Abstract:
A polygonal cylindrically shaped phased array antenna forming a radar has an active aperture that focuses in any of one or more angular azimuth directions without inertia. It further includes adjacent multiple similar polygonal staves joined along their vertical edges to form a right regular polygonal cylinder. Each stave is further decomposed into flat panels, wherein each panel has a plurality of antenna elements positioned in a regular rectangular or triangular lattice. Each panel contains a beam forming network that electronically forms and steers an electromagnetic beam for purposes of transmission and subsequent reception. The panels optionally may operate as autonomous radars which when coherently combined form multiple larger antenna apertures, each capable of operating autonomously. A switching network allows transmit power and all requisite radar and control signals to be sent to and received from a selected set of panels anywhere on the polygonal cylinder.

Description:
RELATED APPLICATION 
   This application claims priority to U.S. Provisional Application No. 60/699,621, filed Jul. 15, 2005, the subject matter thereof incorporated by reference herein in its entirety. 

   FIELD OF THE INVENTION 
   This application relates generally to radar systems, and more particularly to cylindrical phased array radar antennas useful for airborne applications. 
   BACKGROUND OF THE INVENTION 
   In both military and civilian terrain mapping and object tracking there exists a need to enable coverage of an earth-fixed azimuth sector from high-altitude airships whose orientation continuously changes. The high-altitude airships are generally gas filled dirigibles or blimps that have shapes designed for maximizing their aerodynamic performance such as lift, maneuverability and stationary or forward movements. The airship&#39;s distinctive skin materials and craft shape often challenge equipment designers in their efforts to effectively mount information gathering instrumentation, such as radar systems. Still, high-altitude airships are receiving increased attention for use as radar sensor platforms because of the inherent capability of an unobstructed view of large segments of the earth&#39;s surface as well the large volume of available space within and/or around the airship. 
   Information gathering missions tend to require radar coverage over a broad azimuth sector that is fixed with respect to the earth&#39;s surface. However, various factors such as the airship&#39;s need to face into the wind, the variable direction of high altitude winds, and the airship&#39;s need to maintain a minimum airspeed for waste heat convection, forces airship orientation to constantly change with respect to the desired coverage sector. These factors require radar systems that can adapt to the changing attitudes in pitch, elevation, yaw and roll movements. 
   As a result, such high altitude airship radar sensors should not only be capable of providing coverage over the desired sector width, but should also be capable of continually reorienting the position of this sector coverage with respect to the airship. Consequently, radar orientation with respect to the airship provides few satisfactory options. 
   One option illustrated in  FIG. 1   a  is to mount a planar phased-array radar flat antenna  110  inside an airship  102 , such that it maintains coverage in a fixed direction by slowly rotating with respect to the airship as the airship orientation changes with respect to the earth. In this configuration, array normal is approximately centered in the desired coverage sector. Electronic steering is then used to position the beam within the sector. Such an internal planar phased array as shown in  FIG. 1   a  provides a beam output that is restricted to about sixty degrees (60°) relative to array normal. Disadvantages associated with such an approach include the undesirable requirements for heavy mechanical components, including a rotary joint and coupler that are incompatible with lightweight airship applications. Furthermore, such a solution would require an increased propulsion power to compensate for a rotating radar antenna&#39;s angular momentum. Still further, the aforementioned planar phased array cannot provide instantaneous coverage over 360°. Moreover, such a solution would suffer significant beamsteering gain loss (e.g. &gt;9 dB) near coverage limits, thus, severely compromising overall operational performance. 
   Another option illustrated in  FIG. 1   b  is to install a non-planar radar antenna phased array  110  on an airship&#39;s doubly-curved surface as opposed to internally to airship  102  (see  FIG. 1   a ), such that the phased-array conforms to a large fraction of the airship&#39;s outer surface  105 . In such a surface-conformal phased array radar system, a portion of the array whose normal approximately matches the center of the desired coverage sector is activated and then used to form and electronically position the beam within the desired sector. Numerous problems exist with this approach as well. 
   As is known in the art, a collimated beam of radio frequency energy may be formed and steered by controlling the phase of the energy radiated from each one of a plurality of antenna elements in an array thereof. A portion of the array whose normal approximately matches the center of the coverage sector might then be activated and used to form and electronically position the beam within a geographic sector. 
   For example, the curvature of surface  105  varies as a function of position on the airship surface (which is made larger or smaller due to gas expansion and contraction) so that antenna radiator element-to-element separations must also change as a function of position in order to maintain conformality. In addition, non-uniform element-to-element separations degrade the shape, gain, and sidelobes of the electronically scanned beam. Furthermore, range coverage and azimuth beamwidth are non-uniform in azimuth, as the projected aperture changes significantly as a function of azimuth. Accurate beamforming and shaping is therefore difficult because the airship surface expands and contracts significantly due to air density and temperature variations and tends to undulate or flutter due to airflow. 
   Still further, manufacturing and construction costs associated with the above approaches are high, due at least in part because the variable surface curvature requires the sub-panels constituting the array be of many different shapes and designs, creating adherence problems analogous to the well publicized space shuttle tiling problem. 
   Time-varying aperture shape associated with the conformal array approach also causes pulse-to-pulse variations that limit clutter cancellation. Other problems associated with the aforementioned approaches include complicated power and signal distribution, as different parts of the array may be hundreds of meters apart. Changing airship shapes also make calibration difficult, particularly with regard to the difficulty or inability to inject test signals into the antenna elements in the above surface-conformal approach. 
   An alternative mechanism for a radar system useful in a vessel such as a high altitude airship, and which overcomes one or more of the above-identified problems, is highly desired. 
   SUMMARY OF THE INVENTION 
   According to an aspect of the present invention, a radar antenna in the form of a right regular polygonal cylinder has multiple generally flat rectangular panels, each capable of operating as an autonomous electronically scanned radar, and each capable of independently forming, steering, and shaping transmit and receive beams. The flat rectangular panels are joined along vertical edges and tangent to a virtual right circular cylinder such that the panels form a right polygonal cylinder having M panels along the circumference of the cylinder and N panels along the axis of the cylinder, where M is an integer greater than or equal to three and N is an integer greater than or equal to one. A signal switching distribution network allows transmit power and requisite radar and control signals to be sent to and received from selected subsets of the panels. A processor coherently combines the outputs of the selected subsets of the panels to provide an output signal indicative of the requested coverage area. 
   According to another aspect of the present invention, a polygonal cylindrically shaped antenna radar array has an active aperture that focuses in one or more angular azimuth directions without inertia. The array further includes M (M≧3) adjacent, flat rectangular staves of like shape and joined to form a right regular polygonal cylinder. Each of the M staves is further decomposed into N (N≧) identical flat rectangular panels joined along their horizontal edges wherein each panel includes a plurality of antenna elements positioned in rectangular, triangular or hexagonal tessellation of the plane or lattice. Each panel contains a beam forming network that electronically forms and steers an electromagnetic beam for purposes of transmission and subsequent reception. The panels optionally may operate as autonomous radars or coherently, which when electronically combined form multiple larger antenna apertures, each capable of operating autonomously. A switching network allows transmit power and all requisite radar and control signals to be sent to and received from a selected set of panels anywhere on the polygonal cylinder. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. The various features of the drawings are not specified exhaustively. On the contrary, the various features may be arbitrarily expanded or reduced for clarity. Included in the drawing are the following figures: 
       FIG. 1   a  illustrates mounting a trainable planar phased-array radar antenna internal to an airship. 
       FIG. 1   b  illustrates the conformal mounting of a phased-array radar antenna on the doubly-curved surface of an airship. 
       FIG. 2   a  illustrates mounting a polygonal cylindrical antenna array internal to the airship in accordance with an exemplary embodiment of the present invention. 
       FIG. 2   b  illustrates a cut-away view of a polygonal cylindrical antenna array mounted within an airship in accordance with an exemplary embodiment of the present invention. 
       FIG. 2   c  illustrates a cut-away end view of a polygonal cylindrical antenna array mounted within airship in accordance with an exemplary embodiment of the present invention. 
       FIG. 3  illustrates a polygonal cylindrical antenna array mounted on the surface of and supported by an inflatable pressure vessel in accordance with an exemplary embodiment of the present invention. 
       FIG. 4  is a functional block diagram of a phased-array radar based on the polygonal cylinder antenna in accordance with an exemplary embodiment of the present invention. 
       FIG. 5  illustrates a plan view of a phased-array radar antenna having staves along the circumference of the cylindrical surface. 
       FIG. 6   a  illustrates a perspective view of a polygonal cylindrical antenna array having active and inactive staves in accordance with an exemplary embodiment of the present invention. 
       FIG. 6   b  illustrates a plane view of a polygonal cylindrical antenna array having sets of active and inactive staves in accordance with one embodiment of the present invention. 
       FIG. 7   a  illustrates a perspective view of a polygonal cylindrical antenna array having sets of staggered active and inactive staves in accordance with one embodiment of the present invention. 
       FIG. 7   b  illustrates a plane view of a polygonal cylindrical antenna array having sets of staggered active and inactive staves in accordance with one embodiment of the present invention. 
       FIG. 8  is a graphical plot of constant-SNR contour against range, height, and elevation for pencil beam performance of an antenna of the present invention. 
       FIG. 9  is a graphical plot of constant-SNR contour against range, height, and elevation for GMTI performance of an antenna of the present invention. 
       FIG. 10  is a graphical comparison of transmit beams with and without phase shaping of an antenna of the present invention. 
       FIG. 11  is a graphical comparison of receive beams with &amp; without amplitude and phase shaping of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In the figures to be discussed, the circuits and associated blocks and arrows represent functions of the process according to the present invention which may be implemented as electrical circuits and associated wires or data busses, which transport electrical signals. Alternatively, one or more associated arrows may represent communication (e.g., data flow) between software routines, particularly when the present method or apparatus of the present invention is a digital process. In the embodiments of the invention disclosed herein, the airships are gas filled dirigibles, however, the invention as disclosed is not limited in its application to dirigibles, but may be employed in other types of aircraft, satellites or stationary ground radar systems. 
   As previously discussed with regard to  FIG. 1   a  and  FIG. 1   b , an airship  102  surface curvature  117  changes continuously over an entire surface  115 , and an antenna array  110  on such a doubly-curved surface must continuously change its antenna radiating element-to-element spacings to maintain conformality. Variable element  119  spacings and changing surface curvature  117  also degrade the quality of the electronically formed beam and corresponding sidelobe structure. Radars mounted as illustrated in  FIG. 1   b  typically produce non-uniform azimuth beamwidth coverage, with the changing surface curvature  117  being particularly severe near the nose  107  or tail  109  of the vessel. Manufacturing and construction costs for the above designs are high because the variable surface curvature  117  requires that the sub-panels  119  constituting the conformal array be different shapes and designs. Air pressure variations due to changes in air density and/or temperature also cause the (non-rigid) airship outer surface to change shape as the airship  102  expands and contracts. Air flowing past the surface  115  also induces localized shape changes, essentially causing the surface  115  to undulate or flutter. Each of these variables contributes to making it virtually impossible to perform accurate beamforming. Additionally, an unpredictable varying surface shape  117  whose variations change as a function of time also induces pulse-to-pulse errors that limit the radar&#39;s ability to cancel severe mainlobe surface clutter. Power and signal distribution is also a problem, particularly for an airship-surface-conformal array, due to the severe spacing (e.g., hundreds of meters apart) between different parts of the array. 
   Referring now to  FIG. 2   a , there is shown a schematic representation of a polygonal cylinder array  210  according to an exemplary embodiment of the present invention. Antenna  210  may be mounted either internal to the airship  102  (as shown in  FIG. 2   a ) or beneath or above its exterior surface  115  or hull, and provides virtually instantaneous scan capability over a full 360° azimuth without inertia and without scan loss. Such mounting avoids problems associated with the surface-conformal array&#39;s shape change, since the polygonal cylinder array  210  structure is independent of the airship surface  115 . The decoupling from the airship  102  surface  115  and decomposition of antenna panel  206  elements into flat vertical staves  207  simplifies electronic calibration, as the ability to inject test signals into the antenna elements is not impeded by changing physical relationships. Furthermore, such a configuration provides for complete modular maintenance, replacement and repair of the antenna and radar components at the line or depot repair level. 
     FIG. 2   a  shows the airship  102  having a hull in which is housed polygonal cylindrical antenna array  210  comprising a right cylinder  203  having mounted upon its outer surface  205  antenna element panels  206  arranged in columns of staves  207 . The panels  206  may be formed from various shapes such as a triangle, hexagon or rectangle, however, each panel&#39;s outer surface is flat and perpendicular to the axis of the right cylinder. The panels  206  as mounted form the polygonal structure of the cylindrical antenna array  210 . It will be recognized by those skilled in the pertinent arts that the location and manner of mounting the cylindrical antenna array  210  will depend on various factors, including the design and choice of the particular application (e.g. particular vessel or airship), and other design choices including weight, balance, and performance of the radar system to meet its intended objectives, for example. 
     FIG. 2   b  illustrates a cut-away view of the polygonal cylindrical antenna array  210  mounted within an airship  102 . The cylindrical antenna array  210  is shown mounted through support members  320 .  FIG. 2   b  illustrates a cut-away end view of antenna array  210  mounted within the airship  102 . In this embodiment of the invention, the supported support members  320  that attach the outer housing of the cylindrical antenna array  210 , also attach to the inner structure of the airship  102 . 
     FIG. 3  illustrates cut-a-way view of the polygonal cylindrical antenna array  210  mounted on the outer surface of an inflatable pressure vessel  310  whose purpose is to keep the antenna rigid with minimum weight. The pressure vessel and antenna are then mounted inside the airship  102 . The antenna array  210  and pressure vessel  310  are supported by support members  320  that attach the outer surface or housing of the pressure-vessel-mounted cylindrical antenna array  210 , which contains electronic processing circuitry and power systems  325 , to the structure of the inflatable pressure vessel  310 . The inflatable pressure vessel provides lift to the airship  102 , but its primary purpose is to provide a lightweight and rigid support for the cylindrical array  210 . In fact through additional partial inflation pressures over the interior pressure of airship the novel configuration of the rigid cylinder shape achieves a relatively lightweight formation. In addition the antenna cylinder shape is highly scalable in terms of radar frequency, cylinder height, cylinder diameter, panel size, number of staves, and number of rows. 
   The antenna array  210  and supporting electronics may be jointly or separately mounted internally or beneath the airship to permit the radar coverage sector to be instantaneously repositioned to any desired azimuth, thus maintaining coverage of an earth-fixed azimuth sector as the airship changes its orientation with respect to the earth. The invention can also be used as a ground-based radar, independent of its airship application, where instantaneous inertial-less 360° azimuth coverage is desirable. 
     FIG. 4  shows a functional block diagram of a cylindrical polygonal antenna array  500   a  and an electronic radar processing system  500   b  for controlling and processing signals to/from the antenna array according to an exemplary embodiment of the present invention. The processing system includes an analog beamforming portion and a digital beamforming portion, in accordance with an embodiment of the present invention. Each panel  206   11 - 206   mn  of the antenna  210  has a corresponding set of transmit-receive subsystems (“T/R subsystems”)  208   11 - 208   nm . Each set of transmit-receive subsystems (“T/R subsystems”)  208   11 -208 nm  comprises individual T/R modules a  513   1 - 513   r , having optional phase shifters with amplitude control, generate multiple independent and simultaneous beams distributed to one of an associated panels  2061 , of the entire set of panel elements  206   11 - 206   nm . In a receive mode the T/R modules  513   1 - 513   r  are synchronized to the previous transmissions. In one configuration, e.g., the multiple simultaneous Ground Moving Target Indicator (GMTI) radars, the multiple simultaneous transmissions emanate from separate radars or panels  206   11 - 206   nm  on the cylinder  205 . The amplitude of panel  206   11 - 206   mn , both in transmission and reception, may be variably controlled depending on the mission and the need to improve the reliability of signal capture. Amplitude control typically is used to maintain low sidelobes on transmit and receive and in some cases is used in combination with phase control to shape the transmit beam. In another embodiment, the system  500   b  broadens the GMTI radar transmit beams in azimuth using phase spoiling. Each broad transmit beam is filled with multiple simultaneous and narrow receive beams to provide more time on target than would be typically be available with a single transmit-receive beam pair. Each flat rectangular panel  206  may be operated as an independent sub-radar, wherein each panel individual T/R modules a  513   1 - 513   r  has a corresponding element  509 . In some cases these sub-radars are grouped and coherently combined to form multiple special-purpose radars, such as the multiple staggered rows, which serve as multiple independent GMTI radars or are coherently combined to form a single pencil-beam radar for track. 
   Still referring to  FIG. 4  and  FIG. 5 , the antenna  210  array may also optionally utilize a variety of beam shapes during operation. For example, the GMTI search radars use a non-linear phase progression across all element columns on transmit and vector or complex (amplitude and phase) weighting on receive to shape the two-way beam gain in elevation. This shaping is such that signal-to-noise ratio (“SNR”) against a reference surface target at a fixed azimuth is approximately constant for any target range from the horizon into some pre-determined minimum range. The transmit beam is further broadened in azimuth by applying a non-linear phase progression across the horizontal dimension of the transmit aperture. Multiple simultaneous receive beams, each with identical elevation shape and each steered to a different azimuth then fill the broadened transmit beam. Each of the resulting simultaneous two-way beams then has the desired constant SNR property. In GMTI track, however, more panels  206  are combined and the element  206  weighting is chosen to produce a beam that is very narrow in both azimuth and elevation, as for example what is commonly referred to as a pencil beam. 
     FIG. 5  illustrates a plan view of a portion of the cylinder array  210  shown in  FIG. 4  as an 18-stave set, having six rows. The multiple stave beams such as the six staves  560   a - 560   f  are coherently shaped and steered by element level analog beamforming, and then combined by stave-level digital beamforming to form pencil beam  580 . In the illustrated example, each stave offsets 20° relative to its adjacent neighbors, 20° being characteristic of an 18-stave design. In coherently combining the multiple staves via a digital beaming system to be more fully described below, the example pencil beam  580  has been steered to 10°, the maximum electronic steering angle employed by this particular set of staves. The net beam has a higher gain than the individual stave beams  560   a - 560   f  and a narrower beamwidth consistent with the projection of the total 6-stave aperture  585  as projected in the direction of the beam  580 . 
   The panel or stave near-field pattern of the antenna  210  is approximately a projection of the stave or shape  206  in a direction perpendicular to the plane of the panel  206 . The panel beam begins to collimate and diverge at a distance approximately given by D 2 /λ were D is the aperture width in meters and λ is the wavelength in meters. The far-field phase front is planar and subtends an angle with respect to the antenna array  210  face which is a function of the beam steering direction. 
   MIMO (Multiple Input Multiple Output) radar applications may also optionally be employed, where multiple sub-radars each transmit different signals, which are then received by multiple sub-radars. The outputs of these radars are combined depending upon mission assigned to the MIMO radar such as by way of example and not limitation, achieving high probability of detection or resolving targets from background or electronic countermeasures. 
   Again referring to  FIG. 4 , the cylindrical polygon array  500   a  circumscribes the outer periphery of the cylindrical surface  205  of the phased array antenna  210 , panels  206  and for purposes of illustration, comprise a subset of flat active panels  515  and a subset of inactive panels  517  in accordance with one embodiment of the present invention. The panels  206  are arranged as adjacent staves in a generally square matrix around the circumference of the cylinder  203  and along the operational length of the cylinder. The number of matrix elements will be a function of the physical dimensions of the operational circumference, length of the cylinder  205  and size of the panel  206 . The electronic system  500   b  may optionally adjust each antenna  210  panel element  206  amplitude and phase independent of all other elements. This “phase-phase” capability enables each of the panel  206  elements to shape and steer its beam in two dimensions. This in turn enables configuring different radars from sets of sub-radars. Panels  206  are typically broadband, but broadband is not a limitation of the basic invention in that any bandwidth falls within the scope of the invention as disclosed. With regard to beamforming, the beam created by each flat rectangular panel  206  can be individually shaped in azimuth and elevation for very low sidelobes. When the beams from multiple panels  206  are coherently combined by digital beamforming the net beam reliably has low sidelobes. This is in contrast to pure cylinder arrays, which suffer from sidelobe blooming where cylinder curvature blocks some elements from view at wide scan angles so the sidelobes at these angles increase dramatically. In addition, the use of flat panels  206  greatly simplifies and improves calibration, sidelobe control, and beam-pointing accuracy. It also reduces SNR loss at the peak of the beam. 
   As further illustrated in  FIG. 4 , the system  500   b  receives an RF signal from panel  206  having elements  509 , which are digitized and later combined. Depending upon the radar frequency, it may be desirable to decompose each panel  206  into sub-panels, each with its own manifold. Each sub-panel would then have its own manifold and transceiver (transmitter and receiver), such that the transceiver outputs would be in-phase (I) and quadrature-phase (Q) signals. More specifically, the phased array antenna array  210  receives RF energy forming a desired beam pattern by imparting a prescribed amplitude-phase distribution over the wave field emanating from its aperture or panels  206 , each containing a radiating element  509 . An analog portion  547  of system  500  comprises a plurality of a T/R modules  513   1 - 513   r , a plurality of panel RF manifolds  516   1 - 516   r  (one manifold per panel or sub-panel if there are sub-panels) that feed and receive T/R modules  513   1 - 513   r  signals, a plurality of transceivers comprising wave form generators and up conversion apparatuses  514   1 - 514   r , that feed the panel RF manifolds  516   1 - 516   r , and a plurality of receiver and digital demodulators  519   1 - 519   r  that receive radar signals from the panel RF manifolds  516   1 - 516   r . 
   The plurality of T/R modules  513   1 - 513   r  amplify the transmit signals on transmission of the radar signal and amplify the received radar signal during reception. The T/R modules  513   1 - 513   r  also serve to provide an element  509  phase and amplitude control. The panel RF manifolds  516   1 - 516   r  receive amplified element  509  signals and feed the signals to the plurality of receiver and digital demodulators  519   1 - 519   r . The panel RF manifolds  516   1 - 516   r  distribute element  509  signals on transmit and coherently combine element signals on receive. 
   A digital portion  549  of system  500   b  comprises a digital fiber link  507  having to feed the plurality of wave form generators and up conversion apparatuses  514   1 - 514   r  and to receive the plurality of receiver and digital demodulators  519   1 - 519   r  radar return signals. The demodulators within the receiver and digital demodulators  519   1 - 519   r  receive radar return signals which are mixed with a local oscillator  510  to produce a demodulated radar signal. Essentially, the receiver and digital demodulators  519   1 - 519   r  and later associated beamforming networks electronically combine the panel  206  elements  509  to amplify the beamformer RF output and associated downconverters into digitized in-phase (I) and quadrature-phase (Q) signal that are then passed on to a signal processor. 
   A panel selector and distributor  520  both feeds and receives transmission signals from a fiber link  507 . Fiber link  507  receives analog signals and converts the analog signals to a digital signal so as panel selector and distributor  520  receives radar return signals from the fiber link  507  for further processing. The waveform generators and up conversion apparatuses  514   1 - 514   r  and associated downconverters digitize an in-phase (I) and quadrature-phase (Q) signal that is then passed on to the panel selector and distributor  520 . The panel selector and distributor  520  receives input data from the radar controller  530  to select the panels  206  that array as a group determined by the mission. Controller  530  also inputs data directly to the T/R modules  513   1 - 513   r  to establish the element  509  phase and gain control commands. 
   A subsystem  535  receives in-phase (I) and quadrature-phase (Q) signal from the panel selector  520 . The subsystem  535  selects the number of panels  206  and the number of radars configured and sets up the multi-radar in-phase (I) and quadrature-phase (Q) output signals. The digital data from sub system  535  feeds a multi-radar signal and data processing system  534  to achieve proper pulse compression and to choose selected processing modes to overcome the effects of clutter or electronic countermeasures. The multi-radar signal and data processing system  534  output provides input to the radar controller  530  for among other things multi-radar detections and mapping data. Radar controller  530  also receives appropriate input from an air ground command  540 , which in turn is dependent on human-machine interface  550  that allows human intelligence through an air ground link  555  to establish various mission operating parameters. 
   It is understood that the processor, memory and operating system with functionality selection capabilities can be implemented in hardware, software, firmware, or combinations thereof. In a preferred embodiment, the processor functionality selection, threshold processing, panel selection and mode configuration may be implemented in software stored in the memory. It is to be appreciated that, where the functionality selection is implemented in either software, firmware, or both, the processing instructions can be stored and transported on any computer-readable medium for use by or in connection with an instruction execution system, apparatus, or device, such as a computer-based system, processor-containing system, or other system that can fetch the instructions from the instruction execution system, apparatus, or device and execute the instructions. 
   Further, it is understood that the subject invention may reside in the program storage medium that constrains operation of the associated processors(s), and in the method steps that are undertaken by cooperative operation of the processor(s) on the messages within the signal and data processing network. These processes may exist in a variety of forms having elements that are more or less active or passive. For example, they exist as software program(s) comprised of program instructions in source code or object code, executable code or other formats. Any of the above may be embodied on a computer readable medium, which include storage devices and signals, in compressed or uncompressed form. Exemplary computer readable storage devices include conventional computer system RAM (random access memory), ROM (read only memory), EPROM (erasable, programmable ROM), EEPROM (electrically erasable, programmable ROM), flash memory, and magnetic or optical disks or tapes. Exemplary computer readable signals, whether modulated using a carrier or not, are signals that a computer system hosting or running the computer program may be configured to access, including signals downloaded through the Internet or other networks. Examples of the foregoing include distribution of the program(s) on a CD ROM or via Internet download. 
   The same is true of computer networks in general. In the form of processes and apparatus implemented by digital processors, the associated programming medium and computer program code is loaded into and executed by a processor, or may be referenced by a processor that is otherwise programmed, so as to constrain operations of the processor and/or other peripheral elements that cooperate with the processor. Due to such programming, the processor or computer becomes an apparatus that practices the method of the invention as well as an embodiment thereof. When implemented on a general-purpose processor, the computer program code segments configure the processor to create specific logic circuits. Such variations in the nature of the program carrying medium, and in the different configurations by which computational and control and switching elements can be coupled operationally, are all within the scope of the present invention. 
   Referring now to  FIG. 6   a , there is shown a perspective view of the polygonal cylindrical antenna  210  array having selective sets of active staves  515  in accordance with one embodiment of the present invention. Selected sets of panels or staves means that signals and power are sent to various subsets of panels  206   a - 206   n  to form one or more active radars. As previously indicated the polygonal  205  cylinder is mounted inside or beneath the airship with the staves oriented vertically and to form a beam at a given azimuth, a subset of panels or staves  206  whose average normal is closest in azimuth to the desired beam azimuth are electronically identified. Staves  206  or sub-panels whose individual normal deviate from the desired azimuth direction by more than some pre-selected threshold angle are electronically excluded. The  FIG. 5   a  panel selector  520  in combination with panel-level multi-radar  535  then configures the selected staves or sub-panels as a radar whose outputs are coherently combined, and if necessary, appropriate phase progressions are applied to electronically steer the net beam to the desired angle. Since the staves  206  or sub-panels are selected to point approximately in the desired direction, electronic steering need not steer the beam by more than 180/N degrees, where N is the number of staves. As the airship changes orientation with respect the desired earth-fixed azimuth to be probed, an updated set of staves  206  or sub-panels is selected whose average normal is closest in angle to the desired beam position. Electronic steering is then applied again to provide a fine beam correction to position the beam exactly at the desired azimuth. Even if the airship rotates through a full 360 degrees, the selected set of staves or sub-panels moves around the polygonal cylinder to maintain its near-normal orientation with respect to the desired beam direction. As a result, the radar beam can continually probe a given earth-fixed azimuth independent of the airship  102  orientation. 
   In an example of active staves dedicated to single pencil-beam radar,  FIG. 6   b  illustrates a plane view of an antenna array  210  having sets of active staves  515  and inactive staves  517 . In the exemplary embodiment shown for purposes of illustration and not limitation, the 16 rows of 48 panels  206   a - 206   n , each arranged in 48 columns (staves) of 16 panels each, for a total 768 panels. In this example, 16 horizontally adjacent staves  605  are activated to electronically form the steered pencil beam, wherein the bulk of azimuth beamsteering is achieved by selecting a set of 16 horizontally adjacent staves  610  whose local normal is closest to the desired pencil beam azimuth. The final position of the pencil beam is achieved via electronic steering of stave set in azimuth and elevation. Note that no set of 16 staves need steer in azimuth more than 3.75° (½ of 7.5°) from its own local normal. The azimuth sector  610  is covered by staves numbered 6 through 21. When operating in a pencil beam mode the 3 dB beamwidth is given approximately by λ/Dp where λ is the wavelength in meters and Dp is the projected width of the aperture onto a plane perpendicular to the beam steering direction. The beam will broaden from this width if aperture weighting is applied to reduce sidelobes. 
   Referring now to  FIG. 7   a , there is shown a perspective view of a cylindrical antenna array  210  having sets of staggered active panels  515  and inactive panels  517  in accordance with one embodiment of the present invention. Instantaneous coverage of a broad azimuth sector does not suffer a significant gain loss as the beam is electronically steered toward the limits of the coverage sector due the ability to stagger each row of active panels  515 . 
     FIG. 7   b  illustrates a plane view of the antenna array  210  having selected sets of panels which are staggered active panels  515  and inactive panels  517  in accordance with one embodiment of the present invention. The cylinder  210  has N staves and M panels  206  per stave for a total of N*M panels. Selected sets of panels means that signals and power are sent to various subsets of these N*M panels  206  to form one or more active radars. The specific selected sets of panels are chosen dependent upon the radar mission (search, track, fire control, etc.), as by way of example and not limitation, the orientation of the airship with respect to the azimuth covered, and a predetermined radar configuration for satisfying the mission. Panels that are not selected remain neither transmit nor receive. The antenna  210  optionally positions nulls in the sidelobes and mainlobes of the beam to reduce interference and jamming. The nulls in the directions of jammers will be formed adaptively on receive, while nulls in the direction of severe surface clutter are formed deterministically. 
     FIG. 8  is a graphical plot of constant-SNR contour against range, height, and elevation for a pencil beam performance of the antenna  210  of the present invention. The example illustrates the performance of the antenna  210  having dimensions eight (8) meters vertical height aperture arrayed in a pencil beam  815  configuration steered to −5.09° and 300 km located at an exemplar elevation of 22,000 meters above earth&#39;s surface along the ordinate and constant ground range  810  from the antenna  210  along the abscissa. From the location of the antenna  210  are indicated constant elevation angles  820  relative to the antenna  210  and contours of constant SNR that are referenced to the SNR at the horizon. In the illustrative example, shown in  FIG. 8 , the transmit aperture is uniformly weighted in amplitude and the receive aperture of 30 dB Taylor weights are applied to each element column. 
     FIG. 9  is a graphical plot of constant-SNR contour against range, height, and elevation for GMTI performance of an antenna of the present invention, further illustrating the antenna  210  having dimensions eight (8) meters vertical height. The result of beam shaping is plotted against contours of constant SNR  825  referenced to the SNR at the horizon. The lines  810  indicate the constant range from the antenna  210 . The GMTI beam in this example is designed to maintain constant reference-target SNR along earth&#39;s surface from −5.09° (300 km) to −60° (24.6 km). In  FIG. 9  the transmit aperture is again uniformly weighted in amplitude and an exponential phase tapers down columns to broaden and shape the elevation response. The receive aperture has an amplitude and phase which tapers down columns designed to shape the response, while countering deficiencies in the transmit pattern. 
     FIG. 10  is a graphical comparison of transmit beams in the elevation plane with and without phase shaping of an antenna of the present plotted against a normalized gain along the ordinate and elevation angles along the abscissa. The line  830  shows the shaped transmit beam from the phase-weighted aperture, and the line  835  shows the transmit beam from a uniformly weighted aperture steered to −5.09° elevation. The transmit phase taper is chosen to complement the complex receive taper such that constant SNR is maintained along earth&#39;s surface from −5.09° to −60° elevation. 
     FIG. 11  is a graphical comparison of receive beams in the elevation plane with and without amplitude and phase shaping of the present plotted against a normalized gain along the ordinate and elevation angles along the abscissa. The line  850  shows the shaped receive beam from the amplitude and phase weighted aperture, and the line  840  shows the receive beam from a 30 dB Taylor-weighted aperture steered to −5.09° elevation. The receive amplitude and phase tapers are chosen to complement the transmit phase taper such that constant SNR is maintained along the earth&#39;s surface from −5.09° to −60° elevation. 
   While the present invention has been described with reference to the illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is expressly intended that all combinations of those elements that perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Substitutions of elements from one described embodiment to another are also fully intended and contemplated.