Abstract:
A system for a satellite includes a core system and multiple nodes for generating an active phased array. Each node includes a transceiver for wirelessly receiving a transmit signal from the core system, for wirelessly transmitting the transmit signals to a target, for wirelessly receiving the receive signals from the target, and for wirelessly transmitting the receive signal back to the core system. The system also includes a subsystem for inhibiting signal interference between the transmit and receive signals. Each of the nodes may also include local power generation circuitry.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/689,473, filed Jun. 9, 2005 (attorney docket number 34716-8002.US00). 
     
    
     BACKGROUND 
       [0002]    A major advantage of phased array antennas is their ability to steer the beam electronically, eliminating the need for mechanical pointing and alignment. Another benefit is that the beam steering can be performed quickly, which allows tracking of rapidly moving targets, and tracking of multiple targets. The rapid beam steering also facilitates applications where an antenna on a moving platform (e.g. a ship at sea) it to maintain contact with a fixed entity such as a communications or broadcast satellite. 
         [0003]    A common application of phased array antennas is in the implementation of radar systems, especially synthetic aperture radar systems. 
         [0004]    Radio detection and ranging, or radar as it is commonly known, has been in existence since World War II and is used for a wide variety of applications. For example, radars are used for tracking the position of objects such as airplanes, ships and other vehicles or monitoring atmospheric conditions. Imaging radars have been developed for constructing images of terrain or objects. 
         [0005]    Basic radar systems operate by transmitting a radio frequency signal, usually in the form of a short pulse at a target. A basic radar system is limited in both range resolution and azimuth resolution. Various techniques have been developed to overcome the limitations of a basic radar system. For example, to improve range resolution techniques such as pulse compression can be used. 
         [0006]    To improve azimuth resolution without requiring an unacceptably large antenna, the Synthetic Aperture Radar technique has been developed. Synthetic Aperture Radars are now commonly used in both airborne and spaceborne (e.g. an airplane or satellite) based applications. 
         [0007]    Modern Synthetic Aperture Radar systems require operational flexibility by supporting imaging over a wide range of resolutions and image swath widths. This operational flexibility requires the use of an active phased array antenna system. 
         [0008]    Current active phased array systems for spaceborne applications suffer from a number of limitations, which restricts their broader use. The antennas are relatively large, on the order of 10 to 20 meters in length, and 1 to 2 meters in width. To preserve the quality of the beam and maintain it stable requires that the antenna itself be rigid and that it be rigidly supported to keep the antenna flat within the required tolerances. This results in an antenna with a high mass and requires support trusses or other mechanical means to provide the required stiffness when extended. 
         [0009]    The size of the antenna generally prohibits launching the antennas in their operational configuration, as it is too large to fit within the available payload volume of the launch vehicle. The antenna is to be folded and stowed for launch, then deployed once in orbit. Complicated and expensive mechanisms to deploy the antenna and hold it rigid when deployed are to be specially designed. Special purpose mechanisms may also be designed and constructed to securely hold the antenna panels while stowed during launch and ensure that that the antenna is not damaged by the stresses incurred during launch. The high mass of the antenna makes the task of stowing and deploying it much more difficult. 
         [0010]    The elements of the active phased array require a complex set of interconnections between the main bus structure and the antenna elements. Connections are needed for power, control, monitoring and distribution of radio-frequency signals for both transmit and receive. Complicated azimuth and elevation beam forming devices and interconnects are required. These interconnections further add to the overall mass, complexity and cost of the antenna. In addition, the interconnections may be made to bridge the hinges between the panels of the antenna adding to the manufacturing complexity and cost, and reducing the overall reliability. 
         [0011]    The RADARSAT-2 spacecraft is an example of a state-of-the-art Synthetic Aperture Radar System using an active phased array antenna. The antenna in this instance is 15 meters in length and 1.5 meters in width. It consists of two wings, each containing 2 panels with each panel approximately 3.75 meters in length and 1.5 meters in width. Each panel contains 4 columns with each column containing 32 transmit/receive modules each with an associated sub-array with 20 radiating elements. A total of 512 transmit receive modules are used in the antenna. The overall mass of the antenna is approximately 785 kg. The extendible support structure required to deploy the antenna panels and maintain them in place has a mass of approximately 120 kg. The mechanisms used to hold the antenna while stowed, and then release it for deployment, add an additional approximately 120 kg of mass. The total mass required by the antenna is approximately 1025 kg. This large mass in turn drives the design of the spacecraft bus structure and attitude control systems, resulting in a larger, heavier spacecraft. 
         [0012]    The large mass and complex design mean that the overall cost of designing, building and launching this class of spacecraft is high. This restricts the use of this technology to specialized applications and limits the number of spacecraft that can be launched, reducing the frequency of observation and limiting the operational missions that can be supported. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    In the drawings closely related figures have the same number but different alphabetic suffixes. 
           [0014]      FIG. 1  shows an overall view of one spacecraft configuration. 
           [0015]      FIG. 2A  shows a block diagram of an antenna system. 
           [0016]      FIG. 2B  shows a timing diagram for the antenna system. 
           [0017]      FIG. 3  shows a block diagram of an active antenna node. 
           [0018]      FIG. 4  shows a block diagram of radio frequency circuit functions contained within the active antenna node. 
           [0019]      FIG. 5A  shows the rear face of one antenna panel. 
           [0020]      FIG. 5B  shows a detailed view of a portion of the rear face of an antenna panel. 
           [0021]      FIG. 5C  shows a detailed view looking from the edge of a portion of the rear face of an antenna panel. 
           [0022]      FIG. 5D  shows a detailed view of a portion of the front (radiating) face of an antenna panel. 
           [0023]      FIG. 6A  shows a cut-away view of a portion of the front face of an antenna panel. 
           [0024]      FIG. 6B  shows a section view through a portion of an antenna panel. 
           [0025]      FIG. 7  shows targets used for a geometry compensation system and optical paths within a satellite bus for collecting images. 
           [0026]      FIG. 8A  shows a detailed view of a fore boom mounted illuminated target. 
           [0027]      FIG. 8B  shows an arrangement of illuminated targets on two antenna panels. 
           [0028]      FIG. 8C  shows a detail of one of the targets. 
           [0029]      FIG. 9  shows a view of one wing, showing a location of targets on the antenna panels. It shows the view observed by the imaging system (bottom of figure) and arrangement of targets such that nearer targets do not obstruct more distant targets. 
           [0030]      FIG. 10  shows components of the geometry compensation system. Geometry compensation is used to adjust phase settings of antenna elements to compensate for mechanical distortions in the antenna. 
           [0031]      FIG. 11A  shows the spacecraft with the antenna panels and booms stowed for launch. 
           [0032]      FIG. 11B  shows the spacecraft during deployment of one antenna wing and boom. 
           [0033]      FIG. 11C  shows the spacecraft in its operational configuration with both wings and booms deployed. 
           [0034]      FIG. 12A  shows an alternative bus structure configuration. 
           [0035]      FIG. 12B  shows another alternative bus structure configuration. 
           [0036]      FIG. 12C  shows another alternative bus structure configuration. 
           [0037]      FIG. 13  shows a sequence of operations for the active antenna node. 
           [0038]      FIG. 14  shows an overall sequence of operations for an active phased array antenna. 
           [0039]      FIG. 15  shows a timing relationship between active antenna node control signals and signals transmitted and received from the active phased array antenna. 
           [0040]      FIG. 16  shows a sequence of operations for performing geometry compensation. 
           [0041]      FIG. 17  shows a block diagram of the radio frequency circuit functions contained within the active antenna node for an active phased array antenna with multiple polarization capability. 
       
    
    
     DRAWINGS 
     Reference Numerals 
       [0000]    
       
           100  spacecraft bus structure 
           105  antenna panel 
           110  antenna fore wing consisting of one or more antenna panels (four panels are shown in this example) 
           115  antenna aft wing consisting of one or more antenna panels (four panels are shown in this example) 
           120  radiating face of antenna panel 
           125  rear face of antenna panel 
           130  fore boom 
           135  aft boom 
           140  boom antenna assembly 
           145  solar array (to provide bus power) 
           150  phased array antenna (comprised of the fore wing and aft wing) 
           200  equipment housed in the spacecraft bus structure 
           205  spacecraft bus systems (power, control, data handling, etc) 
           210  receiver/exciter 
           215  stable local oscillator 
           220  transmit pulse generator 
           225  receiver 
           230  signal extraction and encoding unit 
           235  broadcast stable local oscillator signal 
           240  two way link with frequency translated transmit and receive signals 
           245  2-wire CAN Bus control bus 
           250  boom mounted antenna for transmit and receive signal distribution 
           255  boom mounted antenna for distribution of the stable local oscillator reference frequency 
           260  control bus 
           265  baseband chirp signal 
           270  antenna controller 
           300  active antenna node 
           305  antenna node solar panel assembly 
           310  battery charge regulator 
           315  rechargeable battery 
           320  power supply and power switching assembly 
           325  antenna for receiving stable local oscillator reference frequency 
           330  reference frequency processing assembly 
           335  antenna for transmit/receive signal 
           340  transmitter assembly 
           345  receiver assembly 
           350  subarray 
           355  antenna node controller 
           360  micro-controller 
           365  digital-to-analog converter means 
           370  phase control signals 
           375  transmit gain control signal 
           380  receive gain control signal 
           385  transmit and receive signals from antenna 
           400  signal routing device (e.g. circulator, switch, coupler, etc) 
           405  variable gain amplifier 
           410  mixer 
           415  high power amplifier 
           420  signal routing device (e.g. circulator, switch, coupler, etc) 
           425  low noise amplifier 
           430  mixer 
           435  variable gain amplifier 
           440  low noise amplifier 
           445  frequency doubler 
           450  direct modulator 
           455  power divider 
           460  phase shifted reference frequency 
           500  node electronics module 
           505  solar cell array 
           510  waveguide slots 
           600  RF Transparent material (e.g. quartz honeycomb) 
           605  panel structure 
           610  bonded aluminum sheet (front face of antenna panel) 
           615  waveguide launcher to inject signal into waveguide 
           700  location of optical assembly and image processing unit 
           705  optical path for antenna wing images 
           710  optical path for boom images 
           715  illuminated targets on antenna panels (not all targets identified) 
           720  illuminated target on fore boom 
           725  illuminated target on aft boom 
           800  example illuminated target on antenna panel 
           1000  optical assembly 
           1005  apertures for fore and aft wings and fore and aft booms 
           1010  image of fore and aft wings and fore and aft booms 
           1015  combined image 
           1020  solid state imaging array 
           1025  image processing unit 
           1030  fore wing target illumination controllers 
           1035  aft wing target illumination controllers 
           1040  fore boom target illumination controller 
           1045  aft boom target illumination controller 
           1050  wing illumination control signals 
           1055  boom illumination control signals 
           1060  interface to antenna controller 
           1100  launch vehicle payload fairing 
           1200  spacecraft bus structure (alternative 1) 
           1205  solar cell array for bus power (alternative 1) 
           1210  spacecraft bus structure (alternative 2) 
           1215  solar cell array for bus power (alternative 2) 
           1220  spacecraft bus structure (alternative 3) 
           1225  solar cell array for bus power (alternative 3) 
           1230  deployable boom assembly 
           1400  CAN Bus timing and control message 
           1405  active antenna node transmit mode enable 
           1410  active antenna anode receive mode enable 
           1700  antenna 
           1702  signal routing device (e.g. circulator, switch, coupler, etc) 
           1074  variable gain amplifier 
           1706  mixer 
           1708  power divider 
           1710  high power amplifier (horizontal polarization) 
           1712  high power amplifier (vertical polarization) 
           1714  signal routing device (e.g. circulator, switch, coupler, etc) 
           1716  horizontally polarized feed assembly 
           1718  vertically polarized feed assembly 
           1720  subarray 
           1722  low noise amplifier 
           1724  mixer 
           1726  variable gain amplifier 
           1728  signal routing device (e.g. circulator, switch, coupler, etc) 
           1730  low noise amplifier 
           1732  mixer 
           1734  variable gain amplifier 
           1736  antenna 
           1738  antenna 
           1740  low noise amplifier 
           1742  power divider 
           1744  frequency doubler 
           1746  direct modulator 
           1748  direct modulator 
           1750  power divider 
           1752  phase control signal 
           1754  phase control signal 
           1756  phase shifted reference frequency (transmitter) 
           1758  phase shifted reference frequency (horizontal receive polarization) 
           1760  phase shifted reference frequency (vertical receive polarization) 
           1762  transmit polarization select signal 
           1764  transmit gain compensation signal 
           1766  receive gain control signal (horizontal polarization) 
           1768  receive gain control signal (vertical polarization) 
           1770  two way link with frequency translated transmit and receive signals 
           1772  one way link with frequency translated receive signal 
       
     
       DETAILED DESCRIPTION 
       [0174]    Embodiments of the invention provide a method and system for constructing a spaceborne active phased array antenna system that retains operational capabilities of traditional phased array antenna systems, but at lower mass, lower manufacturing complexity and hence lower overall mission cost. A space feed distributes signals to active antenna nodes, active antenna nodes contain local power generation and storage capability, construction method producing lightweight antenna panels, and a compensation system measures and compensates for mechanical distortions in the antenna geometry. 
         [0175]    Various embodiments of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various embodiments 
         [0176]    The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention. Certain terms may even be emphasized below; however, any terminology intended to be interpreted in any restricted manner will be overtly and specifically defined as such in this Detailed Description section. 
         [0177]      FIG. 1  shows a configuration of a spacecraft using a lightweight space-fed active phased array antenna system. A phased array antenna  150  is comprised of multiple antenna panels  105 . Each panel has a front surface referred to as a radiating face  120  for transmitting a signal towards a target, and receiving the return signal reflected from the target. A rear face  125  of each panel contains multiple active antenna nodes  300  that form the active phased array. 
         [0178]    The antenna panels  105  are arranged into two groups, which will be referred to as wings. A leading wing  110 , relative to the direction of flight of the spacecraft, is referred to as the fore wing. The other wing  115  is referred to as the aft wing. 
         [0179]    A frequency translated signal to be transmitted is distributed to the fore wing active antenna nodes through a space feed arrangement using antenna  250  contained in a boom antenna assembly  140  mounted on a deployable boom  130 . The signal for the aft wing is distributed using another boom antenna assembly  140  mounted on a similar deployable boom  135 . The antennas located on the two boom antenna assemblies also receive frequency translated signals transmitted from active antenna nodes. The received frequency translated signal contains the return signal from the target received at the radiating face of the phased array antenna. 
         [0180]    Each boom antenna assembly  140  also contains a second antenna  255 . This second antenna is used to broadcast a stable reference frequency to each of the active antenna nodes. 
         [0181]    In the depicted embodiment antennas  250  and  255  are patch antennas, however other types of antenna can also be used. 
         [0182]    A bus structure  100  provides mechanical support for the active phased array antenna system. The bus contains within it systems commonly found on most spacecraft to perform functions including communications, attitude control, spacecraft monitoring and control, thermal control, data handling, propulsion, etc. Solar arrays  145  mounted on the sun facing surfaces of the bus structure provide power for all parts of the spacecraft except active antenna nodes  300  that may provide their own power. 
         [0183]    The block diagram of  FIG. 2A  shows major components of the active phase array antenna system and how they interact with each other. For simplicity only a single antenna panel of a single wing is shown. The other antenna panels are similar in construction and operation. 
         [0184]    A receiver/exciter  210  is contained within the bus structure  100 . The receiver/exciter generates a reference frequency and modulated transmit signals employed for the radar application. The receiver/exciter also receives a return signal from the panel and provides signal extraction and encoding functions to digitize and format received signal data. 
         [0185]    The receiver/exciter interfaces to a spacecraft bus systems  205  to receive power for operation and to transfer received data. An antenna controller  270  in the receiver/exciter is connected to the main spacecraft bus processor through control bus  260  to permit control and monitoring of the antenna system. There are no special requirements for the control bus and it can be implemented using any one of several available technologies such as MIL STD 1553B or CAN Bus. 
         [0186]    The antenna controller  270  provides control and monitoring of all units in the receiver/exciter and the active antenna nodes  300 . 
         [0187]    A stable local oscillator  215  generates a stable, un-modulated reference frequency. This reference frequency is distributed locally to a transmit pulse generator  220  and receiver  225  and is also broadcast to all of the active antenna nodes  300  using antenna  255  in boom antenna assemblies  140 . A single stable local oscillator is used to drive both boom antenna assemblies through a simple power divider. 
         [0188]    The transmit pulse generator  220  produces the waveform of the transmitted pulse. For radar systems this is usually a linearly modulated frequency pulse commonly known as a chirp. Techniques for generating this type of pulse are well known in the current art. 
         [0189]    The chirp is transmitted  240  from the boom antenna assembly  140  to all active antenna nodes  300  in the corresponding wing. Within each active antenna node the chirp is received, converted to the operating frequency of the antenna, adjusted for phase and amplitude, amplified and transmitted from the radiating face of the antenna. 
         [0190]    The active antenna nodes  300  receive the returned signal from the target and re-transmit this signal so that it can be received by the antenna  250  on the boom antenna assembly  140 . 
         [0191]    To avoid interference with other signals, the chirp and the received signals transmitted using the space-feed are converted to a separate carrier frequency according to a defined frequency plan to produce frequency translated versions of the original signals. As an example, a frequency plan for a typical SAR application would be as follows: SAR operating frequency of 5.400 GHz (C-band), stable local oscillator frequency of 2.400 GHz and carrier frequency for the frequency translated transmit chirp  240  and received signals  240  of 10.200 GHz (X-band). The description that follows assumes this example frequency plan. 
         [0192]      FIG. 2B  shows an example of a timing relationship between different signals. The stable local oscillator reference frequency is continuously broadcast  235  to each active antenna node. The transmit pulse generator  220  generates a baseband chirp signal  265  and a modulated chirp signal at X-band that is also broadcast  240  to all active antenna nodes. In the active antenna node, the X-band chirp signal is converted to C-band and is adjusted for phase prior to being transmitted  385  towards the target. The return signal  385  from the target is adjusted for phase and gain and is converted from C-band to X-band and transmitted  240  to the receiver  225 . Gain adjustments  375  and  380  are used to compensate for space feed path differences. Gain adjustment  380  also provides antenna aperture apodization. 
         [0193]    The receiver  225  receives the converted broadcast signal  240 , demodulates it and forwards the baseband signal to the signal extraction and encoding unit  230 . The signal is digitized, encoded and formatted and the resulting digital data is transferred to the spacecraft bus systems  205  for processing, storage and/or transmission to a ground based receiving terminal. 
         [0194]    The phased array antenna  150  is comprised of multiple antenna panels  105 . Each antenna panel contains multiple active antenna nodes  300  mounted on the rear surface  125  of the panel. As an example, an active phased array antenna for a synthetic aperture radar application would contain on the order of 8 antenna panels, with each panel containing on the order of 64 active antenna nodes, for a total of 512 active antenna nodes. 
         [0195]      FIG. 3  shows a block diagram of an active antenna node  300 . The active antenna node contains its own local power generation and storage means to provide power to all its components. To provide power generation, a solar cell array  305  is mounted on the rear face of the antenna panel  125 . In normal operation, the radiating face of the antenna panel  120  will be pointed at the earth at an angle of at least 30 degrees from nadir. At this spacecraft attitude, the solar cell arrays on the rear of the antenna panels will be exposed to the sun when the spacecraft is placed in an appropriate orbit such as a sun-synchronous, dawn-dusk orbit. The spacecraft can be slewed to better orient the solar panels towards the sun for more efficient solar power generation and battery charging. This can occur in periods that do not require operation of the antenna system, such as intervals where SAR imaging is not requested. 
         [0196]    An integrated circuit battery charge regulator  310  regulates the power from the solar cell array  305  and charges a rechargeable battery  315 . A regulated power supply with switching circuits  320  provides power to all other components of the active antenna node and allows elements of the active antenna node, for example the transmitter or receiver, to be independently powered on and off. 
         [0197]    The RF components of the active antenna node consist of two antennas  325  and  335 , reference frequency processing circuit  330 , transmitter circuit  340 , receiver circuit  345  and subarray  350 . Operation of the RF components of the active antenna node is described in the discussion on  FIG. 4  that follows. 
         [0198]    In the depicted embodiment antennas  325  and  335  are patch antennas, however other types of antenna can also be used. 
         [0199]    In the depicted embodiment, subarray  360  is a slotted waveguide subarray, however other arrangements could also be used. One example of an alternative arrangement is a subarray consisting of multiple patch, conformal or planar radiators bonded to the font or back surface of the antenna panel. If bonded to the back, the panel would be RF transparent; this alternative would provide simplicity and reduced mass in mounting and feeding the radiating subarray elements, while also providing structural support. 
         [0200]    Control of the active antenna node can be achieved by using a microcontroller or other programmable logic element such as a field programmable gate array. The depicted embodiment uses a microcontroller  360  such as an Intel 8051 that incorporates a built-in CAN Bus interface. A two-wire CAN Bus interface connection  245  is used to provide control and timing signals from the antenna controller  270  to the active antenna node, and to monitor status of the node. Although an embodiment using a wireless interconnect for this interface could be used, some wiring may still be required to provide conductive paths to dissipate electro-static charge that could accumulate on the antenna panels. A wired bus is both easier to implement and can be used to dissipate this electro-static charge. The microcontroller drives a digital-to-analog converter  365  that generates analog control signals  380 ,  375 ,  370  used to control transmitter gain, receiver gain and phase (both transmit and receive) respectively. 
         [0201]      FIG. 4  shows RF circuits of an active antenna node. Note that filters have been omitted from the diagram to make it simpler. There are no extraordinary requirements for the filters and their use, design and construction is well understood in the current art. Antenna  325  receives the broadcast stable local oscillator signal  235 . This signal is amplified by low noise amplifier  440  and then doubled in frequency using frequency doubler  445 , although other frequency adjustment may be employed. Direct modulator  450  is used to adjust the phase of the signal based on phase control signal  370  from the digital to analog converter  365 . The phase adjusted reference signal is divided using power divider  455  (or switch) and phase adjusted reference signals  460  are routed to both transmitter  340  and receiver  345  sections of the active antenna node. An alternative embodiment could use a phase shifter in place of direct modulator  450 , or two modulators in lieu of the power divider. 
         [0202]    The active antenna node receives the frequency translated chirp signal  240  using antenna  335 . A signal routing device  400  routes the signal to variable gain amplifier  405  whose gain is set by the microcontroller through signal  375 . Mixer  410  converts the signal to the operating frequency of the radar and phase adjusts the signal to form the beam. The signal is amplified using high power amplifier  415  and routed to subarray  350  through signal routing device  420 . 
         [0203]    Signals reflected from the target are received by subarray  350  and routed to the receiver portion of the active antenna node through signal routing device  420 . Low noise amplifier  425  amplifies the signal. Mixer  430  upconverts the signal and adjusts the phase of the signal to form the receive beam. The signal is amplified and its gain adjusted by variable gain amplifier  435 , whose gain is set by the microcontroller through signal  380 . Signal routing device  400  routes the signal to antenna  335  for transmission to receiver  225  in the receiver/exciter  210 . 
         [0204]    An alternative embodiment could use a double or triple balanced mixer in place of either or both mixers  410  and  430 . 
         [0205]    To improve the signal to noise ratio for received signals, the beam pattern of the antenna is made narrower in elevation when in receive mode, resulting in an increased gain in this axis. To maintain coverage of the target area, the beam pattern is swept through the target area from near range to far range. The sweep is timed to point the beam in elevation to receive signals from targets at the near range edge at the start of the sweep, and targets at the far range edge at the end of the sweep. Microcontroller  360  controls the sweeping of the beam by using digital-to-analog converter means  365  to generate control signals  370  to adjust the phase of the received signal. This method of steering the beam during receive maintains the signal to noise ratio with lower transmitted power, allowing for fewer or lower power active antenna nodes to be used, further lowering mass and simplifying construction. 
         [0206]    The active antenna node signals over the space feed should be isolated from the signals transmitted/received from the front face of the antenna panels to/from the target. Such isolation is required to prevent coupling of signals between these two radio frequency links. The embodiment described above uses frequency translation to achieve this isolation. (While in one embodiment such frequency isolation is performed at the nodes rather than the bus structure  100 , an alternative embodiment could employ the reverse.) Other techniques may also be used to achieve this isolation or for inhibiting interference between signals. Possible techniques can include one or a combination of any of the following: electromagnetic shielding, use of different signal polarizations, use of digital signal processing techniques, use of differently coded spread spectrum channels, use of time domain multiplexing alone or in conjunction with local signal storage. 
         [0207]      FIG. 5A  shows an arrangement of active antenna nodes on the rear face  125  of an antenna panel  105 . The number and arrangement of active antenna nodes can be adjusted to suit the needs of the intended application. The arrangement shown is typical for a synthetic aperture radar application. This example arrangement has a total of 64 active antenna nodes per antenna panel, arranged as two columns of 32 active antenna nodes per column. Alternative arrangements are also possible, for example a six panel antenna with a total of 384 active antenna nodes, with panel dimensions adjusted to provide the desired aperture size. 
         [0208]      FIG. 5A  also shows node electronics modules  500  and solar cell arrays  505  for each active antenna node. 
         [0209]      FIG. 5B  shows a detailed view of a portion of the rear of the panel  125  with the node electronics module  500  and the solar cell array  505  identified. 
         [0210]      FIG. 5C  shows the edge view of a portion of the antenna panel with the antenna panel radiating surface  120  and rear surface  125  of the antenna, and the node electronics module  500  identified. 
         [0211]      FIG. 5D  shows the radiating face  120  of the antenna panel with slots  510  for a slotted waveguide subarray visible. The arrangement, size and number of slots is dependent on the operating frequency and operational requirements for the antenna and the means for determining these characteristics is well understood and documented in the prior art. 
         [0212]      FIG. 6A  shows a cutaway view of a portion of an antenna panel to illustrate construction of the slotted waveguide subarray. The antenna panel frame  605  is constructed out of conducting material such as aluminum or conductively plated non-conducting material such as carbon fiber to form the structures for supporting the node electronics modules  500  and to form the cavities for the slotted waveguide subarray. To provide structural support, the cavity of the slotted waveguide subarray may be filled with an RF transparent material  600  such as quartz honeycomb. The quartz honeycomb material is commercially available for space-qualified applications. Other RF transparent materials can also be used. 
         [0213]      FIG. 6B  shows a section thorough the antenna panel. Detail “B” shows construction of the panel with antenna panel frame  605  and RF transparent material  600  identified. An aluminum sheet or conductively plated carbon fiber sheet  610  with slots  510  is bonded to the antenna frame and RF transparent material using a conductive adhesive, forming the radiating face of the antenna and providing structural strength. Detail “A” shows a portion of node electronics module  500  and waveguide launcher element  615  used to couple RF signals between the node electronics module and the slotted waveguide subarray. 
         [0214]    Current active phased array antennas, such as the one used for the RADARSAT-2 mission have a mass on the order of 45 kg per square meter. The combination of constructing antenna panels as described, and the elimination of wiring harnesses for power and RF signal distribution result in the active phased array having a mass on the order of 5 kg per square meter. 
         [0215]    The significant reduction in mass makes it possible to use technology developed by the space industry for the deployment of large solar arrays for spacecraft. This technology can be readily adapted to support and deploy the active phased array antenna. This technology is the lowest cost, most reliable way of deploying large apertures. Many companies have successfully built and deployed large solar arrays and the techniques used are fully qualified and have established heritage. 
         [0216]    In the design and operation of the antenna, compensation is employed for effects introduced by the space feed arrangement. One effect is due to the non-uniform radiation pattern from the antennas on the booms and the active antenna nodes. Another effect is the variation in gain and phase due to the path length differences from the space feed antenna assemblies  140  and the active antenna nodes. This effect is a function of the antenna geometry. 
         [0217]    The radiation patterns can be measured on the ground and compensation at each active antenna node can be computed. Compensation for the effects that are a function of the antenna geometry requires that the geometry be known while the antenna is operating. An ideal active phased array would have a front radiating surface that was planar and not subject to mechanical or thermal distortion. The antenna geometry would be constant and could be measured on the ground prior to launch, and necessary compensation at each active antenna node computed. 
         [0218]    The disadvantage of using solar array technology is that it cannot achieve these ideal characteristics, as the deployed aperture is not stiff and can have mechanical and thermal distortions and oscillations. The expected deviation from ideal due to the distortions and oscillations are in the order of a few centimeters at frequencies of 0.1 Hz or less. This inherent limitation should be overcome by a means that provides geometry compensation of the antenna. 
         [0219]    There are several possible approaches for implementing the geometry compensation means. For example, compensation can be implemented on-board the spacecraft to perform dynamic real-time compensation of antenna distortions. An alternative approach is to implement geometry compensation as a non real-time correction applied on the ground during processing of the acquired radar data. The selected approach depends on the size of the antenna aperture, the antenna dynamics and the application. 
         [0220]    The depicted geometry compensation means uses an optical technique to take multiple images of illuminated targets mounted on the rear face of the antenna panels and on the fore and aft booms to perform dynamic real-time geometry compensation on-board the spacecraft. 
         [0221]      FIG. 7  gives an overview for dynamic geometry compensation of the active phase array antenna. A cavity  700  within the spacecraft bus structure  100  houses optical and electronics assemblies that comprise a dynamic compensation system. Optical paths  705  and  710  are provided from the optical assembly cavity to the fore and aft wings and to the fore and aft booms respectively. Targets  715 ,  720  and  725  are attached to the back of the antenna panel and to the ends of the fore boom and aft booms respectively. The targets contain an internal light source to illuminate the surface of the target facing in the direction of the optical path. The light source can be switched on and off under control of the dynamic geometry compensation system. The shape of the illuminated surface of the targets is selected to facilitate accurate determination of the center of the target&#39;s position in an image of the target. For example a circular shape sized so that the resulting image of the target will be multiple pixels wide allows techniques to locate the centroid of the target&#39;s image to be used to improve position determination. Distortion of the booms and antenna panels in the dimension along their respective lengths is small, and the impact of this distortion is negligible, and the geometry compensation means does not need to measure in this dimension. Distortions are more pronounced in the other two dimensions and their impact is significant. The optical path is arranged to achieve high accuracy in these two dimensions by imaging along the length of the structures being measured. 
         [0222]    To further improve the ability to extract the targets from the imagery, the targets may use solid-state light sources with a narrow spectral bandwidth. Optical filters with the corresponding bandwidth are placed in the optical assembly to filter out light that falls outside the filter&#39;s bandwidth. 
         [0223]      FIG. 8A  shows a detail of the mounting location of target  720  on the fore boom  130 .  FIG. 8B  shows two antenna panels  105 . Each antenna panel, except the panels nearest to the spacecraft bus structure, have 4 targets mounted in the positions shown. The two panels nearest to the spacecraft (not shown) bus structure only have two targets mounted. The mounting positions for the targets for the nearer panel are arranged so as avoid a nearer target obstructing the view to a further target when viewed from the optical assembly. This is illustrated in  FIG. 9  with optical paths shown in dashed lines. Targets are mounted sufficiently above the surface of the antenna panel or boom so that they remain visible when the antenna wing or boom distorts or oscillates.  FIG. 8C  shows an example target  800 . Targets may be folded against the panel when the panels are stowed prior to launch and may deploy using a simple spring or other means after the panels are deployed. 
         [0224]      FIG. 10  shows the optical and electronic components of the geometry compensation system. Optical assembly  1000  receives light  1010  from the fore and aft booms and the fore and aft wings. The optical assembly combines the light from the four apertures so as to form a single, combined image  1015  that is projected onto the imaging surface of a solid state, two dimensional imaging array  1020 . The output of the imaging array is received, processed and interpreted by computer based image processing unit  1025 . Boom target controllers  1040  and  1045  control the illumination of the targets on the fore and aft booms respectively. Panel target controllers  1030  and  1035 , located on each antenna panel of the fore wing and aft wing respectively, control the illumination of the panel targets. 
         [0225]    Control signals  1055  for the boom target controllers are provided by a wired connection from image processing unit  1025 . Control signals  1050  for the panel target controllers are provided by a control signal initiated by image processing unit  1025  and transmitted to each panel target controller using a CAN Bus signal. Alternatively, a coded infrared signal generated by the image processing unit  1025  and directed to and received by the panel target controllers could be used to affect this control function. 
         [0226]    Operation of the geometry compensation system is described below. 
       Operation 
       [0227]    The description above describes the operation of the individual elements of the active phased array antenna system. Here we will describe the overall operation of the system, using as an example a typical spaceborne radar application, such as a synthetic aperture radar that is used for making images for observation of the earth&#39;s surface. 
         [0228]    Prior to launch, the spacecraft is placed in its launch configuration.  FIG. 11A  shows the spacecraft with the fore and aft booms  130 ,  135  and fore and aft wing  110 ,  115  antenna panels in their stowed position, inside the launch vehicle&#39;s payload fairing  1100 . 
         [0229]    After launch and initial checkout, the wings and booms are deployed into their operational configurations.  FIG. 11B  shows the spacecraft on orbit with the fore boom  130  and the fore wing  110  partially deployed.  FIG. 11C  shows the spacecraft in its fully deployed, operational configuration. 
         [0230]    In the example application, and typical of other applications as well, the radar is operated intermittently, being active (collecting image data in this example) over areas of interest and remaining inactive at other times. 
         [0231]    To conserve power, the active phased array antenna system is placed into a standby state with its internal units either switched off completely, or put into a low power state that allows them to respond to commands. In this state, the spacecraft will generally be slewed to an attitude that improves the efficiency of solar power generation. 
         [0232]    The circuits of the units that comprise the receiver/exciter  210  are powered off, except for those elements to respond to signals on control bus  260  that instruct the units to power up and become active. 
         [0233]    A similar approach is used for the phased array antenna. As there are many active antenna nodes in the antenna, each node is designed to consume a minimum of power when not in use. This standby state is achieved by powering down all circuits within the node, except for the battery charging and power supply circuits and the microcontroller. The microcontroller is placed into a very low power standby state that will allow it to respond to a wakeup signal sent to it via the CAN Bus interface. 
         [0234]    To make understanding of the overall operation easier, the operation of an active antenna node will be described first. 
         [0235]      FIG. 13  shows the sequence of events to bring an active antenna node from the inactive state to the operational state. The figure illustrates one embodiment, and alternative approaches and sequences can also be used to accomplish a similar purpose. It is assumed that the node is in the standby state described above at the start of the sequence. 
         [0236]    The microcontroller circuits monitor the CAN Bus for a wakeup signal (step  1 ). When the wakeup signal is received, microcontroller clocks are enabled and it exits the standby mode and resumes execution of its software programs (step  2 ). The microcontroller then begins execution of a self-test sequence that verifies correct operation of the microcontroller itself, and powers up the remaining circuits in the node and determines their operating condition. Temperatures and voltages are also measured to determine if they are within the acceptable range. 
         [0237]    If a significant fault is detected, then the fault is reported to antenna controller  270  (step  5 ) and the node enters a maintenance mode (step  6 ). The maintenance mode puts the node into a safe state and permits further diagnostic testing and the uploading of instructions or software patches to correct the fault. A command on the CAN Bus interface from the antenna controller causes the microcontroller to exit maintenance mode (step  7 ). The microcontroller then returns the node to its low power standby state (step  8 ). 
         [0238]    If no faults are detected, then the node waits for a command to put it into operational mode (step  9 ). If this command is not received within a specified period of time, the node will enter maintenance mode. If the command is received, the node enters operational mode (Step  10 ). In operational mode, the node responds to control and timing messages from the antenna controller and processes the transmitted and received radar signals. Further detail is provided in the discussion on  FIG. 14  below. 
         [0239]    During operational mode, the microcontroller monitors node operation to detect any faults or non-nominal conditions such as a temperature that is too high (step  10 ). If a fault is detected, the node exits operational mode (step  11 ), reports the fault condition (step  5 ) and enters maintenance mode (step  6 ). Operation in maintenance mode is as previously described. 
         [0240]    If no fault was detected while in operational mode, the microcontroller determines if a shutdown signal has been received from the antenna controller (step  12 ). If no shutdown signal has been received, operational mode continues. If a shutdown signal has been received, the microcontroller returns the node to its low power standby state (step  8 ) and the radar operation session is complete at the node. 
         [0241]      FIG. 14  shows the overall operation of the phased array antenna system. It is assumed that the system is in the standby state at the start of the sequence. 
         [0242]    Operation of the radar is scheduled to occur at specific times when the spacecraft is in the correct position in its orbit for the desired imaging operation. The scheduling is accomplished by using time-tagged commands issued from the spacecraft control center on the ground. Shortly before the scheduled start time of an image take, the receiver/exciter  210  hardware located in the spacecraft bus is powered up (step  1 ). The antenna controller  270  sends a wake up signal to the active antenna nodes (step  2 ). The active antenna nodes begin to execute their start-up sequence and self-test activities as described above. 
         [0243]    The antenna controller begins a self-test sequence for the entire phased array antenna system, verifying correct operation of all units mounted in the bus structure and receiving status from the active antenna nodes (step  3 ). If a major fault is detected (step  4 ), the antenna controller reports the fault in antenna telemetry (step  5 ) and the antenna enters maintenance mode (step  6 ). The maintenance mode puts the antenna system into a safe state and permits further diagnostic testing and the uploading of instructions or software patches to correct the fault. When maintenance activities are completed, the antenna controller exits maintenance mode (step  7 ). A shutdown signal is sent to the active antenna nodes (step  8 ) and the receiver/exciter is powered down and returned to its standby state (step  9 ). 
         [0244]    If no fault is detected, then the antenna controller determines if the scheduled activity for the antenna is a maintenance activity or an operational activity (step  10 ). If it is a maintenance activity, then maintenance mode is entered (step  6 ). If not a maintenance activity, the antenna begins its nominal operation. 
         [0245]    The first step of nominal operations is to initialize the active antenna nodes with beam parameters and other operational parameters, for example transmit and receive window timing and duration, required for this image (step  11 ). The geometry compensation process is started to measure the geometry of the antenna and determine the phase and amplitude compensation for each active antenna node (step  12 ). The operation of the geometry compensation process is described below. 
         [0246]    At the scheduled imaging time, the active phased array antenna begins to operate (step  13 ). The operation is controlled by timing and control messages  1400  broadcast on the CAN Bus to all active antenna nodes by the antenna controller  270 . The messages are sent at a transmit pulse repetition frequency. 
         [0247]      FIG. 15  shows an example of timing relationships. The CAN Bus timing and control message is sent shortly before the next transmit pulse. The message defines a timing reference point for the next pulse cycle. The active antenna node microcontroller uses the received timing and control message to establish two timing windows, a transmit timing window represented by the transmit mode enable 1405, and a receive timing window represented by the receive mode enable 1410. These windows are made slightly larger than required to allow for timing jitter in the CAN Bus messages. Precise timing for the transmitted pulse is established by the transmit pulse generator  220 . 
         [0248]    Operation continues (steps  15  and  16 ) until either the scheduled end time is reached (step  14 ) or a major fault is detected (step  17 ). 
         [0249]    In the case of reaching the scheduled end time, the radar operations and geometry compensation processes are terminated (step  19 ). A shutdown signal is sent to the active antenna nodes to return them to their standby state. Components within the receiver/exciter are also powered to conserve battery power (step  9 ). 
         [0250]    In the case that a fault is detected, the fault is reported in the antenna telemetry (step  18 ), the radar operation and geometry compensation processes are terminated (step  19 ) and the antenna system powered down and returned to its standby state (steps  8  and  9 ). 
         [0251]      FIG. 16  shows the sequence of operations for performing geometry compensation and describes how the geometry compensation system operates. Other sequences that collect reference images more or less frequently or collect images of the targets in a different order are possible, but the overall concept remains the same. 
         [0252]    The geometry compensation operation is initiated whenever the active phased array antenna is active. The lights of all targets  715 ,  720  and  725  are switched off (step  1 ) and a reference image is captured and stored (step  2 ). The reference image consists of the superimposed images of the fore and aft booms and the fore and aft wings. Lighting conditions of the booms and wings is not critical. The fore wing panel  1  lights are switched on (step  3 ) and an image is collected (step  4 ). This image also consists of the superimposed images of the fore and aft booms and the fore and aft wings, however the targets on one panel are now illuminated. Note that the specific panel designated as panel  1  is not important, as all panels will be imaged during each cycle. 
         [0253]    The reference image of step  2  is subtracted from the image of step  4  (step  5 ). Since the nominal position of the target is known, only the region of the image around the nominal target position needs to be processed. As the images are taken fractions of a second apart, the differences in the two images will be due solely to the illumination of the targets on fore wing panel  1 . The resulting image will contain only the illuminated targets, effectively extracting the targets from the images. The targets are identified based on their relative position and the position of each target in the image is determined by applying an algorithm to locate the centroid of each target (step  6 ) and computing the two dimensional location. The third dimension is fixed and can be obtained by on-ground measurements prior to launch. The resulting 3-dimensional positions of the targets are stored (step  7 ). 
         [0254]    The lights on panel  1  are turned off (step  8 ) and the process of determining the target positions is repeated for panel  2  (step  9 ). Similarly panel  3  (step  10 ) and panel  4  (step  11 ) measurements are taken. The process of collecting a reference image, turning on the lamps for each panel in turn and determining the target positions is repeated for the four panels of the aft wing (step  12 ). 
         [0255]    A new reference image is collected and stored (step  13 ). The target on the fore boom is illuminated (step  14 ) and the position of the fore boom target is determined (step  15 ). Similarly the position of the aft boom target is determined (step  16 ). To reduce noise in the measurements and improve the overall accuracy, several measurements are taken (step  17 ) and averaged (step  18 ) to produce a final position determination for each target (step  19 ). 
         [0256]    Using these position measurements a geometric model of the antenna is constructed (step  20 ). This model is used to compute the phase errors introduced by mechanical distortions and oscillations in the antenna at each active antenna node position and the phase correction required to compensate for these errors (step  21 ). For each active antenna node, the latest computed phase compensation value is compared to the previously computed value for that node to determine which nodes require updated correction information. The updated correction information is transmitted to those nodes that require it using the CAN Bus interface (step  22 ). 
         [0257]    This process of measuring and updating phase compensation of the antenna nodes operates continuously as long as the antenna is active (step  23 ). 
       DESCRIPTION AND OPERATION OF ADDITIONAL EMBODIMENTS 
       [0258]    The depicted embodiment uses a square cross-section spacecraft bus structure  100 . Different cross sections can be used and may have advantages in certain applications. Three examples of different configurations are given.  FIG. 12A  shows a triangular bus structure  1200  with the solar arrays used to provide bus power mounted on the surface  1205 .  FIG. 12B  shows a variation of the triangular shape that provides more internal volume within the bus structure  1210 . Solar cells to provide bus power may be mounted on surface  1215 .  FIG. 12C  shows an alternate arrangement in which the phased array antenna is mounted outboard of the bus structure  1220 . In this arrangement only a single boom assembly  1230  is required. Solar cells to provide bus power are mounted on surface  1225 . 
         [0259]    One embodiment of the invention produces a radar that operates with the same polarization in both transmit and receive, for example vertical polarization on transmit and vertical polarization on receive. The present system can be implemented to provide a radar capable of operating with selective polarization for transmitted signals, and dual polarizations for received signals. For example, transmit signals can be selected to be either horizontal polarization or vertical polarization, and receive signals can be selected to be horizontal polarization, vertical polarization, or both polarizations simultaneously. A quad-polarization radar can thus be achieved by transmitting horizontal and vertical polarizations on alternate transmit pulses, and simultaneously receiving both horizontal and vertical polarizations on for all pulses. 
         [0260]    The basic concepts and characteristics described in the above embodiment remain, however some modifications may be employed to support the additional polarization, such as a different arrangement for the subarray in the active antenna node. Although a slotted waveguide arrangement can be constructed for dual polarization, it may have the disadvantage of resulting in a thicker antenna panel, increasing the mass and makes the stowing and deployment more difficult. Instead of a slotted waveguide subarray, a thin subarray assembly  1720  consisting of multiple patch radiators bonded to the front surface of the antenna panel. Each patch radiator element is driven by two feed assemblies, one for the horizontal polarization  1716  and the other for the vertical polarization  1718 . The mechanical construction of the antenna panel is simplified by eliminating the conductive cavities under the slotted waveguide. 
         [0261]    On the transmit side, a means is provided to select which of the two feeds is driven on a pulse by pulse basis, with the control signals generated by the microcontroller in the active antenna node. On the receive side, two receive channels are provided, both in the active antenna node and in the receiver/exciter. 
         [0262]      FIG. 17  shows a block diagram of the radio frequency circuit functions contained within the active antenna node for an active phased array antenna with multiple polarization capability. The frequency translated transmit pulse is received by antenna  1700  and directed to the transmitter circuits by signal routing device  1702 . The received signal is first amplified by variable gain amplifier  1704  and then converted to the operating frequency of the radar by mixer  1706 . The amplitude and phase are adjusted using gain control signal  1764  and phase control signal  1752 . High power amplifiers  1710  and  1712  are selectively enabled to drive either the horizontal or vertical feed of the subarray respectively, by polarization select signal  1762 . Signal routing devices  1714  and  1728  connect the transmit signal to the horizontal and vertical feed assemblies  1716  and  1718  respectively. 
         [0263]    The reflected signal returned from the target is received by the patch radiators in the subarray and the horizontal and vertical polarizations are routed to the two separate receive channels by signal routing devices  1714  and  1728 . The horizontal polarization is amplified by low noise amplifier  1722  and frequency converted and phase adjusted by mixer  1724 . The signal is amplified by variable gain amplifier  1726 , and routed by signal routing device  1702  to antenna  1700  for transmission to a boom antenna assembly  140 . The amplitude and phase are adjusted using gain control signal  1766  and phase control signal  1752 . The vertical polarization is similarly processed using signal routing device  1728 , low noise amplifier  1730 , mixer  1732  and variable gain amplifier  1734 . Antenna  1736  is used to transmit the signal to the boom antenna assembly. The amplitude and phase are adjusted using gain control signal  1768  and phase control signal  1754 . 
         [0264]    Since a second receive frequency is to be simultaneously transmitted to the boom antenna assembly, the frequency plan for the space feed is to be extended. Extending the example presented earlier, a frequency plan for a typical multiple polarization SAR application would be as follows: SAR operating frequency of 5.400 GHz (C-band), stable local oscillator frequency of 2.400 GHz, carrier frequency for the frequency translated transmit chirp and horizontal received polarization signal  1770  of 10.200 GHz and carrier frequency for the frequency translated vertical received polarization signal  1772  of 7.8 GHz. 
         [0265]    The broadcast stable local oscillator signal is received by antenna  1738 , amplified by low noise amplifier  1740  and divided into two signals by power divider  1742 . One output of the divider directly provides the reference frequency used for the received vertical polarization. The other output of the divider is doubled in frequency by frequency doubler  1744  to provide the reference frequency used for downconverting the frequency translated chirp and upconverting the received horizontal polarization. The phase of the reference frequencies is adjusted by direct modulators  1748  and  1746  based on control signals  1754  and  1752  respectively. Since transmit and receive do not occur simultaneously, direct modulator  1746  can be used to provide the phase adjusted reference frequency to both the transmitter and horizontal polarization receive circuits through power divider  1750 . Phase control signal  1752  is adjusted during the pulse period to first produce the required phase for the transmit pulse and then the required phase for the received signal. 
         [0266]    Other embodiments of a multiple polarization antenna are possible, however the basic principles remain the same. 
         [0267]    The geometry compensation system can alternatively be implemented using passive targets whose surface is covered by highly directional reflective material. The targets are selectively illuminated by narrow beams of light projected from light sources located in the vicinity of the optical assembly. Light sources with a narrow spectral bandwidth and corresponding filters in the optical path are used. Operation is similar to that described for the targets with the built in light sources, except that the light sources in the bus structure are illuminated in sequence instead of the light sources in the targets. This approach simplifies the design of the targets and eliminates the need for control circuits and power sources for the targets on the antenna panels. The disadvantage is a more complicated optical assembly, because it is to incorporate the light sources close to the optical axis. 
         [0268]    Antenna distortion can be decomposed into two components, a fixed distortion and a varying distortion. The fixed distortion can be measured and compensated for using a classic calibration approach traditionally used in such a system. For example, in a SAR system, a beam pattern can be measured over a well-selected target area and distortion can be determined and removed by applying phase compensation using the same phase shifters used to shape the beams. Compensating for the varying component involves making on-orbit measurements over the period that the antenna is in use and applying a dynamic compensation. Geometry compensation that takes advantage of this characteristic can also be used in place of an optically based compensation approach. 
         [0269]    One alternative is to use ground processing of on-orbit measurements. A method for accomplishing this has been described by Luscombe et al (In orbit Characterisation of the RADARSAT-2 Antenna—Proceedings of the Committee on Earth Observation Standards—Working Group on Calibration and Validation—Synthetic Aperture Radar Workshop 2004). This technique uses a portion of the antenna as a reference to obtain data on relative geometric displacement of a different portion of the antenna (e.g. a row or column) that is being measured. The reference portion initially used is then measured by using a previously measured portion of the antenna as the reference. A complete set of measurements can be taken in a relatively short period of time (&lt;2 seconds typically). In operation, a set of measurements is made immediately prior to and following the collection of data for an image. The measured results are transmitted to the ground and are post-processed to determine the antenna geometry present during the imaging operation. This geometry information is then used to compensate for antenna distortion during the processing of the image data. 
         [0270]    Another alternative means of geometry compensation is to measure temperature at numerous points across the antenna as a means to determine the varying distortion. Classical techniques would be used to determine and compensate for the fixed distortion as described above. A calibration campaign would then be conducted to characterize the antenna distortion as a function of temperature. This calibration campaign would involve repeated measurements of antenna pattern over a well-selected target area. Temperature of the antenna prior to these measurements would be varied, for example by heating the antenna by re-orienting the spacecraft or by using the antenna for varying lengths of imaging prior to taking the measurement (thus dissipating more or less power from Transmit Receive modules into the antenna structure). On-ground analysis of the resulting antenna patterns would yield distortion compensation calibration data. Compensation of antenna distortion could then be applied either as a real time correction on the spacecraft (measure temperatures and apply corresponding phase correction at each point in the antenna) or as part of the on-ground processing of the SAR data. 
         [0271]    In one embodiment of the antenna system, an active lens configuration is used. Because a lens configuration is intrinsically less sensitive to physical antenna distortion than a direct fed array or a reflector, it is particularly suited to either of the above alternative geometry compensation approaches. 
         [0272]    The construction of the active phased array antenna for radar applications takes advantage of the antenna not needing to support simultaneous transmit and receive functions. However, the antenna can be adapted for uses in applications other than radar systems, for example, in a communications system, where simultaneous and continuous transmit and receive is required. The approach is to use two carrier frequencies, on each of the space feed and the active phased array antenna face, one frequency for the signal to be transmitted, and one for the received signal. The basic structure of the active antenna node remains unchanged. An example frequency plan is as follows: Communications link transmit operating frequency of 5.700 GHz, receive frequency of 5.100 GHz, stable local oscillator frequency of 2.400 GHz, carrier frequency for the frequency translated transmit signal of 10.5 GHZ, and frequency translated receive signal 9.900 GHz. 
         [0273]    Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” As used herein, the terms “connected,” “coupled,” or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling of connection between the elements can be physical, logical, or a combination thereof. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or,” in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list. 
         [0274]    The above detailed description of embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise form disclosed above. While specific embodiments of, and examples for, the invention are described above for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. For example, while processes or blocks are presented in a given order, alternative embodiments may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and/or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, or may be performed at different times. 
         [0275]    The teachings of the invention provided herein can be applied to other systems, not necessarily the system described above. The elements and acts of the various embodiments described above can be combined to provide further embodiments. 
         [0276]    All of the above patents and applications and other references, including any that may be listed in accompanying filing papers, are incorporated herein by reference. Aspects of the invention can be modified, if necessary, to employ the systems, functions, and concepts of the various references described above to provide yet further embodiments of the invention. 
         [0277]    These and other changes can be made to the invention in light of the above Detailed Description. While the above description describes certain embodiments of the invention, and describes the best mode contemplated, no matter how detailed the above appears in text, the invention can be practiced in many ways. Details of the system may vary considerably in its implementation details, while still being encompassed by the invention disclosed herein. As noted above, particular terminology used when describing certain features or aspects of the invention should not be taken to imply that the terminology is being redefined herein to be restricted to any specific characteristics, features, or aspects of the invention with which that terminology is associated. In general, the terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification, unless the above Detailed Description section explicitly defines such terms. Accordingly, the actual scope of the invention encompasses not only the disclosed embodiments, but also all equivalent ways of practicing or implementing the invention.