Radar vs. Renewables
November 13, 2018
Introduction

An ongoing issue in the development of utility-scale wind and solar installations is the potential for the planned facilities to interfere with nearby radar and communications systems. According to a 2016 report from the U.S. Department of Energy, the probability for wind installations to pose compatibility issues with nearby radar systems related to air traffic control, weather forecasting, homeland security, and national defense is likely to increase, as is the potential severity of those conflicts.1  Offshore wind farms may affect marine navigation and communications systems, airborne radar, coastal HF radars, and subsurface acoustics.2  Utility-scale solar installations have also been subject to scrutiny for their potential to adversely affect the performance of certain radar systems.3,4

Over the past decade, a variety of renewable energy projects around the United States have faced concerns over radar interference, such as the Cottonwood wind installation near a weather radar in Nebraska,5,6 the proposed Cape Wind offshore project near civil aviation radar in Massachusetts,7  and two solar installations in California’s Antelope Valley located in the area of a military contractor’s facility for radar testing of stealth technology.8

In the case of the Antelope Valley solar facilities, Exponent determined that, given an appropriate radar configuration, either solar facility would be essentially invisible to radar pulses transmitted by the radar testing facility.9,10,11 Both solar farms were approved by respective county officials over the objections of the military contractor, and the U.S. Department of Defense (DoD) reportedly had “no position” on the matter.12,13

Wind turbines and other objects can interfere with and degrade the performance of radar systems in several ways. For example, the National Oceanic and Atmospheric Administration (NOAA) has noted that turbines can affect the performance of weather radar by partially blocking the radar beam, by causing high-energy reflections that damage the radar receiver, or by introducing clutter that “contaminate” reflectivity, velocity, and radar spectrum width data.14 According to NOAA, the ultimate effect on weather radar could be to increase uncertainty and delay in weather forecasts and decrease the accuracy of the derived products such as wind profiles and rainfall estimates.15

Many components of the electrical power system can be analyzed to understand the potential for interference with radar systems and more broadly the potential for electromagnetic interference (EMI) on other surrounding installations.16  EMI analysis typically considers the effects of emitted radiofrequency noise, physical obstruction or reflection of electromagnetic signals, and induced currents or voltages. For example, the potential for interference caused by electrical transmission lines and towers scattering radio waves is well known, as are certain applicable mitigation techniques.17  These phenomena have been studied in the context of disruption to radio communications, such as cell phones, broadcast television and radio, and GPS signals, and are understood to be highly dependent on the frequency of the incident radio waves.

Example: Antelope Valley, California

A pair of solar generating facilities in Los Angeles County (242 MW18) and Kern County (200 MW)19  were objected to by Northrop Grumman, a defense contractor with a radar cross-section (RCS) testing facility near the proposed sites. They argued that the project would “adversely impact the military mission” of the testing center by unacceptably increasing the radar clutter levels and introducing time variation of the clutter levels.20,21

Exponent’s modeling of the solar facilities showed that a properly chosen pulse repetition frequency renders either solar facility essentially invisible to radar pulses transmitted by the RCS test facility and, for one facility, the calculated return power from the solar facility is less than the noise floor figure provided by Northrop Grumman for many other combinations of pulse repetition frequency and radar operating frequency.22,23,24 Both solar farms were approved by respective county officials over Northrop Grumman’s objections, and DoD reportedly had “no position” on the matter.25,26

Example: Amazon Wind Farm U.S. East

The first large-scale wind farm in North Carolina, Amazon Wind Farm U.S. East, developed by Avangrid Renewables, was brought up to full operation following a six-year permitting process that included an extensive review by the U.S. military and criticism from local lawmakers stemming from the proximity of the site to a long-range military radar system in Chesapeake, Virginia.27 To satisfy concerns that the wind farm would potentially affect U.S. national security, the developer engaged with expertsto model the effects of the turbines on the radar system and came to an agreement with the military to reduce the size of the project, alter the positions of turbines, and curtail its operations in certain circumstances.28

Example: Cape Wind

In 2012, the Federal Aviation Administration (FAA) found that that the proposed Cape Wind offshore project would have “no effect on aeronautical operations.”29 This determination30 required that the developer hold $15 million in escrow to pay for an upgraded FAA radar system if 2012 upgrades to the Otis Airfield airport surveillance radar (ASR) system were inadequate.

A 2009 FAA report describes the potential impact of the project to the three FAA radar systems near the proposed site in Nantucket Sound and suggests a number of potential mitigation steps in the form of modifications and upgrades to the FAA radar systems.31 The FAA study used software modeling tools and empirical evidence.

A study commissioned by the U.S. Coast Guard on the effect of the project on the use of marine radars on navigation in the Nantucket Sound area modeled the turbines, marine craft, and typical marine radar specific to the Cape Wind project.32  Based on the results of this study and opportunities for mitigation, the Coast Guard found the proposed project to have a “moderate” impact on marine navigation safety.

Example: Blekinge Baltic Sea Project

A 2.5 GW, 700 turbine project proposed by Swedish developer Eolus Vind AB was blocked in late 2016 by the Swedish government due to its location being in one of the countries’ “strategically most important defense areas.”33  The government found that it was “not possible to combine the defense business with wind turbines” at the proposed offshore location in the Baltic Sea.34 Meanwhile, the Swedish electrical utility Vattenfall has placed turbine orders for the Kriegers Flak project, a 600 MW offshore wind installation in the Baltic Sea.35

Example: Concerns Over Military Mission Readiness

Lawmakers in the U.S. Congress from Texas and New York proposed 2017 legislation to “discourage the construction of new wind turbines” within a 30- or 40-mile radius of a military airfield in an effort to “protect military airfields from electromagnetic radar interference caused by nearby wind turbines.” Similar bills are also under consideration in the Texas legislature.37,38 One Texas state legislator recently called wind farms the “current leading threat to mission readiness at two Texas military installations.”39

Basics of Radar Systems and Radar Cross Section

Radar systems operate by sending out pulses of electromagnetic energy through a highly directional antenna.40 These pulses propagate from the radar through the atmosphere, and small amounts of energy are scattered by targets and clutter. Targets are objects the radar is trying to detect or characterize (such as aircraft), and clutter includes unwanted returns from objects other than a target. The portion of energy that may be scattered back is received by the antenna, and the radar system attempts to either identify targets among the clutter (in the case of a detection problem) or accurately measure the return (in the case of a radar-cross-section measurement problem). Doppler-capable radar systems can measure the speed of the target moving toward or away from the radar by analyzing the change in frequency of the reflected electromagnetic pulse.

An object, such as a wind turbine or collection of solar panels, can potentially interfere with a radar system by blocking emitted radar energy from reaching an intended target area or by reflecting radar energy back toward the radar receiver in a way that the radar system is unable to correctly interpret. The RCS is a measure of an object’s ability to scatter incident electromagnetic field radiation in the direction of a receiver and is defined as the ratio of power scattered by a distant object relative to the incident power illuminating the object. The RCS is normalized such that it is a function of object geometry, incident wave angle, material properties of the scattering object, wave polarization, and excitation frequency.

In general, the RCS is not the same as the physical size of the scattering object. For instance, adding radar-absorbing material to an object will decrease the amount of scattered energy directed back to the receiver, effectively decreasing its RCS relative to the same object without absorbing material. So too, faceted surfaces that reflect incident radiation away from the source decrease the RCS compared to surfaces that are perpendicular to incident radiation. These are among the commonly used techniques to minimize RCS.

The RCS for various objects can vary by orders of magnitude. As such, the RCS is commonly converted to a logarithmic scale and expressed as referenced to an object 1 m2 in size with the units of “dBsm.” RCS values for various objects are listed in Table 1. Every RCS shift of −10 dBsm corresponds to a factor of 10 decrease in RCS. For instance, an insect with 1/1000th (one one-thousandth) the RCS of a human body has an RCS that is 30 dBsm less that the RCS of a human body.

meters (m2) and dBsm 41,42

Table 1

Table 1. Examples of objects and corresponding reported radar cross sections in square

The role of RCS in characterizing the received signal strength for a given transmitter-receiver pair can be explained by means of the radar equation, which incorporates the received power, transmitted power, the gain of the transmitting antenna, the radar cross section of the scattering object, the aperture of the receiving antenna, and the distance (also referred to as the range) to the object from the transmitter/receiver.

The radar equation shows that that for a given object with a given RCS, the reflected power at the receiver varies as the 4th power of range (assuming all other variables are constant). Thus, an object with a given RCS will have a radar return echo that carries 16 times less power when the distance from the object to the transmitter/receiver increases by 2 times.

Generally, an object with substantially larger RCS that is placed much farther from the transmitter/receiver can exhibit the same received signal strength as an object with smaller RCS that is placed much closer to the transmitter/receiver. Additional factors can be included in the monostatic radar equation to account for multipath terrain-dependent (e.g., reflection and/or diffraction from surrounding objects) and medium dependent (e.g., atmospheric) losses. These factors are not included in the above formulation for far-afield clutter since their effect is second order.

Mitigation Techniques

Turbines can result in “significant clutter and interference” when located within the line of sight of radar systems.43  Several existing features of radar systems and of wind turbine installations can be leveraged in some cases to reduce the potential for interference.44  For example, radar antennas are generally highly directional, with most of the energy transmitted in the main beam and a far smaller amount transmitted in side lobes. Pulsed radar systems also can employ a technique called range gating wherein the radar system ignores radar reflections unless they correspond to an object within a given range of distances, as determined by the time it takes for the pulse to travel from the radar antenna to the object and back.

Clutter from wind turbines can be particularly difficult to mitigate due to its fluctuating nature. Variations in wind speed and direction cause the RCS of the turbine blades and nacelle seen by the radar system to change dramatically and high RCS orientations can produce reflections that saturate a radar receiver. Moreover, the rotation of the turbine blades can produce Doppler shifts that exceed the ability for the radar system to uniquely distinguish the measured velocity.

A number of mitigation techniques are under study or in various stages of deployment and can be placed into several broad categories.46  The first category of mitigation techniques includes changing aspects of the wind installations, such as the size, shape, or materials of the wind turbines themselves (in particular the blades) or the layout, number, or specific locations of the turbines, to reduce the individual RCS of each turbine or the aggregate effective RCS and of the installation. A second category of mitigation techniques includes adding or replacing radar hardware, resulting in radar systems with higher resolution and narrower beams located at positions less likely to be subject to potential turbine interference. A third category of mitigation techniques includes making software improvements to radar systems to improve the detection and tracking processes, via advanced image processing and interpolation,47 and potentially combining data from multiple radar systems that cover the same aircraft or area.

Siting Evaluation Processes in the United States

According to a 2008 U.S. Department of Commerce technical report, “the precise impact on radar performance can only be determined on a case-by-case basis.”48 The risk of interference depends on such factors as the geography of the site and its proximity to the radar installation, the quantity and placement as well as the specific geometry, materials, and motion of the turbines or solar panels, and the radar frequency bands and signal processing capabilities. A study would take these factors into account and could potentially include a software-based model in addition to laboratory or onsite measurements.

According to one developer, “nearly a fifth of current operating wind farms are within 25 miles of military installations.”49 There are formal, regulatory-based processes as well as informal processes to review wind turbine siting in the context of government operated radar systems.50 Formal processes include the FAA Obstruction Evaluation/Airport Airspace Analysis (OE/AAA) and the DoD Siting Clearinghouse.

According to the FAA, federal regulations require filing with the FAA for proposed structures or alterations depending on the height, proximity to an airport, location, frequencies emitted from the structure, and other factors.51  For example, these requirements cover “any construction or alteration exceeding 200 feet above ground level.”52 An online Preliminary Screening Tool provides a “preliminary review of potential impacts to Long-Range and Weather Radar(s), Military Training Route(s) and Special Airspace(s) prior to official OE/AAA filing.”

The DoD Siting Clearinghouse formal review process evaluates potential impacts of proposed wind, solar, transmission, and other projects on military activities and explores mitigation options.53  DoD also conducts informal reviews initiated at the request of developers and landowners or other federal, state, or local government agencies. For informal reviews of wind turbine projects, DoD requests such information as the geographic location of each turbine, along with the turbine type, hub height, and blade tip height.54 For informal reviews of solar installations, DoD requests such information as the geographic location of each turbine, solar tower or panel height, and solar layout and acreage.55 In 2014, the Clearinghouse processed over 2,300 projects, with the “vast majority” of projects presenting “no unacceptable impacts to DoD mission.”56

The NOAA Screening Tool enables developers to obtain a preliminary review of potential impacts to weather radar(s) before official OE/AAA filing. The use of this tool is 100% optional and will provide a first level of feedback and single points of contact within NOAA to discuss impacts/mitigation efforts on NEXRAD Weather Radars. The use of this tool does not in any way replace the official FAA processes/procedures.57

References

  1. U.S. Department of Energy “Federal Interagency Wind Turbine Radar Interference Mitigation Strategy” January 2016.
  2. Final Report DE-EE0005380 Assessment of Offshore Wind Farm Effects on Sea Surface, Subsurface and Airborne Electronic Systems 
  3. http://articles.latimes.com/2010/nov/24/business/la-fi-solar-radar-20101124 
  4. http://www.hastingstribune.com/news/wind-turbines-going-up-in-webster-county/article_28f6fd8e-ad4a-11e7-8cd0-1f82560afb1c.html 
  5. http://www.wcvb.com/article/faa-gives-cape-wind-project-key-clearance/8172975 
  6. http://articles.latimes.com/2010/nov/24/business/la-fi-solar-radar-20101124 
  7. http://file.lacounty.gov/SDSInter/bos/bc/153727_official_20101115_project_r2009-02239_bos-package.pdf 
  8. http://www.co.kern.ca.us/planning/pdfs/eirs/RosamondSGS/RosamondSGS_bos_sr_110910_add2.pdf 
  9. http://www.co.kern.ca.us/planning/pdfs/eirs/RosamondSGS/RosamondSGS_ch7_RTC.pdf 
  10. http://articles.latimes.com/2010/nov/24/business/la-fi-solar-radar-20101124 
  11. http://www.reuters.com/article/us-firstsolar-idUSTRE6AM66R20101123?feedType=RSS 
  12. See, for example, NOAA Radar Operations Center https://www.roc.noaa.gov/WSR88D/WindFarm/TurbinesImpactOn.aspx?wid=dev 
  13. See, for example, NOAA Radar Operations Center https://www.roc.noaa.gov/WSR88D/WindFarm/TurbinesImpactOn.aspx?wid=dev 
  14. B.R.T. Cotts, K.L. Graf., and W.H. Bailey. “Electromagnetic Interference Considerations for Electrical Power Systems.” in The Power Grid: Smart, Green, and Reliable, B.W. D’Andrade, Ed. 2016. Elsevier, 2017. pp. 174 – 177.
  15. B.R.T. Cotts, K.L. Graf., and W.H. Bailey. “Electromagnetic Interference Considerations for Electrical Power Systems.” in The Power Grid: Smart, Green, and Reliable, B.W. D’Andrade, Ed. 2016. Elsevier, 2017. pp. 174–177.
  16. http://www.exeloncorp.com/locations/power-plants/antelope-valley-solar-ranch-one 
  17. https://www.pv-tech.org/news/project_focus_sempra_generation_gets_approval_for_200mw_solar_pv_project_in 
  18. http://articles.latimes.com/2010/nov/24/business/la-fi-solar-radar-20101124 
  19. http://www.avhidesert.com/forum/showthread.php?tid=1464 
  20. http://file.lacounty.gov/SDSInter/bos/bc/153727_official_20101115_project_r2009-02239_bos-package.pdf 
  21. http://www.co.kern.ca.us/planning/pdfs/eirs/RosamondSGS/RosamondSGS_bos_sr_110910_add2.pdf 
  22. http://www.co.kern.ca.us/planning/pdfs/eirs/RosamondSGS/RosamondSGS_ch7_RTC.pdf 
  23. http://articles.latimes.com/2010/nov/24/business/la-fi-solar-radar-20101124 
  24. http://www.reuters.com/article/us-firstsolar-idUSTRE6AM66R20101123?feedType=RSS 
  25. https://s3-us-west-2.amazonaws.com/iberdrola-pdfs/pdf/AWFUSE-fact-sheet.pdf
  26. http://bigstory.ap.org/article/c3e7472379f140d4979989e1bc9cf79a/full-go-nc-wind-farm-politicians-claimed-threat 
  27. http://www.citizen-times.com/story/news/2017/01/19/navy-wind-farm-opposed-gop-lawmakers-harm-radar/96802816/
  28. https://web.archive.org/web/20120817022852/http://www.boston.com/metrodesk/2012/08/15/faa-rules-cape-wind-project-poses-hazard-planes/wrds6gRVgtGkk7HlekpZ4O/story.html 
  29. https://www.boem.gov/uploadedFiles/BOEM/Renewable_Energy_Program/Current_Projects/FAA%20Cape%20Wind%20DNH%2008152012.pdf 
  30. http://users.ece.utexas.edu/~ling/US1%20Study_of_Nantucket_Wind_Farm_Report.pdf 
  31. https://www.boem.gov/Renewable-Energy-Program/Studies/FEIS/Appendix-M---USCG-Report.aspx 
  32. http://news.nationalpost.com/news/world/sweden-denies-permit-for-7-4b-offshore-wind-farm-because-the-project-would-interfere-with-its-military 
  33. http://news.nationalpost.com/news/world/sweden-denies-permit-for-7-4b-offshore-wind-farm-because-the-project-would-interfere-with-its-military 
  34. https://maritime-executive.com/article/vattenfall-places-record-offshore-wind-turbine-order 
  35. https://www.cornyn.senate.gov/content/new-cornyn-bill-will-protect-military-airspace-radar-interference-caused-wind-turbines 
  36. https://www.eenews.net/stories/1060050066 
  37. https://legiscan.com/TX/bill/SB277/2017 
  38. https://chriscollins.house.gov/media-center/press-releases/congressman-chris-collins-introduces-legislation-to-blow-back-wind 
  39. http://www.timesrecordnews.com/story/news/politics/2017/04/05/wind-farm-bill-receives-support-opposition-austin/100082994/ 
  40. http://www.senate.texas.gov/members/d25/press/en/p20170419a.pdf 
  41. Portions of this section appear in Exponent report “Impact of the Antelope Valley Solar Ranch on the Tejon Test Facility,” November 2, 2010.
  42. Knott, Eugene F., “Radar Handbook: Radar Cross Section”, McGraw-Hill, 2008.
  43. Office of the Director of Defense Research and Engineering “The Effect of Windmill Farms On Military Readiness” 2006.
  44. Sandia Report SAND2014-19003 “Wind Turbine-Radar Interference Test Summary” September 2014.
  45. See, for example, “QinetiQ Report on Impact of the PdV Wind Farm on RCS Measurements at the Tejon Range” July 20, 2007.a
  46. Gallardo-Hernando et al., “Wind Turbine Clutter” Radar Technology, ed. Kouemou, 2009.
  47. Sandia Report SAND2014-19003 “Wind Turbine-Radar Interference Test Summary” September 2014.
  48. Isom et al., “Detailed Observations of Wind Turbine Clutter with Scanning Weather Radars” Journal of Atmospheric and Oceanic Technology, May 2009.
  49. https://www.its.bldrdoc.gov/publications/download/TR-08-454.pdf 
  50. http://www.watertowndailytimes.com/news03/federal-bill-could-prevent-wind-projects-near-military-bases-20170105 
  51. https://energy.gov/sites/prod/files/2014/10/f18/IFTE%20Industry%20Report_FINAL.pdf 
  52. https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp?action=showNoNoticeRequiredToolForm
  53. https://oeaaa.faa.gov/oeaaa/external/portal.jsp
  54. http://www.acq.osd.mil/dodsc/about/faq.html 
  55. http://www.acq.osd.mil/dodsc/contact/dod-review-process.html
  56. http://www.acq.osd.mil/dodsc/contact/dod-review-process.html
  57. http://www.acq.osd.mil/dodsc/about/faq.html 
  58. http://pikes.peakspatial.org/NOAA/ScreeningTool/ 

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