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'''Wind turbines''' have become a common sight after the increase in demand for renewable energy sources. These turbines are physically large structures, often 50 ft or more in height. As a result, they are often picked up by radar even at long ranges. In this article, a '''Doppler spectral signature''' observed by [[Facilities#CSU-CHILL|CSU-CHILL]] from a collection of wind turbines (also called a '''wind farm''') is presented in several different formats.

==Wind Turbines/Farm==
The wind farm studied in this article is the [http://www.xcelenergy.com/XLWEB/CDA/0,get%20this,1-1-1_1875_4797_4010-3672-0_0_0-0,00.html '''Ponnequin Wind Farm'''], which lies on the Colorado-Wyoming border, near highway 85. The turbines are located roughly 63 km away from the radar. The map below has markers indicating the wind farm and the location of the CSU-CHILL radar. The map is centered on the wind farm, zooming in will reveal several rows of turbines, oriented in a NE-SW direction.

<gmap type="hybrid" lat="40.994581" long="-104.81327" zoom="14" width="600" height="600">
points:
40.994581|-104.81327|Windfarm
40.44625|-104.63708|CSU-CHILL [[Image:CHILL_Radome.jpg|thumb|100px]]
</gmap>

Each of the turbine blades is over 100 ft. long, and the tower height is 180 ft. In order to maximize their efficiency, the turbines are located on a ridge, which puts them in clear view of the CSU-CHILL radar. Switching the map to Terrain mode indicates the ridge structure.

==Radar Observations==
The CSU-CHILL radar was used to observe the echo from these wind turbines. On May 16, 2006,  the radar was used to collect time-series data in the fixed pointing mode. The radar azimuth was adjusted to point to the wind turbines, while the elevation was set to 0&deg;.

The data collection system saved 120 range gates of data in the vicinity of the turbines. Each range gate is 150 meters in length. Approximately 25 seconds of data was collected at a PRF of 957 Hz.

[[Image:Windturbine range vs doppler.png]]

The first image above shows the range vs. Doppler spectrum observed from the radar. Color values indicate the relative signal strength at various different ranges and Doppler velocities. The bright line running from left to right is the ground clutter return signal, at zero velocity. There are two vertical lines visible, this is energy leaking from the spectral peaks resulting due to two wind turbine targets. The first target, with the lower overall energy, is considered for further examination.

[[Image:Windturbine time vs doppler.png]]

The next image shows the spectra from the range gate containing the first target displayed over time. It is observed that there is a significant non-zero return from the turbines. The variation of the spectral peaks with time may be clearly distinguished. The reason for the variations is the '''movement of the turbine blades''' with respect to the radar location.

==Animation==
The animation shown below is a movie file showing a '''short-term spectral analysis''' repeated over the duration of the data collection. One can notice a peak value moving from top to bottom in each of the returns from the wind turbines. The data presented is the same as in the first figure, with range on the X axis and Doppler velocity on the Y axis. Color values represent power at each range gate and Doppler velocity bin.

The '''audio''' presented along with the animation is derived from the first wind turbine echo. The I and Q components of the radar receiver voltage are converted to the left and right audio channels, respectively, of the animation. One can clearly distinguish the repetitive pattern due to movement of the turbine blades.

[ftp://chill.colostate.edu/pub/jgeorge/windturbine_web.avi Wind turbine animation (2.1 MB AVI, MPEG-4, MP3 audio)]

===Youtube version===
<center>
<html>
<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/wS7i_2QTYYA&hl=en&fs=1&rel=0"></param><param name="allowFullScreen" value="true"></param><embed src="http://www.youtube.com/v/wS7i_2QTYYA&hl=en&fs=1&rel=0" type="application/x-shockwave-flash" allowfullscreen="true" width="425" height="344"></embed></object>
</html>
</center>

==Impact of Wind Turbines on Meteorological Observations==
Echoes from most ground clutter targets, such as buildings, trees, communication towers and the landscape may be rejected through the use of various different '''clutter filtering algorithms'''. These algorithms depend on the spectral properties of ground clutter, namely, '''narrow spectrum width''' and '''mean velocity of zero''', to distinguish it from weather echo signals. We can clearly observe, however, that wind turbines do not have a zero mean velocity, and that simple spectral clipping algorithms will no longer eliminate clutter echo from wind turbines.

The problem of suppressing these echoes has not been addressed in the current literature. Wind turbine contamination has been reported from National Weather Service WSR-88D radars in the past, an example from the KCYS radar station in Cheyenne, WY is shown [http://einstein.atmos.colostate.edu/~mcnoldy/NWSradar here]. False alarms due to the turbine echoes may be reduced by enforcing a wide spectral notch in the clutter map for those range gates known to contain wind turbines.

==External Links==
* [http://www.xcelenergy.com/XLWEB/CDA/0,get%20this,1-1-1_1875_4797_4010-3672-0_0_0-0,00.html Xcel Energy's website about the wind farm]
* [http://www.k0lee.com/blog/2007/07/colorados-wind-farms.html Lee Devlin's Weblog: Colorado's Wind Farms] - Contains aerial photographs of the wind farm
* [http://www.awea.org/news/wpa3a.html American Wind Energy Association] - opening of the Ponnequin Wind Farm
* [http://en.wikipedia.org/wiki/Wind_turbine Wikipedia entry on wind turbines]
* [http://einstein.atmos.colostate.edu/~mcnoldy/NWSradar Nexrad image from KCYS (Cheyenne) observing the same wind farm]

[[Category:Technical Notes|004]]