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Turbines can interfere with military radar systems 

Wind-powered turbines at a height of 400 feet over ridge tops have the potential to interfere with military aircraft over Highland County, and earlier this year, officials in the armed services said there was a study under way to determine risk and outline ways that risk could be reduced.

That report was released recently, by the U.S. Department of Defense, and states that while the military endorses renewable energy, whether industrial wind plants cause potential harm to certain fly-over areas must be studied individually (see related story). “The numbers, height and rotation of these wind turbines present technical challenges to the effectiveness of radar systems that must be carefully evaluated on a case-by-case basis to ensure acceptable military readiness is maintained,” the report says.

The results from flight trials conducted by the military show that “state-of-the-art utility-class wind turbines can have a significant impact on the operational capabilities of military air defense radar systems. The results demonstrated that the large radar cross section of a wind turbine combined with the Doppler frequency shift produced by its rotating blades can impact the ability of a radar to discriminate the wind turbine from an aircraft,” the report concluded. “Those tests also demonstrated that the wind farms have the potential to degrade target tracking capabilities as a result of shadowing and clutter effects.”

If built, the commercial utility proposed by Highland New Wind Development LLC here would be the first wind plant in Virginia. Its site on Allegheny Mountain is within the Evers Military Operations Area, used by the Air Force for training, and sits beneath a military training route used by the U.S. Navy for high-speed, low-altitude mission practice. That location may pose a risk, military officials have said.

The military’s obstruction evaluation program has not yet received information from the Federal Aviation Administration about HNWD’s plans; the FAA is charged with coordinating wind facility applications with the overflight patterns of military units.

The military would be concerned whether the project could interfere with radar, both on the ground and inside a cockpit.

Last March, Lt. Col. William Crowe at the Pentagon told The Recorder that Air Force instructors and air space managers will be encouraged to stay in close contact with local planners as wind utilities are introduced, as reiterated in the DOD’s report. In particular, Crowe said, the military is concerned about radar interference after a U.K. study that showed turbines could have an adverse effect.

“For long range radar, (turbines) could be picked up as false targets or could block the signal with a blind spot behind them “¦ radar on board low-flying aircraft look to the front and down for upcoming obstacles like ridges, so we don’t know if that’s a factor, too,” Crowe had said.

The DOD’s report refers to the 1994 U.K. study on the characteristics and impacts of radar interference from a wind utility with 14 300 kw turbines in a radar line of sight. “The significant interference that was being observed in the radar primary surveillance mode of operation had led to a degradation in detection performance,” the report explains. “The primary conclusion of that study was wind turbines cause interference to primary surveillance radars. The responses appear as valid targets on the radar display “¦ As a result of the trial, the (U.K. military) decided it needed to be consulted on all proposals for wind turbines closer than 60 percent of the maximum instrumented range of military radars “¦ In 2004, the policy of carefully scrutinizing wind turbine proposals so far away from operational radars was increasingly being questioned by wind farm developers, especially in light of much less restrictive constraints imposed by other European countries.”

So the military there conducted more studies in 2004 and 2005. They, too, showed “significant obscuration of primary radar returns above wind turbines. This effect was observed independent of the height of the aircraft throughout the full height range used for the trial (2,000- 24,000 feet above mean sea level) and represented the most significant operational effect of wind turbine farms on air defense operations.”
Crowe explained whether the military approves certain facilities depends on optical interference, and each unit is responsible for assessing whether a project would interfere with its missions. “The question is, can we live with it? Each unit will need to determine that and, are there any other mitigations? The unit makes that decision,” he said.

The DOD report says “only three methods so far have been proven to be completely effective” in preventing primary radar systems from being impaired, and the main way is to simply avoid placing turbines in locations that would cause interference.

In mountainous terrain, military aircraft must be a certain number of feet above the ground. In the Evers MOA, that is typically 1,000 feet. They must consider flight safety, electromagnetic interference with on-board navigation, and national security. Aside from the Evers MOA, Crowe said, wind turbines can be a real concern along military training routes. “For those, radar interference and obstacles are a real problem.”

Last fall, Crowe spoke to the National Wind Coordinating Committee to explain Air Force policy. “I told them we accept any project provided it does not interfere with our training. We can fly in another area, and we’ve moved training routes before, but it takes a lot of time and a lot of money.” Crowe said moving an MTR can take up to three years, especially where new environmental assessments are required.

While the military has given up some air spaces it no longer uses, “We know we fly out there,” he said, referring to western Highland.
John Clemens, air space manager with the 1st Fighter Wing, Langley Air Force Base, had told The Recorder the planes don’t always stay above their limit for military training. “The average MOA fly-over is 1,000 feet above ground,” Clemens said. “But on those other MTRs, those planes can go a lot lower. For those guys, a 400-foot tower might be a huge problem.”

The training route closest to HNWD’s site is exactly the kind used by the Navy for high-speed flights at very low altitudes. Highland residents are long familiar with these fighters flying by at nearly eye level with ridgelines.

A military official who had asked not to be identified said the route over Highland is considered a visual route, meaning pilots rely on seeing out the window instead of depending on instruments to guide them, and fly lower and faster than most other routes. “I have no doubt those (turbines) will interfere with them,” he said. “Any type of obstruction or additional interference is a problem and I don’t know what military technicians could possibly do to mitigate it. Is it potentially dangerous? Yes.”

By Anne Adams “¢ Staff Writer


This article is the work of the source indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.

The copyright of this article resides with the author or publisher indicated. As part of its noncommercial educational effort to present the environmental, social, scientific, and economic issues of large-scale wind power development to a global audience seeking such information, National Wind Watch endeavors to observe “fair use” as provided for in section 107 of U.S. Copyright Law and similar “fair dealing” provisions of the copyright laws of other nations. Send requests to excerpt, general inquiries, and comments via e-mail.

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