Abstract:
An apparatus and associated method for improved angular resolution capability of a remote sensing echo system based on utilizing both a first and a secondary echo of a single transmission signal.

Description:
BACKGROUND 
       [0001]    The angular resolution of a conventional remote sensing echo system depends on the linear extent of the echo transmitting/receiving aperture. A single-point transmitting/receiving element (having a linear extent that is small in comparison with the transmitted wavelength) is essentially omni-directional, and does not provide for resolving the target&#39;s angular direction. 
         [0002]    Using multiple single-point transmitting/receiving elements spatially-arranged in an array provides for angular resolution. 
       SUMMARY 
       [0003]    Embodiments of the present invention provide utilizing secondary round-trip reflections of the transmitted sensor signal from a target. In embodiments of the invention, temporal information from the secondary echoes off the target is interpreted as additional spatial information related to the target. Certain embodiments of the invention provide spatial information (such as angular displacement of the target) which is otherwise not readily available. Other embodiments of the invention provide increased spatial target resolution. A particular embodiment for use with a real sensor array, for example, provides a virtual sensor array having a resolution equivalent to the real array of doubled size. 
         [0004]    Secondary reflections, sometimes referred to as “ghost” reflections, are well-known, and typically are encountered when a portion of the signal reflected by the target undergoes an additional reflection from an incidental object, such as an object in proximity to the signal&#39;s return path. A special case occurs when a portion of the return signal reflects off the transmitter/receiver, returns to the target, and is reflected back again to the transmitter/receiver. That is, a portion of the emitted signal does a double round-trip, in the process undergoing three reflections instead of one. The ghost signal in this case appears to be at twice the distance. The target appears to be moving with a velocity that is twice the velocity as indicated for the target object by the primary reflected signal. 
         [0005]    Embodiments of the invention are applicable to electromagnetic remote sensing echo systems (such as Radar) as well as to acoustic remote sensing echo systems (such as Active Sonar and Lidar). Embodiments of the present invention are illustrated herein in the non-limiting case of automotive radar, but further embodiments can also be applied in other areas as well, including, but not limited to: suitable regimes of radar and electromagnetic spectrum sensing, laser sensors, and optical sensors in both the visible and non-visible portions of the spectrum; and suitable regimes of acoustical sensing, sonar, and ultrasound. 
         [0006]    Therefore, according to an embodiment of the present invention, there is provided a method for measuring an angular direction of an object and a method for separation between closely spaced objects, the method including transmitting a signal; receiving a first echo of the signal from the object; redirecting the first echo back towards the object; receiving a second echo from the object; and computing an angular direction of the object using joint measurements of the first echo and the second echo. 
         [0007]    In addition, according to another embodiment of the present invention, there is provided an apparatus for measuring an angular direction of an object, the apparatus including: a signal transmission element operative to transmit a signal towards the object in the vicinity of the apparatus; a signal reception element operative to receive a first echo of the signal from the object; a signal retransmission device operative to return the first echo towards the object, such that the signal receiver receives a second echo from the object; and a processor operative to compute an angular direction of the object based on measurements of the first echo and the second echo. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The subject matter disclosed may best be understood by reference to the following detailed description when read with the accompanying drawings in which: 
           [0009]      FIG. 1  conceptually illustrates a prior art automotive radar pattern. 
           [0010]      FIG. 2A  through  FIG. 2D  conceptually illustrate azimuthal angular direction measurements by signal transmission and reception utilizing a reflector according to an embodiment of the invention. 
           [0011]      FIG. 3  illustrates a simplified geometry for the examples illustrated in  FIG. 2A  through  FIG. 2D . 
           [0012]      FIG. 4  illustrates timing measurements for the examples illustrated in  FIG. 2A  through  FIG. 2D . 
           [0013]      FIG. 5A  conceptually illustrates a sensor array with a retransmission element displaced orthogonally from the array, according to an embodiment of the invention. 
           [0014]      FIG. 5B  conceptually illustrates a portion of a sensor array with a retransmission element interposed within the array, according to another embodiment of the invention. 
           [0015]      FIG. 6  is a flowchart of a method for angular direction measurements according to an embodiment of the invention. 
           [0016]      FIG. 7  is a block diagram of an apparatus for angular direction measurements according to an embodiment of the invention. 
       
    
    
       [0017]    For simplicity and clarity of illustration, elements shown in the figures are not necessarily drawn to scale, and the dimensions of some elements may be exaggerated relative to other elements. In addition, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. 
       DETAILED DESCRIPTION 
       [0018]      FIG. 1  conceptually illustrates a prior art automotive radar pattern  101  emanating from a vehicle radar system  103  and providing coverage of an object  105  ahead of vehicle  103 . The broad lobe shape of radar pattern  101  is such that the angular direction of object  105  cannot be accurately determined by the radar system to within a meaningful angular accuracy. Moreover when two targets are presented within the small angle, they can not be distinguished or resolved, and thus the radar pattern  101  limits the angular resolution of the automotive radar. At best, the automotive radar system can detect the distance of object  105  and the rough location of object  105  as being in front of vehicle  103 , as opposed to being behind or to the side of vehicle  103 . 
         [0019]      FIG. 2A  through  FIG. 2D  conceptually illustrate angular direction measurements for an object  213  by signal transmission and reception from an angular direction sensor system  201  utilizing a transmission element  203 , a reception element  205 , and a signal retransmission device  207  according to an embodiment of the invention. For example, the sensors of angular direction sensor system  201  may include radar, sonar, or lidar receivers or transceivers. Each sensor measures the time of flight of a signal that was transmitted by a transmitter or transceiver of the array and was received by the sensor. 
         [0020]    In a related embodiment of the invention, signal retransmission device  207  is a passive reflector, such as a corner cube. In another related embodiment, signal transmission device  207  is an active repeater. Any such reflector, repeater, or other device that is capable of returning an incident signal is herein referred to as a retransmission device. Any returning of an incident signal, such reflecting, repeating, or otherwise transmitting a signal that was incident on the retransmission device, is herein interchangeably referred to as retransmission or returning of the signal. 
         [0021]    In  FIG. 2A , a signal  211  is emitted by transmission element  203  at a time t 0    209 , such that signal  211  can reach object  213 . Time t 0    209  is directly measurable and is known to angular direction sensor system  201 . 
         [0022]    In  FIG. 2B , an echo  217  of signal  211  is reflected from object  213  at a time t 1    215 , such that echo  217  can reach reception element  205  and signal retransmission device  207 . Echo  217  arrives at reception element  205  at a time t 2    219 . Time t 2    219  is directly measurable and is known to angular direction sensor system  201 . 
         [0023]    In  FIG. 2C , echo  217  of signal  211  arrives at retransmission device  207  at a time t 3    223 , and retransmission device  207  sends out a retransmission  221  of echo  217  also at time t 3    223 , such that retransmission  221  can reach object  213 . 
         [0024]    In  FIG. 2D , an echo  227  of retransmission  221  is reflected from object  213  at a time t 4    225 , such that echo  227  can reach reception element  205 . Echo  227  arrives at reception element  205  at a time t 5    229 . Time t 5    229  is directly measurable and is known to angular direction sensor system  201 . 
         [0025]      FIG. 3  illustrates a simplified geometry for the examples illustrated in  FIG. 2A  through  FIG. 2D . In this simplified geometry, transmission element  203  and reception element  205  are shown as being in the same point, at a vertex  306  of a triangle  301 . In some embodiments of the invention, transmission element  203  and reception element  205  are combined into a single device (e.g. a transceiver). Object  213  is at the upper vertex of triangle  301 , and retransmission device  207  is at a vertex  308  of triangle  301 . A base  303  of triangle  301  has a length corresponding to the predetermined fixed physical distance between transmission element  203 /reception element  205  and retransmission device  207 , which distance is denoted as h. In various embodiments of the invention, retransmission device  207  is spatially displaced from transmission element  203 /reception element  205  by a predetermined distance, such as h. In other embodiments of the invention, reception element  205  a spatial extent that is larger then the displacement distance h (e.g. includes an array of individual receivers that are spatially separated from one another). For example, if reception element  205  includes N elements with inter-element spacing d, its aperture is d·N&gt;h. Therefore, in general, an angular direction sensor system  201  with retransmission device  207  need not have a larger aperture than a prior art device that lacks a retransmission device. Thus, embodiments of the present invention may be used to increase angular resolution of direction sensing system without increasing the aperture of the system. 
         [0026]    An angular direction a  309  represents the azimuthal angular direction of object  213  relative to transmission element  203 /reception element  205 , i.e., the angular direction of a side  305  of triangle  301 . In another embodiment of the invention, a computed angular direction corresponds to the angular direction of a side  307  of triangle  301 . In certain other embodiments of the invention which perform Doppler shift measurements to compute a speed of object  213  relative to angular direction sensor system  201 , an angular direction of object  213  is an angular component of a vector velocity {right arrow over (v)}  311  of object  213 . 
         [0027]      FIG. 4  illustrates a timing chart  401  for the examples illustrated in  FIG. 2A  through  FIG. 2D . On a time axis  405  are shown the times which are directly measurable and known to angular direction sensor system  201 , namely time t 0    209 , time t 2    219 , and time t 5    229 . An amplitude axis  403  conceptually illustrates the amplitudes of the signals involved. Signal  211 , which is transmitted at time t 0    209  ( FIG. 2A ), is shown as an amplitude  407 . Echo  217 , which is received at time t 2    219  ( FIG. 2B ) is a smaller amplitude  409 . Echo  227 , which is received at a time t 5    229  ( FIG. 2D ) is an even smaller amplitude  411 . 
         [0028]    A time interval  413  between time t o 209 and time t   2    219  (t 2 −t 0 ) corresponds to the time for signal  211  to travel to object  213  ( FIG. 2A ) and then for echo  217  to return ( FIG. 2B ). In the simplified geometry of  FIG. 4 , this is the length of side  305  times two, denoted as 2d 1 . In embodiments of the invention utilizing electromagnetic signals (such as radar or lidar systems), then the time-distance relation is expressed as 
         [0000]    
       
         
           
             
               
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         [0000]    where c is the speed of light. In other embodiments utilizing acoustical signals (such as sonar systems), a similar relationship holds, where the speed of light is replaced with the speed of sound. 
         [0029]    If retransmission  211  were performed by transmission element  203 , corresponding to vertex  306  of triangle  301 , then time interval (t 5 −t 2 ) would correspond to a second round-trip of side  305  of triangle  301  and would be equal to time interval (t 2 −t 0 ), as represented in  FIG. 4  by a time  417  on time axis  405 . As shown in  FIG. 2C , however, retransmission  221  is performed by retransmission device  207 , which is spatially displaced in position from transmission element  203  to vertex  308  of triangle  301 . Thus, in general time t 5    229  is temporally displaced by a time increment/decrement Δt  418  to be a time (t 5 −t 2 )  416 , because of the spatial displacement of retransmission device  207  from transmission element  203 . In effect, t 5    229  is the time for a round-trip of side  305  of triangle  301  plus a round-trip of side  307  of triangle  301 . That is, 
         [0000]    
       
         
           
             
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         [0000]    Letting t 0 =0 without loss of generality, the sides of triangle  301  are thus: 
         [0000]    
       
         
           
             
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         [0030]    Therefore, given the time measurements of t 2  and t 5 , the three sides of triangle  301  are known, and triangle  301  can be solved (such as with the cosine rule) to obtain angular direction α  309 . In other embodiments of the invention, angular direction is measured relative to different points. For example, in one embodiment, angular direction is measured from the midpoint of base  303  of triangle  301 . Transformation from angular direction α  309  to an angular direction relative to another desired reference point is easily performed using standard methods. 
         [0031]    The description and equations above relate to time measurements involving pulse signals. Embodiments of the invention utilizing phase difference measurements are presented and discussed below. 
         [0032]    In  FIG. 4 , time (t 2 −t 0 )  417  and time increment/decrement Δt  418  are shown to emphasize the point that in certain applications (such as automotive radar), both d 1  and d 2  are generally much larger than h, so that d 2 ≈d 1 , and thus increment/decrement Δt  418  is generally small. Thus, echo  227  can be expected to arrive at reception element  205  during a time interval  419 . Accordingly, a signal processor can use a rectangular filter to isolate time interval  419  when measuring t 5  to improve recognition of echo  227 , which may be weaker than echo  217 , particularly if retransmission device  207  is a passive reflector. 
         [0033]      FIG. 5A  schematically illustrates a sensor array  501  having a retransmission element  503  according to an embodiment of the invention. Sensor array  501  includes sensors  501   a,    501   b,    501   c,    501   d,    501   e,    501   f,  and  501   g,  arranged horizontally, so that sensor array  501  can determine an azimuthal angular direction of a target object through known phased-array techniques. In  FIG. 5A , retransmission element  503  is shown in an embodiment as displaced orthogonally from sensor array  501 .  FIG. 5B , discussed below, shows a retransmission element interposed collinearly with the sensor array, such as between a pair of adjacent sensors. 
         [0034]    According to an embodiment of the present invention, retransmission element  503  may include a retroreflector (e.g. a corner reflector). According to another embodiment of the invention, retransmission element  503  includes an active repeater. An active repeater involves greater hardware cost than a passive reflector, but produces a stronger retransmitted signal and therefore results in a stronger received echo. 
         [0035]    In various embodiments of the invention, the transmitted sensor signal is a pulse. In additional embodiments, the transmitted signal is a continuous wave. In further embodiments, the frequency of the transmitted signal is swept, resulting in a “chirp”. Other embodiments of the invention feature different waveforms. Thus, depending on the transmitted waveform, embodiments of the invention perform processing of the received signals with techniques involves time discrimination, frequency discrimination, or both time and frequency discrimination. 
         [0036]      FIG. 5B  conceptually illustrates an embodiment of the invention using differential phase measurements. A portion of an array  505  includes sensors  505   a,    505   b,  and  501   c,  arranged horizontally, so that sensor array  505  can determine an azimuthally angular direction of a target object through known phased-array techniques. Sensor  505   a  is element k−1 of array  505 , sensor  505   b  is element k of array  505 , and sensor  505   c  is element k+1 of array  505 . Elements of array  505  have a constant linear spacing  519 , denoted as Δx. Interposed within array  505  is a retransmission element  507 , located a distance  521  from element k (sensor  505   b ). Distance  521  is denoted as Δy. A target  513  is located at a distance  517 , denoted as R, at an angular displacement  515 , denoted as angle θ. Distance R  517  is large compared to the dimensions of array  505 , so that angle θ 515  and distance R  517  are substantially constant across all sensor elements of array  505 . A wavefront phase retardation  523  is illustrated for element k (sensor  505   b ), for a wavelength λ. 
         [0037]    First echo phase retardation φ 1k  at sensor k  505   b  is given by 
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         [0038]    Second echo phase retardation φ 2k  at sensor k  505   b  is given by 
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         [0039]      FIG. 6  is a flowchart of a method for angular direction measurements according to an embodiment of the invention. In a step  601  a signal  603  is transmitted toward an object  605 . In a step  607  an echo  609  is received from object  605 , and measurements of echo  609  are taken and stored in a measurement storage  621 . In a step  611  a signal  613 , which is a retransmission of received echo  609 , is retransmitted toward object  605 . In a step  615  an echo  617  is received from object  605 , and measurements of echo  617  are taken and stored in measurement storage  621 . In a step  619  measurements of echo  609  and echo  617  are used to compute an angular direction  623 . 
         [0040]    Different embodiments of the invention utilize methods similar to the method illustrated in  FIG. 6 , with adjustments as necessary according to the embodiments. For example, in an embodiment of the invention, retransmitting signal  613  is done by a passive reflector; in another embodiment, retransmitting signal  613  is done by an active repeater. In an embodiment of the invention, angular direction  623  is an azimuthal angle; in another embodiment, angular direction  623  is an elevation angle. In an embodiment of the invention, measurements of echo  609  and echo  617  are time measurements; in another embodiment, measurements of echo  609  and echo  617  are frequency measurements. In an embodiment of the invention, angular direction  623  is of a position vector of object  605 ; in another embodiment, angular direction  623  is of a velocity vector of object  605 . 
         [0041]      FIG. 7  is a block diagram of an apparatus  700  for angular direction measurements according to an embodiment of the invention. A signal transmission element  701  is controlled by a processor  707 , which receives input from a signal reception element  703 . In this embodiment, a signal retransmission element  705  is independent of processor  707 , but is part of apparatus  700 . In another embodiment of the invention, signal retransmission element  705  is an active repeater which receives power from apparatus  700 . In certain embodiments, apparatus  700  includes a clock  709 . In other embodiments, apparatus  700  includes a frequency discriminator  711 . In still other embodiments, apparatus  700  includes a signal processing co-processor  713 . The result of computations by processor  707  is an angular direction  715 .