Patent Publication Number: US-6906665-B1

Title: Cluster beam-forming system and method

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
   This application claims priority to U.S. Provisional No. 60/426,485 filed Nov. 15, 2002, which is incorporated by reference herein. 

   BACKGROUND OF THE INVENTION 
   The present invention relates in general to detecting objects and/or areas. More particularly, the invention provides a method and system for cluster beam-forming. Merely by way of example, the invention is described as it applies to a phased array antenna, but it should be recognized that the invention has a broader range of applicability. 
   A phased array antenna has been widely used for communications and radar systems. The phased array antenna usually does not mechanically steer antenna directions, and can provide rapid beam scanning. The phased array antenna can also direct transmission power to an intended target and thereby reduce power loss. The directivity of the phase array antenna can be achieved by properly adjusting the relative phases between signals transmitted or received by different antenna elements. These antenna elements can reinforce the transmitted or received radiation in a desired direction. 
   The phased array antenna usually has a number of subarrays, and each subarray includes multiple antenna elements. For example, a phased array antenna has one hundred and twenty-eight subarrays, and each subarray includes eight antenna elements. Consequently, the phased array antenna has one thousand and twenty-four antenna elements. These antenna elements occupy an area called a phased array antenna panel. The beam pattern of a panel depends on two other beam patterns, namely, the subarray beam pattern and the panel beam pattern. The subarray beam pattern relates to an individual subarray, and the panel beam pattern relates to an array of subarrays. 
     FIG. 1  is a simplified diagram for subarrays of antenna elements and an array of subarrays for a conventional phased array antenna. A phased array antenna  100  includes subarray beam forming systems  110 ,  112 , and  114 , and a panel beam forming system  120 . Each subarray beam forming system  110 ,  112 , or  114  includes a subarray of antenna elements and forms a subarray beam pattern. The panel beam forming system  120  forms a panel beam pattern. The subarray beam patterns and the panel beam pattern determine the beam pattern of the phased array antenna  100 . Each subarray beam forming system includes various electronic components and can electronically steer the reception or transmission direction of the subarray. 
   The phased array antenna system usually has the same number of beam-formers within a subarray as the number of beam-formers within a panel antenna, which is also the number of beams provided by the phased array antenna. Each beam of the phased array antenna system is provided by a designated panel antenna beam-former, which is fed by a set of designated subarray beam-formers. The designated subarray beam forming and panel beam forming systems arc usually set to the same reception or transmission direction to produce a single beam. This approach of having a complete set of designated subarray and panel beam formers point to the same direction to provide a single beam usually requires multiple sets of such antenna beam forming systems to produce multiple electronically scanned beams. 
   Hence it is highly desirable to improve beam-forming techniques. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates in general to detecting objects and/or areas. More particularly, the invention provides a method and system for cluster beam-forming. Merely by way of example, the invention is described as it applies to a phased array antenna, but it should be recognized that the invention has a broader range of applicability. 
   According to a specific embodiment of the present invention, a method for detecting a plurality of objects includes steering a first beam-forming system to a first direction. The first direction is associated with a first object. Additionally, the method includes steering a second beam-forming system to a second direction. The second direction is associated with a second object. Moreover, the method includes receiving a first plurality of signals from at least the first object and the second object at the first beam-forming system, receiving a second plurality of signals from at least the first object and the second object at the second beam-forming system, processing the first plurality of received signals, and processing the second plurality of received signals. Also, the method includes generating a first combined signal based on at least information associated with the first plurality of received signals, and generating a second combined signal based on at least information associated with the second plurality of received signals. Additionally, the method includes dividing the first combined signal into at least a first divided signal and a second divided signal, and dividing the second combined signal into at least a third divided signal and a fourth divided signal. Moreover, the method includes processing at least the first divided signal, the second divided signal, the third divided signal, and the fourth divided signal. Also, the method includes generating a first output signal based on at least information associated the first divided signal and the third divided signal, and generating a second output signal based on at least information associated with the second divided signal and the fourth divided signal. The first output signal is associated with the first object, and the second output signal is associated with the second object. 
   According to another embodiment of the present invention, a method for detecting a plurality of objects includes steering a first beam-forming system to a first direction. The first direction is associated with a first object. Additionally, the method includes steering a second beam-forming system to a second direction. The second direction is associated with a second object. Moreover, the method includes receiving a first plurality of signals from at least the first object and the second object at the first beam-forming system, and receiving a second plurality of signals from at least the first object and the second object at the second beam-forming system. Also, the method includes generating a first combined signal based on at least information associated with the first plurality of received signals, and generating a second combined signal based on at least information associated with the second plurality of received signals. Additionally, the method includes dividing the first combined signal into at least a first divided signal and a second divided signal, and dividing the second combined signal into at least a third divided signal and a fourth divided signal. Moreover, the method includes generating a first output signal based on at least information associated with the first divided signal and the third divided signal, and generating a second output signal based on at least information associated with the second divided signal and the fourth divided signal. The steering a first beam-forming system to a first direction is substantially free from changing a first phase center of the first beam-forming system. The steering a second beam-forming system to a second direction is substantially free from changing a second phase center of the second beam-forming system. The first output signal is associated with the first object, and the second output signal is associated with the second object. 
   According to yet another embodiment of the present invention, a system for detecting a plurality of objects includes a first beam-forming system configured to steer to a first direction, receive a first plurality of signals from at least the first object and a second object, and generate a first combined signal based on at least information associated with the first plurality of received signals. The first direction is associated with a first object. Additionally, the system includes a second beam-forming system configured to steer to a second direction, receive a second plurality of signals from at least the first object and the second object, and generate a second combined signal based on at least information associated with the second plurality of received signals. The second direction is associated with the second object. Moreover, the system includes a first divider system configured to divide the first combined signal into at least a first divided signal and a second divided signal, and a second divider system configured to divide the second combined signal into at least a third divided signal and a fourth divided signal. Also, the system includes a first phase shifter, a second phase shifter, a third phase shifter and a fourth phase shifter configured to process at least the first divided signal, the second divided signal, the third divided signal, and the fourth divided signal respectively. Additionally, the system includes a first combiner system configured to generate a first output signal based on at least information associated the first divided signal and the third divided signal, and a second combiner system configured to generate a second output signal based on at least information associated with the second divided signal and the fourth divided signal. The first output signal is associated with the first object, and the second output signal is associated with the second object. 
   According to yet another embodiment of the present invention, a system for detecting a plurality of objects comprises a first beam-forming system configured to steer to a first direction, receive a first plurality of signals from at least the first object and a second object, generate a first combined signal based on at least information associated with the first plurality of received signals. The first direction is associated with a first object. Additionally, the system includes a second beam-forming system configured to steer to a second direction, receive a second plurality of signals from at least the first object and the second object, and generate a second combined signal based on at least information associated with the second plurality of received signals. The second direction is associated with the second object. Moreover, the system includes a first divider system configured to divide the first combined signal into at least a first divided signal and a second divided signal, and a second divider system configured to divide the second combined signal into at least a third divided signal and a fourth divided signal. Also, the system includes a first combiner system configured to generate a first output signal based on at least information associated the first divided signal and the third divided signal, and a second combiner system configured to generate a second output signal based on at least information associated with the second divided signal and the fourth divided signal. The first beam-forming system is configured to steer electronically without substantially changing a first phase center of the first beam-forming system. The second beam-forming system is configured to steer electronically without substantially changing a second phase center of the second beam-forming system. The first output signal is associated with the first object, and the second output signal is associated with the second object. 
   According to yet another embodiment of the present invention, a method for detecting a change of a phase center includes steering a first antenna to a first direction associated with a first phase center corresponding to the first direction, sending a first signal from a signal source to the first antenna, and receiving the first signal at the first antenna associated with the first direction. Additionally, the method includes steering the first antenna to a second direction associated with a second phase center corresponding to the second direction, sending a second signal from the signal source to the first antenna, and receiving the second signal at the first antenna associated with the second direction. Moreover, the method includes determining a first phase based on at least information associated with the first sent and received signal, determining a second phase based on at least information associated with the second sent and received signal, processing at least information associated with the first phase and the second phase, and determining whether the first phase center is the same as the second phase center based on at least information associated with the first phase and the second phase. 
   According to yet another embodiment of the present invention, a system for detecting a change of a phase center includes a first antenna configured to steer to a first direction and a second direction and receive a first signal and a second signal. The first direction is associated with a first phase center of the first antenna, and the second direction is associated with a second phase center of the first antenna. Additionally, the system includes a signal source configured to send the first signal to the first antenna associated with the first direction and send the second signal to the first antenna associated with the second direction. Moreover, the system includes a processing system configured to determine a first phase based on at least information associated with the first sent and received signal, determine a second phase based on at least information associated with the second sent and received signal, process at least information associated with the first phase and the second phase, and determine whether the first phase center is the same as the second phase center based on at least information associated with the first phase and the second phase. 
   According to yet another embodiment of the present invention, a method for detecting a plurality of objects includes receiving a first input signal, receiving a second input signal, generating a first divided signal and a second divided signal based on at least information associated with the first input signal, and generating a third divided signal and a fourth divided signal based on at least information associated with the second input signal. Additionally, the method includes processing at least the first divided signal, the second divided signal, the third divided signal, and the fourth divided signal. Moreover, the method includes combining at least the first divided signal and the third divided signal into a first combined signal, combining at least the second divided signal and the fourth divided signal into a second combined signal, generating a first plurality of signals based on at least information associated with the first combined signal, and generating a second plurality of signals based on at least information associated with the second combined signal. Also the method includes steering a first beam-forming system to a first direction. The first direction associated with a first object. Additionally, the method includes steering a second beam-forming system to a second direction. The second direction associated with a second object. Moreover, the method includes processing the first plurality of signals, processing the second plurality of signals, transmitting the first plurality of signals to at least the first object and the second object at the first beam-forming system, and transmitting the second plurality of signals to at least the first object and the second object at the second beam-forming system. The first input signal is associated with the first object, and the second input signal is associated with the second object. 
   According to yet another embodiment of the present invention, a system for detecting a plurality of objects includes a first divider system configured to receive a first input signal and generate a first divided signal and a second divided signal based on at least information associated with the first input signal, and a second divider system configured to receive a second input signal and generate a third divided signal and a fourth divided signal based on at least information associated with the second input signal. Additionally, the system includes a first phase shifter, a second phase shifter, a third phase shifter and a fourth phase shifter configured to process at least the first divided signal, the second divided signal, the third divided signal, and the fourth divided signal respectively. Moreover, the system includes a first combiner system configured to combining at least the first divided signal and the third divided signal into a first combined signal, and a second combiner system configured to combine at least the second divided signal and the fourth divided signal into a second combined signal. Also the system includes a first beam-forming system configured to generate a first plurality of signals based on at least information associated with the first combined signal, steer to a first direction associated with a first object, and transmit the first plurality of signals to at least the first object and a second object at the first beam-forming system. Additionally, the system includes a second beam-forming system configured to generate a second plurality of signals based on at least information associated with the second combined signal, steer to a second direction associated with the second object, and transmit the second plurality of signals to at least the first object and the second object at the second beam-forming system. The first input signal is associated with the first object, and the second input signal is associated with the second object. 
   Many benefits may be achieved by way of the present invention over conventional techniques. For example, certain embodiments of the present invention form a plurality of beams using one set of subarray outputs. The beam width of a subarray is usually broader in comparison to that of a beam formed using a plurality of subarrays. These embodiments of the present invention can steer several narrow beams within a region defined by the beam pattern of a subarray. Some embodiments of the present invention reduce hardware complexity. The reduction becomes increasingly pronounced with increasing number of subarrays and system level beams. For example, the reduction of the total number of components and cables in a phased array antenna system that produces 64 beams could be more than 80%. Certain embodiments of the present invention significantly reduce cost and power consumption of a phase array antenna system. 
   Depending upon the embodiment under consideration, one or more of these benefits may be achieved. These benefits and various additional objects, features and advantages of the present invention can be fully appreciated with reference to the detailed description and accompanying drawings that follow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a simplified diagram for subarrays of antenna elements and an array of subarrays for an conventional phased array antenna; 
       FIG. 2  is a simplified diagram for a cluster beam-forming system according to one embodiment of the present invention; 
       FIG. 3  is a simplified diagram for a subarray beam forming system in the cluster beam-forming system according to an embodiment of the present invention; 
       FIG. 4  is a simplified block diagram for a cluster beam-forming method according to one embodiment of the present invention; 
       FIG. 4A  is a simplified block diagram for a cluster beam-forming method according to another embodiment of the present invention. 
       FIG. 5  is a simplified diagram for a phase center verification system according to one embodiment of the present invention; 
       FIG. 6  is a simplified diagram for measured phase as a function of scan angle according to one embodiment of the present invention; 
       FIG. 7  is a simplified diagram for a cluster beam-forming system according to another embodiment of the present invention; 
       FIG. 8  is a simplified diagram for tracking multiple aircrafts according to one embodiment of the present invention; 
       FIG. 9  is a simplified diagram for tracking multiple aircrafts according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention relates in general to detecting objects and/or areas. More particularly, the invention provides a method and system for cluster beam-forming. Merely by way of example, the invention is described as it applies to a phased array antenna, but it should be recognized that the invention has a broader range of applicability. 
     FIG. 2  is a simplified diagram for a cluster beam-forming system according to one embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. A cluster beam-forming system  200  includes subarray beam-forming systems  210 ,  212 , and  214 , and a panel beam forming system  220 . Although the above has been shown using systems  210 ,  212 ,  214 , and  220 , there can be many alternatives, modifications, and variations. For example, some of the systems may be expanded and/or combined. Additional subarray beam forming systems may be added to the cluster beam-forming system  200 . Other systems may be inserted to those noted above. Depending upon the embodiment, the specific systems may be replaced. Further details of these systems are found throughout the present specification and more particularly below. The cluster beam-forming system  200  may be used to transmit signals, receive signals, or transmit and receive signals. 
     FIG. 3  is a simplified diagram for a subarray beam forming system in the cluster beam-forming system according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. An subarray beam forming system  300  includes antenna elements  310 , phase shifters  320 , and a signal combiner and divider system  330 . Each of the phase shifter  320  can provide a phase change ranging from −180° to 180°. The signal combiner and divider system  330  generates or receives a subarray output or input  340 . The subarray beam forming system  300  may also include low-noise amplifiers and a command and control system. The subarray beam forming system  300  can be the subarray beam forming system  210 ,  212 , or  214 . The subarray output or input  340  is applied to or received from the panel beam-forming system  220 . The subarray beam forming system  300  can electronically steer the reception or transmission direction of the subarray without changing physical orientation of the subarray beam forming system. The subarray beam forming system  300  may be used to transmit signals, receive signals, or transmit and receive signals. 
   The panel beam-forming system  220  includes signal combiner and divider systems  230 ,  240 , and  250 , as shown in FIG.  2 . Each of the signal combiner and divider systems  230 ,  240 , and  250  divides one of the outputs  238 ,  248 , and  258  into three signals or combines three signals into one of the outputs  238 ,  248 , and  258 . The outputs  238 ,  248 , and  258  are generated or received by the subarray beam forming system  210 ,  212 , and  214  respectively. The three signals from or to the signal combiner and divider system  230  are received or output by phase shifters  232 ,  234 , and  236  respectively. Similarly, the three signals from or to the signal combiner and divider system  240  are received or output by phase shifters  242 ,  244 , and  246  respectively. The three signals from or to the signal combiner and divider system  250  are received or output by phase shifters  252 ,  254 , and  256  respectively. The phase shifters  232 ,  242 , and  252  send or receive signals to or from a signal combiner and divider system  260  respectively; the phase shifters  234 ,  244 , and  254  send or receive signals to or from a signal combiner and divider system  270 ; and the phase shifters  236 ,  246 , and  256  send or receive signals to or from a signal combiner and divider system  280 . The signal combiner and divider systems  260 ,  270 , and  280  generate or receive beams  262 ,  272 , and  282  respectively. These beams are the electronically scanned beams. 
   As discussed above and further emphasized here,  FIG. 2  is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the signal combiner and divider systems  230 ,  240 , or  250  can divide one signal into a number of signals other than three, or combine a number of signals other than three into one signal. Accordingly, additional signal combiner and divider systems may be added to generate or receive beams in addition to the beams  262 ,  272 , and  282 , or some of the signal combiner and divider system s  230 ,  240 , and  250  may be removed. In  FIG. 2 , the cluster beam forming system uses an one-dimensional array of antenna elements, but the cluster beam forming system can also use two-dimensional array of antenna elements. As another example, the cluster beam forming system may include a subarray beam forming system capable of forming multiple beams. 
     FIG. 4  is a simplified block diagram for a cluster beam-forming method according to one embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. A cluster beam-forming method  400  includes a process  410  for steering subarray beam-forming systems, a process  420  for receiving signals from targets, a process  430  for processing received signals, a process  440  for dividing processed signals, a process  450  for processing divided signals, and a process  460  for generating output beams. Although the above has been shown using a selected sequence of processes, there can be many alternatives, modifications, and variations. For example, some of the processes may be expanded and/or combined. Other processes may be inserted to those noted above. Depending upon the embodiment, the specific sequence of steps may be interchanged with others replaced. Further details of these elements are found throughout the present specification and more particularly below. 
   At the process  410 , the subarray beam-forming systems  210 ,  212 , and  214  are each electronically steered to their respective directions. Each of these directions can be described by an azimuth angle and an elevation angle. For example, the azimuth angle is the horizontal angular separation measured clockwise from the north. For example, East has an azimuth angle of 90 degrees. The elevation angle is the angle in degrees above the horizon. For example, 0 degree corresponds to a direction parallel to the horizon and 90 degrees corresponds to a direction straight up. The directions for the subarray beam-forming systems  210 ,  212 , and  214  can be set independently of each other, and they may be different or the same. For example, the subarray bean-forming systems  210 ,  212 , and  214  track targets A, B, and C respectively. 
   The direction of a subarray beam-forming system is determined in part by the phase states of the phase shifters  320 . The phase states of the phase shifters  320  depend on a set of voltages. The relationship between the set of voltages and the direction is described in a beam-forming calibration table. The beam-forming calibration table is a matrix of numbers, where the first two elements of a row of numbers indicate the azimuth angle and the elevation angle of the beam that should be pointing, and the rest of the elements in that row specify the voltages required to determine the phase states of all the associated phase shifters to cause the beam to point properly. The method and system to implement the circuits to electronically scan the beam of a phased array antenna is well known. 
   At the process  420 , the subarray beam-forming systems  210 ,  212 , and  214  receive signals from various targets. For example, the subarray beam-forming systems  210 ,  212 , and  214  each receive signals from the targets A, B, and C, even though they are each steered towards only one of the targets A, B, and C. 
   At the process  430 , the received signals are processed by the subarray beam-forming systems  210 ,  212 , and  214 . The processing usually includes performing phase shifts on signals received by various antenna elements. For example, the subarray beam forming system  210  is steered to the target A and receives signals from the targets A, B, and C. The received signals from various antenna elements  310  are delayed with respect to each other by the phase shifters  320 , and the phase shifts substantially maximize the sum of the signals received at various antenna elements with respect to target A, depending on the electronic steering of the same subarray beam-forming system. 
   At the process  440 , the processed signals from the subarray beam-forming systems  210 ,  212 , and  214  are each divided into several signals. For example, the processed signal from the system  210  is divided into three signals by the signal combiner and divider system  230 , and these three signals are sent to the phase shifters  232 ,  234 , and  236  respectively. Similarly, the processed signal from the system  212  is divided into three signals sent to the phase shifters  242 ,  244 , and  246 . The processed signal from the system  214  is divided into three signals sent to the phase shifters  252 ,  254 , and  256 . 
   At the process  450 , the divided signals are processed by the panel beam-forming system  220 . The processing usually includes performing phase shifts on signals received by various subarray beam forming systems. For example, the phase shifters  232 ,  242 , and  252  change the relative phases between the signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals received by the subarrays from, for example, target A. Similarly, the phase shifters  234 ,  244 , and  254  change the relative phases between the signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals received by the subarrays from, for example, target B. The phase shifters  236 ,  246 , and  256  change the relative phases between the signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals received by the subarrays from, for example, target C. 
   At the process  460 , output beams are generated by the signal combiner and divider systems  260 ,  270 , and  280 . For example, the signal combiner and divider system  260  receives the processed signals from the phase shifters  232 ,  242 , and  252 , and generates the output beam  262 . Similarly, the signal combiner and divider system  270  receives the processed signals from the phase shifters  234 ,  244 , and  254 , and generates the output beam  272 . The signal combiner and divider system  280  receives the processed signals from the phase shifters  236 ,  246 , and  256 , and generates the output beam  282 . The output beams  262 ,  272  and  282  can be independently scanned to any point within the subarray beam pattern. For example, the output beams  262 ,  272 , and  282  corresponds to the targets A, B, and C respectively. 
     FIG. 4A  is a simplified block diagram for a cluster beam-forming method according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. A cluster beam-forming method  480  includes a process  482  for generating output signals, a process  484  for processing divided signals, a process  486  for combining processed signals, a process  488  for steering subarray beam-forming systems, a process  490  for processing combined signals, and a process  492  for transmitting signals to targets. Although the above has been shown using a selected sequence of processes, there can be many alternatives, modifications, and variations. For example, some of the processes may be expanded and/or combined. Other processes may be inserted to those noted above. Depending upon the embodiment, the specific sequence of steps may be interchanged with others replaced. Further details of these elements are found throughout the present specification and more particularly below. 
   At the process  482 , output signals are generated by the signal combiner and divider systems  260 ,  270 , and  280 . For example, the signal combiner and divider system  260  receives the beam  262  and generate three output signals to the phase shifters  232 ,  242 , and  252  respectively. Similarly, the signal combiner and divider system  270  receives the beam  272  and generate three output signals to the phase shifters  234 ,  244 , and  254  respectively. The signal combiner and divider system  280  receives the beam  282  and generate three output beams to the phase shifters  236 ,  246 , and  256  respectively. The beams  262 ,  272  and  282  can be independently scanned to any point within the subarray beam pattern. For example, the beams  262 ,  272 , and  282  corresponds to the targets A, B, and C respectively. 
   At the process  484 , the output signals are processed by the panel beam-forming system  220 . The processing usually includes performing phase shifts on signals received by various phase shifters. For example, the phase shifters  232 ,  242 , and  252  change the relative phases between the output signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals to be transmitted to, for example, target A. Similarly, the phase shifters  234 ,  244 , and  254  change the relative phases between the output signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals to be transmitted to, for example, target B. The phase shifters  236 ,  246 , and  256  change the relative phases between the output signals sent to these phase shifters and these phase shifters provide the appropriate phase delays to maximize the sum of the signals to be transmitted to, for example, target C. 
   At the process  486 , the processed signals from the phase shifters are combined. For example, the processed signals from the phase shifters  232 ,  234  and  236  are combined into one signal by the signal combiner and divider system  230 , and this signal is sent to the system  210 . Similarly, the processed signals from the phase shifters  242 ,  244 , and  246  are combined into one signal to the system  212 . The processed signals from the phase shifters  252 ,  254 , and  256  are combined into one signal to the system  214 . 
   At the process  488 , the subarray beam-forming systems  210 ,  212 , and  214  are each electronically steered to their respective directions. Each of these directions can be described by an azimuth angle and an elevation angle. The directions for the subarray beam-forming systems  210 ,  212 , and  214  can be set independently of each other, and they may be different or the same. For example, the subarray beam-forming systems  210 ,  212 , and  214  track targets A, B, and C respectively. 
   At the process  490 , the combined signals are processed by the subarray beam-forming systems  210 ,  212 , and  214 . The processing usually includes performing phase shifts on signals received from the signal combiner and divider systems  230 ,  240  and  250 . For example, the subarray beam forming system  210  is steered to the target A and transmits signals to the targets A, B, and C. The signals received from the signal combiner and divider system  230  are delayed with respect to each other by the phase shifters  320 , and the phase shifts substantially maximize the sum of the signals transmitted at various antenna elements with respect to target A, depending on the electronic steering of the same subarray beam-forming system. 
   At the process  492 , the subarray beam-forming systems  210 ,  212 , and  214  transmit signals to various targets. For example, the subarray beam-forming systems  210 ,  212 , and  214  each transmit signals to the targets A, B, and C, even though they are each steered towards only one of the targets A, B, and C. 
   As discussed above and further emphasized here,  FIGS. 4 and 4A  are merely examples, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The processes  410  and  490  for steering subarray beam-forming systems may each be a repetitive process. The subarray beam-forming systems  210 ,  212 , and  214  each track the targets A, B, and C respectively. If the target A moves, the subarray beam-forming system  210  may need to steer to a different direction. According to an embodiment of the present invention, the steering process of a subarray beam-forming system does not change the location of the phase center for the subarray beam-forming system. For example, the beam of the subarray beam-forming system can be commanded to scan electronically in elevation and/or azimuth to following a target, while keep its phase response to another target stable. 
   The antenna phase center is the location of a point associated with an antenna such that, if it is taken as the center of a sphere whose radius extends to the far-field, the phase of a given field component over the surface of the radiation sphere is essentially constant at least over that portion of the surface where the radiation is significant. The antenna phase center may be located physically on the antenna itself or elsewhere. For example the phase center of a dish antenna could be located at a point in space that is in front of the dish. The constant phase over the surface of the radiation usually requires the variation in electrical phase falls within ±5°. The portion of the surface where the radiation is significant usually refers to the portion corresponding to the main beam of the antenna, say, within the 3 dB beamwidth of the antenna. 
   The determination of the phase center of a mechanically scanned antenna is well known. Simply, the antenna is scanned mechanically centered about a point, and if that point was the phase center of antenna, then the electrical phase at the output of the antenna would not change. Based on this method the location of the phase center can be verified or estimated. It is often the case that the location of the phase center is a function of frequency. The process of determining the phase center adds to the antenna cost, if the location of the phase center is not well known before hand or well controlled, and search has to be made over a large volume of uncertainty. 
     FIG. 5  is a simplified diagram for a phase center verification system according to one embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. A phase center verification system  500  includes a network analyzer  510 , a transmit antenna  520 , and an antenna under test  530 . Although the above has been shown using systems  510 ,  520 , and  530 , there can be many alternatives, modifications, and variations. For example, some of the systems may be expanded and/or combined. Other systems may be inserted to those noted above. Depending upon the embodiment, the specific systems may be replaced. Further details of these systems are found throughout the present specification and more particularly below. 
   The network analyzer  510  generates a test signal, which is applied to the transmit antenna  520 . The test signal traverses a distance Z and reaches the antenna under test  530 . The distance Z should be greater than twice the Fresnel length, where the Fresnel length equals 
           D   2     λ     .       
 
D is the longer dimension associated with the antenna under test  530 , and λ is the wavelength of the test signal.
 
   The antenna under test  520  is a phased array antenna or a subarray beam-forming system with an electronically scanned beam. The phased array antenna is commanded to scan its beam electronically over a sequence of pointing direction. The pointing direction is usually described by an azimuth angle and an elevation angle. At each pointing direction, the phased array antenna  530  receives a signal from the transmit antenna  520 . For example, the signal from the transmit antenna  520  is a test signal generated by the network analyzer  510 . The phase of the output signal of the antenna  530  is measured by the network analyzer  510  with respect to the test signal that the network analyzer  510  generates and for each pointing direction. The network analyzer may also process at least information associated with the measured phase and determine whether the phase center remains constant during the scanning process. If the measured phase over a certain range of pointing direction remains essentially constant, then the scanning process keeps constant the phase center of the antenna  530 . The range of pointing directions usually corresponds to the beamwidth of the antenna  530  where the radiation is significant. If the measured phase does not stay constant over a certain range of pointing direction, then the phase center changes during scanning. Consequently, the beam-forming calibration table should be modified until the phase center remains substantially unchanged, such as within ±5°, as a function of beam scanning. 
     FIG. 6  is a simplified diagram for measured phase as a function of scan angle according to one embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The vertical axis is the measured phase of the output signal of the antenna  530 , and the horizontal axis is the elevation scan angel. The plotted curves  610 ,  620 , and  630  illustrate the phase response at the output of the antenna  530  as a function of the antenna beam being scanned electronically in elevation. The curves  610 ,  620 , and  630  correspond to different frequencies of the test signal, and these frequencies equal to 2.22, 2.30 and 2.38 GHz respectively. During measurement, the antenna  530  is not physically moved, but the electronics in the antenna  530  are commanded to change their electronic states. The data show that when the beam is scanned by up to ±15° in elevation, the output phase of the antenna  530  is essentially constant. This verifies that the phase center of the antenna is stable when the beam is scanned. 
     FIG. 7  is a simplified diagram for a cluster beam-forming system according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. A cluster beam-forming system  700  includes at least subarray beam-forming systems  710 ,  712 , and  714 , and a panel beam forming system  720 . Although the above has been shown using systems  710 ,  712 ,  714 , and  720 , there can be many alternatives, modifications, and variations. For example, some of the systems may be expanded and/or combined. Additional two panel beam forming systems may be added to process signals received by the subarray beam-forming systems; and the cluster beam-forming system  700  generates or receives 9 beams corresponding to 9 targets. In another example, additional subarray beam forming systems may be added to the cluster beam-forming system  700 . The cluster beam forming system  700  may be used to transmit signals, receive signals, or transmit and receive signals. Other systems may be inserted to those noted above. Depending upon the embodiment, the specific systems may be replaced. Further details of these systems are found throughout the present specification and more particularly below. 
   The subarray beam-forming systems  710 ,  712 , and  714  each include a subarray beam-forming system substantially similar to the system  210 . The systems  710 ,  712 , and  714  each divide one signal into three outputs or combine three outputs into one signal. The subarray beam-forming systems  710 ,  712 , and  714  each point to three directions. For example, the subarray beam-forming system  710  tracks targets A, B, and C. The subarray beam-forming system  712  tracks targets D, E, and F. The subarray beam-forming system  714  tracks targets G, H, and I. The panel beam-forming system  720  is substantially similar to the panel beam-forming system  220 . For example, the panel beam-forming system  720  includes signal combiner and divider systems  730 ,  740 , and  750 . Each of the signal combiner and divider systems  730 ,  740 , and  750  divides one of the outputs  738 ,  748 , and  758  into three signals or combines three signals into one of the outputs  738 ,  748  and  758 . The signal combiner and divider systems  260 ,  270 , and  280  generate or receive beams  262 ,  272 , and  282  respectively. For example, these beams electronically tracks the targets A, D, and G. 
   The cluster beam-forming method according to another embodiment of the present invention is substantially similar to the cluster beam-forming method  400 . For example, the process for steering subarray beam-forming systems includes pointing each of the subarray beam-forming systems  710 ,  712 , and  714  electronically to their respective three directions. The three directions for any subarray beam-forming systems  710 ,  712 , and  714  can be set independently of each other, and they may be different or the same. For example, the subarray beam-forming system  710 ,  712 , and  714  each track three different targets. 
   The cluster beam-forming method according to another embodiment of the present invention is substantially similar to the cluster beam-forming method  480 . For example, the process for steering subarray beam-forming systems includes pointing each of the subarray beam-forming systems  710 ,  712 , and  714  electronically to their respective three directions. The three directions for any subarray beam-forming systems  710 ,  712 , and  714  can be set independently of each other, and they may be different or the same. For example, the subarray beam-forming system  710 ,  712 , and  714  each track three different targets. 
   As discussed above and further emphasized here,  FIG. 7  is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. For example, the subarray beam-forming systems  710 ,  712 , or  714  can point to a number of directions other than three. The number of directions associated with different subarray beam-forming systems may be different. The number of beams output from or received by the phased array is equal to the number of outputs or inputs of the panel beam-forming system  720 . 
   The present invention has various applications. For example, the cluster beam-forming system according to one embodiment serves as a component of a multi-beam phased array antenna system. As another example, the cluster beam-forming system enables a spacecraft to track multiple aircrafts. 
     FIG. 8  is a simplified diagram for tracking multiple aircrafts according to one embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 8 , three aircrafts  810 ,  812 , and  814  are monitored by a spacecraft. The spacecraft carries a cluster beam-forming system  200 . The cluster beam-forming system  200  includes three subarray beam-forming systems  210 ,  212 , and  214 . The subarray beam-forming system  210  is set to track the aircraft  810 , and receives signals from an area  820 . Similarly, the subarray beam-forming system  212  is set to track the aircraft  812 , and receives signals from an area  822 . The subarray beam-forming system  214  is set to track the aircraft  814 , and receives signals from an area  824 . Each of the areas  820 ,  822 , and  824  covers two or all of the three aircrafts  810 ,  812 , and  814 . With respect to the aircraft  812 , the cluster beam-forming system  200  uses signals received not only by the subarray beam-forming system  212  but also by the subarray beam-forming systems  210  and  214 . The cluster beam-forming system  200  performs phase shifts onto these signals and combine them to generate an output beam corresponding to the aircraft  812  as shown in FIG.  2 . With respect to the aircraft  810 , the cluster beam-forming system  200  uses signals received not only by the subarray beam-forming system  210  but also by the subarray beam-forming system  212 . The cluster beam-forming system  200  performs phase shifts onto these signals and combine them to generate an output beam corresponding to the aircraft  810  as shown in FIG.  2 . Similarly, the cluster beam-forming system  200  generates an output beam corresponding to the aircraft  814 . 
   As discussed above and further emphasized here,  FIG. 8  is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The cluster beam-forming system  700  may also be used to track the aircrafts  810 ,  812 , and  814 . As shown in  FIG. 7 , the cluster beam-forming system  700  can point to up to 9 directions; hence several directions may point to the same aircraft for redundancy. 
     FIG. 9  is a simplified diagram for tracking multiple aircrafts according to another embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in  FIG. 9 , three aircrafts  910 ,  912 , and  914  are monitored by a spacecraft. The spacecraft carries a cluster beam-forming system  200 . The cluster beam-forming system  200  includes three subarray beam-forming systems  210 ,  212 , and  214 . The subarray beam-forming system  210  is set to track the aircraft  910 , and receives signals from an area  920 . Similarly, the subarray beam-forming system  212  is set to track the aircraft  912 , and receives signals from an area  922 . The subarray beam-forming system  214  is set to track the aircraft  914 , and receives signals from an area  924 . The area  920  covers the aircraft  910 , not the aircrafts  912  and  914 . Similarly, the area  922  covers the aircraft  912 , not the aircrafts  910  and  914 . The area  924  covers the aircraft  914 , not the aircrafts  910  and  912 . With respect to the aircraft  910 , the cluster beam-forming system  200  uses only signals received by the subarray beam-forming system  210 . This is implemented by adding a variable attenuator in serial with each phase shifter within the beam forming system  220 . For example, the beam  262  is designated to track aircraft  910 . When the output of subarray  210  provides the highest received signal level among all the subarrays from aircraft  910 , then the attenuation of the attenuator in series with phase shifter  232  is set to a minimum attenuation. When the output of subarray  212  provides a received signal level of aircraft  910  that is a number of dB such as 1 dB below the highest received signal level from aircraft  910 , then the attenuation of the variable attenuator in series with phase shifter  242  is set to a minimum attenuation plus a number of dB of additional attenuation such as 1 dB. When the subarray  214  provides a received signal level of aircraft  910  that is a number of dB such as 2 dB below the highest received signal from aircraft  910 , then the attenuation of the variable attenuator in series with phase shifter  242  is set to a minimum attenuation plus a number of dB of additional attenuation such as 2 dB. Similar approaches are taken for the aircrafts  912  and  914 . 
   As discussed above and further emphasized here,  FIG. 9  is merely an example, which should not unduly limit the scope of the present invention. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The cluster beam-forming system  700  may also be used to track the aircrafts  910 ,  912 , and  914 . As shown in  FIG. 7 , the cluster beam-forming system  700  can point to up to 9 directions; hence several directions may point to the same aircraft for redundancy. 
   The present invention has various advantages. Certain embodiments of the present invention form a plurality of beams using one set of subarray outputs. The beam width of a subarray is usually broader in comparison to that of a beam formed using a plurality of subarrays. These embodiments of the present invention can steer several narrow beams within a region defined by the beam pattern of a subarray. Some embodiments of the present invention reduce hardware complexity. The reduction becomes increasingly pronounced with increasing number of subarrays and system level beams. For example, the reduction of the total number of components and cables in a phased array antenna system that produces 64 beams could be more than 80%. Certain embodiments of the present invention significantly reduce cost and power consumption of a phase array antenna system. 
   Although specific embodiments of the present invention have been described, it will be understood by those of skill in the art that there are other embodiments that are equivalent to the described embodiments. Accordingly, it is to be understood that the invention is not to be limited by the specific illustrated embodiments, but only by the scope of the appended claims.