Radar 2009 a 10 radar clutter.2pdf

IEEE New Hampshire Section
Radar Systems Course 1
Clutter 11/1/2009 IEEE AES Society
Radar Systems Engineering
Lecture 10 Part 2
Radar Clutter
Dr. Robert M. O’Donnell
Guest Lecturer

Clutter 11/1/2009
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)

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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 Radar
S Band
Detection Range - 60 nmi on
a 1 m2 target
Clear Day (No Rain)
Courtesy of FAA

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Display
Center
PPI Display Radar Normal Video
Airport Surveillance Radar
S Band
Detection Range - 60 nmi on
a 1 m2 target
Clear Day (No Rain)
Display
Center
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 Wave
Right Handed Circular (RHC)
Reflected Electromagnetic Wave
Left 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 Characterization
Drizzle – 0.25 mm/hr
Light Rain – 1 mm/hr
Moderate Rain – 4 mm/hr
Heavy Rain – 16 mm/hr
Excessive rain – 40 mm/hr
In Washington DC
0.25 mm/hr exceeded 450 hrs/yr
1 mm/hr exceeded 200 hrs/yr
40 mm/hr exceeded 2.2 hrs/yr0.25 mm/hr
1 mm/hr
16 mm/hr
4 mm/hr
40 mm/hr
Attenuationfromrain-dB/km(oneway)
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
Reflectivity(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)
Refractivity
Fluctuations
(Maximum Observed)

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Reflectivity of Uniform Rain
(σ in dBm2/m3)
Heavy Stratus Clouds –100 –85 –69 –62
Drizzle, 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 Type
S
3.0 GHz
C
5.6
X
9.3
Ku
15.0
Ka
35
W
95
mm
140
* Approximate
Frequency
=
=
+
−
=
=λ
λ
π
=σ ∑
D
93.0
1n
1n
K
DK
2
2
2
62
4
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 dB
Range of Altitude
11,500 ft to 12,800 ft
Altitude
11.5 k-ft 12.1 k-ft 12.8 k-ft
4.52.6 Slant Range, nmi
Amplitude(LinearUnits)
C Band
Azimuth 17°
Elevation 6°
Pulse Width
1.6 μsec
C Band
Azimuth 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 Kt
Doppler Velocity
+ 60 Kt0
Azimuth = 320° Azimuth = 330°
0
– 10
– 20
– 30
– 40
– 50
– 60
– 70
– 80
– 90
dB
– 60 Kt
Doppler 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 ft dB – 60 Kt
Doppler 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
0.5
Velocity Spectrum
Of Rain
Cross Sectional Sketch
of Radar Beam
With Wind Blown Rain
RvΔ
1Rv0Rv2Rv
Doppler Velocity
RelativePower
)h(vW
2Rv
1Rv
Wind Velocity
Vertical
Gradient
of 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
2
Fall
2
Beam
2
Turb
2
Shearv
ψ=σ
βθ=σ
=σ
≤σφ=σ
σ+σ+σ+σ=σ
=
=
=
=
=
=
ow
,
R
k
ψ
β
φθ
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 center
Elevation 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:

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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
Land / Ocean
– Variations

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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%at0Density
75% 50% 25% 10% 1%
Number of birds per sq mi
1 10 100 1000
1
2
3
4
5
FrequencyofOccurrence(%/dB)
Adapted from Pollon, reference 7
Data Characteristics
286 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
Spring
Autumn
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 Eastwood
reference 8

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Altitude Distribution of Migrating Birds
1
2
3
4
5
6
7
0
Height(ftx1000)
0 2 4 6 8 10 12
Percent of Birds Detected
Nocturnal Migrating Birds
Bushy 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, 2006
5:25 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
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
Land / Ocean
– Variations

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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)
RadarCrossSection(m2
)
10-3
10-4
10-2
0 1 2 3
SparrowS-Band
Radar Cross Section vs. TimeJoint Air Force NASA Radar Facility
Wallops 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
NumberofBirdDetections
60
40
80
20
10-5 10-4
10-3 10-2 10-1 1 10
Radar 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
Land / Ocean
– Variations

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Distributions of the Radial Velocity of BirdsFrequencyofOccurrence
FrequencyofOccurrence
L-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
0.15
0.10
0.10
0.15
0.20
0.20
00
0.25
0.30

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Bird Clutter
Land / Ocean
– Variations
• Radar cross section
• Velocity / Doppler distribution
• Effects of birds on radar

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Why Birds Are an Issue for Radars
0 20 40 60 80 100 120 140 160
Birds
Insects
Clear Air Turbulance
DetectableCrossSection(m2)
1
101
10-2
10-1
10-3
10-4
10-1 m
2 at 70 km
10-2 m
2 at 70 km
10-3 m
2 at 70 km
Detection Curve
For 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
4
RN
S σ
∝

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Bird Example from Dallas-Fort Worth
Radar & Beacon
Beacon-Only
Radar Uncorrelated
Radar 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
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

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Homework Problems
• From Skolnik, Reference 6
– Problems 7-2, 7.4, 7.9, 7.11, 7.15, and 7.18

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