Abstract:
A hybrid parabolic reflector phased array antenna system which is stowable in a space vehicle and is deployable in space. The antenna includes a large torus which acts as a support structure for a plurality of small reflector cells called super elements, each including its own reflector and an array of feed elements. The torus supports a stretched reflector mesh and matching back-up catanary wires that provide a mechanism for pulling the reflector surface of the cells down to an exact paraboloid. A set of rigid corner posts for stretching the mesh fabric for forming multiple reflectors is also provided. The torus is also used to support individual super element feed arrays for each reflector. The super elements incrementally scan the beam by group selection of feed elements in each feed array with time delay phase control being used to steer the array factor so as to achieve fine steering. Each of the super elements scans incrementally with a selected group of feed elements varying between three and twelve, which are varied in position relative to the focal axis of the feed array. The groups of feed elements of the feed arrays are also controlled so as to mitigate any undesired grating lobe problem.

Description:
This application claims priority on provisional Application No. 60/177,282 filed on Jan. 21, 2000, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to antennas used for space applications and more particularly to a hybrid parabolic reflector phased array antenna which is stowed in a collapsed state for launch and thereafter deployed to form a relatively large reflector type antenna when in orbit. 
     2. Description of Related Art 
     Extremely large scanning antennas for space applications and having limited scan requirements are well known. As the antenna is moved away from the earth, the scan angles are reduced, while the size of the antenna increases. The problem of deploying and steering very large antennas is formidable. Phased arrays generally have too many elements to be cost effective while reflector antennas have configuration problems in amount of blockage and performance degradation at the edges of scan. 
     Currently, large scanning antennas use parabolic reflectors with clusters of elements at and near the focal point to scan the beam. In order to steer the antenna, a large group of elements are used to transmit and receive. On transmit, phase-only control is preferred, while on receive both phase and amplitude controls are used. Moreover, on transmit, amplitude is uniform while in receive it is normally tapered. In order to distribute the power among many elements to reduce the heat concentration, the feed array is typically displaced forward of the focal point; however, this increases the size of the feed rapidly, with commensurate increase in blockage loss. 
     Apertures comprised of a plurality of reflector super elements, all having feed array generating respective antenna patterns, steer a composite beam pattern near the desired direction. In such apparatus, phase or time delay between elements is then used to fine steer the antenna. With large spacing between elements, however, grating lobes are formed, which is the classic problem of using a large element in a phased array. At beam positions between element pointing positions, there can be major grating lobes that sap the power from the main beam and that, in turn, raise serious clutter problems. 
     SUMMARY 
     The present invention is directed to a hybrid parabolic reflector phased array antenna system which is stowable in a rocket and is deployable in space. The antenna includes a large torus which acts as a support structure for a plurality of small reflector cells called super elements, each including its own reflector and an array of feed elements. The torus supports a stretched reflector mesh and matching back-up catanary wires that provide a mechanism for pulling the reflector surface of the cells down to an exact paraboloid. A set of rigid corner posts for stretching the mesh fabric for forming multiple reflectors is also provided. The torus is also used to support individual super element feed arrays for each reflector. The super elements incrementally scan the beam by group selection of feed elements in each feed array with time delay phase control being used to steer the array factor so as to achieve fine steering. Each of the super elements scans incrementally with a selected group of feed elements varying between three and twelve, which are varied in position relative to the focal axis of the feed array. At intermediate positions, where grating lobes appear, the groups of feed elements are reduced in number and selected so as to steer precisely to this position, thus relieving the grating lobe problem. Other methods of mitigating the grating lobe problem include randomly selecting groups of elements about the optimum position, gradually shifting the selected group of elements from one position to another, randomly positioning the feed arrays about their respective focal points, and overlapping feed distributions to gradually shift the feed center and thus precisely adjusting the feed element pattern to agree with the array factor peak position. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description provided hereinbelow and the accompanying drawings which are provided by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a perspective view generally illustrative of a space borne antenna system including an embodiment of the subject invention; 
     FIG. 2 is a front planar view of the L-band subsystem shown in FIG. 1 which forms the subject invention; 
     FIG. 3 is illustrative of a cross-section of the antenna structure shown in FIG. 2 taken along the lines  3 — 3  thereof; FIG. 4A is a perspective view illustrative of the details of a single super element cell of the antenna structure shown in FIG. 3; 
     FIG. 4 is a perspective view of seven contiguous super element cells for the antenna shown in FIGS. 2 and 3; 
     FIG. 5 is a front planar view of a multi-element feed array in accordance with the subject invention, with a group of seven feed elements being activated; 
     FIG. 6 is an electrical block diagram of control circuitry for selectively activating a selected group of feed elements shown in FIG. 5; 
     FIG. 7 is a diagram illustrative of the physical arrangement of the feed elements which are activated in accordance with the control circuitry shown in FIG. 6; 
     FIGS. 8A,  8 B and  8 C are illustrative of the manner in which seven elements in a feed array can be selectively activated so as to move the group of activated elements over the face of the feed array; 
     FIGS. 9A and 9B are illustrative of the array steering mechanism where feed group selection steers a super element beam at 0° with time delay units also steering the array factor to 0°; 
     FIGS. 10A and 10B are illustrative of the array steering mechanism where feed group selection steers a super element beam to 0° while time delay units steer the array factor to 1.1°. 
     FIGS. 11A and 11B are illustrative of the array steering mechanism where feed group selection steers a super element beam to 2.4°; 
     FIGS. 12A and 12B are illustrative of the array steering mechanism where feed group selection steers a super element beam to 4.8° while time delay units steer the array factor to 6°; 
     FIGS. 13A and 13B are illustrative of an example of the grating lobe problem occurring when feed group selection steers a super element beam to 0° and time delay units steer the array factor in elevation to 1.386°; 
     FIGS. 14A,  14 B and  14 C are illustrative of the method for reducing grating lobes by steering a feed group reduced in number to 1.3860 in elevation where the array factor is scanned to 1.386° in elevation as shown in FIGS. 13B; 
     FIGS. 15A-15D are illustrative of another method of reducing the grating lobe problem as shown in FIG. 13B by randomly selecting feed groups; 
     FIGS. 16A-16D are illustrative of still another method of mitigating the grating lobe problem and involves transitioning between beam positions; and 
     FIGS. 17A-17C are illustrative of yet another method of mitigating the grating lobe problem and comprises random positioning of the feed arrays about respective focal axes. 
     FIGS. 18A-18C are illustrative of still yet another method of mitigating the grating lobe problem which involves overlapping the amplitude distribution of feed elements to steer horizontally between nominal beam positions; 
     FIGS. 19A-19D are illustrative of still yet another method of mitigating the grating lobe problem which involves overlapping the amplitude distribution of feed elements to steer vertically between nominal beam positions; and 
     FIG. 20 is a diagram illustrative of a method of distributing power more evenly across a feed array while steering the element pattern to the same location as the array factor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the figures wherein like reference numerals refer to like parts throughout, FIG. 1 depicts a space borne antenna system  10  including an X-band sub-system  12  and an L-band sub-system  14 . The present invention is directed to the L-band sub-system  14 , the details of which are shown in FIGS. 2-4. The L-band sub-system  14  comprises a relatively large inflatable antenna assembly  18  which includes a torus support structure  20  (FIG. 2) which is, for-example, 50 meters in diameter and supports 91 contiguous reflector super elements  22 . A cross-section of the antenna assembly  18  taken along the lines  3 — 3  of FIG. 2 is shown in FIG.  3 . Each reflector cell  22  as shown in FIGS. 3 and 4A includes a mesh-type parabolic reflector  24  having a hexagonal outline or perimeter  25 . The mesh reflector  24  is supported at its six corners by rigid post members  26  which when the antenna is deployed, stiffen the mesh reflector  24  as well back-up suspension cables  28  which form a web  30  and a set of drop lines  32  which act to pull the mesh-type reflector  24  into a parabolic shape. 
     Each reflector super element  22  also includes a multi-element feed array  34  consisting of, for example, a cluster of thirty seven contiguous feed elements  36  as shown in FIG.  5 . The feed array  34 , moreover, is suspended above the concave surface of the reflector  24  by means of a set of suspension cables  38  which extend between the rigid support posts  26  and the feed array  34 . Although not immediately evident, cable members  38  are also included along the edges of the mesh reflector  24  between the posts  26  so that the mesh does not stretch along the edges when the reflector surface is pulled down by the drop lines  32  and a suspension cable  28 . 
     While FIG. 4A discloses the mechanical details of a single reflector super element cell  22 , FIG. 4B is illustrative of a group of seven contiguous reflector cells  22   1 ,  22   2 , . . .  22   7 . It should be noted that in such an arrangement, one support post  26  in many cases occurs at the intersection of three reflectors  24  of contiguous super element reflector cells such that, for example, at the intersection of three surfaces, a “Y” is formed. The rigid post elements  26  also act to maintain alignment of the various reflector cells  22 . 
     Referring now to FIGS. 5,  6  and  7 , shown thereat are the details of the feed array  34  for each reflector super element  22 . As noted above, each feed array  34  includes thirty seven discrete feed elements  36  which are activated to transmit (Tx) and receive (Rx) power via a switch matrix  35  shown in FIG.  6 . The switch matrix  35  includes seven sets of switches  40   1 ,  40   2 , . . .  40   7  where the first six sets of switches  40   1 , . . .  40   6  includes a set of five single-pole, double-throw switches, while the seventh set of switches  40   7  include seven single-pole, double-throw switches of which only six are used. Thus, each feed element is connected to a respective single-pole, double-throw switch. The seven sets of switches are connected to a 1:7 signal splitter  42 , which is coupled to a circulator  44  which receives transmit signals from a power amplifier module  46  and feeds received signals to a low noise amplifier via a receiver protector device  50 . 
     It can be seen with respect to FIG. 7 that the signal splitter  42  is operable to feed seven elements at a time in a pattern A, B, C, . . . G shown in FIG. 7 to form a cluster or group  37  of feed elements  36  at the position shown in FIG.  5 . This position comprises one of a plurality of positions on the face of the feed array  34 , as shown, for example in FIGS. 8A,  8 B and  8 C. As shown in FIG. 8A, a group of seven feed elements are selected at the center of the array, whereas in FIG. 8B, a group of seven elements are selected to the right of the array which corresponds to that shown in FIGS. 5 and 7, while the group shown in FIG. 8C comprises a group of seven elements  36  located above and to the right of the array. A group  37  of feed elements  36  is not limited to a fixed number of elements, such as seven elements, but can be made to be variable with as many as, for example, twelve feed elements in a group, however, the design of FIG. 6 would change. 
     There are two mechanisms for steering the array. One comprises feed group selection. The other mechanism is time delay steering the array factor. Accordingly, where a plurality of super element reflector cells form a phased array antenna system such as shown in FIGS. 1-3, reflector feed group selection includes selecting a specific feed group for a beam covered region wherein similar groups in each feed are selected, and wherein all of the super element individual reflectors produce a broad element pattern in the same direction. Time delay steering of the array factor results in providing fine steering control and is achieved by time delay units, not shown, which adjust the relative delay between super element reflector cells. Examples of array steering by reflector feed group selection and time delay steering the array factor is shown in FIGS. 9-12. 
     Referring now to FIGS. 9A and 9B they are illustrative of the array steering mechanism where feed array  34  steers a super element beam generated by feed array  37  in FIG. 9A to 0° with time delay units also steering the array factor to 0°. As shown in FIG. 9A, the feed element of group  37  is centered in the feed element array  34 . With no array factor steering being applied, an antenna pattern as shown in FIG. 9B results. In FIG. 9B, reference numeral  52  depicts the super element beam pattern generated by the selected feed element group  37 . The composite antenna pattern of the entire phased array antenna system as shown in FIGS. 2 and 3 includes a main lobe  54 , and pairs of side lobes  55 . Array factor steering is indicated by the position of a pair of grating lobes  56  on either side of the main lobe  54 . 
     FIGS. 10A and 10B are illustrative of the steering mechanism where group selection again steers the super element beam to 0° by centering the selected feed element in group  37  as shown in FIG. 10A, but the array factor is now steered to 1.1° as shown in FIG. 10B by the grating lobes  56 . The main lobe  54  of the composite antenna pattern is also now at 1.1°. 
     Next, considering FIGS. 11A and 11B, FIG. 11A depicts feed group selection steering of the individual super element beam pattern of feed group  37  to 2.4°, but now the array factor is also steered to 2.4°, which is shown in FIG.  11 B and where an individual super element pattern  52  of feed group  37  and the main lobe  54  of the composite antenna pattern are both positioned at 2.4°. 
     Referring now to FIGS. 12A and 12B, shown thereat is a condition where group selection steers the respective super element beam pattern of feed group  39  (FIG. 12A) to 4.8°, while the array factor is steered to 6° as shown by the grating lobes  56  in FIG.  12 B. The main lobe  54  of the composite antenna pattern is now also located at 6°. 
     It should be noted, however, that there exists some scan positions where a grating lobe problem arises particularly as it pertains to the composite pattern. For example, as shown in FIGS. 13A and 13B where feed element group selection (FIG. 13A) steers the super element beam to 0°, but the array factor is steered in elevation to 1.386°. As shown in FIG. 13B, it can be seen that while the main lobe  54  of the composite pattern is located at 1.386°, a grating lobe  58  of the composite pattern which is significant in amplitude (down 5 dB) relative to the main lobe  54  is generated. 
     This undesirable condition can be overcome by in accordance with this invention selecting a reduced feed element group  60 , as shown in FIG. 14A, which consists in a group of only three feed elements  36  and aiming the beam generated by the feed element group  60  (FIG. 14A) at the 1.386° position in elevation. It can be seen in FIG. 14B that the undesirable grating lobe  58  of the composite beam pattern is reduced by almost 20 dB in amplitude relative to the main lobe  54  of the composite beam pattern. 
     One possible variation of such an implementation shown in FIG. 14A is shown in FIG. 14C where six feed elements  36  are configured in a triangular group  62  as shown. 
     Another method of reducing grating lobes  58  of the composite beam is to randomly select feed element groups about the optimum position as shown in FIGS. 15A,  15 B and  15 C where the configuration of the selected feed groups  37   a  of feed array  34 - 1  is centered at 0°, while the feed groups  37   b  and  37   c  of feed arrays  34 - 1  and  34 - 2  as shown in FIGS. 15B and 15C are offset to the left and right relative to group  37   a . Such an arrangement would produce antenna patterns such as shown in FIG. 15D, where the main lobe  54  of the composite pattern is located at 1.38° in elevation; however, the grating lobes  58  are significantly larger than those depicted in FIGS. 14B, being only 10 dB down from the amplitude of the main lobe  54 . 
     FIGS. 16A-16D depict yet another method of mitigating the grating lobe problem. This involves gradual transitioning from one beam position to another. For example, as shown in FIG. 16A, the feed element group  37  is centered, while in FIGS. 16B and 16C, an irregular pattern of feed elements depicts a transition to the final position as shown in FIG.  16 D. In each instance, the same number of feed elements are utilized. 
     Still another method of mitigating the grating lobe problem is shown in FIGS. 17A,  17 B and  17 C and comprises randomly positioning the feed arrays, for example, arrays  34 - 1 ,  34 - 2  and  34 - 3  about the focal point of the respective array which is shown located at the intersection of the X and Y axis. 
     FIGS. 18A-18C and  19 A- 19 D are illustrative of yet another method of mitigating the grating lobe problem, and involve adjusting the amplitude distribution of each element feed. The distributions are overlapped as required to precisely steer the feed array to the same position as the array factor. The drawback is that the feed array amplitudes are not uniform. 
     FIGS. 18A and 18B, for example, depict two nominal distributions for beams at 0° and 2.4°. Overlapping distributions form a composite distribution as shown in FIG. 18C which scans the element pattern horizontally exactly half way between beam positions. Thus for that position, there is no error between element pattern beam peak and array factor beam peak. Accordingly, grating lobes are reduced. 
     With respect to FIGS. 19A,  19 B, and  19 C, shown thereat are three nominal distributions for beams at θ=0°, φ=30° and θ=2.4°, and φ=30° and θ=2.4°. Overlapping distributions form a composite distribution as shown in FIG. 19D to vertically steer the element pattern exactly to the intersection between positions. Thus for that position, there is no error between element pattern beam peak and array factor beam peak, and thus grating lobes are reduced. 
     Referring now to FIG. 20, translating the feed aperture forward of the focal plane  60  to a new location  62  provides a way to distribute the power more evenly across the feed array while precisely steering the element pattern to the same location as the array factor. In this configuration, the amplitude and phase of the feed array elements must be adjusted individually on transmit and receive. In FIG. 20, the precise feed point is determined analytically. An optimum feed is assumed to radiate from that point, and its radiation is projected to the feed plane. The distribution determined at the feed plane is then radiated from there. The benefit of this approach is to distribute the power among all the feed elements. The drawback is that the phase and amplitude must be controlled. 
     From the above, it will be appreciated that the present invention permits the deployment of a Limited Field of View Antenna for Space Borne Applications by forming a plurality of reflector cells in a flexible reflective membrane using rigid support members that abut the flexible membrane at spaced locations and a mechanism, such as tension wires, that pulls the flexible membrane against the rigid support members to forms the reflector cells. 
     Having thus shown and described what is at present considered to be the preferred embodiments of the invention, it should be noted that the same has been made by way of illustration and not limitation. Accordingly, all modifications, alterations and changes coming within the spirit and scope of the invention as set forth in the appended claims are herein meant to be included.