Abstract:
Systems and methods for improving bearing accuracy in a Traffic Collision Avoidance System (TCAS) environment. An interrogation signal is transmitted from an array of antenna elements. A response to the transmitted interrogation signal from a target is received at a first pair of elements of the array. The first pair of elements is separated by at most ½λ of the response signal. A processor determines coarse bearing of the received response. A second pair of elements of the array of antenna elements receives a response to the interrogation signal. The second pair of elements is separated by approximately Nλ of the response signal. N is an integer not equal to zero. A first bearing value to the target is determined based on the determined coarse bearing and the received response at the second pair of elements. The array is mounted on an aircraft or on a ground installation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Currently, Traffic Collision Avoidance System (TCAS) bearing is determined from a TCAS interrogation response signal, by a 4-element antenna array. The elements are spaced equally about the circumference of the antenna array, geometrically at 90-degree intervals. The relative bearing is determined by measuring the phase difference of the response signal between opposite element pairs. 
         [0002]    The length between opposite elements is short (within  1 / 2  wave length of the 1090 MHz XPDR signal), thus the TCAS bearing measurements are vulnerable to noise and susceptible to coupling. Thus, TCAS II (recommends evasive maneuvers) bearing measurement can be inaccurate. Usually, the error is no more than 5 degrees but it can be greater than 30 degrees. The low bearing accuracy introduces a large uncertainty in the TCAS position that increases with range. Thus, a displayed TCAS target symbol can appear to jump, due to these errors/inaccuracies. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention includes systems and methods for improving bearing accuracy in a Traffic Collision Avoidance System (TCAS) environment. In an exemplary method an interrogation signal is transmitted from an array of antenna elements. A response to the transmitted interrogation signal from a target is received at a first pair of elements of the array of antenna elements. The first pair of elements are separated by at most ½λ of the response signal. A processor determines coarse bearing of the received response. A second pair of elements of the array of antenna elements receives a response to the transmitted interrogation signal. The second pair of elements is separated by approximately Nλ of the response signal. N is an integer not equal to zero. A first bearing value for the target is determined based on the determined coarse bearing and the received response at the second pair of elements. 
         [0004]    In other aspects of the invention, the array of antenna elements is mounted on an aircraft or on a ground installation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0006]      FIG. 1  is a schematic diagram of an exemplary system located aboard an aircraft as formed in accordance with the present invention; 
           [0007]      FIG. 2  illustrates antenna configuration for the system shown in  FIG. 1 ; 
           [0008]      FIG. 3  is a flow diagram of an exemplary process performed by the system shown in  FIG. 1 ; 
           [0009]      FIG. 4  is a schematic diagram of an exemplary system located on land formed in accordance with the present invention; and 
           [0010]      FIG. 5  is a flow diagram of an exemplary process performed by the system shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0011]      FIG. 1  illustrates an aircraft  20  that includes a traffic collision and avoidance system (TCAS) formed in accordance with an embodiment of the present invention. The TCAS includes a TCAS processor  24  that is in signal communication with a RF Module  26 , a display  28  (or other output device), and other aircraft systems  34 , such as an air data system (ADS), a flight management system (FMS) or a global positioning system (GPS). The RF Module  26  is attached to a first four-element antenna  30  that is located on top of the aircraft  20  and a second four-element antenna  32  that is located on the bottom of the aircraft  20 . 
         [0012]    In one embodiment, the aircraft  20  includes only one of the two four-element antennas  30 ,  32 . The TCAS processor  24  reduces error in a more accurate bearing value produced by a first pair of the four-element antenna using information associated with a coarse bearing value determined by the other three elements (two pairs) of the four element antenna  30 ,  32 . Thus, a highly accurate bearing is produced using only a single four-element antenna  30 ,  32 . Further errors may be reduced by using both top and bottom four-element antennas  30 ,  32 . This will be described in more detail below. 
         [0013]      FIG. 2  illustrates a sample configuration of the top and bottom four-element antennas  30 ,  32 . All of the elements of both antennas  30 ,  32  are attached to RF Module  26 , which is in communication with the TCAS processor  24 . 
         [0014]    The spacing between top antenna elements E T1 , E T2  and E T1 , E T4  is identical (½λ or less). λ is the wavelength of XPDR response signal carrier in 1090 MHz. Lines connecting E T1 , E T2  and E T1 , E T4  are perpendicular to each other. In one embodiment, the elements E T1 , E T2,  and E T4  are integrated into one directional antenna and E T3  is an omniblade antenna. E T3  is located Nλ spacing to E T1 , and has the same spacing to E T2  and E T4 . 
         [0015]    The bottom antenna elements have the similar setup with the top elements, and with axes determined by E T1 , E T3  and E B1 , E B3  that are perpendicular to each other. 
         [0016]    In this configuration, frequency drift introduced error, and elevation angle error contributed by attitude, range and altitude errors can be removed from fine bearing by utilizing perpendicular unambiguous determination of φ T13  and φ B13 . 
         [0017]      FIG. 3  shows a flowchart of an exemplary process  80  performed by the system shown in  FIG. 1  using the antenna configuration shown in  FIG. 2 . First, at a block  84 , an interrogation signal is transmitted from any one of the antenna elements. At a block  86 , a response to the interrogation signal is received at the antenna elements. Next, at a block  88 , a coarse bearing value determined based on the received response at a first subset of the antenna elements. The coarse bearing value is derived from the phase difference values of the first subset elements. At a block  92 , a phase difference of a second subset of the antenna elements is determined based on the received response to the interrogation signal and the coarse bearing value. Finally, at a block  94 , a fine bearing value is determined based on the determined phase of the second subset of antenna elements. 
         [0018]    The idea for improving the TCAS bearing measurement is to modify the existing directional antenna by extending the distance between one pair of its elements (E B1  and E B3 ; E T1  and E T3 ) with one antenna being a directional antenna and one being an omni-blade non-directional antenna. 
         [0019]    E B1  and E B2  (or E T1  and E T2 ) determine phase deviation in sine component φ 12 =K sin(β); E B1  and E B4  (or E T1  and E T4 ) determine phase deviation in cosine component φ 14 =K cos(β); a coarse bearing can thus be determined, β=tan −1 (K sin(β)/K cos(β)). U.S. Pat. No. 5,122,808 discloses similar bearing determinations and is hereby incorporated by reference. 
         [0020]    By example, E B1  and E B3  measure phase difference φ 13m  ∈ (−π, π). When considering the solutions in the first bearing quadrant (−π/4, π/4), the actual phase difference can be, 
         [0000]      φ 13 ={2 πn+φ   13m , (2 n+ 1)π+φ 13m },
 
         [0021]    n=0, 1, 2 . . . N−1. 
         [0022]    N is the number of wavelengths between E 1  and E 3 . 
         [0023]    Within the plane of the aircraft  20 , the fine bearing resolutions determined by φ 13  can have very high accuracy and the final unambiguity is provided by the coarse bearing value. The same method can apply to the other three quadrants. 
         [0024]    The following equations are simplified for fine bearing determination when the target aircraft and antenna array are on the same plane (elevation angle=0). 
         [0025]    For bottom antenna, 
         [0000]      β=π/4−sin −1 (φ 13 /2 πN ) when coarse bearing falls in (−π/4, 3π/4),
 
         [0000]      β=5π/4+sin −1 (φ 13 /2 πN ) when coarse bearing falls in (3π/4, π),
 
         [0000]      β=−3π/4+sin −1 (φ 13 /2 πN ) when coarse bearing falls in (−π, −π/4).
 
         [0026]    For top antenna, 
         [0000]      β=−π/4+sin −1 (φ 13 /2 πN ) when coarse bearing falls in (−3π/4, π/4),
 
         [0000]      β=3π/4−sin −1 (φ 13 /2 πN ) when coarse bearing falls in (π/4, π),
 
         [0000]      β=−5π/4−sin −1 (φ 13 /2 πN ) when coarse bearing falls in (−π, −3π/4).
 
         [0027]    In practice, elevation angle should be involved in the calculation of fine bearing, if the fine bearing determination process only associate to top or bottom antenna alone. 
         [0028]    If φ T1T3  or (φ B1B3 ) is available at the same time (through dual interrogation or from different interrogation cycles spaced by short interval), fine bearing can be further determined with φ B1B3  and φ T1T3 , and elevation angle is no longer involved in the solution. 
         [0000]      β=π/4−tan −1  (φ B1B3 /φ T1T3 ) where coarse bearing∈ (−π/4, 3π/4),
 
         [0000]      β=5π/4−tan −1 (φ B1B3 /φ T1T3 ) where coarse bearings∈ (3π/4, π),
 
         [0000]      β=−3π/4−tan −1  (φ B1B3 /φ T1T3 ) where coarse bearing∈ (−π, −π/4).
 
         [0029]    Target elevation angle e=f(α, β, γ, r, Δalt),
       where α is the pitch angle of own aircraft,   β is the roll angle of own aircraft,   γ is the coarse bearing target to own,   r is the range target to own,   Δalt is the altitude difference between target to ownship.       
 
         [0035]    Other methods for determining fine bearing, such as by calculating every possible fine bearing by φ T1T3  or φ B1B3  or both, correlating the fine bearings with coarse bearing and finally determining the fine bearing. 
         [0036]    In one embodiment, a TCAS  110  ( FIG. 4 ) is used to replace expensive secondary surveillance radar (SSR) for small airports or as a backup surveillance solution at larger airports. The TCAS  110  provides improved fine bearing outputs. 
         [0037]    The TCAS  110  includes a processor  112  that is in data communication with an RF Module  114  and a display  116  or other output device. The RF Module  114  is connected to all the elements of a four-element inner antenna array  118  and four-element outer antenna array  120 . 
         [0038]    The outer antenna array  120  includes four omniblade (or comparable) antenna elements E O1-4  that surround the inner antenna array  118  on the same plane. The inner antenna array  118  includes elements E I1-4 . Spaces between the antenna elements E O1  and E O3 , E O2  and E O4  are Nλ. The axes determined by the elements E O1  and E O3 , E O2  and E O4  are perpendicular to each other. The inner antenna array  118  may be a conventional TCAS directional finding antenna. 
         [0039]      FIG. 5  shows an exemplary process  130  performed by the TCAS  110  shown in  FIG. 4 . First, at a block  134 , an interrogation signal is transmitted from the inner antenna array  118 . The interrogation signal is preferably transmitted from all 4 elements of the inner antenna array  118  for the purpose of directional interrogation capability. 
         [0040]    At a block  136 , a response to the interrogation signal is received at the first set of antenna elements. Next, at a block  138 , a coarse bearing value is determined based on the received response at the inner antenna array  118 . Then, at block  142 , an interrogation signal is transmitted from the inner antenna array  120 . At a block  144 , a response to the interrogation signal is received at the outer antenna array  120 . Finally, at a block  150 , a fine bearing value is determined (disambiguated) from the response received at the outer antenna array  120  based on the coarse bearing value associated with the inner antenna array  118 . 
         [0041]    In another embodiment, the fine bearing is determined by calculating every possible fine bearing by φ B1B3  and φ B2B4 , correlating the fine bearings with coarse bearing and finally determining the fine bearing. Also, the phase measurement pairs are not limited to φ O1O3  and φ O2B4 , φ O1O4  and φ O1O2 , E O2O1  and E O2O3 , E O2O3  and E O4O3 , and E O1O4  and E O3O4  can also be used for calculation. Higher precision of antenna mounting and less deformation for ground installation will further elevate the accuracy and alleviate coupling bias. 
         [0042]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.