Air Traffic Control at Wind Farms with
TERMA SCANTER 4000/5000
A.C.K.Thomsen#1, O.Marqversen#2, M.Ø.Pedersen#3, C.Moeller-Hundborg#4, E.Nielsen#5, L.J.Jensen#6, K.Hansen#7.
#1-7
Terma A/S, Hovmarken 4, 8520 Lystrup, Denmark
Contact mail: [email protected]
Abstract — The challenges of aircrafts detections in the area of
wind farms are addressed. Requirements for a gapfilling radar
solution is identified and obtained performance with SCANTER
4000 and SCANTER 5000 are described.
I. INTRODUCTION
For many long range ATC-Radars (Air Traffic Control),
wind farms reduce the detectability of aircrafts for several
reasons [1]. First of all, the blades of wind turbines are
moving at high velocities, making it difficult to separate wind
turbine radar echoes from aircraft radar echoes by simple
velocity filters. Secondly, the radar echo from the tubular
wind turbine tower generates range side lobes in a significant
range before and after a wind farm. The range side lobes can
exceed the echo from small aircrafts in the same area and
make the ATC-Radar ‘blind’ in a large area around a wind
farm. The ‘blindness’ is a major problem for civil ATC
operators and military RAP (Recognized Air Picture) creators
and causes wind farm projects to be postponed or cancelled
[4,9]. This paper describes the requirements needed for air
detection in the area of a wind farms and present coverage
performance obtained by Terma SCANTER 4000 and
SCANTER 5000 air coverage radars.
II. QUANTIZATION OF THE PROBLEM
Long range ATC-Radars typically use pulse compression
with chirp lengths of about 100µs or more. The pulse
compression process generates range side lobes in a range
determined by the chirp length. For a typical long range ATC
Radar with 100µs chirp lengths the range side lobes will be
15km before and after a wind farm [1]. The peak level of
range side lobes is typically -40dBp relative to the echo from
the tubular tower (Fig. 5 from [1]). The Radar Cross Section,
RCS, of a tubular tower is reported to be about 60dBsqm
[10,2]. However, when the actual range and aspect angle at
the tubular tower is taken into account as shown in Fig. 4, the
RCS will be lower than 60dBsqm, but typically about
30dBsqm at S-band as shown in Fig 1. At X-band, shown in
Fig. 2, the peak RCS is higher in the far-field, but the roll-off
vs. aspect angle is faster so the actual RCS is expected to be at
20dBsqm. Mind that figures in this paper is not exact figures,
but estimation of the order of magnitude.
The range side lobes will desensitise the radar due to the
signal of the range side lobes. The minimum detectable target
in the area of range side lobes will therefore be about 0dBsqm
(30dBsqm - 40dB + 10dB) for the typical S-band ATC radar if
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the radar has the same antenna gain at the target as at the wind
turbine tower and 10dB detectability factor is required. For 2D
radars (range/azimuth) elevation coverage is often obtained by
shaping the antenna elevation pattern.
Fig. 1 RCS calculation of a perfect conical frustum with an upper diameter of
4m, a base diameter of 6m and height of 100m based on [3]. RCS is
calculated at S-band in far-field (blue), 60km (purple),20km(magenta),
6km(red) and 2km(green) distance vs. aspect angle to centre line of frustum.
Fig. 2 RCS calculation of a perfect conical frustum with an upper diameter of
4m, a base diameter of 6m and height of 100m based on [3]. RCS is
calculated at X-band in far-field (blue), 60km (purple),20km(magenta),
6km(red) and 2km(green) distance vs. aspect angle to centre line of frustum.
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If the elevation pattern is shaped to a co-secant-squared
antenna pattern, optimised for targets at 10,000 feet, the two
way antenna gain on the elevated target can be 10-20 dB
lower than the gain on the tubular tower, as seen in Fig. 3,
where the signal gain for a target at 10,000 feet relative to the
main loop signal gain is shown.
Two way gain on target [dB]
Antenna gain on target in fixed elevated height of 10.000feet
− 10
− 20
− 30
− 40
20
40
60
80
100
Range [km]
Fig. 3 Two way antenna gain relative to peak antenna gain on an elevated
target in 10,000 feet for a co-secant squared antenna as described in [5].
In this case the minimum detectable target due to range side
lobes from the tubular tower reflection will be 10 to 20dBsqm.
Aspect angle [deg]
range
2
1.5
1
0.5
0
− 0.5
5
10
15
20
Range [km]
Fig. 4 Aspect angle relative to the centre line of a vertical wind turbine tower
for an elevated radar antenna positioned at 100mASL.
Fig. 5 Predicted pulse compression range sidelobes from a typical ATC radar
performed by [1].
The described desensitisation so far is caused by only one
wind turbine. When several wind turbines generate range or
azimuth side lobes in the same desensitised cell, the vectors of
all contributions shall be added. This means that if 10 wind
turbines generate side lobes in the same cell, the
desensitisation can in worst case be increased up to 20dB. The
minimum detectable signal will in this case be 30 to 40dBsqm,
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which is far beyond the typical RCS from small aircraft of
0dBsqm.
Desensitising can also be caused by dynamic saturation. If
the azimuth resolution and the chirp length allow many wind
turbines to be present in the same non-compressed pulse, the
receiver must be able to handle the voltage addition of all the
signals. If saturation occurs, the side lobe level will be
increased above the mentioned level and cause additional
desensitisation.
The moving blades of a wind turbine will not be attenuated
by MTI (or MTD) filters. If the range and azimuth resolution
causes the MTI signals from the turbines to merge, or almost
merge, inter-turbine visibility will be impossible. Even with
fine resolution, typical CFAR processing will process the
turbines in a large wind farm as background clutter and
attenuate the whole wind farm area with some extension at
that.
III. REQUIREMENTS FOR INTER-TURBINE DETECTION
Gap filling radars have earlier been proposed to mitigate
desensitisation of prime ATC-Radars [1, 6]. However, in
some of the presented configurations, the gap-filling radar
must be positioned such that no illumination of the wind farm
occurs. This can not always be achieved, especially for offshore wind farms. If the gap-filling radar should be able to
detect small aircraft close to or even above the area of the
wind farm, a number of requirements must be fulfilled. These
requirements for inter-turbine detection can be categorized as
follows:
1) Azimuth antenna resolution as fine as possible for
the required scanning update rate.
2) Range resolution in the order of the physical
extension of the wind turbine.
3) CFAR processing that excludes wind turbines in
the background clutter processing.
4) Instantaneous linear dynamic range of the receiver
higher than the difference of signals from the
vector sum of towers in a non-compressed pulse /
azimuth cell and the minimum target to be
detected.
5) Adaptive sensitivity control to make the receiver
able to place its linear dynamic range right to
handle the actual number of turbines in the noncompressed pulse.
6) Pulse compression range side lobes sufficiently
low, so that the signal strength in the range side
lobes generated by the illuminated number of
towers/blades will be sufficiently lower than the
signal of target to be detected.
7) Azimuth antenna side lobes sufficiently low, so
the vector sum of signals from towers in the
antenna side lobes, but only in the pulse
compressed range cell, will be sufficiently lower
than the signal of target to be detected.
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8) MTI/MTD processing sufficiently advanced to
suppress clutter in the vicinity of the tubular
towers.
9) Tracking algorithms which are not seduced by the
presence of wind turbines.
Quantisation of the listed requirements is of course site and
application dependent, but the requirements are tried to be
quantified in the following:
1) Antenna beam width shall be as narrow as possible, but
is limited by instrumented range, update rate and hits per
beamwidth. If instrumented range is 30nmi, update rate is 12
rpm and 12 hit per beamwidth is required for Doppler
processing, the beamwidth shall be wider than 0.32°. This size
of antenna beamwidth can be obtained by a 7m X-band
antenna or a 21m S-band antenna and is therefore more
convenient to realize at X-band.
2) Range resolution shall be as fine as possible to get interturbine visibility, but as the turbines can have some physical
extension caused by the blades, range resolution do not need
to be finer than 10 to 20m. Range resolution about 20m makes
inter-turbine visibility possible as the distance between the
wind turbines in a wind farm is typically between 200 and
500m.
3) CFAR processing needs to ignore the wind turbines and
detect the clutter between the wind turbines. Methods for this
have been addressed in several papers, such as [7].
4) The maximum signal that the receiver shall be able to
handle is the vector sum of signals from all towers in an area
determined by the non-compressed pulse length times the
antenna azimuth beam width. If the antenna beam width is
0.5° and the distance between wind turbines is 200m, only one
wind turbine will be present in a beamwidth when the distance
from the radar is below 45km. If the non-compressed pulse
length is 20µs and the distance between wind turbines is 200m,
15 wind turbines can contribute to the vector sum of the signal.
However, this requires that all wind turbines are lined up
within an antenna beamwidth (which shall be avoided if
possible). If the RCS for a single wind turbine is 20dBsqm,
the receiver shall be able to handle the vector sum from 15
wind turbines, equal to 44dBsqm. As the signal from an
elevated target can be attenuated due to lower antenna gain, an
elevated target of 1sqm will result in a lower signal equals to
-20dBsqm in the main beam. The instantaneous linear
dynamic range of the receiver shall therefore be more than
64dB.
5) In the dynamic range described in 4), no range
dependency is included. To avoid range dependency to be
added to the required dynamic range of the receiver, adaptive
sensitivity control can be used to place the dynamic range of
the receiver so the maximum signal described in 4) just can be
handled linearly by the receiver and processing.
6) Range side lobes shall be sufficiently low so the vector
sum of range side lobes that can add up in a single cell as
described in 4) will make target detection possible. If the
target size is assumed to be 1sqm, the number of turbines that
can contribute to a signal is 15, the RCS of a wind turbine
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tower is 20dBsqm, the gain on target is 20dB lower than the
gain on the wind turbines and the detectability factor is 10dB,
then the required range side lobe level will be
20+24+20+10=74dBp. Mind that this is a worst case factor
that seldom occurs.
7) Antenna side lobes shall be sufficiently low so the vector
sum of signals from antenna side lobes, that can add up in a
single range-azimuth cell, will make target detection possible.
If the antenna side lobes roll off as shown in Fig 9, wind
turbines in an azimuth band of ±3° can contribute to the
vector sum in a single range-azimuth cell. In worst case 24
turbines can contribute to the sum if the distance between
turbines is 200m, the range is 45km and the turbines are
positioned on an arc with the radar in its centre. This will very
rarely happen due to the fine range resolution and the layout
of the wind turbines. More than 10 turbines will seldom
contribute to the vector sum in the same cell. Given the same
assumption as in 6), but the number of turbines being 10, the
required two-way antenna side lobe level will be
20+20+20+10=70dB or 35dB one-way.
8) MTI/MTD processing shall be able to suppress rain and
sea clutter between the wind turbines without being seduced
by the signal spectrum from the wind turbine tower and blades.
9) The tracking algorithm shall be able to distinguish
between air targets and wind turbines. As the signals from
wind turbines have a broad Doppler spectrum and flashes in
amplitude, the signal from a wind farm can look like a target
that moves between the wind turbine positions from scan to
scan. The tracking algorithm shall therefore be able to classify
“flashing signals from a stationary target” as wind turbines
and remove the plots from the pool for other track associations.
IV. OBTAINED RADAR PERFORMANCE
The nine requirements can be used to evaluate how suitable
a radar system is for detection of aircrafts in a wind farm or
just in the vicinity of a wind farm. As an example the
performance of SCANTER 4000 and 5000 is used in this
section.
1) Antenna beamwidth close to 0.32°. The Terma produced
antenna 15’ LACP-C has an azimuth beamwidth of 0.51°.
This antenna is normally used for long range air surveillance,
but for a gap-filling application with an instrumented range
less than 40km and an update rate that shall match the longrange ACT radar, the 21’ LAHP-C [5] can be used with only
0.36° beamwidth. To minimize the RCS of the wind turbine
tower, vertical polarization shall be avoided and therefore also
circular polarization.
2) Range resolution less than 20m. Terma SCANTER 4000
has a range quantisation of 6m and the resolution can be
configured from 12m and up. In normal air coverage
configuration 24m is used to reduce straddling loss on target,
but for inter-turbine visibility, 12m is preferred. Terma
SCANTER 5000 has a range quantisation of 3m and the
resolution can be configured from 6m and up.
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3) Exclusion of wind turbines in CFAR processing.
SCANTER 4000 and 5000 have a fine resolution CFAR map
that can exclude cells down to 12x12m from the statistic
analysis in the CFAR processing.
4) Instantaneous linear dynamic range higher than 64dB.
The linear dynamic range of SCANTER 4000 and 5000 is
limited by 14bit AD converters. In principle 14bit is equal to
84dB, but a few dB back-off of the maximum signal is needed
to avoid saturation, and the kTB-noise floor needs to be more
than 12dB above the minimum detectable signal of the ADC
to make its quantisation noise insignificant to the kTB-noise.
However, the receiver chain of SCANTER 4000 and 5000 has
distributed sensitivity control where the first 25dB of
attenuation is performed after the LNA. The kTB noise can
thereby be attenuated below the quantisation noise of the
ADC, which makes it possible to use the full dynamic range
of the ADC. The following digital processing makes no
limitation of the dynamic range as all linear processing is
performed in floating point format with 138dB linear dynamic
range.
Fig. 7 Nuttall Window function with 16 dB/octave decay.
It is important that the side lobes are generated by noise and
therefore will be integrated as noise power instead of voltage
signals. This reduces the requirement to 20+12+20+10=62dBp.
5) Adaptive sensitivity control to match the receiver
dynamic range to the actual signal levels. SCANTER 4000
and 5000 detects the actual signal level and can in the next
sweep change the attenuation level of the dynamic STC if
saturation is going to be likely. The actual attenuation of the
signal is reversed in the digital domain to maintain low range
side lobe and MTI attenuation even with fast adjustments of
the dynamic STC. To show how the dynamic STC works, Fig.
6 shows an example of attenuation vs. range and azimuth.
Fig. 8 Measured side lobes of a large cylindrical “thermos bottle” at the
power plant Studstrupværket. At right the A-scope scale is from 0 to 75dB.
Fig. 6 High resolution dynamic STC map vs. range and azimuth
6) Range side lobes in the order of -74dBp. The pulse
compression technique used in SCANTER 4000 and 5000
makes it possible to suppress range sidelobes to the level of
the window function applied. To minimize sidelobe addition
far from the peak signal, a window function with 16dB/octave
roll off in range is used as shown in Fig. 7. However, the
measurable side lobe level is limited by noise in calibration
signals leaving a sidelobe level of noise just below -60dBp as
seen in Fig. 8.
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7) Two way antenna side lobes below -70dBp. Fig. 9 shows
the typical two-way antenna gain vs. azimuth angle of the
Large Aperture, Horizontal Polarized Cosecant Squared
Antenna LAHP-C produced by Terma A/S [5]. As seen the
average peak side lobe level within ±3° is well below the
-70dBp.
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displacement of the wind turbines of only 100x200m, the
signals from two turbines merge into one.
Fig. 14 shows how the finer range resolution of SCANTER
5000 can be used to separate the wind turbines of Tunø Knob
Wind farm and improve the inter-turbine detectability.
Terma 21' LAHP-C 9.17GHz
Two way azimuth side lobe level [dBp]
0
-10
-20
-30
-40
-50
-60
-70
-80
-15
-10
-5
0
5
10
Azimuth [deg], dot: 10x zoom
Fig. 9 Two-way antenna gain of Terma 21’ Large Aperture Antenna vs.
azimuth. Dotted line is zoomed 10 times in angle.
15
Gap filling performance can be obtained both with shore
based radars as shown, but also with in-farm positioned radars,
as described in [6]. The challenges and benefits of in-farm
positioned radar are not addressed in this paper. However, the
gap-filling solution makes additional services available as
– vessel traffic control
– collision warning
– turning warning flash lights on only on aircrafts
approach
– monitoring fishing boats in prohibited areas
8) Clutter handling between wind turbines. A clutter map in
SCANTER 4000 and 5000 makes it possible for the statistic
process to distinguish between clutter in fixed positions, as
land and wind turbines, and distributed clutter. The map has a
resolution of 12x12m which makes it possible to take out the
signals from wind turbines from the statistics and make
normal sea and rain CFAR processing between the turbines.
9) Tracker wind turbine handling. By defining a static
target type which has zero mean speed and maintain track on
this type of target even with low probability of detection (very
fluctuating intensity), the Terma Embedded Tracker
automatically generates an internal track on each wind turbine
and thereby occupies the wind turbine plot to avoid faulty air
target plot-track associations.
V. RECORDING OF INTER-TURBINE DETECTION
Recorded detection performance will be shown in this
section. As an example, Fig. 10 shows a small target passing
through Nysted Wind Farm [11]. Nysted Wind Farm is one of
the largest off-shore wind farms in the world. The video was
recorded from SCANTER 4100, a 12kWp TWT based radar,
on a test platform on the Danish patrol vessel “Lommen”. The
small boat passing between the turbines is fully detected only
with a minor signal reduction when the boat is just behind a
turbine tower caused by shadowing from the tower.
Another example is shown in Fig 11, where a Cessna 172 is
passing close to 10 wind turbines at Tunø Knob in a distance
of 30km from the radar, a 200Wp solid state SCANTER 5000.
As seen the aircraft is just detectable with the same low
probability of detection close to the wind farm as before and
after the wind farm. This shows that the radar has not reduced
sensitivity in the vicinity of the wind farm.
A third example is shown in Fig. 12 and 13 where a Bell
helicopter type 206L is approaching the wind farm at Tunø
Knob. The radar, SCANTER 4000 has been backed off 12 dB
to make the target just detectable as seen on the approach in
Fig. 12. On its passing of the wind farm, shown in Fig. 13, no
indication of desensitivity is seen. Due to the distance from
the radar to the wind farm of 30km and the small
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Fig. 10 Example of radar video at Nysted, Denmark with small target moving
through. Yellow is the radar video of the last scan and red is decaying trails.
Total size of the wind farm is 6x4km
Fig. 11 Example of radar video at Tunø Knob, Denmark with a small aircraft
passing. Yellow is the last video of the last scan and red is decaying trails.
Total size of the wind farm is 0.4x0.8km.
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Fig. 12 Example of radar video at Tunø Knob, Denmark with a Bell 206L
helicopter approaching. Distance between turbines is 100m times 200m.
Distance from radar to wind farm is 30km. Radar is SCANTER 4000 with
15’LACP antenna, but backed off 12dB by 60m extra waveguide run.
VI. CONCLUSIONS
The requirements needed to get aircraft detectability in the
area and vicinity of wind farms have been argued and
quantified. The requirements have been compared with the
actual performance of the Terma developed and produced
radar series SCANTER 4000 and 5000 to justify their
capabilities in this aspect. Actual recorded inter-turbine
detection has been shown and the benefits of this solution
compared to an upgrade solution of standard ATC radars are
listed. It shall be noted that SCANTER 4000 family is an offthe-shelf products that was installed at a customer for the first
time in 2007 and now sold to 6 nations. SCANTER 5000 is a
newly developed family of solid state based radars aimed for
different applications [8] where SCANTER 6000 is a variant
of the SCANTER 5000 complemented with functions for
being installed on moving platforms.
Article [6] demonstrates that the concept of gap-filling
radars have proven to be acceptable by governments, and with
the features listed here, this may even be easier to obtain in
the future.
ACKNOWLEDGEMENT
Acknowledgement to all our colleagues who have
participated in the development of SCANTER 4000,
SCANTER 5000 and LACP antennas.
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Fig. 13 Condition as in Fig 9 zoom in on helicopter passing by.
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FOX16 News, “Jodziewicz, American Wind Energy Association, said
that projects totaling 10,000 megawatts of wind power were built in the
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Fig. 14 Tunø Knob Wind farm on SCANTER 5000 with 6m range resolution
in normal radar view without decaying trails. In the right side of the Radar
Service Tool, the VRM-scope is shown on the top of the A-scope.
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