Patent Publication Number: US-7224322-B1

Title: Balloon antenna

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   The invention described herein may be manufactured and used by or for the government of the United States of America for government purposes without the payment of any royalties thereon or therefor. 

   BACKGROUND OF THE INVENTION 
   Large aperture antennas are needed in satellite based radars to focus a radar beam, due to the limitations on power available on the satellite. In order to place a large aperture antenna in space on a satellite the radar assembly has to be packaged to fit into the launch rocket as well as be assembled or unfolded in space. This is very difficult and prohibitive for large aperture phased arrays. 
   Phased array radar systems are preferred in space based applications due to the ability to electronically steer the array, thereby not requiring a movement of mass. Making a large phased array antenna that can be assembled or unfolded in space is very difficult task. There is a need to provide the effect and coverage of a large aperture phased array antenna while solving the problems of difficulty of installation in space and limited power supply. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a phased array balloon antenna for space based operation according to an embodiment of the invention. 
       FIG. 2  is a diagram of G a     —     dB (d spot )vs, d max /mi. 
       FIG. 3  is a diagram of a phased array aperture. 
   

   DESCRIPTION OF THE EMBODIMENTS 
   Before explaining the disclosed embodiments of the present invention in detail it is to be understood that the invention is not limited in its application to the details of the particular arrangement shown since the invention is capable of other embodiments. Also, the terminology used herein is for the purpose of description and not of limitation. In the figures, the same reference numbers are used to identify the same components. 
   Embodiments of the invention include a phased array balloon antenna and a method for using the antenna. A phased array antenna system is placed on an inner membrane of a balloon. The outer membrane of the balloon has a reflective film suspended across a portion of it&#39;s volume or is lined with a reflective film. The phased array operates as a large aperture antenna by reflecting the radar energy or beam from the phased array antenna off the reflective film so that it has the same effect as if large aperture antenna was in use. This approach permits a very small phased array to operate as a large aperture phased array while utilizing loss power. A conventional phased array antenna coupled to a large reflector will operate for beaming purposes as a large phased array antenna. The balloon system shall be packed deflated and launched with a satellite and shall be deployed by inflating on arrival. 
     FIG. 1  illustrates a space based embodiment of the invention. A balloon antenna  100  is constructed of an inner membrane  120  and an outer membrane.  110 . The outer membrane  110  is constructed so as to be inflatable and to hold a volume of gas. The inner membrane  120  supports a phased array antenna  130 . The outer membrane  110  has a reflector film  150  suspended across a portion of the outer membrane&#39;s volume, having a curved shape determined by an inflation pressure differential between the large compartment  140  and the small compartment  144  on the opposite sides of the reflective film  150 . The reflective film is constructed of a non-gas permeable material. A phased array antenna (represented by  130 ) is connected and mounted on the inner membrane  120 . The phased array  130  is directed outwardly from the center of the balloon towards the inside curved surface of the reflective film  150 . The reflective film  150  operates as a large aperture antenna and as the reflector for the phased array radar  130  system connected to the inner membrane  120 . 
   The phased array radar system antenna  130  transmits an energy towards the reflective film  150 , reflecting the energy from the reflective film  150  outwards and illuminating a target area smaller than the area (with more energy per unit area) that would be illuminated by the phased array antenna operating alone. The phased array  130  shall transmit radar energy towards the reflective film  150  on the outer membrane  110 , reflecting the radar signal  180  onto the target area  160 . The direction of the radar signal  180  is electrically steered by the phased array  130 , which requires no moving parts, only a change in phase between the elements. It is noteworthy that since no mass is being relocated the phase array  130  is suitable for use on a satellite. The amplitude and phase of the radar energy is provided by the phased array  130 . The effective curvature or shape can be adjusted smaller or larger as the circumstances required by adjusting the pressure differential between the large compact  140  and the small compartment  144 . By adjusting the shape of the reflective film (thereby focusing the antenna), the limited power available on a space based radar can be concentrated in a desired area. Therefore, the phased array signal will have the effect on the target as if it were coming from a large aperture antenna. Other embodiments may be constructed utilizing additional reflective films and additional phased arrays within the balloon. In this way embodiments of the invention may be used for tracking and illumination of additional target areas. 
   Another embodiment of the invention includes a method for illuminating an area with radar energy including: providing a balloon antenna comprising at least one inner and an outer membrane, at least one phased array antenna connected each inner membrane, the outer membrane having at least one reflective film and being inflatable; transmitting a radar energy from each phased array antenna towards each reflective film; reflecting the radar energy outwards from the reflective film and illuminating a target area that is smaller than an area illuminated by the phased array antenna; and changing the inflation of at least one compartment within the outer membrane which changes the shape of the reflective films, thereby adjusting the target area illuminated by the phased array balloon antenna. 
   RESULTS OF SIMULATION 
   A calculated simulation was performed to evaluate the performance of an embodiment of the invention. The simulation assumed a radar illuminator in a geosynchronous orbit, having no detection requirements, as could be used as one part of a bistatic system. Based on an illuminated target area of 25 miles in diameter, the member of elements and array size can be found. 
   Constant and Units 
   
     
       
         
           
             
               
                 c 
                 := 
                 
                   
                     3 
                     · 
                     
                       10 
                       8 
                     
                     · 
                     
                       m 
                       sec 
                     
                   
                   ⁢ 
                   Speed 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   Light 
                 
               
             
             
               
                   
               
             
             
               
                 
                   nmi 
                   : 
                 
                 ⁢ 
                 
                     
                 
                 = 
                 
                   1852 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   m 
                 
               
             
           
           
             
               
                 
                   f 
                   := 
                   
                     10 
                     · 
                     GH 
                   
                 
                 ; 
                 
                     
                 
                 ⁢ 
                 
                   λ 
                   := 
                   
                     c 
                     f 
                   
                 
               
             
             
               
                   
               
             
             
               
                 λ 
                 = 
                 
                   0.03 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   m 
                 
               
             
           
         
       
     
   
   Gain vs. Spot Size 
   One of the major concerns when operating from geosynchronous orbit is controlling the illuminated area (spot) size on the ground, which determines the energy per unit area. As power in space is limited it is necessary to limit the illuminated area to maintain detectability. 
   
     
       
         
           
             
               
                 
                   
                     d 
                     spot 
                   
                   := 
                   
                     1 
                     · 
                     mi 
                   
                 
                 , 
                 
                     
                 
                 ⁢ 
                 
                   1.1 
                   · 
                   
                     mi 
                     . 
                     
                         
                     
                     ⁢ 
                     25 
                   
                   · 
                   m 
                 
               
             
             
               
                 
                   
                     d 
                     g 
                   
                   := 
                   1 
                 
                 , 
                 
                     
                 
                 ⁢ 
                 
                   
                     1.1 
                     .. 
                   
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   25 
                 
               
             
             
               
                 
                   H 
                   a 
                 
                 := 
                 22800 
               
             
           
         
       
     
     
       
         
           
             H 
             alt 
           
           := 
           
             22800 
             ⁢ 
             
                 
             
             ⁢ 
             m 
           
         
       
     
     
       
         
           
             
               
                 
                   
                     
                       
                         θ 
                         
                           3 
                           ⁢ 
                           dB 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           d 
                           spot 
                         
                         ) 
                       
                     
                     : 
                     
                       
                         d 
                         spot 
                       
                       
                         H 
                         alt 
                       
                     
                   
                   ⁢ 
                   
                       
                   
                 
               
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       θ 
                       ⁡ 
                       
                         ( 
                         
                           d 
                           s 
                         
                         ) 
                       
                     
                     := 
                     
                       2 
                       · 
                     
                   
                 
               
             
           
           ⁢ 
           
             atan 
             ⁡ 
             
               ( 
               
                 
                   0.5 
                   ⁢ 
                   
                     d 
                     s 
                   
                 
                 
                   H 
                   a 
                 
               
               ) 
             
           
         
       
     
     
       
         
           
             
               D 
               main_ref 
             
             ⁡ 
             
               ( 
               
                 d 
                 spot 
               
               ) 
             
           
           := 
           
             1.27 
             · 
             
               λ 
               
                 
                   θ 
                   
                     3 
                     ⁢ 
                     dB 
                   
                 
                 ⁡ 
                 
                   ( 
                   
                     d 
                     spot 
                   
                   ) 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 η 
                 := 
                 
                   60 
                   ⁢ 
                   % 
                 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   
                     
                       A 
                       main_ref 
                     
                     ⁡ 
                     
                       ( 
                       
                         d 
                         spot 
                       
                       ) 
                     
                   
                   := 
                   
                     
                       π 
                       · 
                       
                         
                           ( 
                           
                             
                               D 
                               main_ref 
                             
                             ⁡ 
                             
                               ( 
                               
                                 d 
                                 spot 
                               
                               ) 
                             
                           
                           ) 
                         
                         2 
                       
                     
                     4 
                   
                 
               
             
           
         
       
     
     
       
         
           
             
               A 
               e 
             
             ⁡ 
             
               ( 
               
                 d 
                 spot 
               
               ) 
             
           
           := 
           
             
               
                 A 
                 main_ref 
               
               ⁡ 
               
                 ( 
                 
                   d 
                   spot 
                 
                 ) 
               
             
             · 
             η 
           
         
       
     
     
       
         
           
             
               
                 
                   
                     G 
                     a 
                   
                   ⁡ 
                   
                     ( 
                     
                       d 
                       spot 
                     
                     ) 
                   
                 
                 := 
                 
                   
                     
                       4 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       π 
                     
                     
                       λ 
                       2 
                     
                   
                   · 
                   
                     
                       A 
                       e 
                     
                     ⁡ 
                     
                       ( 
                       
                         d 
                         spot 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   
                     
                       G 
                       a_dB 
                     
                     ⁡ 
                     
                       ( 
                       
                         d 
                         spot 
                       
                       ) 
                     
                   
                   := 
                   
                     10 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       log 
                       ⁡ 
                       
                         ( 
                         
                           
                             G 
                             a 
                           
                           ⁡ 
                           
                             ( 
                             
                               d 
                               spot 
                             
                             ) 
                           
                         
                         ) 
                       
                     
                   
                 
               
             
           
         
       
     
     
       
         
           
             
               G 
               a_dB 
             
             ⁡ 
             
               ( 
               
                 5 
                 · 
                 mi 
               
               ) 
             
           
           = 
           82.98 
         
       
     
     
       
         
           
             
               G 
               a_dB 
             
             ⁡ 
             
               ( 
               
                 10 
                 · 
                 mi 
               
               ) 
             
           
           = 
           76.959 
         
       
     
     
       
         
           
             
               G 
               a_dB 
             
             ⁡ 
             
               ( 
               
                 15 
                 · 
                 mi 
               
               ) 
             
           
           = 
           73.437 
         
       
     
   
   For 25 miles spot size: 
                     G   a_dB     ⁡     (     25   ·   mi     )       =   69           mil_rad   :=     rad   1000                     A   main_ref     ⁡     (     25   ·   mi     )       =     948.265   ⁢           ⁢     m   2               μ_rad   :=       10     -   6       ·   rad                     D   main_ref     ⁡     (     25   ·   mi     )       =     948.265   ⁢           ⁢     m   2                                   θ     3   ⁢   dB       ⁡     (     25   ·   mi     )       =     0.063   ⁢           ⁢   deg                 θ     3   ⁢   dB       ⁡     (     25   ·   mi     )       =     1.096   ⁢           ⁢   mil_rad                 
Assuming peak power needed for detection
 
   
     
       
         
           
             
               
                 
                   P 
                   peak 
                 
                 := 
                 
                   5000 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   watt 
                 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   
                     N 
                     element 
                   
                   := 
                   
                     floor 
                     ⁡ 
                     
                       ( 
                       
                         
                           P 
                           peak 
                         
                         
                           P 
                           element 
                         
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   
                     N 
                     element 
                   
                   = 
                   625 
                 
               
             
           
           
             
               
                 
                   P 
                   element 
                 
                 := 
                 
                   8 
                   · 
                   watt 
                 
               
             
             
               
                   
               
             
             
               
                   
               
             
           
         
       
     
   
   Beam Pointing Resolution 
   The beam pointing resolution determines the amount of movement on the ground required to keep a target area illuminated by the receiver (such as an attack aircraft). For example, for the 25 mile diameter target area the phased array may move the illuminated area in steps of 5 or 10 miles. 
   
     
       
         
           
             
               
                 
                   
                     ⁢ 
                     
                       ( 
                       s 
                       ) 
                     
                   
                   := 
                   
                     floor 
                     ( 
                     
                       
                         
                           D 
                           main_ref 
                         
                         ⁡ 
                         
                           ( 
                           
                             25 
                             ⁢ 
                             
                                 
                             
                             ⁢ 
                             
                               ·   
                             
                             ⁢ 
                             mi 
                           
                           ) 
                         
                       
                       s 
                     
                     ) 
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 Number 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 of 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Elements 
               
             
           
           
             
               
                 
                   
                     θ 
                     BB 
                   
                   := 
                   
                     
                       50.8 
                       
                         ( 
                         
                           
                             
                               D 
                               main_ref 
                             
                             ⁡ 
                             
                               ( 
                               
                                 25 
                                 · 
                                 mi 
                               
                               ) 
                             
                           
                           λ 
                         
                         ) 
                       
                     
                     · 
                     deg 
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 Broadside 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 Beamwidth 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   ψ 
                   := 
                   
                     1 
                     · 
                     deg 
                   
                 
               
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       S 
                       ground 
                     
                     := 
                     
                       ψ 
                       · 
                       
                         H 
                         alt 
                       
                     
                   
                 
               
               
                 
                   
                       
                   
                   ⁢ 
                   
                     S 
                     ground 
                   
                 
               
             
           
           = 
           
             397.935 
             ⁢ 
             
                 
             
             ⁢ 
             m 
           
         
       
     
   
   Translation on the Ground 
   The remaining calculations are based on basic design methodology of the phased array antenna based on the desired amount of beam translation on the ground. By defining the amount of beam step on the ground (translation), there is the ability to proceed with the design of a Cassegrain system. The calculations assume an F 1  system wherein the diameter of the main reflector (reflector film) is the same as the focal length. Based on this the number of phase elements on the sub reflector is calculated gives a particular phase shifter, i.e. the number of bits in the phase shift. 
   
     
       
         
           
             
               
                 
                   
                     S 
                     g_desired 
                   
                   := 
                   
                     20 
                     · 
                     m 
                   
                 
               
               
                 
                   := 
                   
                     λ 
                     2 
                   
                 
               
             
           
           ⁢ 
           element 
           ⁢ 
           
               
           
           ⁢ 
           spacing 
         
       
     
     
       
         
           
             
               
                 
                   
                     ψ 
                     δ 
                   
                   := 
                   
                     
                       S 
                       
                         g 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         desired 
                       
                     
                     
                       H 
                       alt 
                     
                   
                 
               
               
                 
                   ψ 
                   δ 
                 
               
             
           
           = 
           
             877.193 
             ⁢ 
             
                 
             
             ⁢ 
             μ_rad 
           
         
       
     
   
   For a Cassegrain system the angle scanned by the subreflector is half the total angle scanned by the main reflector.
 
Ψ sr :=1·deg
 
Angle off boresight for subreflector
 
   For an F 1  optical system the diameter at the main reflector is the same as the focal length. 
   
     
       
         
           
             
               
                 
                   D 
                   
                     reflector 
                     ⁢ 
                     
                         
                     
                   
                 
                 := 
                 
                   0.8 
                   ⁢ 
                   
                     
                       D 
                       main_ref 
                     
                     ⁡ 
                     
                       ( 
                       
                         25 
                         · 
                         mi 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   D 
                   reflector 
                 
                 = 
                 
                   27.798 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   m 
                 
               
             
           
           
             
               
                 const 
                 : 
                 1.5 
               
             
             
               
                   
               
             
           
           
             
               
                 focal 
                 : 
                 
                   const 
                   · 
                   
                     D 
                     reflector 
                   
                 
               
             
             
               
                 focal 
                 = 
                 
                   41.697 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   m 
                 
               
             
           
           
             
               
                 := 
                 
                   
                     D 
                     reflector 
                   
                   focal 
                 
               
             
             
               
                 
                   θ 
                   BB 
                 
                 = 
                 
                   38.197 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   deg 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   θ 
                   B 
                 
                 := 
                 
                   
                     θ 
                     BB 
                   
                   
                     cos 
                     ⁡ 
                     
                       ( 
                       
                         ψ 
                         ST 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   θ 
                   B 
                 
                 = 
                 
                   38.203 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   deg 
                 
               
             
             
               
                 Scanned 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 beamwidth 
               
             
           
         
       
     
     
       
         
           
             
               
                 p 
                 := 
                 7 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   # 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   bits 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   phase 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   shifter 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   
                     δ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       θ 
                       ⁡ 
                       
                         ( 
                         s 
                         ) 
                       
                     
                   
                   := 
                   
                     
                       9.0 
                       B 
                     
                     
                       
                         N 
                         ⁡ 
                         
                           ( 
                           s 
                           ) 
                         
                       
                       · 
                       
                         2 
                         P 
                       
                     
                   
                 
                 ⁢ 
                 
                     
                 
               
             
             
               
                 
                     
                 
                 ⁢ 
                 
                   beam 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   pointing 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   resolution 
                 
               
             
           
         
       
     
     
       
         
           
             
               
                 
                   N 
                   elements 
                 
                 := 
                 
                   floor 
                   ( 
                   
                     
                       9 
                       · 
                       
                         θ 
                         B 
                       
                     
                     
                       
                         
                           ψ 
                           s 
                         
                         2 
                       
                       · 
                       
                         2 
                         P 
                       
                     
                   
                   ) 
                 
               
             
             
               
                 
                   N 
                   elements 
                 
                 = 
                 106 
               
             
           
         
       
     
     
       
         
           
             N 
             elements 
           
           ⁢ 
           
             
               
                 
                   
                     · 
                     
                       λ 
                       2 
                     
                   
                   = 
                   
                     1.59 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     m 
                   
                 
               
               
                 
                   Size 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   of 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   subreflector 
                 
               
             
           
         
       
     
   
   It is to be understood that the foregoing detailed description is exemplary and explanatory only and is not to be viewed as being restrictive of embodiments of the invention, as claimed. The invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive. Thus the scope of this invention should be determined by the appended claims, drawings and their legal equivalents.