Wind Turbine Interference-Mitigation Study
A strategy for lessening wind turbines' effects on the performance of an aircraft measurement system at a naval air station on the Chesapeake Bay could inform future research into interference mitigation for other radar applications.
Wind power supplies more than 4.5% of the U.S. electricity portfolio today. Approximately 48,000 wind turbines in 39 U.S. states and Puerto Rico generate this power.1 The U.S. Department of Energy envisions that wind power will supply 10% of the national end-use electricity demand by 2020 and 20% by 2030.2 Despite technological advances that could increase the electrical output of individual turbines, achieving this projection will require a significant growth in the number of U.S. wind farms constructed on land and possibly off shore.
The U.S. government is concerned about the impact of wind farms on the performance of its air surveillance and weather radar systems. Because of their large radar cross sections (RCS), wind turbines are highly visible to radars, and the interference that turbines produce can impede the detectability and characterization of targets of interest. The complex scattering and reflections of radar returns from the rotation of the turbines' blades further exacerbate the problem by spreading the interference across Doppler frequencies (related to the speed and direction of a target) typically occupied exclusively by aircraft. These factors make the mitigation of the wind turbines' effects a challenging problem for radar engineers.
The U.S. Navy asked Lincoln Laboratory to analyze the effects of wind turbines on radar systems that provide critical measurements for aircraft and surface vessel characterization and to propose a strategy for moderating those effects. The ensuing wind turbine interference-mitigation study was carried out for the Advanced Dynamic Aircraft Measurement System (ADAMS) located at the Naval Air Station Patuxent River (Atlantic Test Range) in Maryland.
ADAMS is part of an outdoor dynamic test range that conducts in-flight aircraft RCS signature and measurement testing. The radar system utilizes several different waveforms and operates in frequency bands spanning 150 MHz (very high frequency [VHF]) to 35 GHz (Ka band). Wind turbine backscatter is a particularly stressing source of interference for ADAMS because of the extraordinary precision and sensitivity required for accurate target characterization. Furthermore, interference-mitigation techniques are often frequency specific, so determining solutions that work across many frequencies can be difficult.
The Laboratory's analysis for ADAMS indicated a high risk of wind-turbine interference in the Chesapeake Bay region. This analysis assumed that no degradation of ADAMS' performance or sacrifice to the ADAMS mission was allowed. Thus, a solution to the interference problem had to be effective for all of ADAMS' operating frequencies, waveforms, test geometries, and measurement types (e.g., inverse synthetic aperture imaging, whole-body RCS).
The objective of the study was to provide a mitigation approach for reducing the impact of wind-turbine interference on ADAMS without degrading the measurement uncertainty or sensitivity of the system in order to maintain measurement accuracy. Lincoln Laboratory designed and developed algorithms in simulation and prototyped a hardware implementation to demonstrate the wind turbine interference–mitigation approach; the hardware included an auxiliary parabolic reflector antenna, an arbitrary waveform generator, a digital receiver, and a data recording system, all of which were installed on the ADAMS X-band radar system. The proposed mitigation approach involved
- Improving range resolution. Pulse-compression techniques were used to reduce the size of range bins, which contain radar returns from a given range of distances. This reduction makes it less likely for a target and a turbine that are in close proximity to each other to be placed in the same bin, where their returns would overlap.
- Exploiting spatial and directional differences between the target and interference. When the target and interference exist in different directions, spatial mitigation techniques can be used to "subtract" the signal (primarily the interfering signal) received by an auxiliary antenna from the combined signal (target and interference) received by the main radar antenna. The auxiliary channel also can be used to retain target measurements without significant interference and to discard any measurements that fail to reach objective performance levels.
- Changing the waveforms from pulse to pulse. This alteration of waveforms allows the identification and modification of radar returns in range and Doppler frequency. One method shifts the interference so that it does not overlap the target. Another technique provides a means to spread localized, intense wind-turbine interference across a much larger swath, thereby distributing and reducing the effects of the interference.
The X-Band Prototype
In August 2015, the Laboratory's mitigation approach was demonstrated with the hardware prototype installed on the ADAMS X-band system at the Atlantic Test Range. This demonstration was part of a risk-reduction effort required before the approach could be implemented across all of the ADAMS frequency bands. For the data collection, helicopter and fixed-wing propeller and jet aircraft were used as test targets. A Lincoln Laboratory-developed coherent repeater device aboard a ship in the Chesapeake Bay emulated wind-turbine-like interference.
The X-band prototype demonstrated that the mitigation approach has significant potential to suppress wind-turbine-generated interference; extrapolation of results indicates that the approach could potentially be used on the other ADAMS frequency bands. The Department of Defense continues to investigate the cost, time, and risks associated with a full implementation of the mitigation strategy for ADAMS. Although this strategy was developed with the ADAMS performance requirements and mission in mind, Lincoln Laboratory is investigating how to utilize the mitigation techniques for interference suppression for over-the-horizon and moving-target-indication radars.
1 “Wind Energy Facts at a Glance,” American Wind Energy Association, 26 Jan. 2016, available at http://www.awea.org/Resources/Content.aspx?ItemNumber=5059&navItemNumber=742.
2 “20% Wind Energy by 2030: Increasing Wind Energy’s Contribution to U.S. Electricity Supply,” U.S. Department of Energy, DOE/GO-1020082567, 2008, available at http://energy.gov/eere/wind/20-wind-energy-2030-increasing-wind-energys-contribution-us-electricity-supply.
Posted May 2016top of page