1
11-062007
Quantising the effects of wind turbines on naval and
aeronautical radarsPrinciples used in Belgium
ESAT – Telemic, K.U.Leuven
Presentation for SET-128
11-062007
#2
Outline
Introduction: Primary Radar
Shadow effectsEffects on accuracyReflections and False EchoesEffects of multiple obstaclesDoppler effects
Secondary RadarEffects on accuracy (monopulse)Reflections and False Echoes
Conclusions
2
11-062007
#3
Introduction
18015010080Tot. Height
60504030Blades
8 x 3 x 45 x 2 x 3Gondola
5432.5Tower top
654.54Tower base
1201006050Tower height
“giant”“large”“small”“mini”Sizes in m
4 standard types of Wind Turbines (aeronautical study)
5 orientations gondola (every 45º)3 orientations blades (every 40º)
Large distances: curvature of Earth (normal atmosphere)
11-062007
#4
Primary RadarsProperties of primary radars
Approach• High beam• Low beam• Brussels,
Liege, …
Enroute• Bertem,
St-Hubert
3634Antenna Gain [dBi]
13002800Frequency [MHz]
21Pulse Width [µs]
1.11.53-dB Beamwidth [º]
-21-26Side Lobe Level [dB]
Horizontal pattern
553-dB Beamwidth [º]
-20-24Side Lobe Level [dB]
1.57/3Main lobe elevation [º]
Vertical pattern
-10Tilt [º]
12060Range [NM]
2500200Transmit Power [kW]
PSR ERPSR AP High/Low
3
11-062007
#5
Primary Radars (cont’d)Patterns: example PSR AP low
−20 −15 −10 −5 0 5 10 15 20−30
−25
−20
−15
−10
−5
0
5
10Relative horizontal pattern of PSR approach radar (low beam)
Angle in degrees
Sig
nal i
n dB
Horizontal Vertical−70 −60 −50 −40 −30 −20 −10 0 10
−80
−60
−40
−20
0
20
40
60
80
Relative vertical pattern of a PSR approach radar (low beam)
Ang
le in
deg
rees
Signal in dB
1000 m
11-062007
#6
Primary Radars (cont’d)Examples of the different rays (Rwr=900 m)
−40
−35
−30
−25
−20
−15
−10
−5
0
5
10
−2000 −1800 −1600 −1400 −1200 −1000 −800 −600 −400 −200−50
−40
−30
−20
−10
0
10
20
30
40
50
x distance in m
Variation of the signal behind a large wind turbine
y di
stan
ce in
m
4
11-062007
#7
PSR Shadow effects
Effects computed with UTDLimited to deep-shadow zone behind the WT: several dBPosition of WT changed
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5
Relative signal level of PSR−ER 100 m behind the wind turbine wind turbine at −1.5 degrees
angle in degrees
rela
tive
sign
al le
vel (
dB)
total signal direct signal
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5
Relative signal level of PSR−AP (low beam) 100 m behind the wind turbine wind turbine at −0.0 degrees
angle in degrees
rela
tive
sign
al le
vel (
dB)
total signal direct signal
ApproachEnroute (WT @ -1.5º)
11-062007
#8
PSR Shadow effects (cont’d)
Effects (2):Larger distance: less significant
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5Relative signal level of PSR−AP (low beam) 60 nm behind the wind turbine
angle in degrees
rela
tive
sign
al le
vel (
dB)
total signal direct signal
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5Relative signal level of PSR−ER 120 nm behind the wind turbine
angle in degrees
rela
tive
sign
al le
vel (
dB)
total signal direct signal
Approach: 60 NM
Enroute: 120 NM
5
11-062007
#9
PSR Shadow effects (cont’d)
Effects (3):The larger the object the larger the effectExample: naval radar (9.41 GHz) from Marseille port authorities and silo (h 33 m, Ø 41,16 m)
11-062007
#10
PSR Shadow effects (cont’d)
Effects (4):Computation of the effect of an existing silo, compared with the shadow of the turbines
6
11-062007
#11
PSR: shadow effects (cont’d)
Probability of detectionFree space:
EPICS: R4 L2 (decay of the signal L > 4πR2)
(Non)Probability of Detection:• Noise: N = kTBF
2
3 4(4 )t t r
fsPG GP
Rσλ
π=
22
2
44 ( )
t t o rr
t t o
PG G GPL PG Gλ πσ
π λ⎛ ⎞
= ⎜ ⎟⎝ ⎠
2 2
22
20
0
/ 2'2
00
( )
22 ' ( ' )
T r
Tr
V V VrN
V NVVN
r
VVNDP Ve I dVN
NDP e V e I V V dVN
+−
− −
=
=
∫
∫
11-062007
#12
PSR: shadow effects (cont’d)Small Values (upper bound):
Approximation:
8.172
r
NDPUPPNDPAPPV
N
=
2 2( )2 2 erfc( ) erfc( )
2 2 2
100
r T rV V Vr T rN N
r
T r
V V VNDPUPP e e VN N N
V VN
π−− − −⎡ ⎤= − + −⎢ ⎥⎣ ⎦
<
7
11-062007
#13
PSR: shadow effects (cont’d)
Probability of detection: example
Alternative representation: change of radar cross section (RCS)Pattern no longer visible
1020.log tot
dir
PP
σ⎛ ⎞
∆ = ⎜ ⎟⎝ ⎠
11-062007
#14
Radar accuracy PSR: distanceRadar resolution
300 m Approach600 m Enroute
EffectDelay ≈ distanceCorrect heightVery small error AP 100 m
ER 120 NM
8
11-062007
#15
Radar accuracy PSR: angleWeighted average of all rays
Viewing angle from radar: 0.14°Perfect symmetry (0°, 0°) zero errorFar from WT smaller angular error
−3 −2 −1 0 1 2 3−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Angular error (degrees) 1000 m behind a wind turbine for a PSR approach radar
angle in degrees
angu
lar
erro
r in
deg
rees
−3 −2 −1 0 1 2 3−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Angular error (degrees) 120 nm behind a wind turbine for a PSR enroute radar
angle in degrees
angu
lar
erro
r in
deg
rees
AP 1000 m ER 120 NM
11-062007
#16
Radar accuracy PSR: angle (cont’d)Larger distance (viewing angle 0.07°): Rwr=2000 m
Smaller angular error
Always smaller than viewing angle from the radarAP 1000 m ER 1000 m
−3 −2 −1 0 1 2 3−0.1
−0.08
−0.06
−0.04
−0.02
0
0.02
0.04
0.06
0.08
0.1
Angular error (degrees) 1000 m behind a wind turbine for a PSR approach radar (at 2000 m)
angle in degrees
angu
lar
erro
r in
deg
rees
−3 −2 −1 0 1 2 3−0.1
−0.08
−0.06
−0.04
−0.02
0
0.02
0.04
0.06
0.08
0.1
Angular error (degrees) 1000 m behind a wind turbine for a PSR enroute radar (at 2000 m)
angle in degrees
angu
lar
erro
r in
deg
rees
9
11-062007
#17
Radar accuracy PSR: angle (cont’d)Other WT position
WT at –2ºReflections due to polygonal shape of tower (artifact)
AP 1000 m ER 1000 m
−3 −2 −1 0 1 2 3−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Angular error (degrees) 100 m behind a wind turbine (at −2 degree) for a PSR approach radar
angle in degrees
angu
lar
erro
r in
deg
rees
−3 −2 −1 0 1 2 3−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
Angular error (degrees) 100 m behind a wind turbine (at −2 degree) for a PSR enroute radar
angle in degrees
angu
lar
erro
r in
deg
rees
11-062007
#18
PSR: Reflections and False Echoes
4 different types of false echoes:Reflection WT – aircraft (types 1 and 2)
Type 2 reflections from aircraft in SLL of antennaVisible behind the WTSame speed as aircraft; until closest position where it
stops and disappears from radar screen
10
11-062007
#19
PSR: Reflections and False Echoes
Reflection aircraft - WT (types 3 and 4)Type 4 reflections from WT in side lobe of antenna
Visible behind the aircraftType 3 moves at twice the speed of the plane ; until
closest position where it stops and disappears from radar screen
11-062007
#20
PSR: False Echoes (cont’d)
Type 1:Only in neighbourhood WTExample: approach (7 km)
2 2 2
41 5 4
. . . ..(4 ) .
TX TX WT MA
RX WR
P GRP R
λ σ σπ
=
Parameters (see next)
11
11-062007
#21
PSR: False Echoes (cont’d)
Simulation parameters:Transmitted power:Gain of transmitting antenna = gain of main lobe of receiving antenna: Gain in side lobe of receiving antenna (see types 2 and 4):Frequency = 2.8 GHz (λ = 0.107 m)Minimal detectable level:Radar cross section of a wind turbine, both mono- and bistatic σxW = 400 m2 (no effect of curvature of the Earth was taken into account)Radar cross section of an aircraft, both mono- and bistatic σxA = 1000 m2
200 kWTXP =
34 dBiTXG =
10 dBiRXG =
140 dBWRXP = −
11-062007
#22
PSR: False Echoes (cont’d)
Type 2, 3 and 4:
3 different regions for type 2/4 and 3 (on axis):Aircraft further from radar than WT:Aircraft between radar and WT:Aircraft behind radar:
(1)AR WR iR R R= +
(2)AR WR iR R R= −
(3)AR i WRR R R= −
2
2 4 4 2 2(4 )TX TX RX BW BA
RX WR AR
P G GR RP R R
λ σ σπ
= =2 2 2
43 5 4(4 )TX TX BA MW
RX AR
P GRP R
λ σ σπ
=
12
11-062007
#23
PSR: False Echoes (cont’d)Type 3: PSR-AP
2 2 2
43 5
( )3 3
(4 )
2 5120
TX TX BA MW
RX
crit
P GKP
R K m
λ σ σπ
=
= =
(1) 33 2
3
(2) 33 2
3
2(3)3 3
24
24
2 2
WR WR
WR WR
WR WR
KRR R K
KRR R K
R RR K
=+ +
=± −
⎛ ⎞= − + +⎜ ⎟⎝ ⎠
11 km
5.1 km
11-062007
#24
PSR: False Echoes (cont’d)Type 2/4: PSR-AP
2
2 40
( ) 32 / 4 2
(4 )
4 10673
TX TX RX BW BA
crit
P G GKP
R K m
λ σ σπ
=
= =
All types simultaneously!!
(1) 22/ 4 2
2
(2) 22 / 4 2
2
2(3) 22 / 4
2 /4 /
2 /4 /
2 2
WR
WR WR WR
WR
WR WR WR
WR WR
WR
K RRR R K R
K RRR R K R
R R KRR
=+ +
=± −
⎛ ⎞= − + +⎜ ⎟⎝ ⎠
14 km
10.7 km
13
11-062007
#25
PSR: False Echoes (cont’d)
At/below ‘critical’ distance:Whole zone between radar and WTBehind WT much smaller zone (2-4 km)
Modification for cylindrical towerScattering: 6 dB decrease of signal
1 0 1
1 1 2cos( )l rρ θ
= +
11-062007
#26
PSR: False Echoes (cont’d)
0 ( ) ( )
( )0 ( )
( ) 0
ww t gon gon
t
g wg w gon
g
w g
hh Dhh h
h h Dh
h h D
σ σ σ σ
σ σ
σ
> ⇒ = − +
+− < < ⇒ =
< − ⇒ =
2 2
3.4.
4 2.sin ( / 2)3 cos( ) 17. 6
a
w t a t
DR
Dh h R he
θ
θθ
=
= − ≈ −
Large distances: curvature of EarthVariation of radar cross section
14
11-062007
#27
PSR: False Echoes (cont’d)
Examples of false echo sizes (curved surf.):
PSR-AP PSR-ER
11-062007
#28
−80 −60 −40 −20 0 20 40 60 80−60
−40
−20
0
20
40
60
80
rada
r cr
oss
sect
ion
in d
Bsm
angle in degrees
Monostatic and bistatic radar cross−section of a 20 x 10 m plate at −45 degrees and 1.3 GHz
monostaticbistatic
Flat plate
PSR: False Echoes (cont’d)
RCS extremelyvariable
WT
Aircraft
2
2
4 ( )abπλ
22 abπλ
15
11-062007
#29
PSR: False Echoes (cont’d)
Vertical cross section:
11-062007
#30
PSR: False Echoes (cont’d)
Fortran routine, provided to BelgocontrolProgram inputVariation of regions with respect to distance radar – wind turbineVertical cross section if only 1 distance is requested
INPUT
16
11-062007
#31
PSR: False Echoes (cont’d)
The zones can be put on a marine/land chart
11-062007
#32
PSR: False Echoes (cont’d)
Example on a digital display
17
11-062007
#33
PSR: Multiple obstacles4 case studies:
Parallel case: WT’s on line with radarPerpendicular case: line WT’s perpendicularWT’s on line at 45º (not in this presentation)Minimal distance between 2 obstacles (Vert. Pol.)?
Remarks:Blocks 6 x 6 x 200 m3 @ intermediate distance of 400 mRadar at 1000 m; trajectory = circle of 10 km radiusOmni-directional antenna (dipole), thus all WT illuminated
the same way (≠ radar pattern)Change radar cross section
11-062007
#34
PSR: Multiple obstacles (cont’d)
Parallel case (enroute radar, h= 35 m)Very little effectOnly deep shadowing behind the line
−3 −2 −1 0 1 2 3−40
−35
−30
−25
−20
−15
−10
−5
0
5
Change Radar Cross Section (enroute) cut at h=1000m
angle (degrees)
chan
ge r
adar
cro
ss s
ectio
n (d
B)
1 block 5 blocks 10 blocks20 blocks
18
11-062007
#35
PSR: Multiple obstacles (cont’d)
Perpendicular case:Obstacles visible at angles: Height of obstacles:
400arctan1000n
nα ⎛ ⎞= ± ⎜ ⎟⎝ ⎠
2 2
100002001000 (400 )
nhn
=+
11-062007
#36
PSR: Multiple obstacles (cont’d)Minimal distance between 2 obstacles?
Varied between 10 and 100 m
Cylinder (heptadecahedron) Extension smaller
19
11-062007
#37
PSR: Multiple obstacles (cont’d)Large parks can be handled taking only into account tower
11-062007
#38
PSR: Doppler effectsDoppler of the blades:
Doppler shift varies between ± maximal value• Max. speed value = (ω = 15 rev./min)• Combination of relative Doppler shifts• ca. 1750 Hz for PSR-AP; ca. 800 Hz for PSR-ER
wl ω
0
v2Df c∆ ≈
20
11-062007
#39
PSR: Doppler effectsMakes rotating blades visible on primary radar
Example: Enroute radar of St. Hubert windmill park of St. Ode (sometimes initiating false tracks on Belgocontrol’s ATC systems):
11-062007
#40
PSR: Doppler effects (cont’d)
Doppler obstacle behind WTObstacle can become visible due to Doppler shiftVery low Doppler shifts (some Hertz; Rwo= 2 km; )Filtered out (30 dB) with simple MTI filter
21
11-062007
#41
Secondary Surveillance RadarsRadar parameters
Max. power = ± 1.685 º3 3 dB-BW = 2.3 º3 dB-BW = 12 º
SLL = -30 dBSLL = -28 dBSLL = -15 dB
Gain = 2.13 dB below sumGain = 28 dBiElevation = 8º
Horizontal Difference patternHorizontal Sum patternVertical pattern
−5 −4 −3 −2 −1 0 1 2 3 4 5−35
−30
−25
−20
−15
−10
−5
0
5
Relative horizontal sum and difference pattern of a secondary radar
angle in degrees
rela
tive
sign
al le
vel (
dB)
sum signal diff signal
−70 −60 −50 −40 −30 −20 −10 0 10
−80
−60
−40
−20
0
20
40
60
80
Relative vertical pattern of a SSR radar
Ang
le in
deg
rees
Signal in dB
11-062007
#42
Secondary Radar (cont’d)
Link budget (without antenna gains)
Effect on radar pattern:Shadowing behind WTLarger distance: less significantChanged WT position (right)
-133.5-132.01Allowed Losses
-115-99Receiver sensitivity [dBW]
18.533.01Transmit Power [dBW]
Reply @ 1090 MHzInterrogation @ 1030 MHz
22
11-062007
#43
Variation SSR pattern
Shadowing effects:
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5
Relative SSR signal level 100 m behind the wind turbine wind turbine at −0.0 degrees
angle in degrees
rela
tive
sign
al le
vel (
dB)
total sum signal direct sum signal total diff signal direct diff signal
−3 −2 −1 0 1 2 3−30
−25
−20
−15
−10
−5
0
5
Relative SSR signal level 100 m behind the wind turbine wind turbine at −1.0 degrees
angle in degreesre
lativ
e si
gnal
leve
l (dB
)
total sum signal direct sum signal total diff signal direct diff signal
11-062007
#44
Radar accuracy SSR: distance
EffectDelay ≈ distanceCorrection for heightVery small error
100 m
1000 m
23
11-062007
#45
Radar accuracy SSR: angle
Angular error (monopulse, else see PSR)Iteratively from
• Where γ, β and α are parameters of the radar pattern
MIGHT BE LARGER THAN ACCURACY OF SYSTEMMIGHT BE LARGER THAN ACCURACY OF SYSTEM
2
.sin( . )cos ( . )γ β ϕ
α ϕ∆=
Σ
100 m 120 NM
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
angle in degrees
angl
e er
ror
in d
egre
es
Angular error (degrees) 100 m behind the wind turbine (h = 20 m)
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
angle in degrees
angl
e er
ror
in d
egre
es
Angular error (degrees) 120 nm behind the wind turbine (h = 20 m)
11-062007
#46
Radar accuracy SSR: angle (cont’d)
Larger distance between radar and WT (Rwr=2 km)Smaller angular error
100 m 120 NM
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
angle in degrees
angl
e er
ror
in d
egre
es
Angular error (degrees) 100 m behind the wind turbine (h = 20 m)
−2 −1.5 −1 −0.5 0 0.5 1 1.5 2−0.5
−0.4
−0.3
−0.2
−0.1
0
0.1
0.2
0.3
0.4
0.5
angle in degrees
angl
e er
ror
in d
egre
es
Angular error (degrees) 120 nm behind the wind turbine (h = 20 m)
24
11-062007
#47
ConclusionsPSR:
Accuracy: angular error angle ≤ WT seen from the radarShadowing effect:
• No effects if
• Visible if Da > D & False Echoes : • Everywhere between radar and WT if distance ≤ ‘critical’ distance• Large distances 2 separate volumetric regions
Effects of N obstacles in all cases ≤ N times the effect of one obstacle and in most cases = effect of one obstacleDoppler shift of reflections very small
2 2
6
3( ) ( )8 17.10t r gt tg gr r
a
D Dh h h h h hR
′ ′− = + − + ≤ =
2 2
6 2( )17.10
a aa r t r
t
D Dh h h hD
′ ′ ′ ′≤ − ≤ −
11-062007
#48
Conclusions (cont’d)
Secondary Radar:Shadowing effect: like PSR, but smaller (half the dB value, 1-way communication system)Accuracy: angular error might be ≥ accuracy of system & viewing angleFalse Echoes: • Type 3 = less strict one• Type 4 = much smaller than PSR (smaller gain of
transponder)
25
11-062007
#49
Poetic views of wind turbines
11-062007
#50
Questions?