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24 the region but not affected by the clutter fill in the area Create transponder mandatory zones (TMZs) that would of the wind farm by a technique called data fusion on allow SSR to provide full augmentation for primary in-fill radar. Also referred to as a mosaic radar, this can radar. However, TMZs are uncommon at this time. only be accomplished where the primary radar can selec- Consider a model program where airspace over specified tively turn off specific areas, there is a supplemental radar wind farms is restricted to transponder carrying aircraft. coverage to provide in-fill, and the two radar systems can be pieced together and displayed. WIND TURBINE IMPACT EXAMPLES Improve radar coverage for areas where low-level radar coverage is not required through physical or terrain Several examples are provided here to illustrate wind turbine masking. This would necessitate moving the radar facility impacts and how they were addressed. to a higher elevation or constructing a man-made struc- ture to create an artificial radar horizon. Planning this type of mitigation would require detailed study to ensure Travis Air Force Base--Fairfield, California that the proposed design would resolve the interference issues. Furthermore, this masking technique would Three wind energy development companies are proposing to only be appropriate if some loss of radar coverage in construct a combined 142 wind turbines in Solano County, the area would be acceptable. California. The area currently supports 833 turbines, with Use radar absorbent materials on WTG towers and the closest structure located 4.65 nautical miles southeast of nacelles to reduce the radar cross section of the struc- Travis Air Force Base (TAFB). The 60th Air Mobility Wing ture that produces clutter. However, materials for use (AMW) at TAFB expressed concern that the proposed tur- on blades has not been effectively developed, which bines could interfere with the base's ability to provide safe is particularly problematic because the blades caused and efficient air traffic services to aircraft operating in the the greatest amount of interference. vicinity of the projects. In particular, the AMW focused on Fund research (collaboratively between government and the potential impact caused by wind turbines on the terminal industry) on technical mitigation that collects additional surveillance radar used by air traffic controllers to provide information of existing wind turbine affects, designs radar services to aircraft (Solano County 2010). parameters for gap-filler radar, characterizes wavelengths used in current radar systems to reduce signatures, and The airspace over the project area is complex and includes operations from Buchanan Field, located in Concord, and Rio advances software processing. Vista Municipal Airport (O88), as well as IFR traffic between Develop new and modified radar facilities. For example, the Sacramento and Oakland. The airspace is designated as multi-lateration involves establishing a secondary radar Class E (with the exception of Class D airspace within 5 miles system with a number of strategically located receiver of the AFB), with a floor of 700 ft above ground level. The stations in the area to provide networked radar coverage. airspace does not require radar service, although airspace These receiving stations would identify aircraft equipped in the area is safer and more efficient as a result of TAFB's with transponders and calculate their location through enhanced capabilities [Digital Airport Surveillance Radar triangulation based on the data collected from multiple model-11 (DASR-11), state-of-the-art terminal surveillance locations. This type of system is a more basic and in- radar], which became operational in February 2009. expensive form of a SSR system that might be employed today. In moving to resolve potential issues of concern, the U.S. Create non-auto-initiation zones (NAIZs) with some Transportation Command (parent to the 60th AMW) entered advanced primary radar plots that filter out tracks cre- into a Cooperative Research and Development Agreement ated by the wind turbines while not filtering out tracks (CRADA) with the three wind energy companies, with the characteristic of aircraft. The problem with a NAIZ is objective of determining the "projected impact of wind turbine that although the wind turbine tracks are not displayed, development upon air traffic operations near TAFB" (U.S. the false returns still exist and, depending on frequen- Transportation Command 2010). Other parties of the CRADA cies, could affect the display of nearby aircraft tracks were the U.S. Air Force Flight Standards Agency and the producing incorrect information. Furthermore, aircraft Idaho National Laboratory. Under the CRADA, three specific gaining elevation from low altitudes and emerging into tasks would be completed: (1) obtain reliable, objective data and above an established NAIZ will not be picked up by to assess current air traffic operational radar coverage in the radar until above the NAIZ, which is the primary reason TAFB area; (2) run a simulation to assess the predicted air traf- why NAIZs are generally discouraged. fic operational impact potentially caused by proposed wind Use advanced tracking algorithms to take advantage turbine development; and (3) assess the operational impact on of high-speed computer processing capabilities to con- the TAFB air traffic control areas of Shiloh III, Montezuma duct nontraditional aircraft tracking and data filtering. Wind, and Solano Wind Phase 3 wind projects. Although promising, the accuracy of the analytical methods has yet to be fully tested and there may still be A working group evaluated both baseline (data recorded in a risk of error. October 2009 from TAFB) and simulation data. The overall

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25 result of this work indicated that the construction and opera- between acceptable and unacceptable impact, the work- tion of the three identified projects would not reduce the prob- ing group took into consideration the type and frequency ability of detection more than the 5 percentage point margin of operations over the area to determine a level of degra- identified by the working group to protect the safety and effi- dation of surveillance coverage that would meet the oper- ciency of operations in proximity to the area. The following ational needs of the Air Force. Additionally, the working issues were considered when making this determination: group considered what services would be lost as a result of that degradation and determined that in the best inter- There would be no impact on aircraft utilizing active est of safety and the efficiency of air traffic operations, transponders or transponder-equipped VFR aircraft an average degradation not greater than 5 percentage because wind turbines do not impact secondary radar points below the established baseline values (current signals. performance) of the probability of detection would be Because the FAA established a minimum level of safety acceptable. for Class E airspace that does not require surveillance coverage, degradation of radar coverage caused by wind turbines would not result in a reduction of safety below Heritage Aviation, Burlington, Vermont the minimum standard set by the FAA. However, because radar coverage exists, and that radar coverage increases Heritage Aviation of Burlington, Vermont, erected a 130-ft the safety and efficiency of operations within the air- WTG on airport property that it leases from the Burlington space, degradation of service caused by the wind tur- International Airport (BTV) to generate electricity for its bines could decrease the overall safety and efficiency hangar and facilities (see Figure 18). As part of the project of operations. Therefore, it was necessary to identify approval, Heritage filed a Form 7460 and provided informa- an acceptable level of degradation in radar coverage tion on potential impacts of physical penetration of airspace and, more specifically, the ability to accurately detect and potential impacts on communication systems. nontransponder-equipped aircraft over the area. The number of false targets presently observed by the Westslope Consulting provided an analysis of the potential controllers is expected to be reduced, if not eliminated, impact of the wind turbine on existing Airport Surveillance after a correction to the Standard Terminal Automa- Radar (ASR-11). The physical cross section that might block tion Replacement System (STARS) configuration. (The radar signals and produce false returns was projected to be an STARS system receives data and flight plan information additional 52 ft, given that the existing building height of the and presents the information to air traffic controllers on Heritage hangar is 53 ft and the proposed height of the wind color displays, allowing the controller to monitor and turbine is 98 ft to the top of the tower. The radar impact analy- control air traffic.) This correction was temporarily sis assumed that the cross section of impact was limited to the demonstrated by the working group in December 2009, tower because the blades and nacelle are made of fiberglass- which clearly showed that the use of track eligibility reinforced polyester and the blades are 120 degrees apart and coupled with existing STARS tracking algorithms elim- moving (Westslope 2009). The shadow area is constrained by inated false targets even during significant wind activ- the close distance of the wind turbine and the ASR and because ity over the area. the terrain rises approximately 2 miles beyond the wind turbine. To further assess the level of impact, the working group Using a 6-ft-diameter tower as the obstruction, the shadow considered the number of nonparticipating aircraft likely modeling predicts a cross range of 12 ft directly behind the to be operating at any given time within the lateral lim- tower spreading to 74 ft in width at the hill 2 miles away. The its of the area. Based on the data collection, the number analysis concluded that aircraft would be operating in an envi- of nonparticipating aircraft was estimated to be minimal. ronment in this area because of their need to fly at very low The working group found that approximately 30 primary- altitudes and close to the wind turbine to encounter the radar only flight tracks occurred in October 2009 over the area. Considering all these factors (the airspace classification, operational configuration, air traffic service requirements, and traffic workload), the working group determined that degradation of radar detection resulting from addi- tional wind turbine development in the area could result in a degradation of radar services provided to nonpar- ticipating aircraft; however, given the "see and avoid" requirement, would not constitute a significant degra- dation of air safety. The working group agreed that a minor reduction in probability of detection over the area would not create an unsafe operating environment, but would decrease the safety and efficiency of operations. Because there was no FIGURE 18 Wind turbine at Burlington International Airport, reference point from which to determine the demarcation Vermont (courtesy: Christopher Hill, Heritage Aviation).

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26 shadow cast by the WTG. Furthermore, if an aircraft were to west of the proposed Ivanpah Valley International Airport, pass through the shadow, the detection level would be below a new airport proposed near Las Vegas. The FAA issued a the tolerance level though the modeling report shows the prob- No Hazard Determination for the wind project, which was ability of detection percentage decreasing from 100% to 40% appealed to the U.S. Court of Appeals by Clark County, the in an area approximately one-quarter to one-half of a mile sponsor of the new airport. behind the turbine at turbine height and decreasing at distance. On April 18, 2008, the FAA was ordered to reconsider its FAA approval included clear conditions that the appli- decision to allow the construction of a wind farm near the site cant would be subject to conducting and paying for miti- of the new Las Vegas Airport. The evidence presented indi- gation necessary to address unforeseen degradation to the cated that the turbines would interfere with the airport's ASR-11 system. Specifically, this included ceasing opera- radar systems. Specifically, the court agreed with evidence tion of the wind turbine while mitigation is implemented, presented by an aerospace consultant for the county that each payment for upgrades to the system, and permanent dis- wind turbine would have a radar signature similar to a jumbo mantling of the system at the owner's expense should other jet and that the wind farm would appear on the radar similar mitigation options not succeed (FAA 2009). to a fleet of jumbo jets. The report also stated that the sig- nature could appear and disappear rapidly based on chang- Ivanpah Wind Project ing wind conditions, which would hamper the air traffic controller's ability to control aircraft in the area. The federal A wind energy developer proposed the construction of 83 wind district court determined that the FAA's determination was turbine generators on Table Mountain approximately 10 miles arbitrary and capricious (Clark County 2008).