Abstract:
Systems and methods for improving bearing initialization for a pair of two-element antennas. An exemplary system includes two-element antennas mounted on the bottom and top of an aircraft fuselage, an output device, and a processing device. The processing device receives phase-difference information based on phase of signals received at each element of a two-element antenna, determines if the received phase-difference information is within a predefined low-confidence region, and initializes bearing if the phase-difference information is not within the low-confidence region or the phase-difference information from a predefined number of consecutively received signals meets a predefined consistency requirement.

Description:
BACKGROUND OF THE INVENTION 
     Traffic collision-avoidance systems (TCAS) use two antennas, one on the top and one on the bottom of an aircraft, each of which is used to estimate the relative bearing between one&#39;s own aircraft and an intruder. Each traditional TCAS antenna has four elements, which are placed orthogonally on the same plane, as shown in  FIG. 1 . A TCAS system interrogates the transponder on the other aircraft. The transponders of other aircraft respond with a reply that may contain altitude or other information. The TCAS uses the reply signal and its multi-element directional antenna to estimate the relative bearing of the other aircraft. In some systems, the phase difference of the reply signal received at elements E1 and E3 is proportional to the sine function value of the intruder&#39;s bearing angle, and the phase difference between elements E2 and E4 is proportional to the cosine function value of the intruder&#39;s bearing angle (where the E1/E3 pair and the E2/E4 pair are orthogonal). The system can estimate the bearing from the signals received on one multi-element antenna. Although this is an adequate setup for determining bearing, it is redundant, costly, and comes with a weight penalty because of the two antennas and eight antenna elements that are needed. 
     A bearing calculated from a single reply, squitter, or their combination, might be corrupted at the point of reception by interference (multipath, reflection from airframe or nearby obstacles). Even under stationary settings, such corrupted raw bearings vary to the extent that, on display (if not heavily filtered), it looks like a moving target. However, such movements do not look natural and may decrease confidence in the system. 
     SUMMARY OF THE INVENTION 
     The present invention provides systems and methods for improving bearing initialization for a pair of two-element antennas. An exemplary system includes two-element antennas mounted on the bottom and top of an aircraft fuselage, an output device, and a processing device. The processing device receives phase-difference information based on the phase of signals received at each element of a first of the two-element antennas, determines if the received phase-difference information is within a predefined low-confidence region, and initializes bearing if the phase-difference information is not within the low-confidence region or the phase-difference information from a predefined number of consecutively received signals meets a predefined consistency requirement. 
     The algorithm evaluates the phase from the low-confidence region and it also estimates bearings using the phase from this low-confidence region and evaluates the stability of these bearings as well. When it initializes bearing it requires phase measurements from all four elements—two on the top and two on the bottom. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below, with reference to the following drawings: 
         FIG. 1  illustrates a prior-art antenna setup for a vehicle; 
         FIG. 2-1  is a schematic diagram of an aircraft having a system formed in accordance with an embodiment of the present invention; 
         FIG. 2-2  illustrates an antenna setup for the aircraft shown in  FIG. 2-1 . 
         FIG. 3  is a flow diagram of an exemplary process performed by the system shown in  FIG. 2-1 ; 
         FIG. 4  is a graph of phase/bearing signals associated with the antennas shown in  FIG. 2-2 ; and 
         FIG. 5  is a chart that shows low-confidence regions for one of the antennas of the system in  FIGS. 2-1 ,  2 - 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2-1  illustrates an exemplary aircraft  10  having a traffic collision-avoidance system (TCAS)  20  formed in accordance with an embodiment of the present invention. The TCAS  20  includes a first two-element antenna  24  located on the top of the aircraft  10 , a second two-element antenna  26  on the bottom of the aircraft  10 , and a TCAS processor  30  that is in signal communication with the antennas  24 ,  26 . The processor  30  performs bearing estimation, based on signals received from the antennas  24 ,  26 . The estimated bearing value is sent to another device for output, such as a display device  34 . 
     As shown in  FIG. 2-2 , a first axis that links the two elements E1, E3 of the first antenna  24  may be orthogonal to a second axis that links the two elements E2, E4 of the second antenna  26 . The vertical axis doesn&#39;t necessarily run through the aircraft&#39;s center of gravity. 
       FIG. 3  illustrates a flowchart of an exemplary process  80  performed by the system shown in  FIG. 2-1 . At a decision block  82 , the processor  30  determines if at least one complete set of phases has been received from signals at the top and bottom antenna elements. If a complete set has not been received (from the same time), then, at a block  84 , bearing initialization is deferred until a complete set has been received. If a complete set has been received, then, at a decision block  86 , the processor  30  determines if the phase and/or computed bearing (based on the phase) indicate that the signal (information) is from a low-confidence bearing region. If the phase and/or computed bearing do not indicate the signal is from the low-confidence bearing region, then, at a block  90 , the bearing filter is initialized, using both top and bottom antennas&#39; phase information. If the phase and/or computed bearing do indicate the signal is from the low-confidence bearing region, then, at a decision block  92 , the processor determines if phase-difference information of the bottom antenna is consistent and if the bearing information is consistent. The bearing information is computed from the phase differences from the top and bottom antennas. If the conditions in decision block  92  are not true, then, at a block  98 , bearing filter initialization does not occur because the phase and bearing information is not adequate. If the conditions in decision block  92  are true, then, at a block  96 , bearing-filter initialization occurs using the phase information from the top and bottom antennas. 
     There are other methods for determining consistency, including but not limited to the following methods:
         checking the variation between a group of phase and bearing measurements;   checking the rate of change of a set of phase and bearing estimates; and   checking the variation about the mode, mean, or median about a group of consecutive phase and bearing estimates.       

     Any of the above checking techniques may be used with the low-confidence region method. 
     To Initialize Bearing—there must be at least one bearing with both the top phase measurement and bottom phase measurement. However, while on the ground (or with landing gear extended), the phase measurements from the bottom, when combined with phase measurements from the top antenna, can result in inaccurate bearing—especially in some low-confidence regions. The present invention allows initialization of bearing even when the phase measurements are coming from the bottom antenna in a region where the system cannot rely on one measurement/estimate (low-confidence regions). In these regions—in order to initialize or determine bearing, multiple top/bottom phase measurements are required. If the bottom phase measurements are consistent and the resulting bearing estimates (using the top and bottom phase measurements) are consistent, then the bearing can be initialized (through standard filtering or averaging techniques). 
     As shown in  FIG. 4 , SineComp and CosineComp represent the same data, but for two antennas positioned differently (i.e., top and bottom antennas  24 ,  26 ). The SineComp is for a two-element antenna with dipoles at −45 and 135 degrees (bearing/azimuth-wise relative to the vehicle (i.e., the aircraft  10 ) in which it is mounted). The CosineComp is for a two-element antenna with dipoles at 45 and −135 degrees. Minimum and maximum values of these signals depend on electric phase spacing of the antenna elements. 
     Phase difference vs. bearing graph and chart ( FIGS. 4 and 5 , respectively) indicate that the phase-difference response of the two-element antenna  24  ( 26 ) is not linear and not the same around a 360° circle. At regions where the bearing aligns with the antenna elements, the slope of the phase-difference magnitude is smaller. In these regions, the phase difference changes less with bearing. These regions are considered “dull” regions. At regions whose bearing is not aligned with the antenna elements, phase slope is larger or “sharper” (and almost linear with bearing) with maximum slope sharpness at directions that are 90° to a line connecting antenna elements. The sharp region centers are located at −135 and 45 degree bearing for the SineComp and at −45 and 135 degrees for the CosineComp. Therefore bearing accuracy in the “dull” regions is more sensitive to errors in phase, because a small change in phase results in a large change in bearing. 
     Interference complicates the bearing-filter initialization, especially in the dull regions  110 ,  112 . In actual application, the interference differently impacts top and bottom antennas, with the bottom antenna being impacted to a greater degree. The following explanation is for a case in which the bottom antenna  26  is the SineComp antenna. However, a case in which the bottom antenna is the CosineComp antenna is approached in the same way. 
     Low-confidence regions  110 ,  112  for a SineComp antenna chart,  FIG. 5 , are shown in a divided 360° circle. The maximum phase excursion is not required to be 140 degrees. Phase excursion depends on electrical antenna-element distance and can be anywhere between 0 and 180 degrees. For purposes of this explanation, the antenna with a 140 degree maximum excursion is selected. 
     The low-confidence regions for a SineComp signal are centered about the “dull” regions. Two low-confidence regions  110 ,  112  are shown. One region is bounded by a bearing of −90 to 0 degrees with a corresponding phase difference of 100 to 140 degrees. The other region is bounded by a bearing of 90 to 180 degrees and a phase difference of −140 to −100 degrees. Other factors can be used to determine the extent of the low confidence regions (e.g., aircraft location and empirical data collection on aircraft). 
     Although the low-confidence region can be statically set for all aircraft or a certain aircraft type—it can also be determined dynamically by the system and stored in a table and adjusted over time. In one embodiment, a default low-confidence region is defined and then the system changes the default to be more customized for the own aircraft. For example, “truth bearing” derived from own ship latitude and longitude and heading together with the intruder&#39;s ADS-B/ADS-R/TIS-B reported latitude and longitude can be used to refine the location and extent of the low confidence region. Alternatively, the low confidence region could be defined as part of an installation procedure using test equipment. 
     Under ideal conditions (no interference), in the low-confidence region  110 , bearing (clockwise) is between −90 and 0 degrees (zero being aircraft nose), while its corresponding SineComp antenna-phase difference is between 100 and 140 degrees. For the low-confidence region  112 , bearing (clockwise) is between 90 and 180 degrees, while its corresponding SineComp antenna-phase difference is between −100 and −140 degrees. If a raw bearing (calculated from received phase information (a reply or squitter signal)) is within the low-confidence region  110  or  112  and the corresponding phase difference is within a phase-difference range for the same region, then the raw bearing is considered as a high-confidence raw bearing. 
     In the low-confidence region—the phase has certain limits and so does the bearing. The phase and/or bearing are examined. 
     The bearing filter is initialized when a consistent sequence of high-confidence raw bearings from low confidence region (or bearings from high confidence region) is received. The sequence is consistent when all bearings are within the same low-confidence region  110  or  112 . The sequence length is bounded on the low end by the possibility of incorrect filter initialization and on the high end by the total time required to collect the needed number of signals for initialization. Allowed misses may be used during bearing filter initialization. The miss is a case in which a reply or squitter is received but raw bearing or phase difference information is not available. 
     Bearing initialization occurs when the sequence of raw bearings is outside of the low-confidence regions  110  or  112 . This is because confidence is high that the raw bearing is accurate, for the reasons described above. 
     Once the bearing filter is initialized, it is used to smooth raw bearings and to predict target bearing. Under the interference condition, when the bottom antenna-phase difference is distorted, the bottom antenna-phase difference or raw bearing is not used for bearing tracking. Instead, filter prediction and phase difference from the top antenna  24  are used without any information from the lower antenna  26 . 
     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.