Radar Systems Engineering Lecture 10 Part 1 Radar Clutter

Transcription

1 Radar Systems Engineering Lecture 10 Part 1 Radar Clutter Dr. Robert M. O Donnell Guest Lecturer Radar Systems Course 1

2 Block Diagram of Radar System Target Radar Cross Section Propagation Medium T / R Switch Power Amplifier Transmitter Waveform Generation Antenna Buildings (Radar Clutter) Signal Processor Computer Receiver A / D Converter Pulse Compression Clutter Rejection (Doppler Filtering) User Displays and Radar Control General Purpose Computer Tracking Parameter Estimation Thresholding Detection Data Recording Photo Image Courtesy of US Air Force Used with permission. Radar Systems Course 2

3 Outline Motivation Backscatter from unwanted objects Ground Sea Rain Birds and Insects Radar Systems Course 3

4 Why Study Radar Clutter? Naval Air Defense Scenario Bird Flock Rain Chaff Targets Target Sea Ground Urban Buildings Ground Hills Courtesy MIT Lincoln Laboratory Used with permission Radar Systems Course 4

5 Radars for Which Clutter is a Issue AEGIS SPY 1 Courtesy of ITT Gillfillan Used with permission SPS-48 SPS-49 Courtesy of US Navy Courtesy of US Navy AWACS E-3A OTH Radar Courtesy of Raytheon Used with permission Courtesy of US Navy HAWKEYE E-2C Courtesy of US Air Force Radar Systems Course 5

6 Radars for Which Clutter is a Issue Courtesy of US Air Force JOINT STARS E-8 AEROSTAT RADAR APG-63 V(2) F-16 APG-68 Courtesy of Alphapapa Courtesy of Boeing Used with permission Courtesy of Northrop Grumman Used with permission FAA ARSR-4 TPS-79 WEDGETAIL Courtesy of Lockheed Martin Used with permission Courtesy of Wings777 Radar Systems Course 6

7 How to Handle Noise and Clutter Radar Systems Course 7

8 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 8

9 Typical Air Surveillance Radar (Used for Sample Calculations) Radar Parameters Frequency FAA - Airport Surveillance Radar Instrumented range Peak power Average power Pulse repetition frequency Antenna rotation rate Antenna size S-band ( MHz) 60 nautical miles 1.4 mw 875 W ( Hz) 1040 Hz average 12.8 rpm 4.8 m 2.7 m Courtesy of MIT Lincoln Laboratory Used with permission Antenna gain 33 db Radar Systems Course 9

10 Outline Motivation Backscatter from unwanted objects Ground Sea Rain Birds and Insects Radar Systems Course 10

11 Outline - Ground Clutter Introduction Mean backscatter Frequency Terrain type Polarization Temporal statistics Doppler spectra Radar Systems Course 11

12 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 12

13 Ground Based Radar Displays Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Plan Position Indicator (PPI) Display Map-like Display Radial distance to center Angle of radius vector Threshold crossings Range Azimuth Detections 0 db Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Radar Systems Course 13

14 Photographs of Ground Based Radar s PPI (Different Levels of Attenuation) Mountainous Region of Lakehead, Ontario, Canada PPI Set for 30 nmi. Attenuation Level 0 db Attenuation Level 60 db Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Radar Systems Course 14

15 Photographs of Ground Based Radar s PPI 0 db Different Levels of Attenuation 10 db 40 db 50 db 0 db 20 db 30 db Shrader, W. from Tutorial on MTI Radar presented at Selenia, Rome, Italy. Used with permission. Radar Systems Course db 70 db

16 Geometry of Radar Clutter Radar Elevation View h ct / 2 φ ½ ct sec φ Plan View Radar θ B Clutter Rθ B σ = 0 σ A A = Rθ B [½ ct sec φ] Courtesy of MIT Lincoln Laboratory Used with permission Radar Systems Course 16

17 Calculation of Ground Clutter Typical Value of σ o = -20 db = c T σ Clutter = σ o A= σ o R θ B 2 For ASR-9 (Airport Surveillance Radar) c T R = 60 km 2 = 100m θ B = 1.5o = radians 0.01 m σ 2 Clutter = x 100 m x 60,000 m x radians = 1500 m 2 m 2 For σ Target = 1 m m 2 m 2 INPUT σ Target = σ Clutter OUTPUT σ Target = 20 σ Clutter Small single-engine aircraft Must suppress clutter by a factor of 1500 x 20 = 30,000 = 45 db For good detection Radar Systems Course 17 Courtesy of MIT Lincoln Laboratory Used with permission

18 Joint U.S./Canada Measurement Program Phase One radar VHF, UHF, L-, S-, X-bands Measurements conducted Archival data at Lincoln Laboratory Radar Systems Course sites Data shared with Canada and the United Kingdom Courtesy of MIT Lincoln Laboratory Used with permission

19 Joint U.S./Canada Measurement Program Phase One Radar Radar System Parameters Frequency Band MHz) VHF UHF L-Band S-Band X-Band Antenna Gain (db) Antenna Beamwidth Az (deg) El (deg) Peak Power (kw) Polarization HH,VV HH,VV HH,VV HH,VV HH,VV PRF (Hz) Pulse Width (µs) 0.1, 0.25, and 1 0.1, 0.25, and 1 0.1, 0.25, and 1 0.1, 0.25, and 1 0.1, 0.25, and 1 Waveform A/D Converter Number of Bits Sampling Rate (MHz) Uncoded CW Pulse 13 10, 5, 1 Uncoded CW Pulse 13 10, 5, 1 Uncoded CW Pulse 13 10, 5, 1 Uncoded CW Pulse 13 10, 5, 1 Uncoded CW Pulse 13 10, 5, 1 Courtesy of MIT Lincoln Laboratory Adapted from Billingsley, Reference 2 Radar Systems Course 19

20 Clutter Physics R Depression H Angle Visible Terrain Microshadowing Clutter Coefficient σ o Radar Systems Course 20 Courtesy of MIT Lincoln Laboratory Used with permission

21 Percent 10 5 Forest X Band σ o F 4 Histograms of Measured Clutter Strength σ o F 4 (db) = 24dB Forest VHF 5 σ o F 4 = 11dB Blue Line Is Mean σ F 4 (db) Farmland X Band σ F 4 (db) Farmland VHF Percent 5 σ o F 4 = 23dB 5 σ o F 4 = 55dB σ F 4 (db) Radar Systems Course σ F 4 (db)

22 Clutter Physics Clutter Histogram Weibull Parameters R Depression H Angle Visible Terrain σ o w (f) a w (A) 4 σ o F (db) Microshadowing Clutter Coefficient σ o Radar Systems Course 22 Courtesy of MIT Lincoln Laboratory Used with permission

23 Weibull Probability Density Function p ( ) x = b log ( ) b 1 2 log 2 x b x 50 2 e b 50 x x b x a 50 b w x = Median value of = 1/ a = = σ o F w 4 x Weibull shape parameter In units of m 2 /m 2 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 a w = 1, the Weibull distribution degenerates to an Exponential distribution in power (a Rayleigh distribution in voltage) Radar Systems Course 23

24 Clutter Physics Lobing Clutter Histogram Weibull Parameters a w (A) Free Space R Depression H Angle Visible Terrain σ o w (f) 4 σ o F (db) Multipath Microshadowing Propagation Factor F Clutter Coefficient σ o 4 Clutter Strength = σ o F Radar Systems Course 24 Courtesy of MIT Lincoln Laboratory Used with permission

25 Clutter Physics Lobing Clutter Histogram Weibull Parameters a w (A) Free Space R Depression H Angle Visible Terrain σ o w (f) 4 σ o F (db) Multipath Microshadowing Propagation Factor F Clutter Coefficient σ o 4 Clutter Strength = σ o F 1) Radar Parameters Frequency, f Spatial resolution, A 2) Geometry Depression angle (Range R, Height H) 3) Terrain Type Landform Land cover Radar Systems Course 25 Courtesy of MIT Lincoln Laboratory Used with permission

26 Mean Ground Clutter Strength vs. Frequency 0 10 Mean of σ F 4 (db) Radar Systems Course ,000 10,000 VHF UHF L- S- X-band Frequency (MHz) General Rural (36 Sites) Range Resolution (m) /36 15/36 Key Polarization H V H V Courtesy of MIT Lincoln Laboratory Used with permission

27 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 27

28 Land Clutter Backscatter vs. Terrain Type and Frequency Median Value of σ o F (db) Terrain Type Frequency Band VHF UHF L-Band S-Band X-Band URBAN MOUNTAINS FOREST/HIGH RELIEF (Terrain Slopes > 2 o ) High Depression Angle (> 1 o ) Low Depression Angle ( 0.2 o ) FOREST/LOW RELIEF (Terrain Slopes < 2 o ) High Depression Angle (> 1 o ) Intermediate Depression Angle (0. 4 o to 1 o ) Low Depression Angle ( 0.3 o ) AGRICULTURAL/HIGH RELIEF (Terrain Slopes 2 o ) AGRICULTURAL/LOW RELIEF Moderately Low Relief (1 o < Terrain Slopes < 2 o ) Moderately Low Relief (Terrain Slopes < 1 o ) DESERT, MARSH, GRASSLAND (Few Discretes) High Depression Angle ( 1 o ) Low Depression Angle ( 0.3 o ) Radar Systems Course 28 Adapted from Billingsley, Reference 2

29 Statistical Attributes of X-Band Ground Clutter aw o σ 50 σ o w Adapted from Billingsley, Reference 2 Radar Systems Course 29

30 Weibull Parameters for Ground Clutter Distributions o σ w ( db ) a w Frequency Bands Resolution(m 2 ) Adapted from Billingsley, Reference 2 Radar Systems Course 30

31 L-Band Clutter Experiment Radar Radar System Parameters 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) HH, VV, HV, VH Uncoded CW Pulse 14 2 Courtesy of MIT Lincoln Laboratory Used with permission. Radar Systems Course 31

32 Windblown Clutter Spectral Model r r + 1 β 2 1 r + 1 ( ν) = δ( ν) + P ( ν) P ( ν) = exp( β ν ) ( ν ) Total spectral power density P tot from a cell containing windblown vegetation Ratio of DC power to AC power P tot ac DC spectral power density AC spectral power density ac P tot (ν) in db North Dakota Cropland (Wheat) Measured Data AC Contribution DC Contribution Doppler velocity in m/s Exponential shape parameter Doppler Velocity (m/s) Adapted from Billingsley, Reference 2 Radar Systems Course 32

33 Measured Power Spectra of L-Band Radar Returns from Forest 20 LCE Radar Range 7 km Forest Wind 1-2 mph 6-7 mph mph Relative Power (db) Light Air Windy Breezy Curves are hand drawn lines through data in Billingsley Reference 2 Radar Systems Course Doppler Velocity (m/s) Adapted from Billingsley, Reference 2

34 Modeled Rates of Exponential Decay in the Tails of L-Band Spectra from Wind-Blown Trees Relative Power (db) P ac Light Air β = 23.5 β 2 Breezy β = 17.5 v 0. 2 Windy β = 10.7 ( ν) = exp ( β ν ) L-Band m / Doppler Velocity (m/s) s 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 β empirically developed from measured data 1 β = [ log w ] Radar Systems Course 34 Exponential shape parameter Velocity of wind (statute miles per hour) Adapted from Billingsley, Reference 2

35 Estimated Ground Clutter at Medium Depression Angles (~3 to 70 ) γ max 0 (db) -10 Open Woods Urban Cultivated land γ = ψ = σ o = o σ sin ψ Grazing Angle Backscatter Coefficient -20 Desert Frequency GHz Many data collections indicate that from ~3 to ~70 o degrees is proportional to (Ref 6) σ sin ψ Curves are Skolnik s estimates from Nathanson data (see Reference 6) Radar Systems Course 35

36 High Depression Angle Ground Clutter can be large near vertical incidence σ o 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, σ measured is at exactly 90 o For an ideal smooth reflecting surface, σ G o This is a better approximation for smooth sea than typically more rough land (lower for land) generally > 1 and > than resolution cell size) σ o < σ o (see Reference 6) Antenna Gain Radar Systems Course 36

37 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: σ σ clutter clutter = = Ω 3.78 θ n T B For FAA Airport Surveillance Radar (S-Band, = 10 cm): Ω = n = 12.7 RPM, 76.2 /sec 22 θb = 1.3 T = Ω = θ B n T 0.8 msec. = = = Antenna rotation rate (Hz) Antenna beamwidth (radians) Number of pulses in 3 db antenna beamwidth Time between radar pulses (sec) σ c 1.3 = radians λ 15 Hz Radar Systems Course 37

38 Outline Motivation Backscatter from unwanted objects Ground Sea Rain Birds and Insects Radar Systems Course 38

39 Attributes of Sea Clutter 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 o σ Radar cross section per unit area σ - Cross section per unit area (db) Sea Clutter Radar Cross Section o σ = σ A Area Illuminated by Radar Beam Mean sea backscatter is about 100 times less than ground backscatter Grazing Angle (degrees) Figure by MIT OCW. Radar Systems Course 39

40 World Meteorological Organization Sea State Classification Sea State Wave Height (m) Wind Velocity (knots) Descriptive Term 0 to 1 0 to to 6 Calm, Rippled to to 10 Smooth, Wavelets to to 16 Slight to Moderate to to 21 Moderate to Rough to 4 22 to 27 Very Rough 6 4 to 6 28 to 47 High Sea State 1 Sea State 3 Sea State 5 Radar Systems Course 40 Courtesy of NOAA

41 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 41

42 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; 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 42

43 Normalized Mean Sea Backscatter Coefficient σ 0 (db below 1 m 2 /m 2 ) Grazing Angle = 1 UHF L S C X Ku Ka/W Sea State Polarization 0.5 GHz /95 0 V H 86* 68* 80* 75* 70* 60* 60* 60* 60* 60* 60* 1 V H 70* 84* 65* 73* * 48* 2 V H 63* 82* 58* 65* * 38* 3 V H 58* 73* 54* 60* V H 58* 63* 45 56* * 5 V H 60* 43 50* V H * 32* * 5-dB error not unlikely Adapted from Nathanson, Reference 3 Data Collections and Analyses by NRL underscore this note (See Reference 2, page 15-10) Radar Systems Course 43

44 Sea Clutter Reflectivity vs. Grazing Angle σ - Cross section per unit area (db) Radar Systems Course 44 V - Vertical Polarization H - Horizontal Polarization X- Band (V ) X- and L- Band (V ) L- Band (H ) 220 MHz (H ) 50 MHz (H ) X- and L- Band (V &H) L- Band (V ) Adapted from Skolnik, Reference 6 X- Band (H ) Grazing Angle (degrees) 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

45 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 45

46 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 46

47 Effect of Wind Speed on Sea Clutter (Various Grazing Angles, Polarizations, and Frequencies) +10 σ - Cross section per unit area (db) X- Band (V & H) 90 X- Band (V & H) 60 X- Band (V) 10 X- Band (H) 10 L- Band (H) 10 Radar Systems Course Wind Speed (knots) Adapted from Skolnik, Reference 6

48 Sea Clutter Effects of the Wind and Waves σ o 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, σ o is highly sensitive to wave height At centimeter wavelengths, σ o is highly sensitive to wind speed at the small and intermediate grazing angles σ o 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 48

49 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 49

50 Sea Spikes Figure by MIT OCW. 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 From Lewis and Olin, NRL Radar Systems Course 50

51 Sea Clutter Distributions (Low Grazing Angles) Percent of time Clutter Exceeds Value Wind speed Low Wind speed Medium Area Of Sea Spikes Wind speed High X-band Data Grazing Angle 3 Polarization - Horizontal 90 Radar Systems Course σ - Cross Section per unit area (db) Adapted from Skolnik, reference 4

52 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 Radar Systems Course 52