Patent Publication Number: US-9897695-B2

Title: Digital active array radar

Description:
This application claims the benefit of U.S. Provisional Patent Application No. 61/886,449, filed Oct. 3, 2013, entitled, “DIGITAL ACTIVE ARRAY RADAR,” the entire content of which is incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to radar systems. 
     BACKGROUND 
     Radar systems may be used by aircraft to detect weather, other aircraft in the surrounding airspace, and other objects in the surrounding airspace. In smaller aircraft, such as some unmanned aerial vehicles (UAVs), weight and power requirements may constrain the design of the radar system. 
     SUMMARY 
     The disclosure describes radar systems and techniques for operating radar systems. The radar system described in this disclosure is a frequency modulated continuous wave (FMCW; transmits continuous waveforms rather than pulsed waveforms) radar system that includes a transmit array including a plurality of transmit antenna elements and a separate receive array that includes a plurality of receive antenna elements. In some examples, the transmit array may include a plurality of transmit antenna elements disposed such that the transmit antenna is wide in the horizontal dimension and short in the vertical dimension, or, alternatively, is tall in the vertical dimension and narrow in the horizontal dimension. This may produce a transmit beam that is elongated in a first illumination dimension compared to a second, substantially perpendicular illumination dimension. The radar system may electronically scan the transmit beam in the second illumination dimension to cover a large volume or surface in a reduced amount of time. 
     In some examples, the receive array may include a square or rectangular array of receive antenna elements, such as a 24 receive antenna element by 24 receive antenna element grid or a 20 receive antenna element by 24 receive antenna element grid. In some examples, the receive array may be functionally divided into quadrants to facilitate formation of monopulse tracking beams. Signals from rows of the quadrants of the receive array may be summed individually and digitally manipulated (e.g., using a complex beam weight or another technique) to produce a plurality of receive beams oriented substantially in the first illumination dimension. The radar system may electronically scan the plurality of receive beams in at least the second illumination dimension along with the transmit beam such that the plurality of receive beams scan in parallel with and overlap the transmit beam. 
     In some examples, the disclosure describes a system that includes an FMCW radar system including a transmit array comprising a plurality of transmit antenna elements arranged such that a number of transmit antenna elements in a first transmit array dimension is greater than a number of transmit antenna elements in a second transmit array dimension. In accordance with some of these examples, the transmit array may be configured to output an FMCW transmit beam that illuminates an area with a greater extent in a first illumination dimension than in a second illumination dimension substantially perpendicular to the first illumination dimension. The FMCW radar system also may include transmit electronics module operable to electronically scan the FMCW transmit beam in the second illumination dimension, and a receive array comprising a plurality of receive antenna elements. In some examples, the FMCW radar system further may include a receive electronics module operable to generate, using a plurality of receive signals received from the receive array, a plurality of receive beams within the area illuminated by the FMCW transmit beam and electronically scan each receive beam of the plurality of receive beams in the second illumination dimension such that the scanning of each receive beam is coordinated with the scanning of the FMCW transmit beam in the second illumination dimension. 
     In some examples, the disclosure describes a method including outputting, by a transmit array comprising a plurality of transmit antenna elements, an FMCW transmit beam. In accordance with some of these examples, the plurality of transmit antenna elements may be arranged such that a number of transmit antenna elements in a first transmit array dimension is greater than a number of transmit antenna elements in a second transmit array dimension, and the FMCW transmit beam may illuminate an area with a greater extent in a first illumination dimension than in a second illumination dimension substantially perpendicular to the first illumination dimension. The method also may include electronically scanning, by a transmit electronics module, the FMCW transmit beam in the second illumination dimension, and receiving, by a receive electronics module, a plurality of receive signals from a receive array comprising a plurality of receive antenna elements. In some examples, the method further includes electronically generating and scanning in the second illumination dimension, by the receive electronics module, a plurality of receive beams such that the scanning of each receive beam is coordinated with the scanning of the FMCW transmit beam so that the plurality of receive beams are within the area illuminated by the FMCW transmit beam throughout the scanning of the FMCW transmit beam and the plurality of receive beams in the second illumination dimension. 
     In some examples, the disclosure describes a computer-readable storage medium comprising instructions that, when executed, configure one or more processors to control a transmit array comprising a plurality of transmit antenna elements to output an FMCW transmit beam. In some examples, the plurality of transmit antenna elements may be arranged such that a number of transmit antenna elements in a first transmit array dimension is greater than a number of transmit antenna elements in a second transmit array dimension substantially perpendicular to the first transmit array dimension, and the FMCW transmit beam may illuminate an area with a greater extent in a first illumination dimension than in a second illumination dimension substantially perpendicular to the first illumination dimension. The computer-readable storage medium also may include instructions that, when executed, configure one or more processors to control a transmit electronics module to electronically scan the FMCW transmit beam in the second illumination dimension, and control a receive electronics module to receive a plurality of receive signals from a receive array comprising a plurality of receive antenna elements. Further, the computer-readable storage medium may include instructions that, when executed, configure one or more processors to control the receive electronics module to electronically generate and scan in the second illumination dimension a plurality of receive beams such that the scanning of each receive beam is coordinated with the scanning of the FMCW transmit beam so that the plurality of receive beams are within the area illuminated by the FMCW transmit beam throughout the scanning of the FMCW transmit beam and the plurality of receive beams in the second illumination dimension. 
     In some examples, the disclosure describes an FMCW radar array including a housing, a transmit array comprising a plurality of transmit antenna elements configured to output an FMCW transmit beam, a receive array comprising a plurality of receive antenna elements, and a slotted choke disposed between the transmit array and the receive array. The transmit array and the receive array may be mechanically coupled to the housing. In some examples, the slotted choke comprises a plurality of slots having dimensions selected to provide cancellation of electromagnetic radiation from the frequency modulated continuous wave transmit beam to reduce a magnitude of radiation from the transmit array to which the receive array is indirectly exposed. 
     In some examples, the disclosure describes a system including a plurality of frequency modulated continuous wave transmit beam (FMCW) radar arrays and a radome. Each of the FMCW radar arrays may include a housing, a transmit array comprising a plurality of transmit antenna elements configured to output an FMCW transmit beam, a receive array comprising a plurality of receive antenna elements, and a slotted choke disposed between the transmit array and the receive array. The transmit array and the receive array may be mechanically coupled to the housing. In some examples, the slotted choke comprises a plurality of slots having dimensions selected to provide cancellation of electromagnetic radiation from the frequency modulated continuous wave transmit beam to reduce a magnitude of radiation from the transmit array to which the receive array is indirectly exposed. Additionally, the slotted choke of each of the plurality of FMCW radar arrays may extend from between the transmit array and the receive array to an inner surface of the radome. 
     In some examples, the disclosure describes a method including mechanically coupling a transmit array comprising a plurality of transmit antenna elements configured to output an FMCW transmit beam to a housing of an FMCW radar array. The method also may include mechanically coupling a receive array comprising a plurality of receive antenna elements to the housing. In some examples, a slotted choke may be disposed between the transmit array and the receive array. The slotted choke may include a plurality of slots having dimensions selected to provide cancellation of electromagnetic radiation from the frequency modulated continuous wave transmit beam to reduce a magnitude of electromagnetic radiation from the transmit array which the receive array is indirectly exposed. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are conceptual and schematic diagrams illustrating an example FMCW radar system including a plurality of FMCW radar arrays. 
         FIG. 1C  is a simplified conceptual diagram illustrating back surfaces of housings of FMCW radar arrays. 
         FIG. 2  is a conceptual and schematic diagram illustrating an example FMCW radar array. 
         FIG. 3  is a conceptual diagram illustrating an example FMCW radar array. 
         FIG. 4  is a conceptual diagram illustrating an example transmit beam and a plurality of example receive beams. 
         FIG. 5  is a conceptual block diagram illustrating an example FMCW radar array, including an associated transmit electronics module and an associated receive electronics module. 
         FIG. 6  is a conceptual block diagram illustrating an example receive antenna and associated receive electronics module. 
         FIG. 7  is a conceptual block diagram illustrating an example quadrant of a receive array. 
         FIG. 8  is a functional block diagram illustrating example functions of analog to digital converters and portions of a digital receive electronics module for a quadrant of a receive array. 
         FIG. 9  is a functional block diagram illustrating example functions for producing a plurality of receive beams. 
         FIG. 10  is a conceptual and schematic diagram illustrating an example slotted choke, disposed between a transmit array and a receive array of an FMCW radar array. 
         FIG. 11  is a cross-sectional conceptual diagram illustrating an example slotted choke disposed between a transmit array and a receive array of an FMCW radar array. 
         FIG. 12  is a diagram illustrating example attenuation of a transmit beam adjacent to a receive array due to the presence of a slotted choke between a transmit array and the receive array of an FMCW radar array. 
         FIG. 13  is a conceptual and schematic diagram illustrating an example FMCW radar system including a plurality of FMCW radar arrays. 
         FIG. 14  is a conceptual and schematic diagram illustrating an example FMCW radar array. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure describes radar systems and techniques for operating radar systems. Phased Array Surveillance Systems, particularly for small manned aircraft or UAVs, may advantageously use an efficient and low weight radar system for object sensing and avoidance or weather radar applications. The radar system described in this disclosure is a frequency modulated continuous wave (FMCW; transmits 100% of the time) radar system that includes a transmit array that includes a plurality of transmit elements and a separate receive array that includes a plurality of receive elements. In some examples, the radar system may include a plurality of transmit arrays and a plurality of receive arrays. For example, the radar system may include three transmit arrays and three receive arrays. By orienting the respective arrays at angles relative to each other, a greater range in azimuth, elevation, or both may be covered by the radar system. Additionally or alternatively, a plurality of transmit beams (e.g., one from each transmit array) may be output by the radar system substantially simultaneously or sequentially, which may allow scanning of three areas in parallel or sequentially using the three transmit and receive radar arrays. 
     In some examples, by utilizing an FMCW radar and separating the transmit and receive antennas of the radar system, radar systems in accordance with this disclosure may operate within a relatively low power envelope, such as less than about 550 Watts (W) total power for the entire radar system including three transmit arrays and three receive arrays. 
     In some examples, the transmit array may include a plurality of transmit elements disposed such that the transmit antenna includes a greater number of transmit antenna elements in a first transmit array dimension and a smaller number of transmit antenna elements in a second transmit array dimension that is substantially perpendicular to the first transmit array dimension. For example, the transmit array may include a plurality of transmit elements disposed such that the transmit antenna is wide in the horizontal direction and short in the vertical direction, or is narrow in the horizontal direction and tall in the vertical direction. This may produce a transmit beam that has a greater extent in a first illumination dimension and a lesser extent in a second illumination dimension that is substantially perpendicular to the first illumination dimension. For example, when the transmit array is wide in the horizontal direction and short in the vertical direction, the transmit beam may be tall in elevation and narrow in azimuth. The transmit beam may be electronically scanned in the second illumination dimension (e.g., azimuth) to illuminate a predetermined window. 
     In some examples, the extent of the transmit beam in the first illumination dimension may cover substantially the entire predetermined window in the direction of the first illumination dimension. This may facilitate more time-efficient scanning, as the transmit beam may not need to be scanned in the first illumination dimension to cover the entire predetermined window. In contrast, weather radars that utilize a focused transmit beam that is narrow in both elevation and azimuth scan the transmit beam in a raster pattern to cover the entire azimuth and elevation of the predetermined window, which may require more time than when using the transmit beam described herein. Similarly, an airborne unmanned aerial vehicle (UAV) may search a large range of elevation and azimuth angle extent with a single focused beam looking for other aircraft to avoid collision. If there are numerous targets at various elevations and azimuth locations near the UAV, attempting to continuously detect and track all possible potential collisions with a single focused beam may become difficult. 
     In some examples, the transmit beam may be scanned in the second illumination dimension by applying a phase shift to the signal output to each transmit antenna element, where the applied phase shift varies as a function of time. In some examples, the transmit array and associated electronics may be configured such that the phase shift is applied at intermediate frequency (e.g., tens of megahertz (MHz)), rather than at the radar output frequency (which may be in the gigahertz (GHz) range). This may reduce power losses and simplify design and construction of the transmit array electronic components compared to radar systems in which the phase shift is applied to the signal at the radar output frequency. 
     In some examples, the receive array may include a square or rectangular array of receive elements. For example, the receive array may include an array of receive antenna elements arranged in a 24 receive element by 24 receive element grid or an array of receive antenna elements arranged in a 20 receive element by 24 receive element grid. Signals from full or partial rows of the receive array may be phase-shifted to steer the receive beams in azimuth, summed, and then these summed values may be manipulated (e.g., by applying a complex beam weight or another technique) to produce a plurality of receive beams oriented substantially in the first illumination dimension. The receive beams may be steered (e.g., electronically scanned) in the second illumination dimension along with the transmit beam to cover the predetermined window. Additionally or alternatively, the receive beams may be steered (e.g., electronically scanned) in the first illumination dimension to predetermined locations within the predetermined window. 
     In some examples, separating the transmit array and receive array may provide transmit-to-receive isolation that facilitates use of FMCW radar, FMCW radar technology may permit use of low data rate receive digital beam forming with phase and amplitude adjustments performed at low IF frequencies between about 0 MHz (DC) and about 32 MHz, rather than using microwave or millimeterwave phase shifters and attenuators. FMCW radar technology may enable use of relatively low cost, relatively low power, and relatively low physical volume components for forming multiple digital receive beams, which facilitates formation of multiple simultaneous focused receive beams, which each may conduct separate functions simultaneously. 
     In some examples, by using a plurality of receive beams and/or three transmit arrays/receive arrays in parallel, a dwell time (at a given location) of the transmit beams and/or receive beams may be increased relative to a radar system which uses a single transmit beam and/or a single receive beam. The increased dwell time may result in better sensitivity compared to a system which uses a single transmit beam and/or a single receive beam. Additionally or alternatively, by utilizing an FMCW radar, input power may be lowered compared to a pulsed radar. 
       FIGS. 1A and 1B  are conceptual and schematic diagrams illustrating an example radar system  10  including a plurality of FMCW radar arrays  12   a - 12   c  (collectively, “FMCW radar arrays  12 ”). In the illustrated example, radar system  10  includes a first FMCW radar array  12   a , a second FMCW radar array  12   b , and a third FMCW radar array  12   c . In other examples, radar system  10  may include any other number of FMCW radar arrays  12 , such as at least one FMCW radar array  12  or a plurality of FMCW radar arrays  12 . Each FMCW radar array includes a respective one of housings  13   a - 13   c  (collectively, “housings  13 ”) a respective one of transmit arrays  18   a - 18   c  (collectively, “transmit arrays  18 ”), and a respective one of receive arrays  20   a - 20   c  (collectively, “receive arrays  20 ”). Each one of transmit arrays  18  includes a respective transmit antenna including a plurality of transmit antenna elements. Similarly, each one of receive arrays  20  includes a respective receive antenna including a plurality of receive antenna elements. 
     For example, first FMCW radar array  12   a  includes a first transmit array  18   a  and a first receive array  20   a , second. FMCW radar array  12   b  includes a second transmit array  18   b  and a second receive array  20   b , and third FMCW radar array  12   c  includes a third transmit array (not shown in  FIGS. 1A and 1B ) and a third receive array (not shown in  FIGS. 1A and 1B ). In this way, as radar system  10  includes three FMCW radar arrays  12 , radar system  10  includes three transmit arrays  18  and three receive arrays  20 . 
     For each of FMCW radar arrays  12 , a respective one of transmit arrays  18  and a respective one of receive arrays  20  are mechanically attached or coupled to a respective one of housings  13 . For example, first transmit array  18   a  and first receive array  20   a  are mechanically attached to first housing  13   a . Similarly, second transmit array  18   b  and second receive array  20   b  are mechanically attached to second housing  13   b , and third transmit array  18   c  and third receive array  20   c  are mechanically attached to third housing  13   c . In some examples, as shown in  FIG. 1A , each of housings  13  may include two sidewalls, two end walls, and a back wall. Each of housings  13  generally defines a rectangular box, with one side (the front side) being substantially open. The front side of each of housings  13  may be substantially open, exposing the transmit antenna elements and receive antenna elements (see  FIG. 2 ). 
     In the example illustrated in  FIGS. 1A and 1B , each of the FMCW radar arrays  12  are mechanically coupled to a radar system frame  14 , which is mechanically coupled to a frame  16  of the aircraft on which the radar system  10  is used. In other examples, at least one of the FMCW radar arrays  12  may be mechanically coupled directly to frame  16  of the aircraft on which the radar system  10  is used. 
     As shown in  FIGS. 1A and 1B , back surfaces  15   a - 15   c  (collectively, “back surfaces  15 ”) of housings  13   a - 13   c  may be mechanically attached or coupled to supports of frame  14 . Frame  14  may be shaped to position housings  13   a - 13   c  relative to each other. For example,  FIG. 1C  is a simplified conceptual diagram illustrating back surfaces  15  of housings  13 . As shown in  FIG. 1C , the three back surfaces  15  of housings  13  are disposed at angles with respect to each other. Interior angles  17   a  and  17   b  may be defined between the first back surface  15   a  and second back surface  15   b , and between the second back surface  15   b  and third back surface  15   c . In some examples, interior angles  17   a  and  17   b  may be the same. In other examples, interior angles  17   a  and  17   b  may be the same. Interior angles  17   a  and  17   b  may be between about 90° and about 180°. In some examples, one or both of interior angles  17   a  and  17   b  may be about 120°. 
     By arranging housings  13  at angles with respect to each other in, the transmit array/receive array pairs (e.g., first transmit array  18   a  and first receive array  20   a , second transmit array  18   b  and second receive array  20   b , and third transmit array  18   c  and third receive array  20   c ) are disposed at angles with respect to each other. This may allow radar system  10  to monitor a greater range in azimuth more efficiently than using only a single transmit array/receive array pair. For example, each transmit array/receive array pair may be configured to scan a predetermined window with a predetermined extent in azimuth and elevation. In some examples, the predetermined extent in azimuth may be about ±40° from the plane orthogonal to the face of the transmit array/receive array pair or about ±38° in azimuth. As the three transmit array/receive array pairs are disposed at angles with respect to each other and the predetermined window for each transmit array/receive array pair may overlap with the predetermined window for the adjacent transmit array/receive array pair(s), radar system  10  may allow a total azimuth scan area of between about 220° and about 228° in some examples. The total azimuth scan area may depend at least in part on an overlap in azimuth between scan areas of the three FMCW radar arrays  12 . 
     Each of FMCW radar arrays  12  also includes a slotted choke  22   a ,  22   b ,  22   c  (collectively, “slotted chokes  22 ”). The respective slotted chokes  22  are disposed between a respective one of the transmit antennas  18  and a respective one of the receive antennas  20  in a transmit array/receive array pair. Slotted chokes  22  may be formed of an electrically conductive material or formed of an electrically insulating material coated with an electrically conductive material. Each of slotted chokes  22  may define a plurality of slots, which are sized, shaped, and/or placed to attenuate strength of electromagnetic radiation output by a respective one of the transmit antennas at the adjacent one of the receive antennas. Further details of slotted chokes  22  will be described below with respect to  FIGS. 10-12 . 
     Each of transmit arrays  18  includes a transmit antenna including a plurality of transmit antenna elements.  FIG. 2  is a conceptual and schematic diagram illustrating an example FMCW radar array, e.g., one of FMCW radar arrays  12 . The conceptual and schematic diagram of  FIG. 2  illustrates additional example details of FMCW radar arrays  12  shown in  FIGS. 1A-1C . FMCW radar array  12  includes a transmit array  18  and a receive array  20 . Transmit array  18  includes a transmit antenna including a plurality of transmit antenna elements  24 . In some examples, the plurality of transmit antenna elements  24  may be disposed in a plurality of rows, where the number of transmit antenna elements  24  in each respective row is the same. In some examples, the number of transmit antenna elements  24  in a single row is greater than the number of rows in the transmit antenna. In this way, in some examples, the transmit antenna may be wider than it is tall, and the transmit beam formed by the transmit antenna may be larger in elevation than in azimuth, such as forming an elliptical shape that is taller than it is wide. In some examples, this may allow the transmit beam to cover substantially the entire elevation of the predetermined window in a single scan, as described below with reference to  FIG. 4 . 
     In other examples, the number of transmit antenna elements  24  in a single row is less than the number of rows in the transmit antenna. In this way, in some examples, the transmit antenna may be taller than it is wide, and the transmit beam formed by the transmit antenna may be larger in azimuth than in elevation, such as forming an elliptical shape that is wider than it is tall. In some examples, this may allow the transmit beam to cover substantially the entire azimuth of the predetermined window in a single scan, as described below with reference to  FIG. 4 . 
     In general, the number of transmit antenna elements  24  in a first transmit array dimension (e.g., width or height) may be greater than the number transmit antenna elements  24  in a second transmit array dimension (e.g., height or width). The first transmit array dimension may be substantially perpendicular to the second transmit array dimension. A transmit antenna that includes a greater number of transmit antenna elements in the first transmit array dimension than in the second transmit array dimension may produce a transmit beam that is elongated in a first illumination dimension compared to a second, substantially perpendicular illumination dimension. Although the remainder of this description describes examples in which the transmit antenna includes more transmit antenna elements  24  in a single row that the number of rows of transmit antenna elements  24 , a person having ordinary skill in the art will understand that the first and second transmit array dimensions may be any substantially perpendicular dimensions, and that the first and second illumination dimensions are based on the first and second transmit array dimensions. 
     In the example illustrated in  FIGS. 1A-1C and 2 , the transmit antenna includes two rows of transmit antenna elements  24 , and each row includes twenty-four transmit antenna elements  24 . However, in other examples, the transmit antenna may include one row or more than two rows of transmit antenna elements  24 , and each row of the transmit antenna may include more or fewer than twenty-four transmit antenna elements  24 . In general, the transmit antenna may include at least one row of transmit antenna elements  24 , and each row may include a plurality of transmit antenna elements  24 . Alternatively, or additionally, transmit antenna elements  24  may not be arranged in rows and columns as depicted in  FIG. 3 ; instead, transmit antenna elements  24  may be arranged in another geometric or non-geometric array. In some examples, transmit antenna elements  24  may include aperture coupled microstrip patches. 
     FMCW array  12  also includes a receive array  20  including a plurality of printed boards  26  on which or in which at least some of the electronics and receive antenna elements of receive array  20  are disposed. Each of printed hoards  26  is connected to a master interconnect board  28  by a respective one of connectors  30 . Master interconnect board  28  may be mechanically attached or coupled to housing  13  ( FIGS. 1A and 1B ), e.g., to a back wall of housing  13 . 
     Each of printed boards  26  includes a plurality of receive antenna elements. Although the receive antenna elements are not illustrated in  FIG. 2 , the receive antenna elements are located adjacent to the top edge  31  of each of the respective printed boards  26 . In some examples, the receive antenna elements may be radiating dipoles. In some examples, the receive antenna elements may be aperture coupled microstrip patches. In other examples, receive array  20  may include another physical configuration, such as receive antenna elements that are not adjacent to top edge  31  of each of the respective printed boards  26 , more or fewer printed boards  26 , or a construction similar to transmit array  18 , in which a plurality of receive antenna elements are mounted on or formed in a major surface of a printed board or other substrate. 
       FIG. 3  is a conceptual diagram illustrating another conceptual view of an example FMCW radar array  12 . As in the example of  FIG. 2 , FMCW radar array  12  includes a transmit array  18  and a receive array  20 . Similar to each of FMCW radar arrays  12  shown in  FIGS. 1A and 1B  the example of FMCW radar array  12  shown in  FIG. 3  also includes a mechanical choke  22  disposed between the transmit antenna and the receive antenna. Transmit array  18  and receive array  20  are physically proximate to each other, e.g., located in a single housing (housing  13  shown in  FIGS. 1A and 1B ). 
     Transmit array  18  includes a plurality of transmit antenna elements  24 . In some examples, transmit array  18  includes two rows (oriented horizontally in the example of  FIG. 3 ) of transmit antenna elements  24 , and each row includes twenty-four transmit antenna elements  24 . In general, transmit array  18  may include at least one row of transmit antenna elements  24 , and each row may include a plurality of antenna elements  24 . In some examples, adjacent transmit antenna elements  24  may be spaced apart in the horizontal direction by approximately one-half of the wavelength of the transmit beam generated using transmit array  18 . 
     As shown in  FIG. 3 , receive array  20  may be conceptually divided into quadrants  32   a ,  32   b ,  32   c ,  32   d  (collectively, “quadrants  32 ”). In some examples, receive array  20  is also electrically divided into quadrants  32 , e.g., based on the electrical connections of the receive antenna elements  34  to receive electronics that process the signals detected by receive antenna elements  34 . Receive signals from each of receive array elements  34  may be used to generate monopulse tracking beams using monopulse beam arithmetic, and dividing receive array  20  into quadrants  32  may facilitate generation of monopulse tracking beams, as described below. In some examples, each of quadrants  32  includes the same number of receive antenna elements  34 . For example, in the implementation shown in  FIG. 3 , each of quadrants  32  includes twelve rows of twelve receive antenna elements  34 , for a total of 144 receive antenna elements  34  in each of quadrants  32  (each row is oriented horizontally and each column is oriented vertically in the example of  FIG. 3 ). In other examples, each of quadrants  32  may include  10  rows of receive antenna elements  34 , each row including 12 receive antenna elements  34  (for a total of 120 receive antenna elements in each of quadrants  32 ). Hence, in the illustrated example, receive array  20  includes twenty-four rows of receive antenna elements  34 , and each row includes twenty-four receive antenna elements  34 . In other examples, receive array  20  may include a different number of receive antenna elements  34 . For example, receive array  20  may include more or fewer rows of receive antenna elements  34 , and each row may include more or fewer receive antenna elements  34  than depicted in  FIG. 3 . In general, receive array  20  may include a plurality of rows of receive antenna elements  34  and each row may include a plurality of receive antenna elements  34 . In some examples, adjacent receive antenna elements  34  may be spaced apart in the horizontal direction by approximately one-half of the wavelength of the transmit beam generated using transmit array  18 . 
     In some examples, receive antenna elements  34  may be arranged in a square array of receive antenna elements  34  (e.g., the number of rows of receive antenna elements  34  is the same as the number of receive antenna elements  34  in each row). In other examples, receive antenna elements  34  may be arranged in a rectangular arrant of receive antenna elements  34  (e.g., the number of rows of receive antenna elements  34  is different than the number of receive antenna elements  34  in each row). Additionally or alternatively, in some examples, the number of receive antenna elements  34  in a row of receive array  20  may be different than the number of transmit antenna elements  24  in a row of transmit array  18 . Alternatively, or additionally, receive antenna elements  34  may not be arranged in rows and columns as depicted in  FIG. 3 ; instead, receive antenna elements  34  may be arranged in another geometric or non-geometric array. 
       FIG. 4  is a conceptual diagram illustrating an example transmit beam  42  and a plurality of example receive beams  44 , which may be generated using transmit array  18  and receive array  20 . Transmit beam  42  is depicted as being approximately elliptical in shape, with a greater extent in elevation than in azimuth.  FIG. 4  also depicts a representation of a predetermined area  48  which is to be illuminated by FMCW array  12  ( FIGS. 1-3 ). As shown in  FIG. 4 , transmit beam  42  may be at least as tall in elevation as the elevation of predetermined area  48 , such that transmit beam  42  illuminates the entire elevation of a section of predetermined area  48  without steering or scanning transmit beam  42  in elevation. In other examples, as described above, transmit beam  42  may be wide in azimuth and short in elevation. In general, transmit beam  42  may have a greater extent in a first illumination direction than in a second illumination dimension substantially perpendicular to the first illumination dimension. 
     A transmit electronics module associated with transmit array  18  may be configured to scan, or steer, transmit beam  42  in azimuth (e.g., the second illumination dimension), as indicated by arrow  46 . In some examples, the transmit electronics module may be configured to apply a phase shift to each transmit antenna element of the plurality of transmit antenna elements  24  ( FIG. 3 ) which changes as a function of time, which results in transmit beam  42  being scanned in azimuth. 
     A receive electronics module associated with receive array  20  is configured to electronically generate the plurality of receive beams  44 . Although twelve receive beams  44  are illustrated in  FIG. 4 , in other examples, the receive electronics module may be configured to generate more or fewer receive beams  44  using receive array  20 . For example, the receive electronics module associated with receive array  20  may be configured to generate at least two receive beams  44 . 
     In some examples, the receive electronics module associated with receive array  20  is configured to scan, or steer, each of the plurality of receive beams  44  in the second illumination dimension (e.g., azimuth) in parallel with transmit beam  42 . For example, the receive electronics module associated with receive array  20  may be configured to scan, or steer, each of the plurality of receive beams  44  in the second illumination dimension (e.g., azimuth) such that the plurality of receive beams  44  are scanned at the same rate and to corresponding locations so that the plurality of receive beams  44  are substantially always (e.g., always or nearly always) located within the area illuminated by transmit beam  42 . 
     In some examples, the receive electronics module associated with receive array  20  may be configured to scan, or steer, the plurality of receive beams in the second illumination dimension (e.g., azimuth) by applying a phase shift to the signals received from each respective receive antenna element of the plurality of receive antenna elements  34 . The receive electronics associated with receive array  20  then may process the phase-shifted signals as described below to produce phase-shifted and summed I and Q values for each row of receive antenna elements  34  in each respective quadrant of quadrants  32  ( FIG. 3 ). For example, when each row of receive antenna elements  34  in each respective quadrant of quadrants  32  ( FIG. 3 ) includes twelve elements, the receive electronics module associated with receive array  20  may be configured to generate a single phase-shifted and summed I value and a single phase-shifted and summed Q value for each row of twelve receive antenna elements  34  each time the receive array  20  is sampled. 
     The receive electronics module associated with receive array  20  also may be configured generate the plurality of receive beams  44  at predetermined first illumination dimension (e.g., elevation) positions by applying a complex beam weight to the phase-shifted and summed I and Q values for each row of each of quadrants  32  ( FIG. 3 ). The phase-shifted and summed I and Q values determined by the receive electronics module for a single sample instance may be reused multiple times to generate the corresponding number or receive beams  44  at respective elevation positions. For example, to generate twelve receive beams  44 , the receive electronics module associated with receive array  20  may apply twelve different complex beam weights to the phase-shifted and summed I and Q values for each row of each of quadrants  32  in twelve separate operations. 
     The plurality of complex beam weights may correspond to the number of receive beams  44 . The values for each of the plurality of complex beam weights may be selected to result in the plurality of receive beams being generated at the respective predetermined elevation positions. As shown in  FIG. 4 , in some examples, the elevation positions of the plurality of receive beams  44  may be selected to substantially fully cover (e.g., fully cover or nearly fully cover) the elevation extent of the predetermined area  48  which is to be illuminated. In some examples, the adjacent ones of the plurality of receive beams  44  may partially overlap in elevation. In this way, the receive electronics associated with receive array  20  may generate a plurality of receive beams  44  at predetermined first illumination dimension (e.g., elevation) positions and scan, or steer, the plurality of receive beams  44  in the second illumination dimension (e.g., azimuth). 
     Additionally, because receive array  20  is conceptually (and, optionally, electrically) divided into quadrants  32 , the receive electronics module associated with receive array  20  may be configured to generate monopulse tracking beams. This may be used to facilitate tracking of objects by radar system  10 . By generating a transmit beam  42  and a plurality of receive beams  44 , radar system  10  may perform monopulse tracking for each of receive beams  44 , which may facilitate tracking multiple objects within predetermined area  48 . For example, by digitally combining the I and Q values for the two left quadrants  32   a  and  32   c  together, digitally combining the I and Q values for the two right quadrants  32   b  and  32   d , and determining the difference between I and Q values for the two left quadrants  32   a  and  32   c  and the I and Q values for the two right quadrants  32   b  and  32   d , the receive electronics module may create an azimuth monopulse tracking beam. Similarly, in some examples, by digitally combining the I and Q values for the top two quadrants  32   a  and  32   b , and digitally combining the I and Q values for the bottom two quadrants  32   c  and  32   d , and determining the difference between I and Q values for the two top quadrants  32   a  and  32   b  and the I and Q values fix the two bottom quadrants  32   c  and  32   d , the receive electronics module may create an elevation monopulse tracking beam. In some examples, by digitally combining the I and Q values for respective rows of all 4 quadrants  32 , a reference sum beam may be created for comparison to the azimuth and elevation monopulse tracking beams. This may permit an accurate phase comparison monopulse to be created for each of receive beams  44 . Additionally, as each of FMCW arrays  12  is configured to generate a transmit beam  42  and a plurality of receive beams  44 , which are scanned within a corresponding predetermined window, this may facilitate tracking of multiple objects by radar system  10 . 
     In some examples, instead of being associated with a single receive array  20 , the receive electronics module may be associated with multiple receive arrays  20  (e.g., receive arrays  20   a - 20   c  shown in  FIG. 1 ). In other examples, a first portion of the receive electronics module may be associated with a single receive array, and a second portion of the receive electronics module may be associated with multiple receive arrays (e.g., receive arrays  20   a - 20   c  shown in  FIG. 1 ). For example, a portion of the receive electronics module that performs frequency downconversion and analog beam steering using phase shifts may be associated with a single array (e.g., receive array  20   a  of  FIG. 1 ), and each receive array may include a respective portion that performs frequency downconversion and analog beam steering using phase shifts. Continuing this example, a portion of the receive electronics module applies complex beam weight to the phase-shifted and summed I and Q values for each row of each of quadrants  32  to form the receive beams at predetermined elevation positions and form monopulse tracking beams may be associated with multiple receive arrays (e.g., receive arrays  20   a - 20   c  shown in  FIG. 1 ). In some examples, then, different portions of the receive electronics module may be conceptually associated with different receive arrays  20  or multiple receive arrays, physically associated with different receive arrays  20 , may be physically separate from receive arrays  20 , or the like. 
       FIG. 5  is a conceptual block diagram illustrating an example FMCW radar array  12 , including associated electronics modules. FMCW radar array  12  includes an array controller  66 , which controls operation of FMCW radar array  12 . Array controller  66  is operably coupled to a master radio frequency (RE) source and clock  68 . Array controller  66  may include one or more processors, including one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), or any other equivalent integrated or discrete logic circuitry, as well as any combinations of such components. The term “processor” or “processing circuitry” may generally refer to any of the foregoing logic circuitry, alone or in combination with other logic circuitry, or any other equivalent circuitry. 
     Master RF source and clock  68  generates a base RF signal, for example, at a frequency of about 13 GHz. In some examples, master RF source and clock  68  may include a fractional N synthesizer. Master RF source and clock  68  is operably coupled to a power amplifier  70 , which amplifies the base RF signal and outputs the amplified base RF signal to a power divider  64 . Power amplifier  70  may amplify the base RF signal to overcome reduction in power as the base RF signal is divided for use in each receive signal and transmit signal. Power divider  64  is operably coupled to a first corporate feed  62 , which is associated with a transmit array  18  ( FIGS. 1-3 ) and a second corporate feed  72 , which is associated with a receive array  20  ( FIGS. 1-3 ). 
     Transmit electronics module  52  indicates electronics (e.g., power amplifier  54 , image reject mixer (IRM)  56 , direct digital synthesizer (DDS)-I  58 , and DDS-Q  60 ) conceptually associated with a single transmit antenna element  24 .  FIG. 5  illustrates conceptually the components present for a transmit signal being sent to a single transmit antenna element  24 . As described above with respect to  FIGS. 1-3 , FMCW radar array  12  may include a plurality of transmit antenna elements  24 . FMCW radar array  12  thus may include a plurality of transmit antenna elements  24  and a plurality of transmit electronics module  52  of  FIG. 5 . 
     In some examples, equivalent functionality for a plurality of transmit signals each being sent to a respective transmit antenna element  24  may be embodied in a single physical component. For example, a single power amplifier may include a plurality of channels, and each channel may be connected to a respective transmit antenna element. Hence, when embodied in a physical product, FMCW radar array  12  may include fewer components than those illustrated in  FIG. 5 , as functions of components may be combined and/or a single component may perform a function described with respect to  FIG. 5  for multiple signals being sent to respective transmit antenna elements  24  or receive antenna elements  34 . 
     Array controller  66  is operably connected to respective inputs of DDS-I  58  and DDS-Q  60 , and instructs DDS-I  58  and DDS-Q  60  to generate a phase shift applied to respective intermediate frequency signals. For example, the intermediate frequency may be on the order of tens of megahertz (MHz), such as about 16 MHz, about 32 MHz, or about 64 MHz. DDS-I  58  and DDS-Q  60  output the phase-shifted signals to IRM  56 . IRM  56  receives both the phase-shifted signals from DDS-I  58  and DDS-Q  60  and the base RF signal from first corporate feed  62 . IRM  56  combines the base RF signal and the phase shifted intermediate frequency signals from DDS-I  58  and DDS-Q  60  to produce two phase shifted RF signals, which have frequencies of the base RF signal plus and minus the intermediate frequency, respectively. IRM  56  also attenuates one of the two phase-shifted RF signals and outputs the other of the two phase shifted RF signals to the power amplifier  54 . Power amplifier  54  amplifies the phase shifted RF signal and outputs the signal to transmit antenna element  24 . 
     As described above, the transmit beam generated by transmit antenna element  24  and the other transmit antenna elements  24  in the transmit array  18  ( FIGS. 1-3 ) may be electronically steered by applying a phase shift to the RF signal output by the transmit antenna elements  24 , where the phase shift changes as a function of time. As shown in  FIG. 5 , the phase shift is generated by DDS-I  58  and DDS-Q  60  under control of array controller  66 . Array controller  66  may linearly change the phase shift generated by DDS-I  58  and DDS-Q  60  to linearly scan the transmit beam  42  ( FIG. 4 ) in azimuth. Because the phase shift is generated at intermediate frequency rather than RF, the phase shift operation may be more efficient, and thus may utilize smaller power amplifiers  54  compared to when the phase shift is implemented at RF. DDS-I  58  and DDS-Q  60  also may provide linear frequency modulation. In some examples, the phase shift applied by DDS-I  58  and DDS-Q  60  may be changed at most once per frequency modulation period. In some examples, to cause the transmit beam to dwell at a particular position, DDS-I  58  and DDS-Q may change the phase shift less often, e.g., after multiple frequency modulation periods having a given phase shift. 
     Turning now to the receive portion of FMCW radar array  12 , each of receive antenna elements  34  is coupled to an analog receive electronics module  74 .  FIG. 5  illustrates conceptually the components present for a receive signal being received by a single receive antenna element  24 . As described above with respect to  FIGS. 1-3 . FMCW radar array  12  may include a plurality of receive antenna elements  34 . Although a single receive antenna element  34  and a single analog receive electronics module  74  are depicted in the example of  FIG. 5 , in implementation, receive array  20  includes a plurality of receive antenna elements  34  ( FIG. 3 ). FMCW radar array  12  thus may include a plurality of receive antenna elements  34  and a plurality of analog receive electronics module  74  or a single analog receive electronics module configured to perform the operations described with respect to analog receive electronics module  74  on each of a plurality of receive signals. 
     However, in some examples, equivalent functionality for a plurality of receive signals each being sent to a respective receive antenna element  34  may be embodied in a single physical component. Hence, when embodied in a physical product, FMCW radar array  12  may include fewer components than those illustrated in  FIG. 5 , as functions of components may be combined and/or a single component may perform a function described with respect to  FIG. 5  for multiple signals being sent to respective receive antenna elements  34 . 
     Analog receive electronics module  74  receives the receive signal from receive antenna elements  34  and also receives a base band signal from a second corporate feed  72 . Receive mixer combines the receive signal and the base band signal and outputs the combined signal to a power amplifier  76 .  FIG. 6  is a conceptual block diagram illustrating an example receive antenna element  34  and an example of analog receive electronics module  74 . In the example illustrated in  FIG. 6 , analog receive electronics module  74  includes a receiver mixer  92 , a low noise amplifier (LNA)  94 , a quadrature mixer  110 , and summing operational amplifiers  106  and  108 . Receiver mixer  92  is operably coupled to receive antenna element  34  and receives a signal directly from receive antenna element  34 , with no intervening amplifiers, intervening amplifiers between receive antenna element  34  and receiver mixer  92  may raise the noise floor of the receiver, due to use of FMCW radar and simultaneous transmit and receive. Receiver mixer  92  also receives a signal from second corporate feed  72 , which is at the RF frequency (e.g., about 13 GHz). Because the RF signal output by DDS-I  58  and DDS-Q  60  ( FIG. 5 ) is offset from the RE frequency by the intermediate frequency (e.g., 16 MHz, 32 MHz, or 64 MHz), the signal received by receiver mixer  92  from receive antenna element  34  is offset from the RF frequency signal from second corporate feed  72  by the intermediate frequency. Hence, the signal output from receiver mixer  92  has a frequency of the intermediate frequency (e.g., 16 MHz, 32 MHz, or 64 MHz). The FMCW radar systems described herein thus may be heterodyne FMCW radar systems, and the intermediate frequency at which the receive signals are operated on (for at least part of the analog receive electronics  74 ) are created by heterodyning the signal received from receive antenna element  34  and the RF frequency signal from second corporate feed  72 . 
     Receiver mixer  92  is operably coupled to a LNA  94 , which amplifies the intermediate frequency signal received from receiver mixer  92  and outputs the amplified signal to quadrature mixer  110 . Quadrature mixer  110  splits the receive signal into I and Q components at block  96  and sends the I and Q signals to mixers  98  and  100 , respectively. At first mixer  98 , the I signal down-converted to base band (e.g., between about 0 MHz and about 2 MHz) by combining the I signal with a reference clock signal  109 , which is derived from the second corporate feed  72  signal and may have a frequency that is an integer multiple of the intermediate frequency. At second mixer  100 , the Q signal down-converted to base band (e.g., between about 0 MHz and about 2 MHz) by combining the Q signal with reference clock signal  109 . First mixer  98  is operatively coupled to a first phase shifter  102 , which shifts the phase of the I signal to steer the receive beams in azimuth. Second mixer  100  is operatively coupled to a second phase shifter  104 , which shifts the phase of the Q signal to steer the receive beams in azimuth. 
     As shown in  FIG. 6 , the phase-shifted I signal and the phase-shifter Q signal are output to respective summing operational amplifiers  106  and  108  (e.g., active filter summing operational amplifiers  106  and  108 ). Although not shown in  FIG. 6  (see  FIG. 7 ), first summing operation amplifier  106  may receive phase-shifted I signals corresponding to all receive antenna elements  34  in a row of one of quadrants  32  ( FIG. 3 ). For each row in each of quadrants  32 , a first summing operation amplifier  106  sums the I signals for the respective receive antenna elements  34  in the row of the quadrant. Similarly, second summing operation amplifier  108  may receive phase-shifted Q signals corresponding to all receive antenna elements  34  in a row of one of quadrants  32  ( FIG. 3 ). For each row in each of quadrants, a second summing operation amplifier  108  sums the Q signals for the respective receive antenna elements  34  in the row of the quadrant. The summing operation amplifiers  106  and  108  output the summed I and Q signals for each row of each of quadrants  34  to analog to digital converter  76 . In some examples, in addition to summing the I and Q signals, respectively, summing operation amplifiers  106  and  108  may apply a high pass filter, a low pass filter, or both, to shape the I and Q signals. The gain slopes for the optional high pass filter may be selected based on the application of the FMCW radar system. As examples, for weather detection, the high pass filter slope may be about 20 dB per octave; for ground imaging, the high pass filter slope may be about 30 dB per octave; for airborne target detection, the high pass filter slope may be about 40 dB per octave; or the like. 
       FIG. 7  illustrates another example conceptual block diagram of an analog receive electronics module portion for a row of a receive array  32 . As shown in  FIG. 7 , a row of receive array  32  ( FIG. 3 ) includes a plurality of receive antenna elements  34   a - 34   l  (collectively, “receive antenna elements  34 ”). Although twelve receive antenna elements  34  are illustrated in  FIG. 7 , in other examples, a row of a receive array  32  may include more or fewer receive antenna elements  34 . In general, a row of receive array  32  may include a plurality of receive antenna elements. 
     Each of receive antenna elements  34  is operably connected to a respective receiver mixer of the plurality of receiver mixers  92   a - 92   l  (collectively, “receiver mixers  92 ”). As described with respect to  FIG. 6 , each of receiver mixers  92  may also receive an RF signal from second corporate feed  72 , although this is not shown in  FIG. 7 . Although twelve receiver mixers  92  are illustrated in  FIG. 7 , in other examples, analog receive electronics module  74  may include more or fewer receiver mixers  92 . In some examples, analog receive electronics module  74  may include a respective receiver mixer  92  for each receive antenna element of receive antenna elements  34 . Each of receiver mixers  92  is operably connected to a respective channel of one of LNAs  94   a - 94   c  (collectively, “LNAs  94 ”). 
     LNAs  94  amplify the receive signal and are operably coupled to a respective channel of one of quadrature mixers  110   a - 110   c  (collectively, “quadrature mixers  110 ”). Although three LNAs  94  each with four channels are illustrated in  FIG. 7 , in other examples, each of LNAs  94  may include more or fewer channels, and there may be more or fewer LNAs  94  for a row of receive antenna elements  34 . Similarly, although three quadrature mixers  110  each with four channels are illustrated in  FIG. 7 , in other examples, each of quadrature mixers  110  may include more or fewer channels, and there may be more or fewer quadrature mixers  110  for a row of receive antenna elements  34 . Quadrature mixers  110  may down-convert the receive signal to base band, separate the receive signal into I and Q components, apply a phase shift to the I and Q components, and output the phase-shifted I and Q signals. 
     As shown in  FIG. 7 , quadrature mixers  110  may output the phase-shifted I signals to a first summing operational amplifier  106 , which sums all of the phase-shifted I signals to yield a summed I signal for the row. Similarly, quadrature mixers  110  may output the phase-shifted Q signals to a second summing operational amplifier  108 , which sums all of the phase-shifted Q signals to yield a summed Q signal for the row. First summing operation amplifier  106  outputs the summed I signal to analog-to-digital converter  76 , and second summing operation amplifier  108  outputs the summed Q signal to analog-to-digital converter  76 . Receive array  20  may include components that perform substantially similar functions for each row of receive antenna elements  34  in each quadrant  34  of the receive array  20 . 
     Referring to  FIG. 5 , analog-to-digital converter  76  outputs the digital data streams for the summed I and Q values to a digital receive electronics module  78 . Digital receive electronics module  78  may be configured to generate a plurality of receive beams from the digital data streams for the summed I and Q values received from analog-to-digital converter  76 .  FIGS. 8 and 9  illustrate example aspects of an example digital receive electronics module  78 .  FIG. 8  is a functional block diagram illustrating example functions of analog to digital converters  76   a - 76   l  (collectively analog to digital converters  76 ) and portions of a digital receive electronics module  78  for a quadrant  32  of a receive array  20 .  FIG. 9  is a functional block diagram illustrating example functions for producing a plurality of receive beams from signals received from a respective receive electronics module  74  for each quadrant  32  of a receive array  20 . 
     As shown in  FIG. 8 , a plurality of analog receive electronics module  74   a - 74   l  each outputs a respective summed I signal and a respective summed Q signal to a respective one of analog-to-digital converters  76 . In the example of  FIG. 8 , twelve analog receive electronics module  74  and twelve analog-to-digital converters  76  are depicted. However, in other examples, a quadrant  32  may include more or fewer rows of receive antenna elements  34 , and may accordingly include more or fewer analog receive electronics module  74 . In some examples, a receive array  20  includes an analog receive electronics module  74  for each row of each of quadrants  32 . Similarly, a receive array  20  may include more or fewer analog-to-digital converters  76 , and the number of analog-to-digital converters for a quadrant  32  may be the same as or different than the number of rows of receive antenna elements  34  in the quadrant  32 . 
     Each of the analog-to-digital converters  76  converts an analog summed I signal to a digital I data stream and an analog summed Q signal to a digital Q data stream. Digital receive electronics module  78  then may apply a complex beam weight  112  to the digital I data streams and digital Q data streams and sum  114  the results to generate a weighted I data stream and a weighted Q data stream  116  for the quadrant. The complex beam weight may be selected to result in weighted I and Q data streams  116  being generated that will be used by digital receive electronics module  78  to generate a receive beam at a predetermined elevation position, as described with reference to  FIG. 4 . The number of complex beam weights  112  may be the same as the number of receive beam positions. 
     In some examples, digital receive electronics module  78  may reuse the digital data streams and the digital Q data streams by applying a different complex beam weight  112  to the digital I signals and the digital Q data streams to generate each of a plurality of weighted I and Q data streams  116 . Each of the plurality of complex beam weights  112  may be selected to result in a respective weighted I and Q data stream being generated that is used to form a receive beam at a predetermined elevation position. The complex beam weights  112  may apply both amplitude taper and elevation beam steering to the digital I data streams and the digital Q data streams. The result of the applying the complex beam weights  112  is a plurality of weighted I data streams and a plurality of weighted Q data streams  116 , one weighted I data stream and one weighted Q data stream  116  for each of the complex beam weights  112 . Hence, each of quadrants  32  forms a plurality of weighted I data streams and a plurality of weighted Q data streams  116 , one data stream in I and Q for each of the receive beam positions. To facilitate formation of the monopulse tracking beams, the number of weighted I data streams and weighted Q data streams  116  output by each of quadrants  32  may be the same. 
     As shown in  FIG. 9 , the output weighted I data streams and weighted Q data streams  116  are used by the digital receive electronics module  78  to form monopulse tracking beams at each receive beam position. As shown in  FIG. 9 , each of quadrants  32  outputs a respective plurality of weighted I data streams and plurality of weighted Q data streams  116   a - 116   d  (collectively, “plurality of weighted I data streams and plurality of weighted Q data streams  116 ”). The number of weighted I data streams and the number of weighted Q data streams  116  for each of quadrants  32  corresponds to the number of receive beam positions. 
     Digital receive electronics module  78  sums the first weighted I data stream from the first quadrant  32   a  and the first weighted I data stream from second quadrant  32   b  (the top two quadrants) to form a first top I data stream. Each of the first weighted I data streams may correspond to the same (a first) receive beam position. Similarly, digital receive electronics module  78  sums the first weighted Q data stream from the first quadrant  32   a  and the first weighted Q data stream from second quadrant  32   b  to form a first top Q data stream. Each of the first weighted Q data streams may correspond to the same (the first) receive beam position. Digital receive electronics module  78  repeats this summation for each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   a  from first quadrant  32   a  and each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   b  from second quadrant  32   b . This results in a plurality of top I data streams and a plurality of top Q data streams  124 , with the number of top I data streams and the number of top Q data streams  124  corresponding to the number of receive beam positions. 
     Similarly, digital receive electronics module  78  sums the first weighted I data stream from the first quadrant  32   a  and the first weighted I data stream from third quadrant  32   c  (the left two quadrants) to form a first left I data stream. Each of the first weighted I data streams may correspond to the same (a first) receive beam position. Similarly, digital receive electronics module  78  sums the first weighted Q data stream from the first quadrant  32   a  and the first weighted Q data stream from third quadrant  32   c  to form a first left Q data stream. Each of the first weighted Q data streams may correspond to the same (the first) receive beam position. Digital receive electronics module  78  repeats this summation for each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   a  from first quadrant  32   a  and each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   c  from third quadrant  32   c . This results in a plurality of left I data streams and a plurality of left Q data streams  122 , with the number of left I data streams and the number of left Q data streams  122  corresponding to the number of receive beam positions. 
     Digital receive electronics module  78  performs this process for each for each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   c  from third quadrant  32   c  and each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   d  from fourth quadrant  32   d  to form a plurality of bottom I data streams and a plurality of bottom Q data streams  128 . Digital receive electronics module  78  also performs this process for each for each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   b  from second quadrant  32   b  and each of the plurality of weighted I data streams and each of plurality of weighted Q data streams  116   d  from fourth quadrant  32   d  to form a plurality of right I data streams and a plurality of right Q data streams  126 . 
     Digital receive electronics module  78  performs monopulse arithmetic  130  using the plurality of I and Q data streams  122 ,  124 ,  126 , and  128  to form a monopulse sum beam, a monopulse azimuth delta beam, and a monopulse elevation delta beam for each of the receive beam positions. For example, by summing each of the first I data streams and each of the first Q data streams, digital receive electronics module  78  may form a monopulse sum beam for the first receive beam position. By subtracting the first right I and Q data streams from the first left I and Q data streams, digital receive electronics module  78  may form a monopulse azimuth delta beam for the first receive beam position. By subtracting the first bottom I and Q data streams from the first top I and Q data streams, digital receive electronics module  78  may form a monopulse elevation delta beam for the first receive beam position. Digital receive electronics module  78  may perform similar calculations to form a monopulse sum beam, a monopulse azimuth delta beam, and a monopulse elevation delta beam at each receive beam position using respective ones of the plurality of left, top, right, and bottom I and Q data streams  122 ,  124 ,  126 , and  128 . 
     After digital receive electronics module  78  has formed each of the plurality of monopulse sum beams, each of the plurality of monopulse azimuth delta beams, and each of the plurality of monopulse elevation delta beams (one of each beam for each receive beam position), digital receive electronics module  78  applies a Fast Fourier Transform (FFT) to each respective beam to transform the beam from the frequency domain to the range domain. In some examples, the FFT generates 2048 FFT bins, each bin corresponding to a range bin of about 24 feet (about 8 meters). The monopulse beams may allow monopulse beam tracking of objects in the predetermined window  48  ( FIG. 4 ). 
     In some examples, the receive electronics module, which may include analog receive electronics module  74 , analog-to-digital converter  76 , and digital receive electronics module  78 , may steer the receive beams in azimuth by applying a phase shift to the receive signals from each of receive antenna elements  34  using analog receive electronics module  74 . Analog receive electronics module  74  may sequentially apply different phase shifts to the receive signals from each of receive antenna elements  34  to steer the receive beams in azimuth. At each azimuth position, digital receive electronics module  78  may generate the plurality of receive beams (including monopulse sum, azimuth delta, and elevation delta beams at each receive beam position). In some examples, the elevation position of each of the receive beams may not change as the receive beams are scanned in azimuth. In other words, in some examples, digital receive electronics module  78  applies the same set of complex beam weights to the I digital steams and Q digital streams at least of the azimuth positions. The output of the digital receive electronics module  78  may be used by the radar system for target selection and tracking. 
     By performing most manipulations of the receive signals at baseband frequencies rather than RF and summing the I and Q signals for each row in a quadrant before digitally forming the plurality of receive beams, component count may be reduced and power efficiency may be increased. Additionally or alternatively, less complex and/or inefficient phase shifters may be used compared to when phase shifting is performed at RF. In some examples, this may reduce or substantially eliminate receiver losses and may not utilize receiver amplifiers with their attendant power dissipation, circuit board space, and cost. In some examples, receive array  20  does include a respective low noise amplifier (LNA) between a respective receive antenna element  34  and a respective receiver mixer  92 . If present between the respective receive antenna element  34  and the respective receiver mixer  92 , the LNA may reduce transmit array-to-receive array isolation and the LNA may be saturated by nearby transmit array leakage power. By avoiding LNAs at every receive antenna element, the parts count of receive array  20  may be reduced, which may improve cost, power dissipation, and/or reliability of receive array  20 . Additionally, the formation of multiple receive beams and monopulse tracking beams at each receive beam position may facilitate object tracking by the radar system. 
     FMCW radar arrays  12  also may include physical or mechanical structures that facilitate operation of FMCW radar arrays  12 , including positioning a transmit array  18  and a receive array  20  in relatively close proximity and transmitting a substantially continuous in time FMCW transmit beam. As described above, FMCW radar arrays  12  ( FIGS. 1-3 ) may include a slotted choke  22 .  FIG. 10  is a conceptual and schematic diagram illustrating an example slotted choke  22  disposed between a transmit array  18  and a receive array  20  of an FMCW radar array e.g., FMCW radar array  12  illustrated in  FIGS. 1A and 1B ).  FIG. 11  is a cross-sectional conceptual diagram illustrating an example slotted choke  22  disposed between a transmit array  18  and a receive array  20  of a FMCW radar array. 
     Slotted choke  22  may include a body  144  formed of an electrically-conductive material or an electrically insulative material coated with an electrically conductive material. Slotted choke  22  may define a plurality of slots  142 , which are sized, shaped, and/or placed to attenuate strength, proximate to receive array  20 , of electromagnetic radiation output by the transmit antenna of transmit array  18 . In some examples, at least some of slots  142  may define a depth that is equal to about ¼ of the wavelength of the transmit beam and a width that is equal to about ⅛ of the wavelength of the transmit beam. In some examples, the spacing between adjacent slots  142  may be equal to about ⅛ of the wavelength of the transmit beam. As shown in  FIG. 11 , in some examples, slotted choke  22  may extend to an inner surface  146  of a radome  148  disposed adjacent to the FMCW array. By extending to the inner surface  146  of radome  148 , slotted choke  22  may reduce reflection of electromagnetic radiation off of inner surface  148  of radome  146  toward receive array  20 . 
     As shown in  FIG. 11 , in some examples, slotted choke  22  may be attached or mechanically coupled to one or both of transmit array  18  or receive array  20 . In other examples, slotted choke  22  may be attached or mechanically coupled to housing  13  (FIGS.  1 A and  1 B). In some examples, slotted choke  22  may extend through the plane defined by the outer surfaces of transmit array  18  and receive array  20 , so that no gap exists between the outer surfaces of transmit array  18  and receive array  20  and slotted choke  22 . This may reduce an amount of electromagnetic radiation that may propagate under slotted choke  22  from transmit array  18  to adjacent receive array  20 . 
       FIG. 12  is a diagram illustrating example attenuation of the transmit beam adjacent to the receive array due to the presence of a slotted choke  22  between a transmit array  18  and a receive array  20  of an FMCW radar array. As shown in  FIG. 12 , in some examples, slotted choke  22  may attenuate the electromagnetic radiation from the transmit array  18  by about 60 decibels adjacent to receive array  20 . 
       FIG. 13  is a conceptual and schematic diagram illustrating another example FMCW radar system  150  including a plurality of FMCW radar arrays  152   a - 152   c . FMCW radar system  150  and FMCW radar arrays  152   a - 152   c  may be similar to or substantially the same as FMCW radar system  10  and FMCW radar arrays  12   a - 12   c  of  FIGS. 1A-4C and 2 , aside from the differences described herein. Each of FMCW radar arrays  152   a - 152   c  includes a respective one of housings  153   a - 153   c , a respective one of transmit arrays  158   a - 158   c , a respective one of receive arrays  160   a - 160   c , and a respective one of slotted chokes  162   a - 162   c . Unlike FMCW radar arrays  12   a - 12   c  illustrated in  FIGS. 1A-1C  and FMCW radar array  12  illustrated in  FIG. 2 , FMCW radar arrays  152   a - 152   c  include respective receive arrays  160   a - 160   c  that include a substantially planar outer surface. 
     For example, instead of including a plurality of printed boards  26  that include edge mounted antenna elements ( FIG. 2 ), receive arrays  160   a - 160   c  may include respective printed boards that include receive antenna elements formed on, in, or adjacent to the surface of the printed board. In some examples, receive arrays  160   a - 160   c  may include may include receive antenna elements that comprise aperture coupled microstrip patches. 
       FIG. 14  is a conceptual and schematic diagram illustrating an example FMCW radar array  152 . FMCW radar array  152  of  FIG. 14  may be an example of FMCW radar arrays  152   a - 152   c  illustrated in  FIG. 13 . As shown in  FIG. 14 , in some examples, FMCW radar array  152  may include a plurality of printed board  172 ,  174 , and  176  disposed substantially parallel to each other and to the front surface of FMCW radar array  152 . In some examples, first printed board  172  may be referred to as a patch layer, and may include antenna elements and radio frequency components. In some examples, second printed board  174  may include digital and frequency synthesizer components, including devices, such as field programmable gate arrays (FPGAs) that control scanning and beamforming on receive. In some examples, third printed board  176  may include power supply components and additional signal processing components, along with an interface for connecting FMCW radar array  152  to other FMCW radar arrays and/or components of the aircraft or device on which FMCW radar array  152  is utilized. In some examples, multiple FMCW radar arrays may be connected to common control electronics, which may control operation of the FMCW radar arrays, including, for example, radar pulse synchronization, scanning frequencies, target tracking, or the like. 
     In some examples, a proposed system is a continuous wave (transmits 100% of the time) at 20 W and uses a total input power for three faces of about 550 W. The top transmit element rows use transmitter parts, while the remaining receive element rows use receive only parts. This may reduce costs by reducing the number of high cost transmit components. 
     The techniques described in this disclosure may also be embodied or encoded in an article of manufacture including a computer-readable storage medium encoded with instructions. Instructions embedded or encoded in an article of manufacture including a computer-readable storage medium encoded, may cause one or more programmable processors, or other processors, to implement one or more of the techniques described herein, such as when instructions included or encoded in the computer-readable storage medium are executed by the one or more processors. Computer readable storage media may include random access memory (RAM), read only memory (ROM), programmable read only memory (PROM), erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), flash memory, a hard disk, a compact disc ROM (CD-ROM), a floppy disk, a cassette, magnetic media, optical media, or other computer readable media. In some examples, an article of manufacture may include one or more computer-readable storage media. 
     In some examples, a computer-readable storage medium may include a non-transitory medium. The term “non-transitory” may indicate that the storage medium is not embodied in a carrier wave or a propagated signal, in certain examples, a non-transitory storage medium may store data that can, over time, change (e.g., in RAM or cache). 
     Various examples have been described. These and other examples are within the scope of the following claims.