Abstract:
A bistatic radar receiver is located on a wind turbine and surrounded by multiple bistatic transmitters to detect and precisely track the positions of nearby birds. Bird target reflections from multiple transmitters are received by the radar receiver and their position and track determined from the transmitter locations, receiver location, and measured transmitter-to-target-to-receiver ranges. Target position and altitude accuracy is similar to GPS. When birds are detected to be on a collision course with the wind turbine, a deterrent is activated to scare them away. Deterrents can be flashing strobe lights, intense sound, air cannon, or any other effective bird deterrent.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the detection and prevention of imminent bird strikes to wind turbines or other avian hazards such as oil sands tailing ponds and power lines. 
     BACKGROUND OF THE INVENTION 
     According to the American Bird Conservancy, national estimates show between 88,000 and 320,000 birds are killed by wind turbines every year. Among these birds killed are many Federally protected birds such as bald and golden eagles. In an effort to reduce these bird kills, the number, locations, and blade speeds of wind turbines are being restricted which impacts the development of wind energy. 
     Accordingly, there is a need for a low cost method of reducing avian mortality without having to impede the siting and efficiency of wind turbines. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a bird strike prevention system is presented which will greatly reduce the incidence of bird collisions with wind turbines or other hazards. 
     Briefly, to achieve the desired object of the present invention, avian radar is combined with bird deterrent techniques to detect, track, and deter away all birds on a collision coarse with a protected wind turbine. Whenever a high probability of bird collision is detected, a bright flashing strobe light or high intensity sound is directed towards the advancing birds thus deterring them away from the wind turbine. 
     As an alternative to deterring birds, the wind turbine blades can be feathered thus stopping their rotation. For large wind farms, multiple wind turbine avian radars can be networked together to provide an early warning of arriving birds thus providing more time to feather the blades. 
     The avian radar consists of a bistatic radar receiver antenna mounted on the wind turbine and multiple bistatic transmitters sited around the wind turbine. An avian target is illuminated by multiple bistatic transmit signals and the reflected signals are received by the bistatic radar receiver antenna. The location of the avian target is determined from the known locations of the transmitters, receiver antenna, and measured transmitter signal ranges. Target position accuracy is primarily set by radar range resolution and is not reduced by antenna beamwidth or target range. Target position accuracy determination is similar to that obtained with the Global Positioning System (GPS). 
     The bistatic radar receiver antenna is composed of a dipole or other antenna mounted atop the wind turbine. The bistatic radar receiver is mounted at any convenient location such as near the antenna or at the base of the wind turbine tower. Antenna signals are received, digitized, and processed in the receiver to obtain avian target positional information. 
     In the preferred implementation, low power CW Pseudo Random Noise (PN) coded bistatic transmitters are used to provide either non-coherent or coherent target processing. Targets are tracked as they approach the wind turbine. Target identification is determined based on target reflectivity, altitude, velocity, spectral width, and track patterns. 
     Other objects and advantages of the present invention will become obvious as the preferred embodiments are described and discussed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates wind turbine  20  whose top mounted antenna beam is directed towards avian target  405 . 
         FIG. 2  illustrates 4 bistatic radar transmitters  401  deployed around wind turbine  20  and illuminating avian target  405 . 
         FIG. 3  is a block diagram of the bistatic radar receiver. 
         FIG. 4  is an overall block diagram of the subject invention. 
         FIG. 5  is a block diagram of the avian radar signal detection processor. 
         FIG. 6  illustrates the ambiguity diagram of various radar signal waveforms. 
         FIG. 7  illustrates the application of this avian radar system to oil sands tailing ponds. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The goal of the present invention is to provide a low cost avian radar with very advanced features. The radar&#39;s primary feature is precise three-dimensional (3D) target position determination. High resolution avian position information is required to avoid activating the deterrents or feathering the blades for birds that are not actually on a high probability collision course with the wind turbine. 
     Previously, an avian radar was described in patent application Ser. No. 12/661,595 “Three Dimensional Radar Method and Apparatus”, filed 18 Mar. 2010 which is incorporated herein by reference. That invention described an avian radar in the context of an airport environment. A number of concepts from that invention are modified and applied to the present invention&#39;s avian radar in a wind turbine environment. 
       FIG. 1  illustrates wind turbine  20  composed of tower  5 , turbine housing  10 , and propeller blades  15 . Bistatic radar receiver antenna  403  is mounted above turbine housing  10  to obtain an unobstructed view of the radar coverage volume surrounding wind turbine  20 . Bistatic radar receiver  402  is mounted in the base of tower  5  for easy maintenance access. Antenna  403  receive pattern  25  is illustrated as pointing towards avian target  405 . Bird deterrent strobe lights (not shown) can be mounted on tower  5  or turbine housing  10 . Air cannons (not shown) could either be mounted somewhere on wind turbine  20 , or more conveniently, on the ground near wind turbine  20 . 
       FIG. 2  illustrates 4 bistatic radar transmitters  401  deployed around wind turbine  20 . Bistatic radar receiver antenna  403  is mounted atop wind turbine  20 . Avian target  405  is somewhere within the design range of the bistatic transmitter array. Bistatic transmitter signals  410  illuminate target  405  at height  406  and their reflected signals  415  are received by bistatic radar receiver antenna  403 . The location of target  405  is determined from the known locations of bistatic radar transmitters  401 , bistatic radar receiver antenna  403 , and measured bistatic transmitter signal ranges. The measured bistatic transmitter signal range is the sum of bistatic radar transmitter signal  410  path length plus reflected signal  415  path length. 
     The locus of all points, such that the total length from a particular bistatic radar transmitter  401  to target  405  and then to bistatic radar receiver antenna  403  is a constant equal to the signal range for that bistatic radar transmitter  401 , defines a 3D line arc. Likewise additional 3D line arcs are defined for all other bistatic radar transmitters  401  signal ranges. The common intersection point of all these line arcs defines the position of target  405 . 
     This process is one of triangulation somewhat similar to position location using GPS. GPS positions are based on solving a set of simultaneous equations by trilateration. Trilateration is a method for determining the intersections of three sphere surfaces given the centers and radii of the three spheres. It also is basically a method of determining position by triangulation as is well known by those skilled in the art of GPS navigation. Triangulation works good for a single target but becomes more complicated when multiple targets are present. Below, a method is described to determine target locations when multiple targets are present. 
     An alternative solution to triangulation is to construct a lookup table that relates the center of each 3D resolution volume cell  409  with its corresponding measured bistatic transmitter signal ranges. For each 3D resolution volume cell  409  location, a cell list is constructed that contains the signal range value for each bistatic radar transmitter  401 . The signal range value is equal to the total path length from that bistatic radar transmitter  401  to 3D resolution volume cell  409  location to bistatic radar receiver antenna  403 . Each cell list is associated with 3D resolution volume cell  409  location using a lookup table. 
     The minimum size of 3D resolution volume cell  409  is based on the radar&#39;s range resolution which is defined by the radar waveform range ambiguity function. The actual size of the 3D resolution volume cell  409  is selected by the required position accuracy of the avian radar. 
     Each 3D resolution volume cell  409  of interest is checked to see if the reflected signal, from a target at that location, for each bistatic radar transmitter  401  matches the lookup table values for that location. If so, then a target  405  is declared at that location. Target  405  size is inferred from the reflected signal  415  amplitudes and other information. 
     Bistatic radar transmitters  401  transmit signal can all be on the same frequency but modulated with different PN codes similar to GPS signals. For each 3D resolution volume cell  409  examined, a PN code cross correlation with the reflected target signals is performed for each bistatic radar transmitter  401  signal using the lookup table cell list signal range values for those bistatic transmitters. That is, the signal range value defines the cross correlation range delay value to use for each bistatic transmitter. If no cross correlation responses are obtained, or if the cross correlation response falls below a predetermined target detection threshold, that 3D resolution volume cell is declared target free. If a cross correlation response is obtained for all or a sufficient number of bistatic radar transmitters  401 , a target and its amplitude are declared for that 3D resolution volume cell  409 . If a large number of bistatic radar transmitters  401  are implemented, a sufficient number of cross correlation responses may be somewhat less than the maximum possible since some targets may not be in range of all bistatic radar transmitters  401 . 
     The advantage of checking specific 3D resolution volume cells  409  is that only the cells of interest need to be checked. The cells checked may initially be “skin” cells around a specific volume to be monitored since targets must penetrate the skin to enter the monitored volume. Once a target is detected in the skin, additional cells around the target location are monitored to track the target as it moves through the monitored volume. 
     Bistatic radar transmitter  401  transmit frequency, power level, bandwidth, and PN code parameters depend on many design factors as is well known by those skilled in the art. Also, more than 4 bistatic radar transmitters can be used if desired. 
     The timing, frequency, and code sequence zero range time of each bistatic radar transmitter  401  is locked to GPS time as is the corresponding timing and cross correlation codes of bistatic radar receiver  402 . This is required so that bistatic radar receiver  402  knows the PN code zero range time of each bistatic radar transmitter  401 . 
     Locking each bistatic radar transmitter  401  to GPS time is a major innovation and advantage of this invention because it allows bistatic radar transmitters  401  to operate autonomously. That is, no communication is required between bistatic radar receiver  402  and bistatic radar transmitters  401 . Bistatic radar transmitters  401  can be sited remotely and powered from solar cell charged batteries if desired. This allows bistatic radar transmitters  401  to be sited accordingly to the best bistatic radar transmitter array geometry which eliminates infrastructure power wiring to hard to access locations around wind turbine  20  or within the overall wind farm. 
     The antenna pattern of each bistatic radar transmitter within the wind farm can be hemispherical to provide omnidirectional coverage. Long range bistatic radar transmitters placed outside the wind farm can have an antenna pattern that directs its signal towards the wind farm. The purpose of these transmitters is to obtain early warning of bird flocks approaching the wind farm. 
     To reduce ground clutter, bistatic radar transmitters can be designed for low elevation pattern attenuation using techniques developed for GPS receiving antennas. GPS receiving antennas reduce multipath reflections by designing an antenna pattern that has a sharp increase in pattern attenuation at very low elevation angles. One technique used is to place the GPS antenna in the center of a circular ground plane mount containing a series of concentric metal rings around the GPS antenna. The location and vertical height of the metal rings are designed to attenuate the low elevation signals that cause multipath. Attenuating the bistatic radar transmitter low elevation transmit energy reduces ground clutter illumination which in turn reduces bistatic radar receiver  402  low elevation clutter reception. 
     Bistatic radar receiver antenna  403  signals are received using receiver  130  illustrated in  FIG. 3 . Preferred receiver antenna  403  is a simple hemispherical coverage vertical dipole. However, multiple or sectored receiver antennas can also be used. Antenna  403  signals are amplified by low noise amplifier (LNA)  131 , filtered using band pass filter (BPF)  132 , downconverted to intermediate frequency (IF) using mixer  133  and local oscillator (LO)  134 , image and anti-alias filtered using IF filter  135 , amplified and buffered using amplifier  136 , and digitized using analog to digital converter (ADC)  137 . 
     A block diagram of the avian radar system of the subject invention is illustrated in  FIG. 4 . The output from receiver  130  is applied to signal detection processor  160  illustrated in  FIG. 5  which consist of a parallel bank of Doppler matched filters  161  and signal detector  162 . GPS receiver  165  provides precise bistatic timing required for signal detector  162  to determine zero range of each bistatic radar transmitter  401 . 
     Each bistatic radar transmitter  401  consists of GPS receiver  165 , transmitter  192 , and antenna  193 . 
     Received signals from receiver  130  are matched filtered and coherently detected. Detected targets are tagged in terms of their 3D resolution volume cell  409 , amplitude, Doppler frequency, and spectral width. Target processor  170  applies tracking and spectral width algorithms to eliminate non-avian targets, identify probable bird type, and form target tracks for deterrent processor  180 . Deterrent processor  180  determines if an avian target is on a high probability collision course with wind turbine  20 . If so, the correct deterrent device (air cannon  187  or strobe light  188 ) is activated. 
     Deterrent processor  180  also formats target data for network interface card (NIC)  182  for transfer over network  183  (along with data  189  from other avian radars) to central processor  184 . Central processor  184  implements a wide area avian radar to track and monitor birds throughout the entire wind farm on display  185 . 
     By implementing a wide area avian radar, individual avian radars throughout the wind farm can receive handoff tracking information from central processor  184  to indicate which resolution volume cells  409  that should be monitored by local avian radars along a bird&#39;s path as it moves through the wind farm. Central processor  184  can also command wind turbines  20  along the predicted path of bird flocks to feather their blades in advance of the flock&#39;s arrival at each wind turbine. 
     Radar is a very mature field with many choices available for frequency, transmit power, and waveform design. Both pulsed and CW waveforms can be used as is well known by anyone skilled in the art. Radar detection is a function of transmit energy, irrespective of the transmit signal waveform design. However, range resolution and Doppler resolution are determined by the actual waveform design as illustrated in  FIG. 6 . 
       FIG. 6  illustrates the ambiguity function of common radar waveforms. Range and Doppler resolution are defined by the radar signal ambiguity function which is simply the cross correlation between the transmit pulse and its range and Doppler shifted versions as is well known by those skilled in the art. 
     Long pulse  200  is characterized by low range resolution  205  and high Doppler resolution  206 . Short pulse  210  is characterized by high range resolution  207  and low Doppler resolution  208 . PN coded pulse compression waveform  225  has both high range and Doppler resolution as illustrated by thumbtack response  209 . 
     Although pulse type radar signals can be used, the most appropriate radar signal for the bistatic radar described in this invention is a CW radar signal using PN coded pulse compression waveform  225 . A CW radar does not have the close range reception dead zone caused by transmit pulse blanking required in pulse type radars. 
     The selection of transmit power, waveform design, and all radar hardware implementation tradeoffs are well known by practicing radar engineers for bistatic radars. The radar cross section for a large variety of birds at different frequencies and aspect angles are available in the literature. The nominal radar cross section of a pigeon is 0.01 square meters. 
     Although many different transmit frequencies could be chosen for an avian radar, S-band is more appropriate than X-band because the radar cross section of birds is larger and rain penetration is better at S-band frequencies than at X-band frequencies. 
     Detecting pulse compression waveforms is very computationally intensive and processing every coverage volume 3D resolution volume cell  409  is difficult. To vastly reduce the processing requirements, only a thin outer surface of the coverage volume needs to be processed continually. A target can only enter the interior coverage volume by first passing through its outer surface. By continually processing a thin surface skin, all targets entering the coverage volume are detected. Once detected, they can be tracked as they move within the interior coverage volume. Targets in critical areas, for example close to wind turbines  20 , can be tracked to within their 3D resolution volume  409 . 
     This avian radar is very well suited for the use of parallel processing. Through the use of lookup tables that relate 3D resolution volume cells  409  with their corresponding bistatic transmitter signal ranges, multiple processors can be programmed to simultaneously examine different collections of radar bins or 3D resolution volume cells  409 . Radar bins containing targets can be passed to other processors that locate all targets in that radar bin to their 3D resolution volume cell  409 . Using multiple parallel processors, almost any desired radar resolution and update performance can be obtained simply by adding processing resources. 
     Although this radar has been described to protect wind turbines and wind farms, it can also be used to protect birds from other hazards such as oil sands tailing ponds, electrical substations, and power lines. 
       FIG. 7  illustrates the application of this wide area avian radar to oil sands tailing ponds. These ponds are filled with residual oil and toxic waste from the oil sands extraction process. If ducks land on these ponds, they become covered with oil and die. 
       FIG. 7  illustrates large pond  700  around which a number of bistatic radar transmitters  401  are deployed. Bistatic radar receiver antennas  403  are deployed in and around pond  700 . Air cannons (not shown) are also deployed near or at each receiver antenna  403  site. When ducks are detected, the air cannons fire as the ducks approach the water, thus deterring them. 
     The advantage of this type of installation is that the deterrents and radar detection can be spread out across a large pond. The high bird positional accuracy of this avian radar allows the air cannons to be mounted on rotating mounts and pointed towards the landing birds. Air cannons are most effective when fired only when birds are about to land. If air cannon blasts are frequent or continuous, the birds get used to the noise and simply ignore the blasts. 
     Other structures, such as electrical substations and power lines can easily be protected to prevent bird kills by distributing these avian radar units and deterrents around the substation or along power line paths. Airports could also be protected using a networked array of avian radars, as described in this invention, instead of using the single centralized receiver described in patent application Ser. No. 12/661,595. 
     Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention. This invention can also be used for many applications other than avian radar. Potential applications include intrusion detection, border security, or various military uses.