Abstract:
A nulling direct radiating array ( 30 ) that includes a main phased array ( 32 ) and a plurality of auxiliary arrays ( 70, 72, 74 ) symmetrically disposed about the main array ( 32 ). The main array ( 32 ) includes a plurality of antenna elements ( 34 ) and a beam forming system ( 56 ) that generates one or more channels made up of several pixel beams. The pixel beams from the main array ( 32 ) are connected to a nulling processor ( 108 ) along with the combined signal from the antenna elements ( 80 ) of the auxiliary arrays ( 70, 72, 74 ). An adaptive weighting network ( 112 ) and an adaptive weight generator ( 114 ) within the nulling processor ( 108 ) determine whether a jamming signal exists in the channel beam, and weight the pixel beams from the main array ( 32 ) accordingly to block the jamming signal. The auxiliary arrays ( 70, 72, 74 ) provide a wider beam aperture that is able to more narrowly define the null in the radiation pattern of the main array ( 32 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a nulling antenna system and, more particularly, to a nulling direct radiating array antenna that employs auxiliary antenna arrays positioned around a main array for increasing the nulling resolution. 
     2. Discussion of the Related Art 
     Various communications systems, such as certain telephone systems, cable television systems, internet systems, and military communications systems, make use of satellites orbiting the Earth to transfer communication signals. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and the satellite retransmits the signal to another satellite or to the Earth as a downlink communications signal to cover a desirable reception area depending on the particular use. The satellite is equipped with an antenna system including an array of antenna feeds that receive the uplink signals and transmit the downlink signals to the Earth. 
     Satellite-based phased array antenna systems have been developed that provide signals to communication areas using pixel beams designed to cover specific areas on the Earth&#39;s surface. Typically, the pixel beams are organized into a matrix of evenly shaped and spaced beams to provide a total coverage area for a large geographical area, such as the visible Earth. One particular phased array suitable for this purpose is the “Enhanced Direct Radiating Array” disclosed in U.S. patent application Ser. No. 09/443,526, filed Nov. 19, 1999, assigned to the assignee of this application and herein incorporated by reference. 
     FIG. 1 is a hexagonal coverage area  10  including cells  12  defined by a phased array antenna system, where each cell  12  represents a pixel beam. The antenna system may provide a plurality of communications channels where each channel includes a plurality of pixel beams. In this example, each channel includes a hexagonal group  14  of seven cells  12 , where each cell  12  in each group  14  is labeled A-G. The particular user may be located in the center cell  12  of the group  14 , where the perimeter cells  12  in the group  14  provide for increased communications performance. Communications signals from locations in the group  14  are received by the antenna system on the satellite, and then retransmitted to another group  14  for communications purposes. The phased array antenna system provides beam steering for all of the groups  14 . 
     Intentional and unintentional jamming of satellite uplink signals occurs in various situations. For example, in a military situation, satellite communications are used to transmit signals and information to and from a warfare theatre or hostile environment. The reception area for the uplink communications signals in the hostile environment may be jammed by the enemy using a high powered transmitter. If the jamming signal comes from with-in the channel area for the uplink signal, it is referred to as in-beam jamming, and if it comes from outside of the channel area for the uplink signal, it is referred to as out-of-beam jamming. The jamming signal must be at the frequency of the uplink signal to be effective for jamming purposes. Jamming signals can also come from unintentional or friendly sources that inadvertently interfere with the satellite uplink signals. 
     In order to eliminate or reduce the effects of jamming signals in both hostile and friendly scenarios, it is known to employ nulling antenna systems that detect the presence of a jamming signal, and provide an antenna null in the antenna radiation or reception pattern so that the jamming signal does not significantly affect the uplink signal. Particularly, nulling antenna systems are able to determine the direction of the jamming signal and create a null or void in the radiation pattern of the antenna so that it in effect does not see the jamming signal. In order to be able to block or null the jamming signal so that it does not affect the ability to transmit the downlink signal, it is necessary to determine the location of the signal, whether it be from an in-beam or out-of-beam jamming source, and then provide the null at that location. 
     An adaptive weighting system is generally used in nulling antenna systems to sample the received pixel beams in a particular channel to determine if a jamming signal is present. The weighting system then weights the pixel beams in the channel to block the jamming signal. The weighting system generally includes a correlator to correlate each of the pixel beams with the combined beam for the channel to determine if a jamming signal is present. Once the correlator determines that a jamming signal is present, algorithms are used to determine the location of the jamming signal. The algorithm goes through each pixel beam separately using a weighting function to determine where the jamming signal is being received from. The weighting function provides the null by inverting the phase of the received signal at the appropriate location. When the weighting of the pixel beams blocks the jamming signal and the image is cleared up, the antenna system knows where the jamming signal is being received from, and can make weighting adjustments accordingly. Various algorithms that perform this function are known to those skilled in the art. 
     The nulled area of the radiation pattern of the antenna has a width and a depth which determines its effectiveness in nulling the jamming signal. However, creating a null in the radiation pattern of the antenna also creates a “blind spot” in the uplink signal. Therefore, it is desirable to limit the size of the null while still blocking the jamming signal. In other words, it would be desirable to provide higher nulling resolution to tightly define the null in the radiation pattern so that more of the uplink signal can be processed by the antenna system. This would minimize the area of the radiation pattern that is nulled, and still provide effective anti-jamming. In this manner, it is possible to provide communication to a wider area around the jamming source. 
     It is known by antenna theory to narrow the antenna radiation pattern by increasing the aperture size of the antenna, i.e., providing more antenna elements. However, adding more antenna elements to increase the aperture size significantly increases the cost and complexity of the antenna system. It would be desirable to increase the aperture of the nulling antenna, without significantly increasing the number of elements to provide more effective nulling capabilities. It is therefore an objection of the present invention to provide such a nulling antenna. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a nulling direct radiating array is disclosed that includes a main phased array and a plurality of auxiliary arrays symmetrically disposed around and spaced apart from the main array. The main array includes a plurality of antenna elements and a beam forming system that generates one or more channels made up of pixel beams. The pixel beams from the main array are connected to a nulling processor along with the combined signal from the antenna elements of the auxiliary arrays. An adaptive weighting network and an adaptive weight generator within the nulling processor determine whether a jamming signal exists in the channel, and weights the pixel beams from the main array accordingly to block the jamming signal. The auxiliary arrays provide a wider beam aperture that is able to more narrowly define the null in the radiation pattern of the main array. 
     Additional objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plurality of satellite pixel beam cells arranged in a particular field-of-view pattern; 
     FIG. 2 is a plan view of an antenna system including an EDRA and a plurality of auxiliary arrays, according to an embodiment of the present invention; 
     FIG. 3 is a schematic block diagram of a nulling direct radiating array, according to an embodiment of the present invention; and 
     FIG. 4 is a schematic block diagram of a nulling direct radiating antenna, according to another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following discussion of the preferred embodiments directed to a nulling direct radiating array is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the discussion below of the nulling direct radiating array is for satellite communication purposes, but as will be appreciated by those skilled in the art, can be used for anti-jamming purposes in other types of communication systems. 
     FIG. 2 is a plan view of a nulling antenna system  16  including an EDRA  18  and auxiliary antenna arrays  20 - 24  symmetrically positioned around the EDRA  18 . Each of the arrays  18 - 24  includes a plurality of antenna elements  26 , some of which are shown here. The EDRA  18  is used for receiving and transmitting communications signals to and from the Earth, and the combination of the EDRA  18  and the auxiliary antenna arrays  20 - 24  is used as a nulling array for anti-jamming purposes. In this example, the EDRA  18  is hexagonal in shape and the arrays  20 - 24  are square in shape. However, for different applications, the EDRA  18  and the arrays  20 - 24  can have other shapes. The arrays  20 - 24  are provided a certain radius R a  away from the center of the EDRA  18 . In one example, R a  is about the diameter of the EDRA  18 , but can be other values depending on the particular application. Additionally, the arrays  20 - 24  are positioned symmetrically around the EDRA  18  for performance purposes. More or less auxiliary arrays  20 - 24  can be provided, but at system expense or performance. 
     The wider the aperture of an antenna, the narrower its radiation and reception pattern. In order to provide a nulling radiation pattern that only nulls the specific location in a communications radiation pattern where a jamming signal is located and doesn&#39;t significantly interfere with the communications signal at other locations, it is necessary that the nulling radiation pattern be narrow. To accomplish this, it is desirable to increase the aperture width of the nulling antenna arrays. A channel group  14  is identified around a particular communications user on the Earth by the EDRA  18 . The radiation pattern of the nulling array is directed towards the jamming signal identified within that group  14  if it is in-beam jamming, and out of the group  14  if it is out-of-beam jamming. The nulling radiation pattern is subtracted from the communications radiation pattern by inverting its phase so that the jamming signal is nulled from the communications signal. 
     FIG. 3 is a schematic block diagram of a nulling antenna array system  30 , according to an embodiment of the present invention. The system  30  includes an EDRA  32  of the type discussed above. In this example, the EDRA has  720  antenna elements  34 , and provides a full Earth field-of-view. The EDRA  32  receives uplink communications signals from the Earth, and provides phase weighting and beam steering of the received signals to combine a certain number of the signals into pixel beams directed in a certain direction. In this example, seven pixel beams combine to form one communications channel. Each channel defines a group  14  on the Earth. 
     Each antenna element  34  is connected to a receiver front end  36 . The front end  36  includes a low noise amplifier (LNA)  40  that amplifies the received signal. The amplified signal is applied to a mixer  42  for frequency down-conversion purposes to an intermediate frequency (IF). A local oscillator (LO) signal is applied to a distribution board  46  that distributes the LO signal to each of the mixers  42  to be mixed with the amplified signal. In order to maintain coherence between all of the mixers  42 , the distribution board  46  further includes phase shifters  48  that align the LO signals in phase prior to the LO signals being applied to the mixers  42 . The down-converted IF signals from the mixers  42  are applied to an attenuator  50  within the distribution board  46 . The attenuators  50  provide amplitude tapering to control beam side lobes, as is well understood in the art. 
     The down-converted signals from the antenna elements  34  are then applied to a back end unit  56 . The back end unit  56  performs beam steering functions in three steps. A Butler matrix  54  receives the down-converted antenna element signals and converts them to a plurality of pixel beams. In other words, the Butler matrix  54  converts the received signals from an element space to a beam space to allow the EDRA  32  to receive signals anywhere on the Earth. In this example, the Butler matrix  54  transforms the  720  antenna element signals into 448 pixel beams. The Butler matrix  54  also provides phase combining of the antenna element signals. 
     Each of the pixel beams from the Butler matrix  54  is then applied to a separate  24 -way splitter  60  in a beam forming matrix  58 . Each of the 24-way splitters  60  splits its pixel beam twenty-four times and sends a separate one of the beams to a 448-to-7 switch  62 . Each switch  62  receives one pixel beam from each splitter  60  to combine the pixel beams into  24  seven beam channels. The seven pixel beam outputs from each switch  62  are applied to a combiner  64  that combines the signals into a single beam channel. A more detailed discussion of the operation of the EDRA can be found in the &#39;526 application referred to above. 
     According to the present invention, the nulling antenna array system  30  includes three conventional phased arrays (CPAs)  70 ,  72  and  74 , representing the auxiliary arrays  20 - 24  above. Only the CPA  70  will be discussed herein, with the understanding that the other arrays  72  and  74  operate in the same manner. The CPA  70  includes a plurality of antenna elements  80 . In one embodiment, there are one-tenth the number of antenna elements in the CPAs  70 - 74  as there are in the EDRA  32 . However, this is by way of a non-limiting example in that the number of antenna elements in the CPAs  70 - 74 , as well as the actual number of CPAs, may be different for different applications. A front end  82  of the array  70  includes an LNA  84  and a mixer  86  that operate in the same manner as discussed above. An LO signal is applied to a distribution board  88  that distributes the LO signal to the mixers  86  to convert the high frequency signals received by the elements  80  into intermediate frequency signals. 
     Beam steering is provided in a receiver back-end  90  by IF phasers  92 . The IF phasers  92  provide the relative phase differences between the various antenna elements  82  so that the signals received from anywhere on the Earth are in phase relationship to each other. Attenuators  94  provide tapering for side lobe control, and IF power combiners  96  combine all of the received signals into a single combined signal. Because the CPAs  70 - 74  are relatively far apart, additional phase delaying may be necessary. Therefore, a time delay line (TDL)  98  is provided to delay the combined signal from the combiners  96  so that the signals from the CPAs  70 - 74  are aligned in phase. In other words, the TDL  98  provides phase alignment for signals that are greater than 360° apart. 
     In this example, only one of the  24  channels from the EDRA  32  is capable of providing nulling. Particularly, the seven pixel beams from the switch  106  and the combined beams from the CPAs  70 - 74  are hardwired to a nulling processor  108 . The processor  108  includes an adaptive weighting network  112  and an adaptive weight generator  114 . The pixel beams from the switch  106  and the combined signals from each of the combiners  96  in the CPAs  70 - 74  are applied as ten inputs to the adaptive weighting network  112 . Ten signal couplers  116  are provided to couple a portion of the signals off of each line applied to the adaptive weighting network  112  and apply the coupled signal to the adaptive weight generator  114 . The seven pixel beams from the switch  106  that make up the communications channel are separated and combined in the adaptive weighting network  112 . A combiner  110  for the nulling array combines the seven pixel beams from the switch  106  and the auxiliary beams from the arrays  70 - 74 . 
     The adaptive weight generator  114  goes through a known mathematical algorithm to determine if a jamming signal does exist, and if so where it is located. The adaptive weight generator  114  provides a weighting for each input line based on this determination that is applied to the adaptive weighting network  112 . For example, if the adaptive weight network determines that a jamming signal is on one of the pixel beams from the switch  106 , it will weight that line to zero so that it does not influence the overall signal. Any combination of pixel beams can be weighted in this manner. The adaptive weighting network  12  provides the adaptive weighting by inverting the phase of the nulling signal and combining it with the communications signal on the channel from the switch  106  so that the jamming signal is nulled. 
     The adaptive weighting network  112  receives the weighting from the adaptive weight generator  114  and applies the weighting on the received signals from the switch  106  and the CPAs  70 - 74 . An optional feedback path  120  can be applied from the combined output of the combiner  110  to the adaptive weight generator  114  so the adaptive weight generator  114  establishes that the jamming signal has in fact been nulled. The signal outputs from the adaptive weighting network  112  are applied to the combiner  110  that provides the weighted beam output. The adaptive weighting network  112  and the adaptive weight generator  114  can be digital or analog depending on the particular embodiment. The discussion above of the nulling processor  108  is by way of example. The present invention can use any suitable nulling processing known in the art. 
     The discussion above with reference to the nulling antenna array system  30  only nulls one of the  24  channels from the EDRA  32 . In an alternate embodiment, any number of the available channels can have nulling capabilities. To show this embodiment, FIG. 4 depicts a general nulling antenna array system  130  that provides nulling capabilities for each of the channels from the EDRA  32 . In the system  130 , like components to the system  30  are identified with the same reference numeral. In this embodiment, there are M number of channels, where K number of pixel beams make up a channel. The number of channels is general, so that the 24-way splitters  60  are replaced with M-way splitters  134 . Likewise, because each M channel includes K number of pixel beams, the switches  62  have been replaced with 448-to-K switches  136 . Further, the number of auxiliary arrays is general so there are N number of auxiliary arrays  132 . 
     Each of the switches  136  is attached to a nulling processor  108  in the same manner as the switch  106  above. Further, each array  132  includes M number of back-ends  90 , one for each M channel, where the signals from the distribution board  82  are split by an M-way splitter  140 . A combined output from the array  132  is provided to each nulling processor  108 , where each combined output is applied to a TDL  98 . Therefore, each M channel from the back-end unit  56  can be nulled separately. 
     The foregoing discussion discloses and describes merely embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.