Abstract:
The beam steering antenna method for unmanned vehicles includes circuits that automatically execute an algorithm in an unmanned vehicle (UV) that enhances the communication link by steering the beam of a patch antenna array to the direction of the maximum received signal strength (RSS) utilizing a received signal strength indicator (RSSI) module. The algorithm can be used on both unmanned ground vehicles (UGV) and unmanned aerial vehicles (UAV). The algorithm was tested through a simulation environment that integrates a virtual feasible aircraft trajectory and an antenna radiation pattern generator. The designed algorithm is simple and fast enough to be executed in real time using a very small hardware platform that can fit inside a small size, low payload vehicle.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to phased antenna arrays and beam steering antennas, and particularly to a beam steering antenna method for unmanned vehicles to steer the antenna beam to improve communication links based on detected RSSI (received signal strength indication) of the other station&#39;s signal. 
         [0003]    2. Description of the Related Art 
         [0004]    Microstrip patch antennas usually comprise just one square or circular metal antenna element attached to a low-loss dielectric substrate. The substrate is mounted on a larger ground plane, which serves as the return path for current induced on the patch element. The microstrip patch antenna performs optimally when it is sized such that the cavity beneath the patch resonates in its fundamental mode (TM 100  or TM 010 ) at the frequency of interest. This occurs when the resonant dimension of the patch is approximately one half-wavelength long within the dielectric substrate. Circularly polarized reception is possible when both the TM 100  and TM 010  modes (transverse magnetic modes) are excited with equal strength, but with a 90° phase shift. Circular polarization is important, since most navigation satellites transmit circularly polarized radiation, and therefore a circularly polarized receive antenna is preferred for optimal system performance. Microstrip patch antennas inherently possess a narrow bandwidth and are suitable for beam steering applications. 
         [0005]    Unmanned aerial vehicles (UAV) are automated vehicles that communicate with ground stations via both a control link and a data link. The control link has a low frequency of operation, and therefore is suitable for long range communications. The data link is used for transmitting sensory data to the ground station, and usually operates at high frequency, e.g., in the 2.45 GHz band. The data link has short range because of its high frequency. It is possible to extend the range of the data link by using a high gain antenna array in the UAV that radiates a directive beam focused towards the ground station. However, this requires that the UAV be able to localize and track the location of the ground station in order to steer the beam and keep the beam directed at the ground station. GPS technology is not adequate for this purpose, since GPS signals can be jammed or suffer from interference. 
         [0006]    Thus, a beam steering antenna method for unmanned vehicles solving the aforementioned problems is desired. 
       SUMMARY OF THE INVENTION 
       [0007]    The beam steering antenna method for unmanned vehicles is an automated method for maintaining a communications link between an unmanned vehicle and a control station. The unmanned vehicle has an array of phased antennas embedded therein and a circuit for making RSSI (received signal strength indication) level measurements connected to the antenna array. The method uses a signal processing circuit to scan signals received from the control station, measuring the RSSI levels in an elliptical pattern around the array according to a search algorithm, and determining the direction of the received signal by adjusting the phase parameters of the antennas. The method maintains the communication link by adjusting the directivity of the antenna array by adjusting the phase of the signals at the antennas. The search algorithm can find the direction of the maximum incoming signal in less than 400 ms. 
         [0008]    These and other features of the present invention will become readily apparent upon further review of the following specification and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a diagrammatic perspective view of a UAV with onboard processor executing a beam steering antenna method for unmanned vehicles according to the present invention. 
           [0010]      FIG. 2  is a top plan view of a 2×6 patch antenna array utilized with the beam steering antenna method for unmanned vehicles according to the present invention. 
           [0011]      FIG. 3  is a 3-dimensional plot of the radiation pattern of a single patch antenna of the array of  FIG. 2 . 
           [0012]      FIG. 4A  is a plot showing the total radiation pattern of the 2×6 patch array of  FIG. 2  at zero steering angles. 
           [0013]      FIG. 4B  is a contour plot showing the total radiation pattern of the 2×6 patch array of  FIG. 2  at zero steering angles. 
           [0014]      FIG. 5A  shows a two-dimensional θ cut of the radiation pattern of the 2×6 patch array of  FIG. 2  at different beam steering angles (θ b =45°, φ b =60°). 
           [0015]      FIG. 5B  shows a two-dimensional θ cut of the radiation pattern of the 2×6 patch array of  FIG. 2  at different beam steering angles (θ b =45°, φ b =60°). 
           [0016]      FIG. 6  shows the search space in a beam steering antenna method for unmanned vehicles according to the present invention, where the contour plot represents the RSS distribution. 
           [0017]      FIG. 7  is a flowchart of the beam steering antenna method for unmanned vehicles according to the present invention, where the initialization step prepares for constructing the ellipse whose edge will contain all the points where an RSS reading will be taken. 
           [0018]      FIG. 8  is a plot showing simulation results for 300 different virtual aircraft trajectories in testing of a beam steering antenna method for unmanned vehicles according to the present invention. 
           [0019]      FIG. 9  is a plot showing the errors in the RSS in testing of the beam steering antenna method for unmanned vehicles according to the present invention. 
       
    
    
       [0020]    Similar reference characters denote corresponding features consistently throughout the attached drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]    The beam steering antenna method for unmanned vehicles includes (as shown in  FIG. 7 ) an initialization step  901 , followed by a construction step  902  that constructs an ellipse  804  whose edge will contain all the points  809  where an RSS (received signal strength) reading needs to be taken and evaluated by the beam steering algorithm. RSS measurements are collected at step  903 . A comparison is performed at step  904 . A termination condition check is performed at step  905 . If the search is to continue, ellipse parameters are updated at step  907  and fed back to be used in the ellipse construction step  902 . If the search is to be terminated, the tracking routine is executed at step  906 . 
         [0022]    The ellipse has 4 parameters that need to be defined before construction. These parameters are r θ   802 , r φ   805 , the ellipse center, and n, where n is the number of points on the ellipse edge. For example, in the example shown in  FIG. 6 , the value of n is 8. These four parameters are defined initially in the initialization block  901  according to the antenna array characteristics. 
         [0023]      FIG. 1  shows an exemplary UAV  104  in its vehicle-centered coordinate system. The antenna array is embedded inside the wing structure  105  so that the patches are facing downwards along the z-axis. The antenna beam  102  is directed and steered for any given spherical steering angles θ b    103  and φ b    101 , where the subscript b is short for beam. The steering angle φ b  can have any value between 0° and 360° while the other steering angle θ b  can only have values between 0° and 180°. 
         [0024]      FIG. 2  shows an example of a designed 2×6 patch antenna array, where its length  203  is 396 mm and its width  204  is 116 mm. The inter element spacing is 30 mm, and the dimensions of the patches  201  are 36×28 mm. The array has 12 patch elements  201  and a common ground plane  202 . These dimensions were made according to a specific UAV size. They can be redesigned to accommodate any vehicle size. 
         [0025]      FIG. 3  shows the radiation pattern of a single patch antenna  301  simulated using an electromagnetic simulation tool. The back lobe  302  is very small compared to the main lobe  303  which is nearly spherical, thus enabling a smooth beam steering capabilities for the array shown in  FIG. 2 . 
         [0026]      FIG. 4A  shows the total radiation pattern of the 2×6 patch array at zero steering angles (θ b =0, φ b =0). The circular axis  401  represents the angle φ and the mesh shading and structure represents the total gain. 
         [0027]      FIG. 4B  shows the same radiation pattern shown in  FIG. 4A  as a contour plot, where the contour lines&#39; shading represents the total gain in dB according to the legend bar. The circular axis  404  represents the angle φ while the concentric circles  402  represent the angle θ. 
         [0028]      FIG. 5A  shows a two-dimensional θ cut of the radiation pattern at different beam steering angles (θ b =45°, φ b =60°). The circular axis  502  represents the angle θ, while φ is fixed at 60°. The concentric circles scale represents the gain in dB. Notice that at these steering angles for the particular cut taken, the antenna produces two different lobes  504  and  501  with nearly the same gain and narrow half power beam width (HPBW). 
         [0029]      FIG. 5B  shows another two-dimensional cut, yet it is a φ cut this time, where the beam steering angles  507  are (θ b =45°, φ b =60°) and θ  506  is fixed at 45°. Notice the presence of an only one main lobe  505  and a wide HPBW. 
         [0030]      FIG. 6  shows the search space, where the contour plot represents the RS S distribution. The search space spans in the φ-axis  806  (φ span ) for 360° and in the θ-axis  807  (θ span ) for 90°. The contour line  801  represents the area of maximum RSS, where the point  803  represents the absolute maximum RSS point, which is the target of the present search algorithm. 
         [0031]    With respect to the ellipse construction step  902  of algorithm  700  ( FIG. 7 ), in order to define the parameters of the first ellipse, the minimum half-power beamwidth “HPBW min ” is selected out of the two values, HPBW φ , and HPBW θ . Then, using the value of HPBW min  in equation (1), the initial value for r is obtained: 
         [0000]    
       
         
           
             
               
                 
                   
                     r 
                     
                       φ 
                       initial 
                     
                   
                   = 
                   
                     
                       
                         1 
                         2 
                       
                        
                       
                         [ 
                         
                           
                             φ 
                             span 
                           
                           - 
                           
                             
                               HPBW 
                               min 
                             
                             2 
                           
                         
                         ] 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0032]    Equation (1) guarantees that the ellipse initially covers the region containing the maximum RSS. 
         [0033]    The value of r θ     initial    is accordingly calculated using equation (2), where this ratio must hold in all the algorithm iterations in order to match the search space at hand: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       r 
                       θ 
                     
                     
                       r 
                       φ 
                     
                   
                   = 
                   
                     
                       
                         θ 
                         span 
                       
                       
                         φ 
                         span 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0034]    The ellipse center is assumed initially at the center of the search space. The last ellipse parameter that requires initialization is n, which is calculated initially according to equation (3). However, it must then be approximated to the nearest highest number divisible by 4. This approximation is done so that the produced points  809  will be symmetric around the ellipse center  808 : 
         [0000]    
       
         
           
             
               
                 
                   
                     n 
                     initial 
                   
                   = 
                   
                     
                       r 
                       
                         φ 
                         initial 
                       
                     
                     
                       
                         1 
                         2 
                       
                        
                       
                         HPBW 
                         min 
                       
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0035]    After the algorithm is initialized at step  901 , the first ellipse points are calculated at step  902 , and an RSS reading is made at step  903  and associated with each point  809  shown in  FIG. 6 . Then, a comparison between all of these values is made at step  904  and the highest RSS point  808  shown in  FIG. 6  is selected to be the center of the next ellipse to be constructed in the next algorithm iteration, if the algorithm wasn&#39;t terminated by the termination condition check made in every iteration at step  905 . 
         [0036]    The termination condition of step  905  compares the RSS recorded from the previous iteration and the RSS produced from the current iteration, and if that difference exceeds 0.4 dB, it proceeds with the next iteration  909 . Otherwise, the improvement is not considered significant enough, and the algorithm is terminated  908 . Another termination condition is constrained by the time consumed by the algorithm, and it is set at 2000 ms as a maximum allowable time for the algorithm. These parameters can be tuned based on the environment and the application at hand. 
         [0037]    If the termination conditions  905  are not satisfied and further iterations are needed  909 , the new ellipse to be constructed will have different parameters, and this is why these parameters are updated  907  before constructing the new ellipse  902 . Ellipse parameters are updated every iteration according to equations (4), (5), and (6) as follows: 
         [0000]        r   φ ( k+ 1)= r   φ ( k )* f   r ,   (4)
 
         [0000]    where k is the iteration counter and f r  represents a reduction factor whose value is in the range [0-1], and it needs to be tuned for better results. In our case, it was (0.8). Then: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       r 
                       θ 
                     
                      
                     
                       ( 
                       
                         k 
                         + 
                         1 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         θ 
                         span 
                       
                       
                         φ 
                         span 
                       
                     
                     * 
                     
                       
                         
                           r 
                           φ 
                         
                          
                         
                           ( 
                           
                             k 
                             + 
                             1 
                           
                           ) 
                         
                       
                       . 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0038]    The center of the new ellipse  808  is the point at which the maximum RSS is obtained in the previous iteration. Then, the number of ellipse points n is updated for the next iteration according to equation (6): 
         [0000]        n ( k+ 1)= n ( k )* f   n ,   (6)
 
         [0000]    where f n  represents another reduction factor whose value can be tuned. In our case it was (0.5). At the end of the search procedure, the algorithm will provide its best estimation for the steering angles θ b  and φ b  that gives the maximum RSS. 
         [0039]    When one of the termination conditions is satisfied, the search process is terminated  908 , and the tracking routine takes over at step  906 . The tracking routine  906  is a simple version of the search algorithm. The tracking routine conducts only one search iteration based on the last recorded values for the ellipse parameters. This is executed every fixed amount of tune according to the wireless communications protocol followed by the transceiver onboard the UAV  104  such that it doesn&#39;t interfere with the data packets being transmitted. If the transceiver is not in the transmitting mode, then the search algorithm can be executed smoothly without interfering with the received data because it needs only the RSS values coming out of the receiver. 
         [0040]    If the RSS degraded dramatically in a short period of time, the tracking routine at step  906  will deduce that the antenna beam  102  has become misaligned with the maximum RSS, i.e., at step  910  the software reports that the beam has lost track of the maximum RSS. Thus, the algorithm initiates another search process to produce more accurate values for the steering angles. 
         [0041]      FIG. 8  shows the simulation results for 300 different virtual aircraft trajectories. For each one of the 300 runs, we have two RSS values. One RSS value  1002  represents the RSS achieved by the algorithm, and the other RSS value  1001  represents the maximum achievable RSS produced by a perfect beam steering. 
         [0042]      FIG. 9  shows the errors in the RSS. This is the difference between the maximum achievable RSS and the RSS achieved by the algorithm. Notice that the average error value is −0.25 dB, and the maximum error  1101  doesn&#39;t exceed −3 dBs. 
         [0043]    The average Time of convergence (TOC) for the 300 runs, i.e., the average time period consumed by the algorithm before producing its output was demonstrated to be approximately 403.3 ms. 
         [0044]    It will be understood that the diagrams in the Figures depicting the beam steering antenna method are exemplary only, and may be embodied in a dedicated electronic device having a microprocessor, microcontroller, digital signal processor, application specific integrated circuit, field programmable gate array, any combination of the aforementioned devices, or other device that combines the functionality of the beam steering method onto a single chip or multiple chips programmed to carry out the method steps described herein, or may be embodied in a general purpose computer having the appropriate peripherals attached thereto and software stored on a non-transitory computer readable media that can be loaded into main memory and executed by a processing unit to carry out the functionality of the inventive apparatus and steps of the inventive method described herein. 
         [0045]    It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.