Patent Publication Number: US-10324176-B1

Title: Mainlobe detection process for monopulse antenna systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     N/A 
     BACKGROUND 
     A monopulse antenna system is commonly used to implement radar tracking or to track intentional radiators. As its name implies, a monopulse antenna system employs a single pulse to identify the presence of an object in the field of view. This is possible due to the use of multiple antennas which can detect angle information from the arriving signal. 
       FIG. 1  illustrates an example block diagram of a typical dual-axis monopulse antenna system. As shown, a dual-axis monopulse antenna system may include four individual antennas (A, B, C, and D) each of which are driven by the same arriving signal. If an object is in the field of view of the monopulse antenna system, each individual antenna will receive the reflected (or transmitted) signal. These received signals (which are referred to as A, B, C, and D respectively) are then fed to a comparator network. Although not shown, some intermediate processing may be performed on the received signals prior to inputting them to the comparator network. 
     Due primarily to the slight differences in the positions/orientations of the individual antennas, the characteristics of the received signals will vary. The comparator network can detect these variations to allow the relative location of the object with respect to the boresight axis to be determined. In particular, the comparator network can generate three tracking channels: (1) a sum (Σ) of the four received signals; (2) an azimuth difference (Δ az ); and (3) an elevation difference (Δ el ). As one of skill in the art would understand how these tracking channels can be employed to identify and track the position of an object, no further description will be provided. 
     As with most antennas, monopulse antennas produce a mainlobe (or main beam) and various sidelobes. In many situations, it will be possible to detect the presence of an object (or intentional radiator) whenever the object is positioned within the mainlobe or within one of the sidelobes due to the relatively high gain of some sidelobes. Therefore, even if the monopulse antenna is not pointed directly at the object, it may still receive a strong enough signal to detect the object&#39;s presence. However, if the object is within a sidelobe, and if the comparator network detects a sum channel peak or a difference channel null, the antenna will incorrectly assume that it is pointing directly at the object. 
     In typical monopulse antenna systems, an open-loop GPS and navigation data backbone is employed to perform coarse tracking. In other words, GPS data of the object to be tracked is supplied to the monopulse antenna system to allow the monopulse antenna system to initially point the antenna in the general direction of the object. Using GPS data in this way also requires that the antenna be physically oriented with respect to true north which can be a tedious process. 
     Additionally, as part of this tracking system, a modem lock signal will typically be employed as an indicator to the system that tracking is occurring. At sufficiently large target ranges, because the mainlobe gain is higher than the sidelobe gains, a SNR sufficient to establish the modem lock should only exist when the object is within the antenna&#39;s mainlobe. However, the fact that a modem lock can be established does not necessarily imply that the object is within the antenna&#39;s mainlobe. In many situations, an adequate SNR for establishing a modem lock may exist even though the object is positioned within the antenna&#39;s sidelobe. In such situations, the monopulse antenna system will track the object using a sidelobe when the desired outcome is to track within the mainlobe. This increases the risk of dropping the link due to marginal signal-to-noise ratio performance as the target moves away. Accordingly, a modem lock is a poor indicator of mainlobe tracking. 
     BRIEF SUMMARY 
     The present invention extends to techniques for implementing a mainlobe detection process in a monopulse antenna system. The mainlobe detection process can include a number of tests that are performed to define when the monopulse antenna system will transition from open loop scanning to closed loop scanning and then to tracking. Once tracking has commenced and all tests have passed, it ensures that the target is within the mainlobe of the monopulse antenna. The present invention also extends to a hybrid tracking technique in which a magnitude-only tracking method is performed prior to phase tracking to allow phase tracking to facilitate the auto-correction of phase-alignment errors between the monopulse antenna system and the monopulse antenna feed comparator network. Magnitude-only tracking can be performed initially to locate the difference channel nulls and to identify the average magnitudes of the difference-to-sum channel tracking ratios at these nulls. Phase tracking can then be performed. During phase tracking, phase corrections can be repeatedly applied to the azimuth and elevation difference channels to attempt to offset any phase error that may exist. During this process, the magnitudes of the tracking ratios near the difference channel nulls are used to determine how the phase corrections should be adjusted. 
     In one embodiment, the present invention is implemented as a method for detecting the mainlobe in a monopulse antenna system. During open loop scanning, an initial power-level test can be performed by determining whether a sum channel power exceeds one or both of an azimuth difference channel power or an elevation difference channel power. In response to the initial power-level test passing, closed loop scanning can be commenced. During closed loop scanning, a track-lock test can be performed by determining whether one or both of an azimuth ratio or an elevation ratio are below a track-lock threshold. In response to the track-lock test passing, tracking can then be commenced. 
     In another embodiment, the present invention is implemented as a monopulse antenna system that includes: a monopulse antenna comprising a number of monopulse antenna elements; a comparator network that generates a sum channel, an azimuth difference channel, and an elevation difference channel from a signal received at the monopulse antenna; and a monopulse detector assembly that receives the sum channel, the azimuth difference channel, and the elevation difference channel from the comparator network and generates an azimuth ratio and an elevation ratio from the channels. The monopulse detector assembly is configured to: perform an initial power-level test during open loop scanning, the initial power-level test comprising determining whether a sum channel power exceeds each of an azimuth difference channel power and an elevation difference channel power; in response to the initial power-level test passing, commence closed loop scanning; during closed loop scanning, perform a track-lock test, the track-lock test comprising determining whether an azimuth ratio and an elevation ratio are both below a track-lock threshold; and in response to the track-lock test passing, commence tracking. 
     In another embodiment, the present invention is implemented as a method for detecting the mainlobe in a monopulse antenna system and then tracking in the mainlobe. During open loop scanning, an initial power-level test is performed. The initial power-level test comprises determining whether a sum channel power exceeds each of an azimuth difference channel power and an elevation difference channel power. In response to the initial power-level test passing, closed loop scanning is commenced. During closed loop scanning, a track-lock test is performed. The track-lock test comprises determining whether an azimuth ratio and an elevation ratio are both below a track-lock threshold. In response to the track-lock test passing, hybrid tracking is commenced in which magnitude-only tracking is initially performed and then phase tracking is performed. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which: 
         FIG. 1  provides a block diagram of a typical monopulse antenna system; 
         FIG. 2  illustrates a block diagram of a monopulse antenna system that is configured in accordance with one or more embodiments of the present invention; 
         FIGS. 3A and 3B  illustrate a flow chart of a mainlobe detection process; 
         FIG. 4  provides a state diagram of the mainlobe detection process; 
         FIGS. 5A-5C  provide various charts representing how the mainlobe detection process can detect the mainlobe and the process results unique to the mainlobe; 
         FIGS. 6A-6C  provide various charts depicting how an antenna feed phase alignment error may exist; 
         FIGS. 7A-7B  illustrate a state diagram of a hybrid tracking process; 
         FIG. 8  illustrates a block diagram of a monopulse antenna system that is configured to implement the hybrid tracking process in accordance with one or more embodiments of the present invention; and 
         FIGS. 9A and 9B  illustrate an example of how a phase shifter can be updated to apply a different phase correction during the hybrid tracking process. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  provides a block diagram of the internal circuitry of a monopulse antenna system  200  that can be configured to perform the techniques of the present invention. Monopulse antenna system  200  includes a monopulse antenna  201  that is configured to produce signals A, B, C, and D which are then fed to a comparator  202  to generate the sum (Σ), azimuth difference (Δ az ), and elevation difference (Δ el ) channels as is known in the art. The three channels produced by comparator  202  are then fed into a monopulse detector assembly (MDA)  203  which produces two ratios from the channels: (1) an azimuth ratio (Ratio az ) indicative of the azimuth error (i.e., an object&#39;s azimuth angle of deviation from the boresight axis); and (2) an elevation ratio (Ratio el ) indicative of the elevation error (i.e., the object&#39;s elevation angle of deviation from the boresight axis). In some embodiments, these ratios can be generated in accordance with the following equations: 
     
       
         
           
             
               Ratio 
               az 
             
             = 
             
               
                 Δ 
                 az 
               
               Σ 
             
           
         
       
       
         
           
             
               Ratio 
               el 
             
             = 
             
               
                 Δ 
                 el 
               
               Σ 
             
           
         
       
     
     MDA  203  can be configured to implement the techniques of the present invention to ensure that monopulse antenna  201  is oriented such that the object to be tracked will be positioned within the mainlobe rather than a sidelobe. In other words, MDA  203  can generate steering signals to control the orientation of monopulse antenna  201  in accordance with the techniques of the present invention as will be described below. Monopulse antenna system  200  may also typically include a processor  204  which can interface with MDA  203  to provide control signals to and to receive state and tracking information from MDA  203 . Also, processor  204  can be configured to interface with monopulse antenna  201  for the purpose of providing steering signals. Although not shown, monopulse antenna system  200  may include a steering component with which MDA  203  and processor  204  interface for purposes of steering monopulse antenna  201 . 
     By way of overview, the present invention utilizes a series of tests during the steering of the monopulse antenna to ensure that tracking is only performed on the mainlobe. Initially, the monopulse antenna can be steered in an open loop scan mode. Open loop refers to the fact that the monopulse antenna is steered independently of the RF signal it receives. For example, while in open loop scan mode, the monopulse antenna can be steered relatively quickly along a spiral or zig zag pattern in an attempt to locate an object. Processor  204  can provide the steering signals to monopulse antenna  201  when in the open loop scan mode. In contrast, closed loop scanning refers to steering the antenna based on the azimuth and elevation tracking errors that are produced from the channel measurements. Therefore, closed-loop tracking often involves relatively slow/small movements in comparison to open loop scanning. Because closed loop scanning is based on the tracking channels, MDA  203  can be tasked with steering monopulse antenna  201  during closed loop scanning. In either case, the monopulse antenna can be steered either mechanically (i.e., by physically moving the monopulse antenna) or electrically (e.g., by using phased array techniques). 
     Continuing the overview, while processor  204  steers monopulse antenna  201  in open loop scanning, MDA  203  can process the three tracking channels to determine whether an object appears in the field of view of monopulse antenna  201  (e.g., based on a comparison of the sum channel power to the difference channel powers). Once MDA  203  detects that an object is present within the field of view (defined as the initial power-level test conditions), control of the steering can be turned over to MDA  203  to allow MDA  203  to determine whether the object is located within the mainlobe of monopulse antenna  201 . If MDA  203  determines that the object is within the mainlobe, it declares track lock and continues to track the object. However, if MDA  203  determines that the object is within a sidelobe, it can pass steering control back to processor  204  to recommence open loop scanning. This process can be repeated as necessary until the monopulse antenna  201  is steered towards the object such that tracking can occur on the mainlobe. 
       FIGS. 3A and 3B  provide a flowchart of this mainlobe detection process. As indicated above, MDA  203  is primarily tasked with performing the steps of this process. Therefore,  FIGS. 3A and 3B  generally represent the steps performed by MDA  203 . However, processor  204  can also be involved in providing various control inputs and in monitoring the state of the process. The interactions that may occur between MDA  203  and processor  204  are described more fully below with respect to the state diagram of  FIG. 4 . 
     The mainlobe detection process can commence when open loop scanning is enabled. For example, processor  204  (or more specifically, software executing on processor  204 ) could instruct MDA  203  to commence the mainlobe detection process. When open loop scanning is commenced, an initial power-level test can be iteratively performed by MDA  203 . As shown, this initial power-level test can include (1) determining whether the sum channel power is greater than each of the difference channel powers. In some embodiments, this initial power-level test may also include (2) determining whether the azimuth ratio and the elevation ratio are each less than an initial power-level threshold. 
     When an object is present within the field of view of monopulse antenna  201 , the sum channel power will exceed both the azimuth channel power and the elevation channel power. Therefore, when MDA  203  detects that this condition is met, it can transition into a closed loop scanning mode. The azimuth and elevation ratios represent the ratio of the difference between the sum channel power and the azimuth and elevation channel powers respectively and can therefore serve to define a threshold for when the initial power-level condition will cause a transition into the closed loop scanning mode. In other words, this second condition of the initial power-level test can prevent closed loop scanning from being performed when the sum channel power only slightly exceeds the difference channel powers. 
     This initial power-level threshold (as well as the track-lock threshold described below) can be a configurable parameter so that the “sensitivity” of the process can be fine-tuned for a given environment. For example, in environments when multipath reflections may occur (e.g., when monopulse antenna system  200  is positioned above the object to be tracked and will therefore receive reflections from the earth&#39;s surface), a higher value for the initial power-level threshold may be desirable. 
     As mentioned above, the transition from open loop scanning to closed loop scanning involves allowing MDA  203  to control the steering of monopulse antenna  201  based on the RF signal received by the antenna (i.e., based on the values of the three channels). The fact that the initial power-level test has succeeded indicates that an object is present within a lobe of monopulse antenna  201 . However, at this point, it is still unknown whether the object is within the mainlobe or within one of the sidelobes. 
       FIG. 5A  provides a plot identifying the areas where the sum channel gain exceeds both difference channel gains. This plot can be interpreted as the composite (including sum, azimuth, and elevation channels) gain profile that would be seen when looking directly at monopulse antenna  201 . Accordingly, the mainlobe is centered at 0° azimuth and 0° elevation and a number of sidelobes are positioned around the mainlobe as indicated. It is noted that this plot is simplified for illustrative purposes to remove various weaker sidelobes. In this example, if an object was positioned within the mainlobe or any of the sidelobes, the received signal would exhibit characteristics that would cause the initial power-level test to pass. In other words, even when the object is in one of the labeled sidelobes, the sum channel power would still exceed both difference channel powers. Both the azimuth and elevation ratios should also be below the initial power-level threshold when the object is in one of the labeled sidelobes. Accordingly, the initial power-level test functions as a “power test” for identifying when an object is located within a lobe. 
     To ensure that MDA  203  does not commence tracking the object when the object is within a sidelobe, MDA  203  can be configured to perform two additional tests. Initially, upon commencing closed loop scanning, MDA  203  can perform a track-lock test. As with the initial power-level test, the track-lock test can involve: (1) ensuring that the sum channel power remains greater than both the difference channel powers; and (2) determining whether the azimuth ratio and the elevation ratio are each less than a track-lock threshold. This track-lock threshold can be substantially less than the initial power-level threshold in order to increase confidence that the object is within the mainlobe. By way of example only, the initial power-level threshold may be 50% and the track-lock threshold may be 5%. However, as indicated above, these thresholds can be configurable to fine-tune the system for a particular environment. 
     This track-lock test is iteratively performed while MDA  203  steers monopulse antenna  201  based on the received values of the tracking channels. In other words, MDA  203  makes small adjustments to the orientation of monopulse antenna  201  to attempt to center the object within the lobe. As this centering is performed, the sum channel power relative to the difference channel powers should increase (i.e., MDA  203  will steer monopulse antenna  201  to minimize the azimuth and elevation ratios). 
     Each time the track-lock test is performed and fails, the timeout parameter can be incremented. The timeout parameter is used to set a test parameter to determine if the system will pass the test in a suitable amount of time. When testing near the mainlobe, the test will always pass under a certain time period. However, for locations other than the mainlobe, the test may never pass, but closed-loop tracking would continue were it not for a timeout period. If the timeout value reaches a particular value (e.g., 100), it can be assumed that the object is within a sidelobe, or certainly not in the mainlobe. More specifically, if the object is within a sidelobe, there may be no orientation within this sidelobe that will cause the track-lock test to pass. As a result, MDA  203  can pass steering control back to processor  204  to recommence the open loop scanning process to attempt to locate the mainlobe. In contrast, if the track-lock test passes, it implies that the sum channel power greatly exceeds the difference channel powers, and, as such, the object may likely be within the mainlobe and tracking can be commenced. 
       FIG. 5B  provides a plot illustrating the area where both the azimuth and elevation ratios are below the track-lock threshold (e.g., below 5%). As shown, in this case, the conditions of the track-lock test are met only within a small portion (the + shaped portion) of the mainlobe. Accordingly, if during open loop scanning, the object comes within one of the sidelobes and passes the initial power-level test, closed loop scanning will commence within that sidelobe. Because the ratios required by the track-lock test do not exist within the sidelobe, the track-lock test will eventually time out causing open loop scanning to be resumed. In contrast, if during open loop scanning, the object comes within the mainlobe, the subsequent closed loop scanning will cause monopulse antenna  201  to eventually be oriented directly towards the object (i.e., oriented so that the object is within the + shaped region of  FIG. 5B ) thereby causing the track-lock test to succeed. 
     The initial power-level and track-lock tests can be performed very quickly such that a number of closed loop scanning intervals can be performed in a relatively short amount of time. Therefore, even if the track-lock test is performed on a number of sidelobes prior to reaching the mainlobe, mainlobe tracking can ultimately be achieved in a reasonable amount of time. The values of the initial power-level and track-lock thresholds can be set to control how quickly this acquisition may occur. 
     In some embodiments, once the initial power-level and track-lock tests have passed, MDA  203  can commence tracking the object using the current values of the azimuth and elevation ratios. In some embodiments, this tracking can be performed in a “magnitude-only mode” in which the magnitudes alone of the azimuth and elevation ratios (or tracking error) are used to steer monopulse antenna  201  (i.e., tracking is performed independently of the phase). During this tracking, a mainlobe-check test can be continuously performed to ensure that the object remains within the mainlobe. As shown in  FIG. 3B , this mainlobe-check test can be the same as the initial power-level condition of the initial power-level test, namely, whether the sum channel power remains greater than both the difference channel powers. If, during tracking, the object moves outside of the mainlobe, this mainlobe-check test will fail thereby causing MDA  203  to return steering control back to processor  204  which may resume the open loop scanning process in an attempt to again orient the mainlobe towards the object. 
     Even with a low value for the track-lock threshold, there may still be some very infrequent scenarios where the track-lock test will pass when the object is located within the sidelobe (e.g., when the target is relatively close to monopulse antenna  201  and strong receive signals are present). The discriminator for these circumstances is built-in to the inherent phase response of the antenna feed comparator network, and can only be exploited when in phase tracking mode. In other words, when closed-loop tracking attempts to pass the track-lock test, MDA  203  could be commanded to be run in either magnitude-only mode or phase mode for tracking. Only in phase tracking mode can the MDA discriminate against situations where the azimuth or elevation ratio magnitudes are less than the track-lock threshold when the target is actually on a sidelobe. As an overview, this phase tracking can employ the phase of the azimuth and elevation tracking ratios (as represented by the sign of the azimuth and elevation ratios) to steer monopulse antenna  201  towards a center of the mainlobe. Due to the differences between the phase pattern within the mainlobe and the phase patterns within the sidelobes, when an object is being tracked within a sidelobe, this phase tracking will cause monopulse antenna  201  to be steered away from the center of the sidelobe. This will ultimately cause the mainlobe check test to fail thereby causing open loop scanning to be resumed. 
       FIG. 5C  provides a plot illustrating the phase pattern that exists within the mainlobe and the various sidelobes that are represented in  FIG. 5A . As is known, each of the A, B, C, and D signals produced by a monopulse antenna will have a phase. In theory, if the object is located directly in the center of the mainlobe and the phase response of the monopulse antenna system is properly aligned, each of these signals will have the same phase. However, if the object&#39;s location is offset from this “zero phase” position, and phase is taken into account, the azimuth and elevation difference ratios can be either positive or negative. For example, if the object is located at 0.5 degrees azimuth, the phase differences in the A, B, C, and D signals may cause the azimuth difference ratio to be positive, while if the object is located at −0.5 degrees azimuth, the phase differences in the A, B, C, and D signals may cause the azimuth difference ratio to be negative. A similar transition in the phase may occur along the elevation axis. 
     MDA  203  can employ these azimuth and elevation phase transitions during tracking. More particularly, MDA  203  can steer monopulse antenna  201  based on the sign of the azimuth and elevation ratios (or equally based on the sign of the azimuth and elevation difference channels). With reference to  FIG. 5C , if the tracked object is located in either of the right-sided quadrants (relative to the azimuth angle 0° reference), the azimuth ratio may have a non-zero positive value, and if the tracked object is located in either of the left-sided quadrants (relative to the azimuth angle 0° reference), the azimuth ratio may have a non-zero negative value. Similarly, if the tracked object is located in either of the top quadrants (relative to the elevation angle 0° reference), the elevation ratio may have a non-zero positive value, and if the tracked object is located in either of the bottom quadrants (relative to the elevation angle 0° reference), the elevation ratio may have a non-zero negative value. MDA  203  can be configured to steer monopulse antenna  201  based on the signs of these ratios. The tables below provide one example of how MDA  203  can be configured to steer monopulse antenna  201  when performing phase tracking. It is noted that the sign of the ratios will depend on how the azimuth and elevation difference channels are produced (e.g., whether the azimuth difference channel is produced as A+C−B−D or B+D−A−C) and therefore the steering directions are relative to how the difference channels are produced. 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Sign of Azimuth Ratio 
                 Direction to Steer 
               
               
                   
                   
               
               
                   
                 + 
                 Right 
               
               
                   
                 − 
                 Left 
               
               
                   
                   
               
               
                   
                 Sign of Elevation Ratio 
                 Direction to Steer 
               
               
                   
                   
               
               
                   
                 + 
                 Up 
               
               
                   
                 − 
                 Down 
               
               
                   
                   
               
            
           
         
       
     
     As can be seen, based on these rules during phase tracking, MDA  203  will continuously steer monopulse antenna  201  to cause the object to be located at the phase transition point in each axis (i.e., where the azimuth and elevation ratios approach zero) which, in theory, should exist at the point where the ratios are minimized (i.e., at the point where the difference between the sum channel gain and the difference channel gains is maximized). Due to this tracking, monopulse antenna  201  will remain oriented properly towards the object such that the mainlobe-check test will repeatedly succeed. 
     Turning again to  FIG. 5C , it is noted that the phase pattern within the sidelobes is not the same as the phase pattern within the mainlobe. For this reason, if phase tracking is being performed on a sidelobe, MDA  203  will cause monopulse antenna  201  to steer away from the object. This would force a failure during the mainlobe check test. For example, if the object is located within the top left (or white) quadrant of the upper sidelobe, the elevation and azimuth ratios will both have a positive sign. Based on the rules above, this will cause MDA  203  to steer monopulse antenna  201  in an upward and rightward direction. In other words, the signs of the ratios will cause MDA  203  to believe the object is above and to the right of the center point of the lobe when in fact the object is above and to the left of the center point. As a result, after MDA  203  steers monopulse antenna  201 , the object will eventually be located outside the upper sidelobe where the sum channel power will not be greater than the difference channel powers thereby causing the mainlobe check test to fail. In this way, MDA  203  employs phase to cause the mainlobe check test to fail when tracking is performed on a sidelobe thereby causing open loop scanning to be resumed in order to locate the mainlobe. 
     In some embodiments, the track-lock test may include an additional condition which monitors the variation in the sum channel power. When on the mainlobe, there should, in theory, be very little variation in the sum channel power across readings. In contrast, when on a sidelobe, the sum channel power may vary substantially. MDA  203  can therefore record the sum channel power level at each sampling and use these recorded levels to verify whether the power level has varied in excess of some threshold (e.g., whether the sum channel power level has varied by more than 2 dB over a series of ten samples). If the sum channel power varies in excess of this threshold, the track-lock test can fail thereby causing open loop scanning to be resumed. This variance threshold can be a configurable parameter as with the other thresholds. For example, in multipath conditions or for a fast-moving target, the sum channel power may vary significantly by comparison when on the mainlobe. In such cases, the variance threshold can be set to a higher level to prevent the track-lock test from incorrectly failing. A primary benefit of employing the variance threshold in the track-lock condition is that it may cause the track-lock test to fail before the timeout has been reached. In this way, less time will be spent doing closed loop scanning within a sidelobe. 
       FIG. 4  illustrates a state diagram for the above described process and represents how MDA  203  and processor  204  can interact. A number of control and state parameters can be employed to enable processor  204  to monitor and control MDA  203  during the process. These parameters include a ScanFlag parameter which when set will enable the scanning process, a ScanMode parameter which defines whether open or closed loop scanning will be performed, a Timeout parameter which tracks the number of iterations of the track-lock test, a TestFlag parameter which identifies the success or failure of the current test, and a TrackLock parameter which represents whether the target is currently being tracked. In some embodiments, processor  204  can set the value of the ScanFlag and ScanMode parameter while MDA  203  can set the value of the TestFlag and TrackLock parameters as will be described below. This state diagram and the parameters are exemplary and are only intended to represent one way in which MDA  203  and processor  204  could be configured to interact. 
     Whenever the ScanFlag parameter is set to 0, monopulse antenna system  200  will be in an idle state (i.e., neither scanning nor tracking). Whenever it is desired to commence the mainlobe detection process, processor  204  can set the ScanFlag parameter to 1 thereby transitioning monopulse antenna system  200  into the scanning state. As part of this transition, processor  204  can set the ScanMode parameter to 0 thereby indicating that open loop scanning should be performed (i.e., instructing MDA  203  that it will not control the steering of antenna  201 ). As open loop scanning is commenced, the MDA  203  can commence performing the initial power-level test while open loop scanning is occurring. 
     Monopulse antenna system  200  will remain in the initial power-level test state until either the initial power-level test succeeds or the scan is completed. As long as the ScanFlag parameter remains set, the system may reset the scan and continue performing the scan pattern. In contrast, if the test succeeds, MDA  203  can set the TestFlag parameter to 1 thereby causing the system to transition into the track-lock test state. In response to the TestFlag parameter being set to 1, processor  204  can set the ScanMode parameter to 1 to indicate to MDA  203  that it should commence closed loop tracking. Also, the Timeout and TestFlag parameters can be reset so that they can be used during the track-lock test. 
     The track-lock test is performed in a similar manner as the initial power-level test. However, each time the track-lock test is performed, the Timeout parameter can be incremented. If the Timeout parameter reaches a defined value x, the system will transition into the timeout state. If the ScanFlag parameter remains set, processor  204  can set the ScanMode parameter to 0 to cause open loop scanning to be resumed and the process restarted. In contrast, if the track-lock test succeeds, MDA  203  can set the TestFlag parameter to 1 which will cause the system to transition into the tracking state. As part of this transition, the TestFlag parameter can again be reset. 
     Once in the tracking state, MDA  203  can commence tracking the object using one of the modes described above (magnitude-only or phase mode) or even a hybrid tracking mode (for the purpose of antenna feed phase correction) which will be described below. While tracking, the system will repeatedly check the mainlobe-check test state. If the mainlobe-check test succeeds, MDA  203  can set the TestFlag parameter to 1 causing the system to remain in the tracking state and also set the TrackLock parameter to 1 to indicate to processor  204  that tracking is occurring. In contrast, if the mainlobe-check test fails, the TestFlag parameter can be set to 0 to cause the system to transition to the lost lock state. The TrackLock parameter can also be set to 0 to indicate that the object is no longer being tracked. When in the lost lock state, the system will return to open loop scanning as long as the ScanFlag parameter remains set. As part of this transition, processor  204  can set the ScanMode parameter to 0 to inform that system that the MDA tracking commands can be ignored. 
     The above described phase tracking is based on the premise that the phase transitions from positive to negative at the same location where the corresponding ratio is minimized. Hence, phase alignment to the phase response of the antenna comparator feed network is required for correct phase tracking. Based on this premise, by steering monopulse antenna  201  to these “phase zero crossings,” MDA  203  will also be steering monopulse antenna  201  to the point where the difference between the sum channel power and the difference channel powers is maximized. However, there are situations where the phase zero crossings may not coincide with the ratio nulls making phase tracking less effective or even completely erroneous. 
       FIG. 6A  provides a plot illustrating a case where the azimuth phase zero crossing coincides with the azimuth ratio null. In contrast,  FIG. 6B  provides a plot illustrating a case where the azimuth phase zero crossing is shifted to the right of the azimuth ratio null. Similar plots could be created for elevation but are not shown for sake of simplicity. 
     In these plots, the center horizontal line represents where the phase of the azimuth ratio transitions between positive and negative. The positive and negative values of the phase correspond directly to the sign of the azimuth or elevation ratio used to steer the antenna in phase tracking mode. In  FIG. 6A , this phase zero crossing occurs at 0° azimuth which is also where the magnitude of the azimuth ratio is minimized. If it is assumed that the magnitude and phase of the elevation ratio similarly coincide, phase tracking would cause monopulse antenna  201  to be steered towards 0° azimuth and 0° elevation. In other words, the plot in  FIG. 6A  corresponds to what is shown in  FIG. 5C . 
     In contrast, in  FIG. 6B , the phase of the azimuth ratio is shifted to the right. Specifically, the phase zero crossing is shown as occurring at approximately 0.8° azimuth where the magnitude of the azimuth ratio is not minimized. In this scenario, phase tracking would cause monopulse antenna  201  to be steered away from the null in the azimuth ratio. More particularly, even if monopulse antenna  201  is initially oriented directly towards the azimuth null at 0°, the sign of the azimuth ratio will be negative indicating that the object is positioned to the left of the bore axis. If in phase mode, MDA  203  would steer monopulse antenna  201  to the left, away from the object. This leftward steering would continue until monopulse antenna  201  is offset by 0.8° to the left of the object. As shown in  FIG. 6B , at this offset, the azimuth ratio is greatly increased which would negatively impact tracking. A similar result would happen if the phase of the elevation ratio is shifted from the elevation ratio null. 
     In a worst case scenario, the phase may be shifted so far that the phase is reversed at the corresponding ratio null.  FIG. 6C  illustrates this case for the azimuth ratio. As shown, the phase is positive to the left of the null and negative to the right of the null. Therefore, when phase tracking is employed, MDA  203  would steer monopulse antenna  201  away from the null and outside the mainlobe mistakenly believing that it is actually steering it towards the null. In essence, when the phase is shifted too much, the mainlobe will appear as a sidelobe during phase tracking thereby causing open loop scanning to be resumed even though the mainlobe had been located. 
     To correct for these phase shift scenarios, MDA  203  can be configured to perform “hybrid mode” tracking which employs a combination of both the magnitude-only tracking mode and the phase tracking mode of the azimuth and elevation ratios. In general, using phase mode for tracking is typically preferred to magnitude-only mode since it offers higher accuracy, is faster, has increased bandwidth response, and greater pull-in range. Hybrid mode tracking allows phase to be used even when a misalignment in the phase is occurring. 
     As an overview, in hybrid mode tracking, MDA  203  can initially perform magnitude-only tracking to steer monopulse antenna  201  towards the azimuth and elevation nulls. During this magnitude-only tracking, MDA  203  can generate various thresholds that will later be employed during phase tracking to ensure that a phase misalignment does not cause monopulse antenna  201  to be steered substantially away from the nulls. Then, MDA  203  can perform phase tracking including performing various calculations to correct (or offset) a phase misalignment. If, during phase tracking, it is determined that monopulse antenna  201  has become substantially misaligned, magnitude-only tracking can be resumed until the nulls are again located. This process of switching between magnitude-only tracking and phase tracking can be repeated as necessary to ensure that phase tracking can be accurately performed even when a phase misalignment is occurring. 
     Prior to describing hybrid mode tracking in detail, it is to be understood that, during either magnitude-only or phase tracking, MDA  203  will constantly make slight adjustments to the boresight angle of monopulse antenna  201  (i.e., steer the antenna) in an effort to align the boresight angle with the target (which may be moving relative to the antenna). In magnitude-only tracking, MDA  203  will steer monopulse antenna  201  to attempt to minimize the azimuth and elevation ratios without using the phase response to determine the sign of the tracking error, whereas, in phase tracking, MDA  203  will steer monopulse antenna  201  to attempt to locate the phase zero crossings in the azimuth and elevation ratios. Accordingly, after each steering iteration, MDA  203  will generate current azimuth and elevation ratios and then calculate, from these current ratios, how to subsequently steer monopulse antenna  201 . 
       FIGS. 7A and 7B  provide a state diagram for hybrid mode tracking in accordance with embodiments of the present invention.  FIG. 7A  illustrates the portion of the state diagram representing magnitude-only tracking, while  FIG. 7B  illustrates the portion of the state diagram representing phase tracking. This state diagram can represent the process that is performed in the Tracking state shown in  FIG. 4 . As indicated above, in hybrid mode tracking, magnitude-only tracking can be performed to initially locate the nulls and then phase tracking can perform phase correction if needed. 
     Turning to  FIG. 4 , when the track-lock test passes, MDA  203  will commence tracking in the tracking state. When hybrid-mode tracking is enabled, as shown in  FIG. 7A , this can include transitioning from the track-lock test state to an initialize state in which a number of tracking parameters are initialized (as represented by MagTrackReset( )). These tracking parameters include a MagCnt parameter which tracks the number of magnitude tracking iterations that have been performed, an AzRatioMag parameter which stores a value representing the sum of the absolute value of the magnitudes of the azimuth ratio within the MagCnt period, an ElRatioMag parameter which stores a corresponding elevation value representing the sum of the absolute value of the magnitudes of the elevation ratios within the MagCnt period, an AzRatioDir parameter which stores a value representing a sum of the signs of the azimuth ratio, and an ElRatioDir parameter which stores a value representing a sum of the signs of the elevation ratio. Each of these parameters can be initialized to zero (e.g., by calling MagTrackReseto) as part of transitioning into the magnitude-only tracking state. 
     As mentioned above, magnitude-only tracking entails calculating the magnitude of current values of the azimuth ratio (az_ratio) and the elevation ratio (el_ratio) and then steering monopulse antenna  201  to minimize the ratio. At each iteration of this steering (i.e., each time MDA  203  causes the boresight angle of monopulse antenna  201  to be changed), the absolute values of the azimuth and elevation ratios can be added to the AzRatioMag and ElRatioMag parameters respectively. Therefore, at any given time, each of these two parameters will store a sum of the magnitudes of the corresponding ratios that have been calculated during the previous iterations of magnitude tracking. The purpose of this summation is to eventually compute an average ratio magnitude value that can be used for future comparison. Also, at each iteration of this steering, the AzRatioDir and ElRatioDir parameters can be updated by adding 1 or −1 to the value of the parameter based on the sign of the current azimuth or elevation ratio respectively. In particular, when the corresponding ratio has a positive value (e.g. when the target is to the right of or above the boresight angle), the parameter can be incremented by 1, whereas when the corresponding ratio has a negative value (e.g., when the target is to the left of or below the boresight angle), the parameter can be decremented by 1. In theory, for a perfect tracking system, the result of this calculation when tracking perfectly at the null would average out to 0. At each iteration, a value of the MagCnt parameter can also be incremented by 1. 
     This loop can be repeated until a specified number of iterations have been performed. For example, the MagTrackHistory parameter can be set to a desired value (e.g., 250). Then, once the MagCnt parameter reaches the value of the MagTrackHistory parameter, the phase mode transition (PMT) test can be performed to determine whether the process should transition into phase tracking. Assuming the value of the MagTrackHistory parameter is 250, 250 iterations of magnitude tracking will be performed prior to performing the PMT test. In this case, both the AzRatioMag and ElRatioMag parameters would equal the sum of the magnitudes of the 250 previous azimuth and elevation ratios, respectively. At this point, they will each be divided by MagCnt to obtain the average ratio value for the azimuth and elevation channels. These parameters can be employed later during phase tracking as will be described below. 
     The PMT test can be employed to determine whether monopulse antenna  201  has been steered sufficiently towards the azimuth and elevation nulls. For example, in some embodiments, the PMT test can determine whether the absolute values of both the AzRatioDir and ElRatioDir parameters are below a defined threshold (e.g., IF ((Abs(AzRatioDir)&lt;Threshold) and (Abs(ElRatioDir)&lt;Threshold)). For example, when the MagTrackHistory parameter is set to 250, the Threshold used in the PMT test can be 125. This Threshold can be a configurable parameter to allow monopulse antenna system  200  to be configured for a specific environment or for a specific target. For example, faster moving targets may require a larger threshold. 
     If the null has been found, MDA  203  would typically cause monopulse antenna  201  to dither about this null such that the sign of the azimuth and elevation ratios (using the magnitude-only mode tracking definition of a sign-change) will periodically switch back and forth. Therefore, when the null has been found, the values of the AzRatioDir and ElRatioDir should be minimized and the PMT test will pass causing phase tracking to be commenced. In contrast, if the PMT test fails, the process can be repeated by resetting the values of each of the parameters (e.g., by calling MagTrackReset( ) to set MagCnt, AzRatioMag, ElRatioMag, AzRatioDir, and ElRatioDir equal to 0) and returning to the magnitude tracking state as shown in  FIG. 7A . 
     As an example of magnitude-only tracking, if the target is initially to the right of the boresight angle, the azimuth ratio will have a magnitude value of Y. In this scenario, and having known the previous direction, MDA  203  will steer monopulse antenna  201  slightly to the right, reducing the azimuth ratio eventually to a magnitude of Z, which represents the minimum azimuth ratio magnitude achieved by the azimuth difference channel null. This process will be repeated until the azimuth ratio magnitude becomes greater than Z (i.e., once the target is to the left of the boresight angle). Therefore, as monopulse antenna  201  is steered towards the null, the value of AzRatioDir will be positive for a number of iterations. If the azimuth ratio is positive for 250 consecutive iterations (meaning that the null has yet to be reached), the value of the AzRatioDir parameter will be 250 therefore causing The PMT test to fail and magnitude tracking to be continued for 250 more iterations. In contrast, if the azimuth ratio is positive for 100 consecutive iterations (which would result in the AzRatioDir parameter reaching a value of 100) and then commences switching back and forth between increasingly larger magnitudes, the AzRatioDir would remain close to 100 (or less) such that The PMT test would pass. In this way, the AzRatioDir and ElRatioDir parameters can be used to determine when the nulls have been found during magnitude-only mode and therefore that phase tracking can be commenced. 
     Turning now to  FIG. 7B , upon transitioning to phase tracking, a number of parameters can be initialized for use in the phase tracking process. These parameters include a SgnCnt parameter which tracks the number of phase tracking iterations that have been performed, a phase lock counter (PLC) parameter that is used to determine when phase lock has occurred, a sign history (SH) parameter which is used to define how many iterations of phase tracking will be performed before a phase error calculation will be performed, a PhaseLock parameter which defines whether phase lock has occurred, an AzRatioPhase parameter which stores a value representing the sum of the absolute value of the magnitudes of the azimuth ratios within a SgnCnt period, and an ElRatioPhase parameter which stores a value representing the sum of the absolute value of the magnitudes of the elevation ratios within a SgnCnt period. In this example, it is assumed that the SH parameter is initially set to 10 while the other parameters are initially reset to 0. 
     As compared to the magnitude-only tracking method described above, during traditional phase tracking, MDA  203  will steer monopulse antenna  201  based on the current phases (or signs) of the azimuth and elevation ratios. Assuming there is no phase error, the phase response of the difference channel signals should be zero at the nulls, and therefore, MDA  203  will steer monopulse antenna  201  to attempt to remain near these phase zero crossings. This can be accomplished by steering the monopulse antenna by an azimuth step and an elevation step at each iteration. The direction of each step will be based on the sign of the corresponding azimuth and elevation tracking ratios. Therefore, MDA  203  will cause monopulse antenna  201  to dither about the phase zero crossings (i.e., it would repeatedly steer the antenna in stepped increments in one direction until the sign changes and then return in stepped increments in the opposite direction). However, if there is a phase error, the phase zero crossings will not align with the nulls which would result in this dithering occurring away from the nulls, or, if the phase error is significant, in the eventual failure of the mainlobe-check test and the return to open loop scanning. 
     To account for these phase error scenarios, the phase error correction process depicted in  FIG. 7B  can be implemented. Prior to describing the phase error correction process, it is to be understood that the phase error that is to be corrected exists in the signal received at the individual antenna elements as opposed to phase drift that may occur on each individual channel as the received signals are processed within monopulse antenna system  200 , or even the sum-to-difference channel phase alignment to compute the difference channel ratio. The phase drift that can occur on each individual channel may be due to different phase errors that are introduced on a component-by-component basis during the independent processing of the channels (e.g., an LNA used to produce the sum channel may introduce a different phase error than an LNA used to produce the azimuth difference channel). Such phase drift is also exacerbated over frequency and temperature. This phase error can be overcome by some amount of hardware unit calibration and is not discussed in this patent. Furthermore, phase alignment is essential to computing azimuth and elevation tracking ratios. The crucial phase alignment in this case is the sum channel signal phase relative to the difference channel signal phase. Techniques exist for accounting for this type of phase alignment when digitally combining the sum and difference channels (see e.g., U.S. patent application Ser. No. 14/572,470) but such techniques are not the subject of the present invention. However, the phase error correction process of the present invention can be used in conjunction with such phase drift calibration or sum-to-difference channel phase alignment/correction techniques. In short, the present invention can allow phase tracking to be employed even when the signals received at the individual antenna elements are not phase aligned with the down-stream RF and digital processing. 
     To enable phase errors to be corrected, monopulse antenna system  200  can include a phase shifter  801  and  802  in the azimuth difference channel and elevation difference channel respectively as shown in  FIG. 8 . Phase shifters  801  and  802  can preferably be implemented on an FPGA that forms part of MDA  203 . Therefore, even though for illustrative purposes  FIG. 8  depicts phase shifters  801  and  802  as separate from MDA  203  (which is one possible way in which the phase shifters  801  and  802  could be implemented), for speed and efficiency reasons, FPGA-implemented phase shifters may typically be employed. The process depicted in  FIG. 7B  will calculate a phase shift that should be applied by each of phase shifters  801 ,  802  to account for a phase error. MDA  203  can calculate the required phase shift in a systematic method until the current phase shift corrects for all measured phase error. MDA  203  can perform this process independently for each difference channel as will now be described in detail. 
     As shown in  FIG. 7B , after the conditions of the PMT test are met, MDA  203  will transition into phase tracking. As part of this transition, the SH parameter can be set to an initial value (e.g., 10) while the PLC, PhaseLock, AzRatioPhase, and ElRatioPhase parameters can be set to 0. As indicated above, during phase tracking, MDA  203  will steer monopulse antenna  201  in azimuth and elevation steps based on the phase (or sign) of the current azimuth and elevation ratios. Initially, it can be assumed that no phase error exists (i.e., that the phase zero crossings align with the nulls in the ratios) and therefore that phase shifters  801 ,  802  are configured to apply no phase shift (i.e., a 0° shift) to the corresponding difference channel. 
     As shown, at each iteration of phase tracking, MDA  203  can add the absolute value of the azimuth and elevation ratios to the AzRatioPhase and ElRatioPhase parameters respectively. Also, at each iteration, MDA  203  will compare the current magnitude of the azimuth and elevation ratios to a Radius Threshold (RT) parameter. The RT parameter can be a configurable parameter that controls when MDA  203  will return to magnitude-only tracking. For example, RT can be set to 0.25 (assuming the ratios during magnitude-only tracking were lower than this amount) such that, whenever either the azimuth or elevation ratio exceeds 0.25 (which would indicate that monopulse antenna  201  is at potential risk for failing the mainlobe-check test), MDA  203  will resume magnitude-only tracking to return to tracking near the difference channel nulls. 
     As long as the azimuth and elevation ratios remain below RT, MDA  203  will continue to sum the magnitudes of the ratios until SgnCnt (which is incremented with each iteration) equals SH. As mentioned above, SH can initially be set to 10 such that 10 iterations will initially be performed. After these 10 iterations, the null-alignment test (NAT) will be evaluated based on the average values of the AzRatioPhase and ElRatioPhase parameters (based on SgnCnt iterations) and the stored values of the AzRatioMag and ElRatioMag parameters. In other words, MDA  203  can compare the magnitudes of the ratios during phase tracking to the magnitudes of the ratios during the previous magnitude tracking to determine whether the current magnitudes have increased beyond a threshold. 
     For example, the NAT can be: IF AzRatioPhase/SH&lt;(AzRatioMag/MTH) and IF ElRatioPhase/SH&lt;(ElRatioMag/MTH). The NAT can therefore determine whether the average magnitude of the ratios during SH iterations of phase tracking is less than the average magnitude of the ratios during the MTH iterations of magnitude tracking that were performed prior to commencing phase tracking. In some embodiments, the average magnitude of the magnitude-only mode ratios can be scaled by a factor of X (e.g., IF AzRatioPhase/SH&lt;(AzRatioMag/MTH)*X and IF ElRatioPhase/SH&lt;(ElRatioMag/MTH)*X. As an example, X can be set to 1.25. The primary purpose of the NAT is to determine whether phase tracking has kept monopulse antenna  201  oriented towards the nulls. Since the average ratios from magnitude-only tracking should represent the average ratios at the nulls, the NAT will identify whether phase tracking caused monopulse antenna  201  to be steered away from the nulls. Scaling by X can prevent the NAT from failing when there are only slight differences between the average magnitude and phase ratios. 
     If the NAT passes, it is assumed that monopulse antenna  201  is still near the nulls and a determination of whether a phase lock has already been established can be performed. In contrast, if the NAT fails, it is assumed that monopulse antenna  201  has been steered away from the null(s) due to the phase zero crossing(s) not aligning with the null(s). If the NAT fails, a phase correction can be calculated and applied to the appropriate difference channels. It is noted that a phase error may exist only in one ratio. Therefore, MDA  203  can perform the phase error correction process independently on each ratio. In particular, the NAT may be performed independently on each ratio leading to an independent calculation of a phase correction for each difference channel. However, for ease of illustration, it will be assumed that a similar phase error exists in both difference channels such that the phase error correction process will proceed in sync for both ratios. 
     Assuming the NAT passes for both ratios, MDA  203  will proceed to determine whether a phase lock has been established. Each time the NAT passes, PLC can be incremented by 1 and SH can be incremented by 50 (or some other reasonable value) unless SH has already reached 1000 (or some other reasonable value) in which case SH will remain at 1000. Then, a phase lock determination can be made. In this case, if PLC is greater than 10 and SH equals 1000, it can be determined that phase lock has occurred. Therefore, for phase lock to occur, the average magnitudes of the ratios must remain below the average magnitudes (or scaled average magnitudes) of the ratios that existed during magnitude-only tracking for a large number of iterations. This would be the case if phase tracking is keeping monopulse antenna  201  at the nulls. If phase lock has occurred, the PhaseLock parameter can be set to 1 and phase tracking can be continued without any adjustment to the current phase corrections (i.e., without updating phase shifters  801 ,  802 ). 
     In contrast, if phase lock has not yet occurred, updated phase corrections can be calculated at phase adjustment block  701   a . The specific manner in which an updated phase correction can be calculated will be described below. Because the NAT has passed thereby implying that monopulse  201  is still at or near the nulls, a relatively small adjustment can be made to the phase corrections. In other words, it can be assumed that the current phase corrections are substantially offsetting any phase error that may exist but that it may be possible to improve the phase corrections. MDA  203  can therefore continue to make these slight phase adjustments in an attempt to completely offset any measureable phase error. 
     Also, the direction of this phase adjustment can be based on whether the magnitude of the corresponding ratio is increasing or decreasing. In other words, if the magnitude of AzRatioPhase (using azimuth as an example) computed at the SH interval is larger than the AzRatioPhase during the previous SH period, it can be assumed that the current phase correction does not perfectly align the phase zero crossing with the null and therefore the phase correction should be adjusted in a negative direction (i.e., leftward or downward with respect to the example orientation used above). In practice, phase adjustment block  701   a  will make repeated small adjustments to the phase correction in one direction until the phase correction causes the phase zero crossing to be positioned on the opposite side of the null (from the perspective of MDA  203 ). At this point, phase adjustment block  701   a  will make repeated adjustments in the other direction. Assuming monopulse antenna  201  remains in the nulls, this process will continue until phase lock is established. 
     Whenever the NAT fails, the process will flow to either phase adjustment block  701   b  if phase lock has been established (i.e., if the PhaseLock parameter is set to 1) or to phase adjustment block  701   c  if phase lock has not been established. In each of phase adjustment blocks  701   b ,  701   c , updated phase corrections can be calculated. With regards to phase adjustment block  701   b , because phase lock had been established, the adjustment to the phase correction can be small but slightly greater than the adjustment made in phase adjustment block  701   a , whereas, with regard to phase adjustment block  701   c , because phase lock has not been established, a relatively large adjustment can be made. Again, these adjustments to the phase corrections can be made to attempt to more accurately offset the actual phase error. The primary role of the phase error correction process therefore is to repeatedly increment the phase corrections until the measureable phase error is minimized. Whenever the NAT fails, SH can be set to 15 (or some other reasonable value) and SgnCnt can be reset to cause 15 iterations to be performed prior to again evaluating the NAT. 
     Finally, if either of the ratios exceeds RT, MDA  203  can transition back to magnitude-only tracking including calculating new phase corrections at phase adjustment block  701   d . In this case, it can be assumed that the current phase corrections are not accurate and should be substantially updated. Therefore, phase adjustment block  701   d  can apply a large (e.g., 90°) adjustment to the phase corrections prior to resuming magnitude tracking. The adjustments made by phase adjustment block  701   d  will remain during magnitude-only tracking and until a subsequent adjustment is made once phase tracking is again resumed. Once phase tracking is resumed, the same process will be performed to again locate a phase correction that will offset the phase error. 
     If the phase error correction process reaches phase adjustment blocks  701   a  or  701   b , it can be assumed that the current phase corrections are relatively accurate. Therefore, phase adjustment blocks  701   a ,  701   b  can be configured to make small adjustments to the phase corrections.  FIGS. 9A and 9B  generally illustrate how this can be done for the azimuth difference channel. In  FIG. 9A , it is assumed that phase shifter  801  is currently configured to apply a phase shift of 10° to the azimuth difference channel. While this 10° phase shift is being applied, MDA  203  can perform the phase error correction process including calculating the magnitude of the azimuth ratio at each steering iteration. As represented by StoreCurrRatios( ) in  FIG. 7B , MDA  203  can be configured to retain the values of the previous AzRatioPhase and ElRatioPhase computed during the previous SH loop period (e.g., as values of PrevAzRatioPhase and PrevElRatioPhase parameters) to allow MDA  203  to determine whether the ratios are increasing or decreasing in subsequent periods. In other words, the phase corrections calculated in phase adjustments blocks  701   a  and  701   b  are based on a comparison of the current ratio average to the corresponding previous ratio average. In this example, it will be assumed that the current AzRatioPhase (computed after reaching SH SgnCnt iterations) averages to a magnitude of 0.14, but the prevAzRatioPhase magnitude during the previous SH period had an average of 0.12, thereby indicating that monopulse antenna  201  is currently being steered away from the azimuth null. Therefore, the computed phase adjustment block  701   a  can determine that the phase correction being applied by phase shifter  801  should be reduced. 
     Assuming that phase adjustment block  701   a  is configured to make approximately a 2.8° adjustment (represented by 8 steps of a 10-bit digital phase-shifter), MDA  203  can apply this adjustment as −2.8° to the current phase shift (i.e., 8 steps or 
                 (     π   64     )     *   NewDirection     ,         
where NewDirection is + when the ratio is decreasing and − when the ratio is increasing). Accordingly,  FIG. 9B  shows the result of the example described in  FIG. 9A , where MDA  203  outputs a signal to phase shifter  801  causing phase shifter  801  to apply a phase shift of 7.2°. A similar calculation, but with a different phase step size will be performed by phase adjustment block  701   b . For example, phase adjustment block  701   b  could be configured to apply an 11.25° adjustment to the current phase correction where this adjustment will be added to or subtracted from the current phase correction depending on whether the azimuth ratio is decreasing or increasing respectively
 
               (       i   .   e   .     ,     32   ⁢           ⁢   steps   ⁢           ⁢   or   ⁢           ⁢     (     π   16     )     *   NewDirection       )     .         
This calculation of the adjustment to the phase correction can be performed independently for each of the azimuth and elevation ratios so that a proper adjustment can be supplied to each of phase shifters  801 ,  802 . In short, phase adjustment blocks  701   a ,  701   b  can both be configured to apply a fixed adjustment to the current phase shift in a direction that is dependent on whether the corresponding ratio is increasing or decreasing.
 
     Phase adjustment blocks  701   c ,  701   d  can be configured to calculate an adjustment in a slightly different manner. Rather than employing previous ratio magnitude averages to determine whether a ratio is increasing or decreasing, each of phase adjustment blocks  701   c ,  701   d  can employ the sin( ) of the current ratio to determine the direction of the adjustment as well as to scale the amount of the adjustment. For example, phase adjustment block  701   c  can calculate the adjustment as 64 steps of a 10-bit digital phase shifter 
               (     π   8     )     *     sin   ⁡     (   ratio   )             
and phase adjustment block  701   d  can calculate the adjustment as 256 steps of a 10-bit phase shifter
 
     
       
         
           
             
               ( 
               
                 π 
                 2 
               
               ) 
             
             * 
             
               
                 sin 
                 ⁡ 
                 
                   ( 
                   ratio 
                   ) 
                 
               
               . 
             
           
         
       
     
     In summary, phase adjustment blocks  701   a - 701   c  can each be configured to calculate an adjustment to the phase corrections that are currently being applied by phase shifters  801 ,  802  in an attempt to offset any phase error that may exist during subsequent phase tracking. In contrast, phase adjustment block  701   d  can apply a large adjustment to the phase corrections prior to transitioning back to magnitude tracking since these current phase corrections will have caused phase tracking to incorrectly steer monopulse antenna  201  away from the nulls. 
     When a phase error exists, phase tracking will initially steer monopulse antenna  201  away from the null. As this steering is occurring, MDA  203  will detect that the ratios are increasing when the NAT fails and can then adjust the phase correction being applied by phase shifter  801  and/or  802 . These repeated adjustments to the phase correction should ultimately offset the phase error thereby aligning the phase zero crossings with the nulls. Once this alignment occurs, phase lock will be established and the NAT will be applied on a less frequent interval. Absent any significant change to the phase error, this phase lock will be retained thereby allowing MDA  203  to perform phase tracking throughout the tracking process. In other words, once phase lock is obtained, no more phase corrections are required unless there is some tracking event that causes the tracking to revert back to magnitude-only tracking mode. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description.