Abstract:
A ground based avian radar receive antenna is implemented using a vertically oriented offset parabolic cylindrical antenna. The desired azimuth beamwidth is determined by the width of the parabolic cylinder reflector surface and the desired elevation beamwidth by the height of the parabolic cylinder reflector surface. A vertical array of antenna elements is mounted along the vertical focal line to provide electronic scanning in elevation. Low sidelobe levels are obtained using tapered antenna element illumination. Low cost modular construction with high reflector accuracy is obtained by attaching a thin metal reflector to thin ribs machined or stamped in the shape of the parabolic cylinder reflector surface. The antenna is enclosed in a radome and mechanically rotated 360 degrees in azimuth.

Description:
RELATED INVENTION 
     The present invention claims priority under 35 U.S.C. §119(e) to: “3D Radar Antenna Method and Apparatus” Provisional U.S. Patent Application Ser. No. 61/271,546, filed 22 Jul. 2009 which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the field of three-dimensional (3D) radar. More specifically, the present invention relates to the field of 3D avian radar for detecting, determining height, and tracking birds. 
     BACKGROUND OF THE INVENTION 
     Aviation experts say bird-plane collisions happen daily. Nearly 200 people have been killed or injured since 1990 in accidents involving aircraft and wildlife. It is estimated that bird strikes cost military and commercial aviation over $2 billion each year due to damage and other costs. 
     Airports take a variety of measures to reduce bird populations near major airports. In a few cases, ground based avian radars are used to detect flying birds near aircraft approach and departure paths. Currently, most ground based avian radars consist of modified marine radars using a long narrow horizontal antenna that is rotated in the horizontal plane to provide 360 degree azimuth coverage. These antennas provide narrow azimuth beamwidths and wide elevation beamwidths. Almost no target height information is provided due to the wide elevation beamwidths. 
     Accordingly, it is the object of the present invention to disclose methods and apparatus which provide a new and improved low cost 3D avian radar with target altitude determination capability. 
     SUMMARY OF THE INVENTION 
     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. application Ser. No. 12/661/595 describes a 3D ground based radar whose receiving antenna consists of a vertical array of horizontal fixed scan narrow azimuth beamwidth slotted waveguide antenna elements. These horizontal elements are electronically combined to provide a narrow azimuth and elevation beamwidth antenna that can be mechanically scanned in azimuth and electronically scanned in elevation. The present invention describes an improved receiving antenna method and apparatus. 
     Briefly, to achieve the desired object of the present invention, a vertically oriented offset parabolic cylindrical receiving antenna is implemented. Using an array of antenna elements along its vertical offset continuous focal line allows a modular high performance electronically elevation scanned antenna to be built at low cost. 
     The desired azimuth beamwidth is controlled by the width of the parabolic cylinder and the desired elevation beamwidth by the height of the parabolic cylinder. A ribbed structure covered with a thin metal reflector is used to achieve the reflector surface accuracy required to obtain very low sidelobe performance. The antenna can be built in short modular vertical segments and stacked vertically to provide different elevation beamwidths. 
     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 the horizontal cross section of a vertically oriented offset parabolic cylinder antenna with vertical focal line  15 . 
         FIG. 2  illustrates an azimuth tapered antenna illumination pattern  25  illuminating parabolic cylinder reflector surface  5  of the vertically oriented offset parabolic cylinder antenna. 
         FIG. 3  illustrates a parabolic cylinder antenna pattern with sidelobe levels below −50 dB. 
         FIG. 4  is an overhead view of the offset parabolic cylinder antenna construction illustrating its horizontal cross section. 
         FIG. 5  is a frontal view of the offset parabolic cylinder antenna construction. 
         FIG. 6  illustrates a block diagram of the receivers attacked to each antenna element. 
         FIG. 7  illustrates the offset parabolic cylinder antenna tilted up 15 degrees, mounted on a rotating pedestal inside a radome, and mechanically rotated 360 degrees in azimuth. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The goal of the present invention is to provide a low cost high gain receiving antenna with very low sidelobes that can be scanned mechanically in azimuth and scanned electronically in elevation. 
     A horizontal cross section of a vertically oriented offset parabolic cylinder antenna with vertical focal line  15  is illustrated in  FIG. 1 . A parabolic curve is formed in the x, y plane and extended in the z plane normal to the drawing to create a parabolic cylinder. Parabolic cylinder reflector surface  10  is eliminated leaving parabolic cylinder reflector surface  5 . Incoming waves  20  are focused onto focal line  15  and collected by an array of antenna elements along focal line  15  using azimuth illumination pattern  25 . Removing parabolic cylinder reflector surface  10  forms an offset parabolic cylinder antenna in which antenna elements do not obstruct incoming waves  20 . 
     As is well known by those skilled in the art, the gain of an offset parabolic cylinder antenna is governed by its aperture area. Its efficiency and sidelobe performance are governed by antenna element illumination pattern  25  and by the accuracy of parabolic cylinder reflector surface  5 . To obtain maximum efficiency, parabolic cylinder reflector surface  5  must be uniformly illuminated over its surface with no spillover energy beyond its edge. However, such uniform illumination produces only −13 dB sidelobes. To produce low antenna azimuth sidelobe levels, antenna element illumination pattern  25  must be tapered as illustrated in  FIG. 2 . Tapers in which edge illumination is on the order of −10 dB below center illumination are typically implemented as a compromise between efficiency and sidelobe level. 
     Parabolic cylinder antenna sidelobe levels below −50 dB, as illustrated in  FIG. 3 , are possible using the correct illumination pattern, parabolic surface accuracy, and antenna element position. There is always a tradeoff between antenna efficiency and sidelobe level. Since the antenna in the present invention is receive only, low sidelobes are more important than antenna efficiency. Therefore, optimizing antenna element illumination pattern  25  for low sidelobes is most important. Low sidelobes minimize the radar signal levels reflected from targets and clutter at azimuth angles outside the main azimuth beamwidth  27 . 
     Very high parabolic cylinder reflector surface  5  and focal line  15  accuracy of offset parabolic cylinder antenna  60  can be provided at low cost using the structure illustrated in  FIG. 4 .  FIG. 4  is an overhead view of the offset parabolic cylinder antenna  60  construction illustrating its horizontal cross section. A series of thin ribs  30 , in which the parabolic curve can be machined or stamped to great accuracy, are stacked vertically to form a support for parabolic cylinder reflector surface  5  which can be tack welded (or fastened in any other way) to ribs  30 . Thin rib supports  35  hold ribs  30  in precise position with respect to each other. For narrow offset parabolic cylinder antenna  60  widths, only a single thin rib support  35  is required on each end. For wide widths, additional thin rib supports  35  can be placed along the straight rear side of thin ribs  30 . Focal line supports  40  hold thin ribs  30 , parabolic cylinder reflector surface  5 , and thin rib supports  35  in precise position along focal line  15 . Printed circuit board (PCB)  50 , containing antenna elements  45  positioned along focal line  15 , are attached to focal line supports  40 . 
     For the relatively narrowband radar signals contemplated for this avian radar, antenna elements  45  can be implemented at low cost using narrowband microwave patch antennas printed directly on PCB  50 . Antennas with horizontal, vertical, or circular polarization can be implemented. However, horizontal polarization is most appropriate for avian targets because birds are wider in their horizontal dimension than in their vertical dimension thus maximizing their radar reflectivity using horizontal polarization. However, any appropriate antenna elements  45  can be implemented that meets the required sidelobe level and cost goals. 
     A frontal view of offset parabolic cylinder antenna  60  is illustrated in  FIG. 5 . Multiple antenna elements  45  are positioned along focal line  15  and separated from each other on the order of a half wavelength. Antenna elements  45  are grouped into sets of 8, 16, or any other convenient number and mounted on PCBs  50 . Doing so provides a modular structure in which varying numbers of antenna elements  45  can be implemented. Conversely, the entire offset parabolic cylinder antenna  60  can be constructed as multiple short modular vertical sections which are stacked vertically to form the completed antenna assembly. Doing so allows narrower elevation beamwidths to be conveniently implemented simply by stacking more vertical sections. As is well known by those skilled in the art, narrower elevation beamwidths require larger elevation apertures. 
     The signal produced by each antenna element  45  will be received using an identical antenna element  45  receiver  130  illustrated in  FIG. 6  (same receiver as illustrated in  FIG. 3  of patent application Ser. No. 12/661,595). That is, the signal from each antenna element  45  is amplified by low noise amplifier (LNA)  131 , filtered using bandpass 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 . The digitized signals of all receivers  130  on PCB  50  will be multiplex together using data multiplexer  138 . 
     Antenna element  45  and LNA  131  can be designed as an active antenna to eliminate antenna to LNA connection loss. Doing so optimizes receiver noise figure and doesn&#39;t require a 50 ohm impedance match between the antenna and LNA. Data Multiplexer  138  places the data from all PCB  50  receivers  130  onto a single digital output  115 . Coax or fiberoptic cable will be used to route digital output  115  to further circuitry in the 3D radar where the data from each receiver  130  will be demultiplexed, digitally beam formed, and processed as explained in patent application Ser. No. 12/661,595. Multiplexing the data from multiple antenna elements  45  eliminates the need for multiple cables and connectors, thus lowering implementation cost. 
     Offset parabolic cylinder antenna  60  will be tilted up 15 degrees, mounted on a rotating pedestal, mechanically rotated 360 degrees in azimuth about rotation axis  315 , and enclosed in radome enclosure  305  as illustrated in  FIG. 7 . Alternatively, radome enclosure  305  could be replaced by a non-radome enclosure in which only the area in front of offset parabolic cylinder antenna  60  is constructed of radome material. Instead of rotating the antenna inside the enclosure, the antenna is attached to the enclosure and the entire antenna and enclosure rotated about rotation axis  315 . 
     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. 
     For example, each PCB  50  could include all receiver, scaler, quadrature downconverter, and beamformers described in patent application Ser. No. 12/661,595. The beamformer outputs from each PCB  50  could then be combined in the signal detection processor, also described in patent application Ser. No. 12/661,595. Doing so would greatly reduce the data transfer requirements between the circuitry on PCBs  50  and the signal processor circuitry on another PCB. 
     As another example, modular vertical sections could be made short enough to contain a single antenna element and receiver. The number of modules stacked would determine the elevation beamwidth. Any convenient means such as coax, fiber optic, or wireless communications could be used to collect the data from all modules and transfer the digital data to the 3D radar for beamforming and further processing. 
     Also, this antenna could be used for many applications other than avian radar simply by changing the illumination pattern  25  of antenna element  45  and the aperture size of offset parabolic cylinder antenna  60 . 
     Many other simple modifications are also possible without departing from the spirit of the invention.