IEEE New Hampshire SectionRadar Systems Course 1Clutter 11/1/2009 IEEE AES Society
Radar Systems Engineering Lecture 10 Part 2
Radar Clutter
Dr. Robert M. O’DonnellIEEE New Hampshire Section
Guest Lecturer
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Block Diagram of Radar SystemTransmitter
WaveformGeneration
PowerAmplifier
T / RSwitch
Antenna
PropagationMedium
Target RadarCross Section
Photo ImageCourtesy of US Air ForceUsed with permission.
PulseCompressionReceiver Clutter Rejection
(Doppler Filtering)A / D
Converter
General Purpose Computer
Tracking
DataRecording
ParameterEstimation Detection
Signal Processor Computer
Thresholding
User Displays and Radar Control
Buildings(Radar Clutter)
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Outline
• Motivation
• Backscatter from unwanted objects
– Ground
– Sea
– Rain
– Birds and Insects
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Attributes of Rain Clutter
• Rain both attenuates and reflects radar signals
• Problems caused by rain lessen dramatically with longer wavelengths (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 spread
Transmitted Electromagnetic Wave
Reflected Electromagnetic Wave
Rain drop
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10 nmi Range Rings on PPI Display
August 1975, FAA Test Center
Atlantic City, New Jersey
PPI Display Radar Normal Video
Airport Surveillance RadarS Band
Detection Range -
60 nmi on a 1 m2
target
Clear Day (No Rain)
Courtesy of FAA
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10 nmi Range Rings on PPI Display
August 1975, FAA Test Center
Atlantic City, New Jersey
PPI Display Radar Normal Video
Airport Surveillance RadarS Band
Detection Range -
60 nmi on a 1 m2
target
Clear Day (No Rain)
10 nmi Range Rings on PPI Display
August 1975, FAA Test Center
Atlantic City, New Jersey
Day of Heavy Rain
Courtesy of FAA Courtesy of FAA
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Reflectivity of Uniform Rain (σ
in dBm2/m3)
• Rain reflectivity increases as f 4 (or 1 / λ4)
– Rain clutter is an issue at S-Band and a significant one at higher frequencies
Figure by MIT OCW.
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Effect of Circular Polarization on Rain Backscatter
• Assumption: Rain drops are spherical• Circular polarization is transmitted (assume RHC),
– Reflected energy has opposite sense of circular polarization (LHC)• Radar configured to receive only the sense of polarization that is
transmitted (RHC)– Then, rain backscatter will be rejected (~ 15 dB)
• Most atmospheric targets are complex scatterers and return both senses of polarization; equally (RHC & LHC)
– Target echo will be significantly attenuated
Transmitted Electromagnetic WaveRight Handed Circular (RHC)
Reflected Electromagnetic WaveLeft Handed Circular (LHC)
Rain drop
Phase change at reflection
point in raindrop
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Attenuation in Rain
Adapted from Skolnik, Reference 6
Rainfall CharacterizationDrizzle –
0.25 mm/hrLight Rain –
1 mm/hrModerate Rain –
4 mm/hrHeavy Rain –
16 mm/hrExcessive rain –
40 mm/hr
In Washington DC0.25 mm/hr exceeded 450 hrs/yr
1 mm/hr exceeded 200 hrs/yr4 mm/hr exceeded 60 hrs/yr
16 mm/hr exceeded 8 hrs/yr40 mm/hr exceeded 2.2 hrs/yr0.25 mm/hr
1 mm/hr
16 mm/hr
4 mm/hr
40 mm/hr
Atte
nuat
ion
from
rain
-dB
/km
(one
way
)
10-4
10-3
10-2
10-1
1
10
Wavelength (cm)0 1 2 3 4 5 6 7 8 9 10
Attenuation at 18°
C
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Reflectivity vs. Frequency
Ref
lect
ivity
(mi-1
)
10-16
10-14
10-12
10-10
10-8
10-6
10-4
100 1,000 10,000 100,000
Frequency (MHz)
Rain (15 mm/hr)
1 m2
on ASR radar(10kft at 30 nmi)
Insects(Maximum Observed)
RefractivityFluctuations
(Maximum Observed)
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Reflectivity of Uniform Rain (σ
in dBm2/m3)
Heavy Stratus Clouds
–100
–85
–69
–62Drizzle, 0.25 mm/hr
–102
–91
–81
–71
–58
–45*
–50*Light Rain, 1 mm/hr
–92
–81.5
–72
–62
–49
–43*
–39*Moderate, 4 mm/hr
–83
–72
–62
–53
–41
–38*
–38*Heavy Rain, 16 mm/hr
–73
–62
–53
–45
–33
–35*
–37*
Rain TypeS
3.0 GHz
C
5.6
X
9.3
Ku
15.0
Ka
35
W
95
mm
140
* Approximate
Frequency
==
+−
=
=λλπ
=σ ∑
D93.0
1n1nK
DK
2
22
624
5
Reflectivity
Wavelength
Complex Index of Refraction
For Rain
Droplet Diameter Date Table Adapted from Nathanson, Reference 3
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Heavy Uniform Rain – Backscatter Coefficient
*
Theoretical Rainfall Rate
11,000 ft
Altitude
7 dB
21.5Slant Range, nmi
1.75 nmi
4.0
40 mm*/hr
10 mm*/hr
3 dB
9 dB
0.2 nmi
2.2 dBRange of Altitude
11,500 ft to 12,800 ft
Altitude11.5 k-ft 12.1 k-ft 12.8 k-ft
4.52.6 Slant Range, nmi
Am
plitu
de (L
inea
r Uni
ts)
C BandAzimuth 17°Elevation 6°Pulse Width
1.6 μsec
C BandAzimuth 336°Elevation 34°Pulse Width
0.2 μsec
Adapted from Nathanson, Reference 3
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Measured S-Band Doppler Spectra of Rain
– 60 KtDoppler Velocity
+ 60 Kt0
Azimuth = 320° Azimuth = 330°0
– 10– 20– 30– 40– 50– 60– 70– 80– 90
dB
–
60 KtDoppler Velocity
+ 60 Kt0
Azimuth = 90°0
– 10– 20– 30– 40– 50– 60– 70– 80– 90
0 + 60 Kt–
60 Kt
Doppler Velocity
0– 10– 20– 30– 40– 50– 60– 70– 80– 90
• Rain is not Gaussian• Mean velocity varies as storm moves
by radar• In these examples the rainfall rate
was approximately 20 mm/hr• Winds 30 kts on ground, 50 kts at
6000 ftdB
–
60 KtDoppler Velocity
+ 60 Kt0
0– 10– 20– 30– 40– 50– 60– 70– 80– 90
Velocity Spread
6 kts
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Effects of Wind Shear on the Doppler Spectrum
Adapted from Nathanson, Reference 3
0.5
Velocity SpectrumOf Rain
Cross Sectional Sketchof Radar Beam
With Wind Blown Rain
RvΔ
1Rv0Rv2RvDoppler Velocity
Rel
ativ
e Po
wer
)h(vW
2Rv
1Rv
Wind Velocity
VerticalGradientof Wind
(Wind Shear)
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Nathanson Rain Spectrum Model
• Nathanson model for velocity spread of rain
( ) ( )( )
( )( )s/msin0.1
s/msinw42.0
s/m0.1
0.6s/mRk42.0
Fall
oBeam
Turb
ShearShear
2Fall
2Beam
2Turb
2Shearv
ψ=σ
βθ=σ
=σ
≤σφ=σ
σ+σ+σ+σ=σ
====
==
ow
,Rk
ψβφθ
Wind Shear Gradient (m/s/km) (~4.0 averaged over 360°)Slant range (km)
Horizontal and vertical two way beam widths (radians)
Azimuth rel. to beam direction at beam centerElevation angle
Wind speed (m/s)
s/m0.1
s/m0.3
Turb
Shear
≈σ
≈σ
s/m0.1
s/m25.0
Fall
Beam
≈σ
≈σ s/m3.3v ≈σ
• Typical Values:
Adapted from Nathanson, Reference 3
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Outline
• Motivation
• Backscatter from unwanted objects
– Ground
– Sea
– Rain
– Birds and Insects
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Bird Clutter
• General properties
• Bird populations and density– Migration / Localized travel
Land / Ocean– Variations
Geography, Height, Diurnal, Seasonal etc
• Radar Cross Section– Mean / Fluctuation properties
• Velocity / Doppler Distribution
• Effects of Birds on radar– Sensitivity Time Control (STC)
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General Properties of Birds
• Good RCS model for bird– Flask full of salt water– Expanding and contracting body, at frequency of wing
beat, is the dominant contributor to individual bird radar cross section fluctuations
• Since many birds are often in the same range-azimuth cell, the net total backscatter is the sum of contribution from each of the birds, each one moving in and out of phase with respect to each other.
Erlenmeyer Flask Snow Goose Sea Gull
Courtesy of pbonenfant Courtesy of jurvetson Courtesy of tk-link
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General Properties of Birds
• Since birds move at relatively low velocities, their speed, if measured, can be used to preferentially threshold out the low velocity birds.
– Direct measurement of Doppler velocity– Velocity from successive measurement of spatial position
Range and angle• Even though the radar echo of birds is relatively small, birds can
overload a radar with false targets because:– Often bird densities are quite large, and– Bird cross sections often fluctuate to large values.
• A huge amount of relevant research has been done over the last 20 years to quantify:
– The populations of bird species, their migration routes, and bird densities, etc., using US Weather radar data (NEXRAD)
– Major Laboratory efforts over at least the last 20 years at Clemson University and Cornell University
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Bird Clutter
• General properties
• Bird populations and density– Migration / Localized travel
Land / Ocean– Variations
Geography, Height, Diurnal, Seasonal etc
• Radar Cross Section– Mean / Fluctuation properties
• Velocity / Doppler Distribution
• Effects of Birds on radar– Sensitivity Time Control (STC)
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Bird Breeding Areas and Migration Routes
Virginia RailNorthern FlickerGadwall
Along the Gulf Coast, during the breeding season, wading and sea
bird colonies exist that have many tens of thousands of birds. Ten thousand birds are quite common. These birds are large; weighing up to 2 lbs and having wingspreads from 1 to 6 feet.
Figure by MIT OCW.
Photos courtesy of vsmithuk, sbmontana, and khosla.
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Bird Breeding Areas and Migration Routes
Northern HarrierBlack TernSpotted Towhee
Figure by MIT OCW.
Photos courtesy amkhosla, Changhua Coast Conservation Action, and amkhosla.
In the lower Mississippi Valley, over 60 blackbird roosts have been identified with greater than 1 million birds each. Many smaller roosts also exits. These birds disperse several tens of miles for feeding each day.
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Density of Migrating North American Birds
13%
at 0
Den
sity
75% 50% 25% 10% 1%
Number of birds per sq mi1 10 100 1000
1
2
3
4
5
Freq
uenc
y of
Occ
urre
nce
(% /
dB)
Adapted from Pollon, reference 7
Data Characteristics286 Sites
1209 Observations~3000 Count-hours
Count = #/mi2/hr
Evening of 3
-
4 October 1952
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Migratory Bird Patterns (Off the US New England Coast)
Bird migrations have been tracked by radars from the Northeast United States to South America and the Caribbean have on Bermuda at altitudes of 17 kft
Direction of Bird Migration
Circles note coverage of 2 radars, one at tip of Cape Cod, the other, offshore on a “Texas tower”
Adapted from Eastwood reference 8
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Bird Migration across the Mediterranean Sea
SpringAutumn
45°
10°W 0° 10°E 20°E 30°E
35°
600 nmi.
For about 2 1/2 months in the Spring and Autumn, there is heavy bird migration, to and from, Europe
and AfricaAdapted from Eastwoodreference 8
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Altitude Distribution of Migrating Birds
1
2
3
4
5
6
7
0
Hei
ght (
ft x
1000
)
0 2 4 6 8 10 12Percent of Birds Detected
Nocturnal Migrating BirdsBushy Hill, England
Spring 1966
Altitude distributions differ for migrating and non-migrating birds
The presence of cloud cover effects the bird height distribution
Distance of their migration can influence migration altitude (NE United States to South America)
Over land vs. over sea migration
Day vs. night migration
Non-migrating birds stay closer to the ground
Adapted from Eastwood, reference 8
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Example of “Ring Roost”
Phenomena
• Radar observations with C-Band, WSR-88 (NEXRAD) NOAA, Pencil Beam Radar located at Green Bay, Wisconsin
“Ring Roosts”
are flocks of birds leaving their roosting location for their daily foraging for food just
before sunrise
Data collected on August 10, 20065:25 to 6:15 AM
About 50 minutes of data is compressed into ~1.5 sec duration
and replayed in a loop
Note intensity scale in dBZ
Courtesy of NOAA
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Spring Bird Migration from Cuba to US
• Radar observations with C-Band, WSR-88 (NEXRAD) NOAA, Pencil Beam Radar located at Key West, Florida
Data collected on April 28, 2002~1 -
3 AM
About 2 hours of data is compressed into ~3 sec duration
and replayed in a loop
Note intensity scale in dBZ
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Bird Clutter
• General properties
• Bird populations and density– Migration / Localized travel
Land / Ocean– Variations
Geography, Height, Diurnal, Seasonal etc
• Radar Cross Section– Mean / Fluctuation properties
• Velocity / Doppler Distribution
• Effects of Birds on radar– Sensitivity Time Control (STC)
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Bird RCS Measurements
• In the late 1960s, Konrad, Hicks, and Dobson of JHU/APL accurately measured the radar cross section (RCS) of single birds and the RCS fluctuation properties.
– Bird RCS fit a log-normal quite well– Like the Weibull distribution, it is a 2 parameter model that
fits data with long tails
Time (min)
Rad
ar C
ross
Sec
tion
(m2 )
10-3
10-4
10-2
0 1 2 3
SparrowS-Band
Radar Cross Section vs. TimeJoint Air Force
NASA Radar FacilityWallops Island, VA
UHF , S-Band and X-Band Radars
Adapted from Konrad, reference 12
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Summary of Measured Bird Cross Section* Data
Grackle (male) 15.7 27 0.73
Grackle (female) 15.4
23.2
0.41
Sparrow 1.85
14.9
0.025
Pigeon
14.5
80.0
10.5
X-Band S-Band UHF
Adapted from Konrad, reference 12
Units of RCS measurement cm2
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Distribution of Bird Radar Cross Section
0
Num
ber o
f Bird
Det
ectio
ns
60
40
80
20
10-5 10-4 10-3 10-2 10-1 1 10Radar Cross Section (m2) Adapted from Eastwood, reference 8
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Radar Cross Section Model
• Wavelength dependence• Fluctuation statistics of cross section (log normal)
Mean
Standard Deviation of Cross Section
Log of Cross Section
Wavelength
(dBsm)
(dB)
X
–33
6
S
–27
6
L
–28 7.5
UHF
–47
15
VHF
–57
17
Adapted from Pollon, Reference 7
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Bird Clutter
• General properties
• Bird populations and density– Migration / Localized travel
Land / Ocean– Variations
Geography, Height, Diurnal, Seasonal etc
• Radar Cross Section– Mean / Fluctuation properties
• Velocity / Doppler Distribution
• Effects of Birds on radar– Sensitivity Time Control (STC)
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Distributions of the Radial Velocity of BirdsFr
eque
ncy
of O
ccur
renc
e
Freq
uenc
y of
Occ
urre
nceL-Band
X-Band
Radial Velocity (m/sec) Radial Velocity (m/sec)
0 5.7 11.4 17.1 22.8 28.5 0 5.7 11.4 17.1 22.8 28.5
0.05 0.05
0.150.10
0.10
0.15 0.20
0.20
00
0.25
0.30
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Bird Clutter
• General properties
• Bird populations and density– Migration / Localized travel
Land / Ocean– Variations
Geography, Height, Diurnal, Seasonal etc
• Radar cross section– Mean / Fluctuation properties
• Velocity / Doppler distribution
• Effects of birds on radar– Sensitivity Time Control (STC)
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Why Birds Are an Issue for Radars
0 20 40 60 80 100 120 140
160
Birds
Insects
Clear Air TurbulanceDet
ecta
ble
Cro
ss S
ectio
n (m
2 )
1
101
10-2
10-1
10-3
10-4
10-1 m2 at 70 km
10-2 m2 at 70 km
10-3 m2 at 70 km
Detection CurveFor an ASR
Range (km)
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Sensitivity Time Control
• These two targets have the same detectability, because in the radar equation:
• This false target issue can be mitigated by attenuating to the received signal by a factor which varies as 1/R4
– Can also be accomplished by injecting 1/R4
noise to the receive channel
• Radars that utilize range ambiguous waveforms, cannot use STC, because long range targets which alias down in range, would be adversely attenuated by the STC
– For these waveforms, other techniques are used to mitigate the false target problem due to birds
Bird at 89 nmi, RCS = 0.0015 m2
Aircraft at 200 nmi, RCS = 1 m2
4RNS σ∝
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Bird Example from Dallas-Fort Worth
Radar & BeaconBeacon-OnlyRadar UncorrelatedRadar Correlated
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Bird Clutter Issues -
Summary
• Birds are actually moving point targets– Velocity usually less than 60 knots
• Mean radar cross section is small, but a fraction of bird returns fluctuate up to a high level (aircraft like)
– Cross section is resonant at S-Band and L-Band
• The density of birds varies a lot and can be quite large – 10 to 1000 birds / square mile
• Birds cause a false target problem in many radars– This can be a significant issue for when attempting to detect
targets with very low cross sections
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Insects
• Insects can cause false detections and prevent detection of desired targets
• Density of insects can be many orders of magnitude greater than that of birds
• Insect flight path generally follows that of the wind
• Cross section can be represented as a spherical drop of water of the same mass
• Insect echoes broad side are 10 to 1,000 times than when viewed end on
Figure by MIT OCW. Adapted from Skolnik Reference 6
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Mayfly Hatching
• Radar observations with C-Band, WSR-88 (NEXRAD) NOAA, Pencil Beam Radar located at La Crosse, Wisconsin (SW WI)
Data collection -
June 30, 2006
La Crosse is the breeding ground of the mayfly population of the
world
~10s of billions of them hatch, live, and die, over a 1 ½
day period, each year in late June /
early
July
Mississippi River
Ephemeroptera (mayfly)
Courtesy of National Weather Service Courtesy of urtica
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Summary
• A number of different types of radar clutter returns have been described
– Ground, sea, rain, and birds
• These environmental and manmade phenomena will produce a variety of discrete and diffuse, moving and stationary false targets, unless they are dealt with effectively
• A number of signal and data processing techniques can be used to suppress the effect of these radar clutter returns.
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References
1.
Billingsley, J. B. , Ground Clutter Measurements for Surface Sited Radar, MIT Lincoln Laboratory, TR-916 Rev 1, (1991)
2.
Billingsley, J. B., Low Angle Radar Land Clutter, Artech House, Needham, MA, (2005)
3.
Nathanson,F. , Radar Design Principles, McGraw Hill, New York,2nd
Ed., (1999)
4.
Skolnik, M., Radar Handbook, McGraw Hill, New York,3rd
Ed., (2008)
5.
Barton, D., Radar System Analysis, McGraw Hill, Artech House, Needham, MA, (1976)
6.
Skolnik, M., Introduction to Radar Systems, McGraw Hill, New York, 3rd
Ed. (2000)
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References -
Continued
7. Pollon, G., “Distributions of Radar Angels,”
IEEE Trans. AESS, Volume AES-8, No. 6, November 1972, pp. 721–726
8. Eastwood, E., Radar Ornithology, Methuen & Co, London, (1967)
9. Riley,J. R., “Radar Cross Section of Insects,”
Proceedings of IEEE, February 1985, pp. 228–232
10. Vaughn, C. R., “Birds and Insects as Radar Targets: A Review,”
Proceedings of IEEE, February 1985, pp. 205–227
11. Billingsley, J. B. , Ground Clutter Measurements for Surface Sited Radar, MIT Lincoln Laboratory, TR-786 Rev 1, (1993)
12. Konrad, et al, “Radar Characteristics of birds in Flight”, Science, vol 159, January 19, 1968