT Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 18 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Joint U.S./Canada Measurement Program • Phase One radar – VHF, UHF, L-, S-, X-bands • Measurements conducted 1982 – 1984 • Archival data at Lincoln Laboratory • 42 sites • Data shared with Canada and the United Kingdom Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 19 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Joint U.S./Canada Measurement Program Radar System ParametersPhase One Radar Frequency Band MHz) Antenna Gain (dB) Antenna Beamwidth Az (deg) El (deg) Peak Power (kW) Polarization PRF (Hz) Pulse Width (µs) Waveform A/D Converter Number of Bits Sampling Rate (MHz) VHF 13 13 42 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 UHF 25 5 15 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 L-Band 28.5 3 10 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 S-Band 35.5 1 4 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 X-Band 38.5 1 4 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 Courtesy of MIT Lincoln Laboratory Adapted from Billingsley, Reference 2
Radar Systems Course 20 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 21 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society dB11F4o −=σ Histograms of Measured Clutter Strength σoF4 (dB) -80 -70 -60 -50 -40 -30 -20 -10 σ°F4 (dB) dB55F4o −=σ Farmland VHF -80 -70 -60 -50 -40 -30 -20 -10 σ°F4 (dB) 0 5 10 Percent Farmland X Band dB23F4o −=σ 10 Forest X Band dB24F4o −=σ -70 -60 -50 -40 -30 -20 -10 0 σ°F4 (dB) 0 50 90 99 5 Percent 0 0 5 5 10 10 Forest VHF σ°F4 (dB) 5050 90 999099 50 90 99 -70 -60 -50 -40 -30 -20 -10 0 Blue Line Is Mean
Radar Systems Course 22 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 23 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Weibull Probability Density Function • The Weibull and Log Normal distributions are used to model ground clutter, because they are too parameter distributions which will allow for skewness (long tails) in the distribution of ground clutter • For , the Weibull distribution degenerates to an Exponential distribution in power (a Rayleigh distribution in voltage) 1aw = ( ) ( ) b 50 b 2 x xlog b 50 1b 2 e x x2logb xp −− ⋅ ⋅⋅ = 4o w w 50 Fx a a/1b xx σ= = = = Median value of In units of m2/m2 Weibull shape parameter
Radar Systems Course 24 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o Lobing Free Space Multipath Propagation Factor F 4 Clutter Strength = Fσo Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 25 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o 1) Radar Parameters • Frequency, f • Spatial resolution, A 2) Geometry • Depression angle (Range R, Height H) 3) Terrain Type • Landform • Land cover Lobing Free Space Multipath Propagation Factor F 4 Clutter Strength = Fσo Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 26 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Mean Ground Clutter Strength vs. Frequency Meanofσ°F4(dB) VHF UHF L- S- X-band 10,000 100 1,000 Frequency (MHz) General Rural (36 Sites) Range Resolution (m) Key Polarization H150 V150 H15/36 V15/36 0 –10 –20 –30 –40 –50 –60 –70 –80 Courtesy of MIT Lincoln Laboratory Used with permission
Radar Systems Course 27 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Major Clutter Variables in Data Collection • Terrain type – Forest – Urban – Farmland – Mountains – Farmland – Desert, marsh, or grassland (few discrete scatterers) • Terrain slope: – High (>2°) – Low (<2°) Moderately low (1° to 2°) Very low (<1°) • Depression angle – High 1° to 2° – Intermediate 0.3° to 1° – Low <0.3°
Radar Systems Course 28 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Land Clutter Backscatter vs. Terrain Type and Frequency Median Value of σo F (dB) Frequency Band Terrain Type VHF UHF L-Band S-Band X-Band URBAN MOUNTAINS FOREST/HIGH RELIEF (Terrain Slopes > 2o ) High Depression Angle (> 1o ) Low Depression Angle (≤ 0.2o ) FOREST/LOW RELIEF (Terrain Slopes < 2o ) High Depression Angle (> 1o ) Intermediate Depression Angle (0. 4o to 1o ) Low Depression Angle (≤ 0.3o ) AGRICULTURAL/HIGH RELIEF (Terrain Slopes ≥ 2o ) AGRICULTURAL/LOW RELIEF Moderately Low Relief (1o < Terrain Slopes < 2o ) Moderately Low Relief (Terrain Slopes < 1o ) DESERT, MARSH, GRASSLAND (Few Discretes) High Depression Angle (≥ 1o ) Low Depression Angle (≤ 0.3o ) -20.9 -7.6 -10.5 -19.5 -14.2 -26.2 -43.6 -32.4 -27.5 -56.0 -38.2 -66.8 -16.0 -10.6 -16.1 -16.8 -15.7 -29.2 -44.1 -27.3 -30.9 -41.1 -39.4 -74.0 -12.6 -17.5 -18.2 -22.6 -20.8 -28.6 -41.4 -26.9 -28.1 -31.6 -39.6 -68.6 -10.1 -21.4 -23.6 -24.6 -29.3 -32.1 -38.9 -34.8 -32.5 -30.9 -37.9 -54.4 -10.8 -21.6 -19.9 -25.0 -26.5 -29.7 -35.4 -28.8 -28.4 -31.5 -25.6 -42.0 Adapted from Billingsley, Reference 2
Radar Systems Course 29 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Statistical Attributes of X-Band Ground Clutter o w o wa σσ50 Adapted from Billingsley, Reference 2
Radar Systems Course 30 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Weibull Parameters for Ground Clutter Distributions 103 aw 106 Frequency Bands Resolution(m2) )dB(o wσ Adapted from Billingsley, Reference 2
Radar Systems Course 31 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society L-Band Clutter Experiment Radar Radar System Parameters Courtesy of MIT Lincoln Laboratory Used with permission . Frequency Band (MHz) Antenna Gain (dB) Antenna Beamwidth Az (deg) El (deg) Peak Power (kW) Polarization PRF (Hz) Pulse Width (µs) Waveform A/D Converter Number of Bits Sampling Rate (MHz) L-Band (1230) 32 6 3 8 HH, VV, HV, VH 500 1 Uncoded CW Pulse 14 2
Radar Systems Course 32 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Windblown Clutter Spectral Model ( )νtotP Exponential shape parameter ( ) ( ) ( ) ( ) ( )νβ− β =ν ν + +νδ + =ν exp 2 P P 1r 1 1r r P ac actot AC spectral power density Ratio of DC power to AC power Doppler velocity in m/s DC spectral power density • Total spectral power density from a cell containing windblown vegetation Doppler Velocity (m/s) North Dakota Cropland (Wheat) Measured Data Ptot(ν)indB DC ContributionAC Contribution Adapted from Billingsley, Reference 2
Radar Systems Course 33 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Measured Power Spectra of L-Band Radar Returns from Forest Curves are hand drawn lines through data in Billingsley Reference 2 Adapted from Billingsley, Reference 2 -3 -2 -1 0 1 2 3 RelativePower(dB) -60 -40 -20 0 20 1-2 mph 6-7 mph 18-20 mph Windy Light Air Breezy LCE Radar Range 7 km Forest Wind Doppler Velocity (m/s)
Radar Systems Course 34 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Modeled Rates of Exponential Decay in the Tails of L-Band Spectra from Wind-Blown Trees ( ) ( )νβ− β =ν exp 2 Pac Exponential shape parameter W indy β = 10.7 0 L-Band –20 –60 –40 RelativePower(dB) 0 0.5 1.0 1.5 Breezyβ=17.5 LightAirβ=23.5 Doppler Velocity (m/s) s/m.v 20≥ • Exponential decay model agrees very well with measured data – X-Band to L-band – Variety of wind conditions Light thru heavy wind – Over wide dynamic range > 50 dB • Previously used Gaussian and power law models break down at wide dynamic ranges • Model parameter empirically developed from measured data [ ]4147.0wlog105.0 10 1 +=β− Velocity of wind (statute miles per hour) β Adapted from Billingsley, Reference 2
Radar Systems Course 35 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Estimated Ground Clutter at Medium Depression Angles (~3 to 70°) Frequency GHz =σ =ψ ψ σ =γ o o sin Backscatter Coefficient Grazing Angle maxγ Urban 0 (dB) -30 -10 -20 0.2 0.5 1.0 2.0 5.0 10.0 20.0 40.0 Open Woods Cultivated land Desert Curves are Skolnik’s estimates from Nathanson data (see Reference 6)Many data collections indicate that from ~3 to ~70 degrees is proportional to (Ref 6) o σ ψsin
Radar Systems Course 36 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society High Depression Angle Ground Clutter • can be large near vertical incidence • In this angle regime the reflected energy is due to backscatter from small flat surfaces on the ground • The total backscatter is the sum of contributions from the different depression angles within the antenna’s beam width – For vertical incidence, measured is at exactly 90° • For an ideal smooth reflecting surface, – This is a better approximation for smooth sea than typically more rough land (lower for land) – generally > 1 and > than resolution cell size) (see Reference 6) oσ oσ oσ< Go ≈σ Antenna Gain oσ
Radar Systems Course 37 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Ground Clutter Spectrum Spread Due to Mechanical Scanning of Antenna • Backscatter from ground modulated by varying gain of antenna pattern as beam scans by ground clutter • Ground clutters Doppler spread: • For FAA Airport Surveillance Radar (S-Band, = 10 cm): Tn 265.0 78.3 clutter B clutter =σ θ Ω =σ = = =θ =Ω T n B Antenna rotation rate (Hz) Antenna beamwidth (radians) Number of pulses in 3 dB antenna beamwidth Time between radar pulses (sec) λ =θ =Ω B = = T n 1.3° ≈σc 1.3° = 0.023 radians 12.7 RPM, 76.2°/sec 22 0.8 msec. 15 Hz
Radar Systems Course 38 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
Radar Systems Course 39 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Figure by MIT OCW. Attributes of Sea Clutter Mean sea backscatter is about 100 times less than ground backscatter o σ • Mean cross section of sea clutter depends on many variables – Radar frequency – Wind and weather Sea State – Grazing angle – Radar Polarization – Range resolution – Cross range resolution • Sea clutter is characterized by – Radar cross section per unit area Ao σ=σ Area Illuminated by Radar Beam Sea Clutter Radar Cross Section Grazing Angle (degrees) σ°-Crosssectionperunitarea(dB)
Radar Systems Course 40 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society World Meteorological Organization Sea State Classification Sea State Wave Height (m) Wind Velocity (knots) Descriptive Term 0 to 1 0 to 0.1 0 to 6 Calm, Rippled 2 0.1 to 0.5 7 to 10 Smooth, Wavelets 3 0.6 to 1.2 11 to 16 Slight to Moderate 4 1.2 to 2.4 17 to 21 Moderate to Rough 5 2.4 to 4 22 to 27 Very Rough 6 4 to 6 28 to 47 High Sea State 1 Sea State 3 Sea State 5 Courtesy of NOAA
Radar Systems Course 41 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter • Environmental parameters – Wave height – Wind speed – The length of time and distance (Fetch) over which the wind has been blowing – Direction of the waves relative to the radar beam – Whether the sea is building up or decreasing – The presence of swell as well as sea waves – The presence of contaminants that might affect the surface tension • Radar parameters – Frequency – Polarization – Grazing angle – Range and cross range resolution • The data has “A curse of dimensionality” – The sea backscatter depends on a large number of variables Adapted from Nathanson, Reference 3
Radar Systems Course 42 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Nathanson Data Compilation of Mean Backscatter Data • Models compiled from experimental data – Upwind, downwind, and crosswind data averaged over – Adjusted from incidence/depression angle to grazing angle – Median values adjusted to mean values – Monostatic radar data; 0.5–5.9 μs pulse; Rayleigh distributions • Original data set (1968), 25 references • Present data set (1991), about 60 references • Grazing angles: –0.1°, 0.3°, 1.0°, 3.0°, 10.0°, 30.0°, 60.0° Adapted from Nathanson, Reference 3
Radar Systems Course 43 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Normalized Mean Sea Backscatter Coefficient σ0 (dB below 1 m2/m2) 0 V 68* 60* 60* 60* H 86* 80* 75* 70* 60* 60* 60* 1 V 70* 65* 56 53 50 50 48* H 84* 73* 66 56 51 48 48* 2 V 63* 58* 53 47 44 42 40* H 82* 65* 55 48 46 41 38* 3 V 58* 54* 48 43 39 37 34 H 73* 60* 48 43 40 37 36 4 V 58* 45 42 39 37 35 32 H 63* 56* 45 39 36 34 34* 5 V 43 38 35 33 34 31 H 60* 50* 42 36 34 34 6 V 33 31* 32 H 41 32* 32 * 5-dB error not unlikely Grazing Angle = 1° UHF L S C X Ku Ka/W Sea State Polarization 0.5 GHz 1.25 3.0 5.6 9.3 17 35/95 Data Collections and Analyses by NRL underscore this note (See Reference 2, page 15-10) Adapted from Nathanson, Reference 3
Radar Systems Course 44 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Reflectivity vs. Grazing Angle • Sea Clutter is independent of polarization and frequency for grazing angles greater than ~45° • In general, backscatter from the sea is less using horizontal polarization than vertical polarization • For low grazing angles and horizontal polarization, the sea clutter backscatter increases as the wavelength is increased Grazing Angle (degrees) 0 10 20 30 40 50 60 70 80 90 -60 -40 -20 0 20 X- and L- Band (V &H) V - Vertical Polarization H - Horizontal Polarization X- and L- Band (V ) L- Band (V ) X- Band (V ) X- Band (H ) L- Band (H ) 220 MHz (H ) 50 MHz (H ) σ°-Crosssectionperunitarea(dB) Adapted from Skolnik, Reference 6
Radar Systems Course 45 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Amplitude Distributions • The distributions for sea echo are between Rayleigh and log normal – Log of sea backscatter is normally distributed • Generally, sea echo for HH polarization deviates from Rayleigh more than it does for VV polarization • For a cell dimension less than about 50 m, sea waves are resolved; the echo is clearly non-Rayleigh • The distributions depend on sea state. The echo usually becomes more Rayleigh-like for the higher seas. • For small cells and small grazing angles, sea clutter is approximately log normal for horizontal polarization Adapted from Skolnik, Reference 6
Radar Systems Course 46 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society More attributes of Sea Clutter • Sea clutter has a mean Doppler velocity and spread – Velocity of waves relative to radar (ship) Wind speed and direction – Sea state • Sea “spikes” – Low grazing angles – Short radar pulse widths
Radar Systems Course 47 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Effect of Wind Speed on Sea Clutter Wind Speed (knots) 4 5 7 10 15 20 30 40 50 σ°-Crosssectionperunitarea(dB) 0 -20 -10 -50 -30 -40 +10 X- Band (V & H) 90° X- Band (V & H) 60° X- Band (V) 10° X- Band (H) 10° L- Band (H) 10° (Various Grazing Angles, Polarizations, and Frequencies) Adapted from Skolnik, Reference 6
Radar Systems Course 48 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Effects of the Wind and Waves • σo increases with increases in wind speed and wave height except at near-vertical incidence • Wind speed and wave height, and wind direction and wave direction are not always highly correlated. • At small grazing angles, σo is highly sensitive to wave height • At centimeter wavelengths, σo is highly sensitive to wind speed at the small and intermediate grazing angles • σo is greatest looking into the wind and waves. – For small grazing angles, the upwind/downwind ratio is often as much as 5 dB and values of 10 dB have been reported Adapted from Skolnik, Reference 6
Radar Systems Course 49 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society More attributes of Sea Clutter • Sea clutter has a mean Doppler velocity and spread – Velocity of waves relative to radar (ship) Wind speed and direction – Sea state • Sea “spikes” – Low grazing angles – Short radar pulse widths
Radar Systems Course 50 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Spikes From Lewis and Olin, NRL •Grazing angle 1.5 deg. •Horizontal polarization • At low grazing angles, sharp sea clutter peaks, known as “sea spikes”, begin to appear • These sea spikes can cause excessive false detections Figure by MIT OCW.
Radar Systems Course 51 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Distributions (Low Grazing Angles) Wind speed Low Wind speed Medium Wind speed High Area Of Sea Spikes -50 -40 -30 -20 -10 0 +10 +20 PercentoftimeClutterExceedsValue X-band Data Grazing Angle 3° Polarization - Horizontal σ° - Cross Section per unit area (dB) 90 30 50 70 10 5 2 1 Adapted from Skolnik, reference 4
Radar Systems Course 52 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Summary • Mean backscatter from sea is about 100 times less than that of ground – Amplitude of backscatter depends on Sea State and a number of other factors Radar wavelength, grazing angle, polarization, etc. • The platform motion of ship based radars and the motion of the sea due to wind give sea clutter a mean Doppler velocity • Sea spikes can cause a false target problem – Occur at low grazing angles and moderate to high wind speeds

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Radar 2009 a 10 radar clutter1

  • 1.
    IEEE New HampshireSection Radar Systems Course 1 Clutter 11/1/2009 IEEE AES Society Radar Systems Engineering Lecture 10 Part 1 Radar Clutter Dr. Robert M. O’Donnell IEEE New Hampshire Section Guest Lecturer
  • 2.
    Radar Systems Course2 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Block Diagram of Radar System Transmitter Waveform Generation Power Amplifier T / R Switch Antenna Propagation Medium Target Radar Cross Section Photo Image Courtesy of US Air Force Used with permission. Pulse Compression Receiver Clutter Rejection (Doppler Filtering) A / D Converter General Purpose Computer Tracking Data Recording Parameter Estimation Detection Signal Processor Computer Thresholding User Displays and Radar Control Buildings (Radar Clutter)
  • 3.
    Radar Systems Course3 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
  • 4.
    Radar Systems Course4 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Why Study Radar Clutter? Bird Flock Ground Urban Buildings Chaff Targets Target Rain Ground Hills Sea Courtesy MIT Lincoln Laboratory Used with permission Naval Air Defense Scenario
  • 5.
    Radar Systems Course5 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Radars for Which Clutter is a Issue SPS-48 OTH Radar AEGIS SPY 1 AWACS E-3A SPS-49 HAWKEYE E-2C Courtesy of US Navy Courtesy of US Navy Courtesy of US Air Force Courtesy of ITT Gillfillan Used with permission Courtesy of US Navy Courtesy of Raytheon Used with permission
  • 6.
    Radar Systems Course6 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Radars for Which Clutter is a Issue F-16 APG-68 FAA ARSR-4 TPS-79 JOINT STARS E-8 AEROSTAT RADAR APG-63 V(2) Courtesy of Alphapapa Courtesy of Northrop Grumman Used with permission Courtesy of US Air Force Courtesy of Boeing Used with permission WEDGETAIL Courtesy of Wings777Courtesy of Lockheed Martin Used with permission
  • 7.
    Radar Systems Course7 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society How to Handle Noise and Clutter
  • 8.
    Radar Systems Course8 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society How to Handle Noise and Clutter If he doesn’t take his arm off my shoulder I’m going to hide his stash of Hershey Bars !! Why does Steve always talk me into doing ridiculous stunts like this ?
  • 9.
    Radar Systems Course9 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Typical Air Surveillance Radar (Used for Sample Calculations) Frequency S-band (2700–2900 MHz) Instrumented range 60 nautical miles Peak power 1.4 mw Average power 875 W Pulse repetition (700–1200 Hz) frequency 1040 Hz average Antenna rotation rate 12.8 rpm Antenna size 4.8 m × 2.7 m Antenna gain 33 dB Radar Parameters FAA - Airport Surveillance Radar Courtesy of MIT Lincoln Laboratory Used with permission
  • 10.
    Radar Systems Course10 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
  • 11.
    Radar Systems Course11 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline - Ground Clutter • Introduction • Mean backscatter – Frequency – Terrain type – Polarization • Temporal statistics • Doppler spectra
  • 12.
    Radar Systems Course12 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society 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 of ground clutter Ship Aircraft
  • 13.
    Radar Systems Course13 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Ground Based Radar Displays 0 dB Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Plan Position Indicator (PPI) Display Map-like Display Radial distance to center Range Angle of radius vector Azimuth Threshold crossings Detections
  • 14.
    Radar Systems Course14 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Photographs of Ground Based Radar’s PPI (Different Levels of Attenuation) Attenuation Level 0 dB Attenuation Level 60 dB Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission.
  • 15.
    Radar Systems Course15 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Photographs of Ground Based Radar’s PPI 0 dB 0 dB 10 dB 20 dB 30 dB 40 dB 50 dB 60 dB 70 dB Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Different Levels of Attenuation
  • 16.
    Radar Systems Course16 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Geometry of Radar Clutter Elevation View Radar h φ cT / 2 ½ cT sec φ Plan View Radar Clutter BRθ 0 A σ σ = Bθ A = RθB [½ cT sec φ] Courtesy of MIT Lincoln Laboratory Used with permission
  • 17.
    Radar Systems Course17 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Calculation of Ground Clutter • σ Clutter = σo AA = σo c T 2 R θB • Typical Value of σo = -20 dB = 0.01 m2 m2 – For ASR-9 (Airport Surveillance Radar) R = 60 km c T 2 = 100m θB = 1.5o = 0.026 radians • σ Clutter = x 100 m x 60,000 m x 0.026 radians = 1500 m2 ∴ Must suppress clutter by a factor of 1500 x 20 = 30,000 = 45 dB 0.01 m2 m2 For σ Target = 1 m2 For good detection σ Clutter σ Target = 1 1500 σ Clutter σ Target = 20 Small single-engine aircraft INPUT OUTPUT Courtesy of MIT Lincoln Laboratory Used with permission
  • 18.
    Radar Systems Course18 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Joint U.S./Canada Measurement Program • Phase One radar – VHF, UHF, L-, S-, X-bands • Measurements conducted 1982 – 1984 • Archival data at Lincoln Laboratory • 42 sites • Data shared with Canada and the United Kingdom Courtesy of MIT Lincoln Laboratory Used with permission
  • 19.
    Radar Systems Course19 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Joint U.S./Canada Measurement Program Radar System ParametersPhase One Radar Frequency Band MHz) Antenna Gain (dB) Antenna Beamwidth Az (deg) El (deg) Peak Power (kW) Polarization PRF (Hz) Pulse Width (µs) Waveform A/D Converter Number of Bits Sampling Rate (MHz) VHF 13 13 42 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 UHF 25 5 15 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 L-Band 28.5 3 10 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 S-Band 35.5 1 4 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 X-Band 38.5 1 4 10 HH,VV 500 0.1, 0.25, and 1 Uncoded CW Pulse 13 10, 5, 1 Courtesy of MIT Lincoln Laboratory Adapted from Billingsley, Reference 2
  • 20.
    Radar Systems Course20 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Courtesy of MIT Lincoln Laboratory Used with permission
  • 21.
    Radar Systems Course21 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society dB11F4o −=σ Histograms of Measured Clutter Strength σoF4 (dB) -80 -70 -60 -50 -40 -30 -20 -10 σ°F4 (dB) dB55F4o −=σ Farmland VHF -80 -70 -60 -50 -40 -30 -20 -10 σ°F4 (dB) 0 5 10 Percent Farmland X Band dB23F4o −=σ 10 Forest X Band dB24F4o −=σ -70 -60 -50 -40 -30 -20 -10 0 σ°F4 (dB) 0 50 90 99 5 Percent 0 0 5 5 10 10 Forest VHF σ°F4 (dB) 5050 90 999099 50 90 99 -70 -60 -50 -40 -30 -20 -10 0 Blue Line Is Mean
  • 22.
    Radar Systems Course22 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o Courtesy of MIT Lincoln Laboratory Used with permission
  • 23.
    Radar Systems Course23 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Weibull Probability Density Function • The Weibull and Log Normal distributions are used to model ground clutter, because they are too parameter distributions which will allow for skewness (long tails) in the distribution of ground clutter • For , the Weibull distribution degenerates to an Exponential distribution in power (a Rayleigh distribution in voltage) 1aw = ( ) ( ) b 50 b 2 x xlog b 50 1b 2 e x x2logb xp −− ⋅ ⋅⋅ = 4o w w 50 Fx a a/1b xx σ= = = = Median value of In units of m2/m2 Weibull shape parameter
  • 24.
    Radar Systems Course24 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o Lobing Free Space Multipath Propagation Factor F 4 Clutter Strength = Fσo Courtesy of MIT Lincoln Laboratory Used with permission
  • 25.
    Radar Systems Course25 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Clutter Physics Depression Angle Microshadowing Visible Terrain Clutter Coefficient σo R H Clutter Histogram Weibull Parameters wa (A) w (f)σ o 4 F (dB)σ o 1) Radar Parameters • Frequency, f • Spatial resolution, A 2) Geometry • Depression angle (Range R, Height H) 3) Terrain Type • Landform • Land cover Lobing Free Space Multipath Propagation Factor F 4 Clutter Strength = Fσo Courtesy of MIT Lincoln Laboratory Used with permission
  • 26.
    Radar Systems Course26 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Mean Ground Clutter Strength vs. Frequency Meanofσ°F4(dB) VHF UHF L- S- X-band 10,000 100 1,000 Frequency (MHz) General Rural (36 Sites) Range Resolution (m) Key Polarization H150 V150 H15/36 V15/36 0 –10 –20 –30 –40 –50 –60 –70 –80 Courtesy of MIT Lincoln Laboratory Used with permission
  • 27.
    Radar Systems Course27 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Major Clutter Variables in Data Collection • Terrain type – Forest – Urban – Farmland – Mountains – Farmland – Desert, marsh, or grassland (few discrete scatterers) • Terrain slope: – High (>2°) – Low (<2°) Moderately low (1° to 2°) Very low (<1°) • Depression angle – High 1° to 2° – Intermediate 0.3° to 1° – Low <0.3°
  • 28.
    Radar Systems Course28 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Land Clutter Backscatter vs. Terrain Type and Frequency Median Value of σo F (dB) Frequency Band Terrain Type VHF UHF L-Band S-Band X-Band URBAN MOUNTAINS FOREST/HIGH RELIEF (Terrain Slopes > 2o ) High Depression Angle (> 1o ) Low Depression Angle (≤ 0.2o ) FOREST/LOW RELIEF (Terrain Slopes < 2o ) High Depression Angle (> 1o ) Intermediate Depression Angle (0. 4o to 1o ) Low Depression Angle (≤ 0.3o ) AGRICULTURAL/HIGH RELIEF (Terrain Slopes ≥ 2o ) AGRICULTURAL/LOW RELIEF Moderately Low Relief (1o < Terrain Slopes < 2o ) Moderately Low Relief (Terrain Slopes < 1o ) DESERT, MARSH, GRASSLAND (Few Discretes) High Depression Angle (≥ 1o ) Low Depression Angle (≤ 0.3o ) -20.9 -7.6 -10.5 -19.5 -14.2 -26.2 -43.6 -32.4 -27.5 -56.0 -38.2 -66.8 -16.0 -10.6 -16.1 -16.8 -15.7 -29.2 -44.1 -27.3 -30.9 -41.1 -39.4 -74.0 -12.6 -17.5 -18.2 -22.6 -20.8 -28.6 -41.4 -26.9 -28.1 -31.6 -39.6 -68.6 -10.1 -21.4 -23.6 -24.6 -29.3 -32.1 -38.9 -34.8 -32.5 -30.9 -37.9 -54.4 -10.8 -21.6 -19.9 -25.0 -26.5 -29.7 -35.4 -28.8 -28.4 -31.5 -25.6 -42.0 Adapted from Billingsley, Reference 2
  • 29.
    Radar Systems Course29 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Statistical Attributes of X-Band Ground Clutter o w o wa σσ50 Adapted from Billingsley, Reference 2
  • 30.
    Radar Systems Course30 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Weibull Parameters for Ground Clutter Distributions 103 aw 106 Frequency Bands Resolution(m2) )dB(o wσ Adapted from Billingsley, Reference 2
  • 31.
    Radar Systems Course31 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society L-Band Clutter Experiment Radar Radar System Parameters Courtesy of MIT Lincoln Laboratory Used with permission . Frequency Band (MHz) Antenna Gain (dB) Antenna Beamwidth Az (deg) El (deg) Peak Power (kW) Polarization PRF (Hz) Pulse Width (µs) Waveform A/D Converter Number of Bits Sampling Rate (MHz) L-Band (1230) 32 6 3 8 HH, VV, HV, VH 500 1 Uncoded CW Pulse 14 2
  • 32.
    Radar Systems Course32 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Windblown Clutter Spectral Model ( )νtotP Exponential shape parameter ( ) ( ) ( ) ( ) ( )νβ− β =ν ν + +νδ + =ν exp 2 P P 1r 1 1r r P ac actot AC spectral power density Ratio of DC power to AC power Doppler velocity in m/s DC spectral power density • Total spectral power density from a cell containing windblown vegetation Doppler Velocity (m/s) North Dakota Cropland (Wheat) Measured Data Ptot(ν)indB DC ContributionAC Contribution Adapted from Billingsley, Reference 2
  • 33.
    Radar Systems Course33 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Measured Power Spectra of L-Band Radar Returns from Forest Curves are hand drawn lines through data in Billingsley Reference 2 Adapted from Billingsley, Reference 2 -3 -2 -1 0 1 2 3 RelativePower(dB) -60 -40 -20 0 20 1-2 mph 6-7 mph 18-20 mph Windy Light Air Breezy LCE Radar Range 7 km Forest Wind Doppler Velocity (m/s)
  • 34.
    Radar Systems Course34 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Modeled Rates of Exponential Decay in the Tails of L-Band Spectra from Wind-Blown Trees ( ) ( )νβ− β =ν exp 2 Pac Exponential shape parameter W indy β = 10.7 0 L-Band –20 –60 –40 RelativePower(dB) 0 0.5 1.0 1.5 Breezyβ=17.5 LightAirβ=23.5 Doppler Velocity (m/s) s/m.v 20≥ • Exponential decay model agrees very well with measured data – X-Band to L-band – Variety of wind conditions Light thru heavy wind – Over wide dynamic range > 50 dB • Previously used Gaussian and power law models break down at wide dynamic ranges • Model parameter empirically developed from measured data [ ]4147.0wlog105.0 10 1 +=β− Velocity of wind (statute miles per hour) β Adapted from Billingsley, Reference 2
  • 35.
    Radar Systems Course35 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Estimated Ground Clutter at Medium Depression Angles (~3 to 70°) Frequency GHz =σ =ψ ψ σ =γ o o sin Backscatter Coefficient Grazing Angle maxγ Urban 0 (dB) -30 -10 -20 0.2 0.5 1.0 2.0 5.0 10.0 20.0 40.0 Open Woods Cultivated land Desert Curves are Skolnik’s estimates from Nathanson data (see Reference 6)Many data collections indicate that from ~3 to ~70 degrees is proportional to (Ref 6) o σ ψsin
  • 36.
    Radar Systems Course36 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society High Depression Angle Ground Clutter • can be large near vertical incidence • In this angle regime the reflected energy is due to backscatter from small flat surfaces on the ground • The total backscatter is the sum of contributions from the different depression angles within the antenna’s beam width – For vertical incidence, measured is at exactly 90° • For an ideal smooth reflecting surface, – This is a better approximation for smooth sea than typically more rough land (lower for land) – generally > 1 and > than resolution cell size) (see Reference 6) oσ oσ oσ< Go ≈σ Antenna Gain oσ
  • 37.
    Radar Systems Course37 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Ground Clutter Spectrum Spread Due to Mechanical Scanning of Antenna • Backscatter from ground modulated by varying gain of antenna pattern as beam scans by ground clutter • Ground clutters Doppler spread: • For FAA Airport Surveillance Radar (S-Band, = 10 cm): Tn 265.0 78.3 clutter B clutter =σ θ Ω =σ = = =θ =Ω T n B Antenna rotation rate (Hz) Antenna beamwidth (radians) Number of pulses in 3 dB antenna beamwidth Time between radar pulses (sec) λ =θ =Ω B = = T n 1.3° ≈σc 1.3° = 0.023 radians 12.7 RPM, 76.2°/sec 22 0.8 msec. 15 Hz
  • 38.
    Radar Systems Course38 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Outline • Motivation • Backscatter from unwanted objects – Ground – Sea – Rain – Birds and Insects
  • 39.
    Radar Systems Course39 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Figure by MIT OCW. Attributes of Sea Clutter Mean sea backscatter is about 100 times less than ground backscatter o σ • Mean cross section of sea clutter depends on many variables – Radar frequency – Wind and weather Sea State – Grazing angle – Radar Polarization – Range resolution – Cross range resolution • Sea clutter is characterized by – Radar cross section per unit area Ao σ=σ Area Illuminated by Radar Beam Sea Clutter Radar Cross Section Grazing Angle (degrees) σ°-Crosssectionperunitarea(dB)
  • 40.
    Radar Systems Course40 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society World Meteorological Organization Sea State Classification Sea State Wave Height (m) Wind Velocity (knots) Descriptive Term 0 to 1 0 to 0.1 0 to 6 Calm, Rippled 2 0.1 to 0.5 7 to 10 Smooth, Wavelets 3 0.6 to 1.2 11 to 16 Slight to Moderate 4 1.2 to 2.4 17 to 21 Moderate to Rough 5 2.4 to 4 22 to 27 Very Rough 6 4 to 6 28 to 47 High Sea State 1 Sea State 3 Sea State 5 Courtesy of NOAA
  • 41.
    Radar Systems Course41 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter • Environmental parameters – Wave height – Wind speed – The length of time and distance (Fetch) over which the wind has been blowing – Direction of the waves relative to the radar beam – Whether the sea is building up or decreasing – The presence of swell as well as sea waves – The presence of contaminants that might affect the surface tension • Radar parameters – Frequency – Polarization – Grazing angle – Range and cross range resolution • The data has “A curse of dimensionality” – The sea backscatter depends on a large number of variables Adapted from Nathanson, Reference 3
  • 42.
    Radar Systems Course42 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Nathanson Data Compilation of Mean Backscatter Data • Models compiled from experimental data – Upwind, downwind, and crosswind data averaged over – Adjusted from incidence/depression angle to grazing angle – Median values adjusted to mean values – Monostatic radar data; 0.5–5.9 μs pulse; Rayleigh distributions • Original data set (1968), 25 references • Present data set (1991), about 60 references • Grazing angles: –0.1°, 0.3°, 1.0°, 3.0°, 10.0°, 30.0°, 60.0° Adapted from Nathanson, Reference 3
  • 43.
    Radar Systems Course43 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Normalized Mean Sea Backscatter Coefficient σ0 (dB below 1 m2/m2) 0 V 68* 60* 60* 60* H 86* 80* 75* 70* 60* 60* 60* 1 V 70* 65* 56 53 50 50 48* H 84* 73* 66 56 51 48 48* 2 V 63* 58* 53 47 44 42 40* H 82* 65* 55 48 46 41 38* 3 V 58* 54* 48 43 39 37 34 H 73* 60* 48 43 40 37 36 4 V 58* 45 42 39 37 35 32 H 63* 56* 45 39 36 34 34* 5 V 43 38 35 33 34 31 H 60* 50* 42 36 34 34 6 V 33 31* 32 H 41 32* 32 * 5-dB error not unlikely Grazing Angle = 1° UHF L S C X Ku Ka/W Sea State Polarization 0.5 GHz 1.25 3.0 5.6 9.3 17 35/95 Data Collections and Analyses by NRL underscore this note (See Reference 2, page 15-10) Adapted from Nathanson, Reference 3
  • 44.
    Radar Systems Course44 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Reflectivity vs. Grazing Angle • Sea Clutter is independent of polarization and frequency for grazing angles greater than ~45° • In general, backscatter from the sea is less using horizontal polarization than vertical polarization • For low grazing angles and horizontal polarization, the sea clutter backscatter increases as the wavelength is increased Grazing Angle (degrees) 0 10 20 30 40 50 60 70 80 90 -60 -40 -20 0 20 X- and L- Band (V &H) V - Vertical Polarization H - Horizontal Polarization X- and L- Band (V ) L- Band (V ) X- Band (V ) X- Band (H ) L- Band (H ) 220 MHz (H ) 50 MHz (H ) σ°-Crosssectionperunitarea(dB) Adapted from Skolnik, Reference 6
  • 45.
    Radar Systems Course45 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Amplitude Distributions • The distributions for sea echo are between Rayleigh and log normal – Log of sea backscatter is normally distributed • Generally, sea echo for HH polarization deviates from Rayleigh more than it does for VV polarization • For a cell dimension less than about 50 m, sea waves are resolved; the echo is clearly non-Rayleigh • The distributions depend on sea state. The echo usually becomes more Rayleigh-like for the higher seas. • For small cells and small grazing angles, sea clutter is approximately log normal for horizontal polarization Adapted from Skolnik, Reference 6
  • 46.
    Radar Systems Course46 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society More attributes of Sea Clutter • Sea clutter has a mean Doppler velocity and spread – Velocity of waves relative to radar (ship) Wind speed and direction – Sea state • Sea “spikes” – Low grazing angles – Short radar pulse widths
  • 47.
    Radar Systems Course47 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Effect of Wind Speed on Sea Clutter Wind Speed (knots) 4 5 7 10 15 20 30 40 50 σ°-Crosssectionperunitarea(dB) 0 -20 -10 -50 -30 -40 +10 X- Band (V & H) 90° X- Band (V & H) 60° X- Band (V) 10° X- Band (H) 10° L- Band (H) 10° (Various Grazing Angles, Polarizations, and Frequencies) Adapted from Skolnik, Reference 6
  • 48.
    Radar Systems Course48 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Effects of the Wind and Waves • σo increases with increases in wind speed and wave height except at near-vertical incidence • Wind speed and wave height, and wind direction and wave direction are not always highly correlated. • At small grazing angles, σo is highly sensitive to wave height • At centimeter wavelengths, σo is highly sensitive to wind speed at the small and intermediate grazing angles • σo is greatest looking into the wind and waves. – For small grazing angles, the upwind/downwind ratio is often as much as 5 dB and values of 10 dB have been reported Adapted from Skolnik, Reference 6
  • 49.
    Radar Systems Course49 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society More attributes of Sea Clutter • Sea clutter has a mean Doppler velocity and spread – Velocity of waves relative to radar (ship) Wind speed and direction – Sea state • Sea “spikes” – Low grazing angles – Short radar pulse widths
  • 50.
    Radar Systems Course50 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Spikes From Lewis and Olin, NRL •Grazing angle 1.5 deg. •Horizontal polarization • At low grazing angles, sharp sea clutter peaks, known as “sea spikes”, begin to appear • These sea spikes can cause excessive false detections Figure by MIT OCW.
  • 51.
    Radar Systems Course51 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Distributions (Low Grazing Angles) Wind speed Low Wind speed Medium Wind speed High Area Of Sea Spikes -50 -40 -30 -20 -10 0 +10 +20 PercentoftimeClutterExceedsValue X-band Data Grazing Angle 3° Polarization - Horizontal σ° - Cross Section per unit area (dB) 90 30 50 70 10 5 2 1 Adapted from Skolnik, reference 4
  • 52.
    Radar Systems Course52 Clutter 11/1/2009 IEEE New Hampshire Section IEEE AES Society Sea Clutter Summary • Mean backscatter from sea is about 100 times less than that of ground – Amplitude of backscatter depends on Sea State and a number of other factors Radar wavelength, grazing angle, polarization, etc. • The platform motion of ship based radars and the motion of the sea due to wind give sea clutter a mean Doppler velocity • Sea spikes can cause a false target problem – Occur at low grazing angles and moderate to high wind speeds