Abstract:
An airborne radar antenna system for detecting a target in a volume includes a tethered aerostat and an antenna that is supported above ground by the aerostat. The aerostat-based antenna is used for transmitting and receiving a radar beam into the volume to detect the target. Additionally, the system includes a ground-based transmitter that generates a beacon signal which monitors the antenna configuration at the aerostat. A computer then evaluates the beacon signal to create an error signal which is used to maintain a predetermined configuration for the antenna. The system also includes mechanisms for orienting the radar beam along preselected beam paths between the antenna and the volume.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention pertains generally to radar antennas. In particular the present invention pertains to ground-based radar systems that incorporate airborne antennas. More particularly the present invention pertains generally, but not exclusively, to target acquisition systems which employ airborne antennas that are supported by aerostats.  
         BACKGROUND OF THE INVENTION  
         [0002]    Several considerations must always be addressed during the design and development of any effective radar system. In particular, and of special concern for the present invention, is the configuration of an antenna that can be used for a ground based radar system, and the way in which it is to be operationally deployed with an aerostat. For this concern, both technical and operational considerations need to be addressed. For example, technical consideration that can affect the target acquisition capability of a radar antenna include its size, its rigidity, its ability to direct a transmitted radar beam along a desired beam path and, of course, its power. Further, important operational considerations involve the location of the antenna, its steerability and, depending on its mission profile, the ease with which it can be set up for deployment and dismantled for subsequent relocation and redeployment.  
           [0003]    It happens that target acquisition radar systems require effectively unobstructed line-of-sight beam paths. Thus, radar systems, in general, are adversely affected by “clutter” in the form of unwanted echoes from terrain features and man-made structures in the immediate vicinity of the antenna. Accordingly, for ground-based radar systems, an obvious solution is to somehow elevate the radar antenna.  
           [0004]    Towers, or other types of vertical structures, are quite commonly used for the purpose of elevating radar antennas to a location where they can be effective. For situations wherein a relatively high degree of mobility is required, however, it may be more cumbersome and time consuming to erect and dismantle antenna towers than is operationally warranted. In such situations, it has been proposed that an aerostat be used as a platform for the antenna. The use of an aerostat for this purpose, however, introduces additional considerations of antenna weight which would otherwise be of much less concern. For instance, the necessary rigidity for an antenna is typically provided by a structure which, even when made of a relatively lightweight material, still has substantial weight. Also, because transmit apertures for antennas are heavier than their associated receive apertures, it may be desirable to reduce the size, and consequently the weight, of the transmit aperture for an aerostat based antenna. The result of such an antenna configuration is that the transmit beamwidth effectively grows larger (i.e. a “floodlight” beam). Consequently, because target detection probability remains a function of energy on target, there is a diminution in target detection ability.  
           [0005]    In light of the above, it is an object of the present invention to provide an airborne radar antenna system for detecting a target in a volume that includes an antenna made of a light weight material, such as printed circuits on a flexible mylar sheet. Another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that is capable of effectively using a smaller transmit aperture than its receive aperture. Still another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that can be effectively deployed with an inflatable aerostat. Yet another object of the present invention is to provide an airborne radar antenna system for detecting a target in a volume that is easy to use, relatively simple to manufacture, and comparatively cost effective.  
         SUMMARY OF THE PREFERRED EMBODIMENTS  
         [0006]    In accordance with the present invention, an airborne radar antenna system for detecting a target in a volume includes at least one inflatable aerostat, and a same number of tethers that respectively anchor each aerostat to points on the ground. A radar antenna, for transmitting and receiving a radar beam, is supported by each aerostat at respective locations above ground level. It is contemplated for the present invention that the radar antenna is preferably one square meter in size and less than approximately seventy kilograms (70 kg). Importantly, the radar antenna may be made of a flexible material. For example, the antenna can be made of a flexible sheet on which the required antenna elements have been printed. The flexible sheet can then be mounted on a rigid frame which, in turn, is supported by the aerostat. Thus, the antenna can be supported by the aerostat in any of several ways. These include mounting the antenna inside the buoyancy chamber of the aerostat. Alternatively, the antenna can be mounted on the surface of the aerostat&#39;s buoyancy chamber or in an enclosure that is suspended beneath the aerostat.  
           [0007]    Included in the system of the present invention is a ground-based transmitter that is positioned at a distance from the aerostat. The specific purpose of this transmitter is to radiate a beacon signal toward the antenna at the aerostat. A computer is then used to evaluate the beacon signal as it is received by the antenna for purposes of creating an error signal. Importantly, this error signal is indicative of any deviations or distortions that may be experienced by the flexible antenna from its desired configuration. Accordingly, with this error signal, system mechanisms can then be activated to electronically or mechanically reconfigure or calibrate the antenna element, as necessary, to orient and direct the radar beam along a predetermined beam path toward the volume. Additionally, system mechanisms can be incorporated for rotating or spinning the antenna element to sweep the radar beam through the volume.  
           [0008]    Also included in the system of the present invention is a ground station that is established to house the computer and any other subsystems that are required to control the antenna for its target acquisition mission. In order to affect this control, a communications link is provided that connects the computer and other subsystems at the ground station with the antenna at the aerostat. The present invention contemplates that any communication, whether it is a two-way or one-way communication, between the ground station and the antenna, can be established through the communications link. By way of example, DC power from a power source at the ground station can be sent, through the communications link, and to the antenna for transmitting and receiving a radar beam. Preferably, the communications link is an optical fiber that is incorporated with the tether. The communications link, however, may be a wireless or an optical link of any type known in the pertinent art.  
           [0009]    It can happen that for certain applications, it is desirable, or necessary, for the receive aperture of the antenna to be a different size than the antenna&#39;s transmit aperture. If so, for instances wherein a first aperture (of area A 1 ) is used for transmitting the radar beam, and a second aperture (of area A 2 ) is used for receiving a return signal from said radar beam, and wherein A 1 =nA 2  with n&gt;1, the present invention envisions filling the transmitter beam with multiple-simultaneous receive beams and having an appropriately increased dwell time on the return signal for target detection by the receive aperture (A 2 ). Specifically, if “x” seconds are required to detect the target for an antenna configuration wherein the value of “n” is one (n=1), the system of the present invention contemplates increasing the dwell time of the antenna to “nx” seconds for receiving the return. The result is an equivalent volumetric search rate with a reduced transmit aperture resulting in reduced size, weight, and (relative) cost. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which:  
         [0011]    [0011]FIG. 1 is a schematic view of the radar system of the present invention showing a typical deployment of the aerostat-based antennas;  
         [0012]    [0012]FIG. 2 is a view of an embodiment of an aerostat as used for the present invention with portions broken away for clarity; and  
         [0013]    [0013]FIG. 3 is a schematic view of the electronic components used for the operation of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring initially to FIG. 1, a radar system in accordance with the present invention is shown and is generally designated  10 . As shown, the system  10  includes a ground station  12  and a plurality of airborne aerostats, of which the aerostats  14   a, b  and  c  are only exemplary. For purposes of the present invention, the ground station  12  can be a fixed installation (as shown), or it can be a mobile facility (e.g. a truck) which is capable of being relocated, if required. In any event, the ground station  12  is intended to manage and command the system  10  as a centralized facility. Insofar as the aerostats  14   a, b  and  c  are concerned, they are preferably made of an elastic material which will allow for helium volume changes that occur as a result of temperature and atmospheric pressure changes.  
         [0015]    Using the aerostat  14   a  for purposes of disclosing the system  10  of the present invention, it will be seen that in one contemplated configuration, the aerostat  14  can include an enclosure  16  that is suspended by interconnecting line  18  beneath the aerostat  14   a.  Further, for all of its possible configurations, the aerostat  14   a  (via enclosure  16 , if used) is preferably anchored to the ground by a tether  20   a.  More specifically, a ground-based mechanism, such as a winch  22 , is used to vary the length of the tether  20   a  and thereby adjust the distance  24  at which the aerostat  14   a  is elevated above ground level. Typically, the distance  24  can be varied from around twenty feet to around five hundred feet (20-500 ft.). Alternatively, it is recognized that higher altitude aerostats are capable of operations at 10,000 ft. and higher.  
         [0016]    Referring now to FIG. 2 it can be seen that the present invention envisions mounting antennas  26  inside the respective aerostats  14   a, b  and  c.  It should be noted, however, that the present invention also envisions mounting the antennas  26  inside respective enclosures  16 , if used. As also envisioned by the present invention, in order to minimize weight requirements the antennas  26  will each include printed circuits  28  which are printed on flexible panels  30  that are mounted on frames  32 . The overall weight for each of the antennas  26  of the present invention is envisioned to be less than approximately seventy kilograms (70 kg). Further, as shown in FIG. 2, in order to increase the directional capability of the antenna  26 , the present invention contemplates the use of two panels  30   a  and  30   b,  with respective printed circuits  28   a  and  28   b  mounted on respective frames  32   a  and  32   b,  which are oriented substantially perpendicular to each other.  
         [0017]    Referring back to FIG. 1, it is seen that a communications link  34  connects the antenna  26  of the system  10  to the ground station  12 . Preferably, the communications link  34  is an optical fiber which is incorporated directly into the tether  20 . The communications link  34  may, however, be any other type of link well known in the pertinent art that is useful for connecting a radar antenna  26  to a ground station  12 , such as a wireless communications link.  
         [0018]    It is important to note that any communication, whether it is a two-way or a one-way communication, between the antenna  26  and the ground station  12  can be accomplished through the communications link  34 . For example, the present invention can include a camera means that is attached to the aerostat  14  and is in electronic communication with the antenna  26 . A radar video of the target  44  that is captured by the camera means can be then sent from the antenna  26 , through the communications link  34 , to a video display monitor at ground station  12 . Another example is that DC power generated by a power source at the ground station  12  can be sent through the communications link  34  and up the tether  20  to the antenna  26  for any desired purposes, such as to operate the camera means.  
         [0019]    Still referring to FIG. 1, it is seen that the system  10  also includes a ground-based beacon  36  which is used to provide a reference for electronically calibrating the printed circuits  28 . The purpose here would be to establish an effective array for the antenna  26 . Specifically, each aerostat  14   a, b  and  c  can have a respective beacon  36   a, b  or  c  positioned on the ground near the aerostat  14  to radiate a beacon signal  38  to the antenna  26 . This beacon signal  38  can then be passed via the communications link  34  to the ground station  12  where it will be processed for the purposes stated above.  
         [0020]    The general intention of the system  10  is to locate, and elevate an antenna  26  at a selected ground location, or to otherwise establish a distribution of such elevated antennas  26 . In either case, the purpose is to radiate a radar beam  40  along a predetermined beam path  42  to detect a target  44 . As implied above, the control of this operation is accomplished at the ground station  12 . In FIG. 3, a general layout of the system  10  is presented which shows that centralized control of the antenna  26  is provided at the ground station  12 . Specifically, this control relies on a computer  46  which operates in concert with a signal processor  48 . Further, as also indicated in FIG. 3, internal communications between the computer  46 , an antenna control  50 , and an antenna corrections function  52  at the ground station  12  provide for necessary operational reconfigurations of the antenna  26 .  
         [0021]    In order to comply with weight restrictions for an aerostat based radar antenna  26 , it may be desirable to reduce the size of the transmit aperture  54  of the antenna  26  relative to its receive aperture  56 . If so, for a situation wherein the antenna  26  establishes an aperture (of area A 1 ) for transmitting the radar beam  40  (i.e. aperture  54 ), and an aperture (of area A 2 ) for receiving a return signal from the radar beam  40  (i.e. aperture  56 ), and wherein A 1 =nA 2  with n&gt;1, the transmitter beam is filled with multiple-simultaneous receive beams and the dwell time of the antenna  26  can be appropriately adjusted. Specifically, if “x” seconds are required to detect the target  44  when n=1, the system  10  of the present invention envisions increasing the dwell time of the antenna  26  to “nx” seconds for receiving the return when n&gt;1.  
         [0022]    While the particular Distributed Elevated Radar Antenna System as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.