Radar Systems Engineering Lect 10 (2)

1. 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

2. 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)

3. 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

4. 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

5. 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

6. 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

7. Radar Systems Course 7
Clutter 11/1/2009
IEEE New Hampshire Section
IEEE AES Society
How to Handle Noise and Clutter

8. 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 ?

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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.

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. 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°

28. 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

29. 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

30. 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

31. 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

32. 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

33. 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)

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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)

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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

49. 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

50. 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.

51. 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

52. 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