Abstract:
A hybrid image gathering and data transmission system is provided. The system includes at least one parabolic reflector to gather, disseminate and direct electromagnetic radiation. A beam splitter using Fresnel Zone Plate Array (FZPA) is configured and arranged to receive and or transmit the electromagnetic radiation from or to the at least one parabolic reflector and separately focus microwave radiation and visual radiation. The beam splitter provides a gain in the microwave radiation and visual radiation. A Radio Frequency (RF) receiver/transmitter receives and transmits the microwave radiation from or to the beam splitter and a Focal Plane Array (FPA) receives the visible radiation from the beam splitter. A processor is in communication with the RF receiver and the FPA. The processor processes signals received by the RF receiver and the FPA and provide processed data to be transmitted to a remote location.

Description:
BACKGROUND 
       [0001]    Earth observation using low cost, low earth orbit satellites for both military and civilian applications has proliferated rapidly in recent years. Finer resolution is desired while imaging large areas during each pass of the satellite which results in a large amount of data generation. This data is typically down linked to the user in the field as soon as possible to be of value. In areas of interest multiple revisits may be required to gather desired information. However, limited available link time to a ground station can hamper operations. Two types of sensing systems are typically employed to observe an area of interest during different times of day and conditions. An optical system imaging in the visible wave spectrum can be used during the daytime on a clear day. The optical system provides a fine resolution of the area of interest but is ineffective during the night or if clouds, fog, smoke, or dust are present in the atmosphere. A microwave system that images in the Radio Frequency (RF) spectrum can be used when the conditions are not ideal for the optical system. However, the resolution of the microwave system is not as fine as the optical system. Including an optical system and a microwave system in the same satellite is very cost prohibitive because of the weight and space needed for the separate receiving and processing systems. 
         [0002]    For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a hybrid optical and microwave system that is effective and efficient and requires a relatively small footprint. 
       SUMMARY OF INVENTION 
       [0003]    The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. 
         [0004]    In one embodiment, a hybrid image gathering system is provided. The system includes a t least one parabolic reflector, a beam splitter, a Radio Frequency (RF) receiver, a Focal Plane Array (FPA) and a processor. The at least one parabolic reflector is configured to direct incident electromagnetic radiation. The beam splitter is configured and arranged to receive the incident electromagnetic radiation from the at least one parabolic reflector and separately focus microwave radiation and visual radiation from the incident electromagnetic radiation. The beam splitter is further configured and arranged to provide a gain in the microwave radiation and visual radiation. The RF receiver is configured and arranged to receive microwave radiation from the beam splitter. The FPA is configured and arranged to receive the visible radiation from the beam splitter. The processor is in communication with the RF receiver and the FPA. The processor is configured and arranged to process signals received by the RF receiver and the FPA for transmission. 
         [0005]    In another embodiment, another hybrid image gathering system is provided. The system includes an electromagnetic radiation directing system, a beam splitter, a Radio Frequency (RF) receive/transmitter, a Focal Plane Array (FPA) and a processor. The electromagnetic radiation directing system is configured and arranged to direct electromagnet radiation. A beam splitter is positioned to receive incident electromagnetic radiation from the electromagnetic radiation directing system. The beam splitter is configured to separate out microwave radiation and visible radiation from the incident radiation. The beam splitter is further positioned to transmit outgoing processed data. The RF receiver/transmitter is configured and arranged to receive microwave radiation from the beam splitter and to transmit microwave radiation to the beam splitter. The FPA is configured and arranged to receive the visible radiation from the beam splitter. The processor is in communication with the RF receiver and the FPA. The processor is configured and arranged to process signals received by the RF receiver and the FPA and communicate the processed data to the RF receiver/transmitter for transmission to a remote location. 
         [0006]    In still another embodiment, a method of monitoring an area is provided. The method includes; separating out microwave radiation and visible radiation from incident electromagnetic radiation; directing the microwave radiation to a RF receiver; directing the visible radiation to a focal plane array; processing signals from the RF receiver and the focal plane array; and communicating the processed signals to a user at a remote location. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The present invention can be more easily understood and further advantages and uses thereof will be more readily apparent, when considered in view of the detailed description and the following figures in which: 
           [0008]      FIG. 1  illustrates a Fresnel Zone plate of the prior art; 
           [0009]      FIG. 2  illustrates a satellite of an embodiment of the present invention; 
           [0010]      FIG. 3  illustrates a beam splitting portion of the satellite of  FIG. 3 ; 
           [0011]      FIG. 4  is a graph illustrating properties of an elliptical Fresnel Zone Plate used in an embodiment of the present invention; 
           [0012]      FIG. 5  is an illustration of an elliptical Fresnel Zone Plate used in an embodiment of the present invention; 
           [0013]      FIG. 6  is a block diagram of a hybrid optical and microwave imaging satellite system of one embodiment of the present invention; and 
           [0014]      FIG. 7  illustrates a Dichroic beam splitter used in an embodiment of the present invention. 
       
    
    
       [0015]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. Reference characters denote like elements throughout Figures and text. 
       DETAILED DESCRIPTION 
       [0016]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims and equivalents thereof. 
         [0017]    Embodiments of the present invention combine an optical and microwave imaging/data transmission system into a satellite. Embodiments of this hybrid system implement a parabolic aperture and the focusing capability of a beam splitter, such as a Fresnel Zone Plate (FZP). Embodiments provide a system with desirable gain with a small overall footprint. Moreover, embodiments provide an ability to substantially increase the data transfer rate of earth imaging satellites without increasing the footprint of the satellite by making optical and a RF aperture one and the same. As stated above, this is done by adding a beam splitter, such as a FZP. 
         [0018]    Typically both the RF and visible systems must work with very low energy electromagnetic signals from distant objects. Therefore, the receiving antennas used to collect the signals should have the largest feasible collection area or aperture as possible. Increasing the aperture size is also very desirable because it results in a relatively small focal length requirement which more efficiently utilizes the available volume in a launch vehicle. With embodiments, using a hybrid system of parabolic aperture and the focusing capability of the FZP antenna, a desirable gain with a smaller overall footprint of the satellite is possible. RF apertures are necessarily large to provide the desired gain over a large bandwidth. Optical reflectors, on the other hand, are typically flatter due to the difficulty of fabricating curved surfaces over large diameters. Cassegrainian configurations are typically used to fold the optical path in order to make the design more compact Embodiments of the present invention provide a system that compromises between the size of the reflector aperture and the complexity of the multiple folded optical wave path by inserting a beam splitter with focusing capability before the Focal Plane Array (FPA). The beam splitter may be fabricated by forming an array of reflecting metallic mirror segments of glass, quartz or other microwave transmissive substrate. In this case, the microwave energy is transmitted through gaps between the mirror segments. Such an arrangement is generally described as a FZP discussed above. Referring to  FIG. 1  a FZP illustration  100  of the prior art is illustrated. The FZP includes a thin support substrate  102  and zone plate metal rings  104 . In this FZP illustration, a source  106  is shown generating electromagnetic waves (or electromagnetic radiation). The overall concept stems from the fact that the spherical waves from the feed create constant phase zones on the planar surface that are circular. The FZP is normally a planar device where the incoming radiation is normal to the plane and produces lens-like focusing of electromagnetic waves (or electromagnetic radiation). It transforms a normally incident plane wave into a converging wave, concentrating the radiation field in a small region about a point which is the focal point. FZP has an interesting property that it can focus both in the transmission and reflection modes. These properties of the FZP are used in embodiments in two ways. First, by using a FZP as beam splitter, the incoming radiation can be separated as either an optical wave front or a microwave radiation and measured accordingly. Second, the focusing capability of the FZP is exploited to add signal gain to the incoming radiation for measurement. This gain is achieved over and above the gain derived from the parabolic aperture. Thus, the overall effect is to either increase the strength of the signal or reduce the size of the aperture. The additional gain that can be derived from the FZP is a function of several parameters as described below. In embodiments, in order to split the beam into optical and microwave radiation to be measurable with appropriate devices, the beam splitter must be orientated at an inclination to the axial direction. This is shown in  FIG. 3  and described below. Therefore, it is required to design the FZP where the positioning of the maximum in the power radiation pattern is in the direction of the focal point. The kind of FZP having this property is an elliptical FZP as discussed below. This requires the parabolic secondary reflector to be used to generate plane waves for interaction with the FZP. 
         [0019]    Referring to  FIG. 2 , a satellite  200  including a hybrid optical and microwavable imaging system is illustrated. The imaging system includes a parabolic primary reflector  202  that reflects incident electromagnetic waves  220 . The incident electromagnetic waves  220  are reflected by the primary reflector  202  as primary reflected electromagnetic waves  225  to a parabolic secondary reflector  204 . The parabolic secondary reflector  204  in turn reflects the waves as secondary reflected electromagnetic waves  230  into a beam splitting portion  302  of the hybrid optical and microwavable imaging system. The beam splitting portion  302  is described in the close up section  300  further described below. The satellite  200 , in this embodiment, further includes a processing portion  210  that is used to process signals from the hybrid optical and microwavable imaging system as well as other process, such as but not limited to, operations of the satellite  200  and the positioning of the satellite  200 . The satellite  200  also includes a function portion  212  that is used to at least position the satellite  200  under direction of the processing portion  210  and a power system  214  that powers the portions of the satellite  200 . The satellite  200  includes a satellite ground link system (SGLS)  208  that is in communication with the processing portion  210 . The SGLS  208  provides task, telemetry and communication functions for the satellite  200 . 
         [0020]    Close up section  300  illustrates the beam splitting portion  302  of the satellite  200 . As illustrated, the secondary reflected electromagnetic waves  230  pass through an opening  304  in the beam splitting portion  302  of the satellite  200 . The secondary reflected electromagnetic waves  230  are incident on the FZP beam splitter  306 . In this embodiment, a surface of the FZP beam splitter  306  is positioned at a  30  degree angle in relation to the secondary reflected electromagnetic waves  230 . The FZP beam splitter  306  reflects waves in the visible spectrum, optical waves  320 , of the secondary reflected electromagnetic waves  230  to a Focal Plane Array (FPA)  308  that senses the optical radiation. The FPA  308  is in communication with the processing portion  210  of the satellite  200 . The FZP beam splitter  306  further refracts the waves in the RF spectrum (microwaves  325 ) in the secondary reflected electromagnetic waves  230  to a RF receiver  310  that senses the RF radiation. The RF receiver  310  is in communication with the processing portion  210  of the satellite  200 . Both the FPA  308  and the RF receiver  310  are in communication with a processor  610  in the processing portion  210  of the satellite  200 . As discussed above, additional gain is derived from the FZP. The additional gain is a function of several parameters as shown in  FIG. 4 . The primary parameters in  FIG. 4  are D/λ (ratio of the diameter of the FPZA and the wavelength of the radiation) and F/λ (ratio of the focal length of the FPZA and the wavelength of the radiation). The other parameters are N (Number of interferometric rings) and FG (focusing gain). 
         [0021]    In order to split the beam into optical and microwave radiation with their signals being measurable with the respective FPA  308  and RF receiver  310 , the beam splitter  306  must be orientated at an inclination to the axial direction as shown in  FIG. 3 . Therefore it is required to design the FZP beam splitter  306  where the position of the maximum in the power radiation pattern is in the direction of the focal points  311  and  315  for RF and visible spectrum respectively. The kind of FZP having this property is an elliptical FZP  306  as shown in  FIG. 5  as opposed to a FZP with circular rings shown in prior art  FIG. 1 . Using the elliptical FZP  306  requires a parabolic secondary reflector  204  (as shown in  FIG. 2 ) to be used to generate plane waves for interaction with the FZP  306 . 
         [0022]    Referring to  FIG. 6 , a block diagram illustration of a hybrid optical and microwave imaging satellite system is provided. As illustrated, the system includes a directing system  602  that directs the incoming and outgoing electromagnetic radiation to and from the beam splitter  604 . As illustrated above, the directing system may include one or more parabolic reflectors. The beam splitter  604  splits the incoming electromagnetic radiation sending visible radiation to the Focal Plane Array  606  and microwave radiation to the RF receiver  608 . Alternatively, the beam splitter returns the outgoing RF radiation from the RF receiver  608 , which in this case acts as a transmitter. Hence in one embodiment,  608  is an RF receiver/transmitter. Further illustrated in  FIG. 6  is a processor  610  (or controller) that is in communication with the Focal Plane Array  606  and the RF receiver  608 . The processor  610  is configured to process signals received from the Focal Plane Array  606  and the RF receiver  608 . The processor  610  is furthering communication with the satellite ground link system  612  which provides communication between the satellite and a control station on the ground. The processor communicates its processed information regarding the signals from the Focal Plane Array  606  and the RF receiver  608  either through the satellite ground link system (SGLS)  612  or through the main parabolic aperture as appropriate. 
         [0023]    As discussed above, in one embodiment the beam splitter is a FZP  306 . However, in another embodiment, the beam splitter  604  is covered with a RF transmissive and optically reflective dichroic coating. This beam splitter embodiment is illustrated in  FIG. 7  and would be incorporated in satellite  200  described above. In this embodiment, the beam splitter  604  is positioned at approximate a 45 degree angle to the incident electromagnetic radiation. In this embodiment, however, no gain is realized on top of the gain obtained with the use of the primary parabolic aperture. 
         [0024]    In embodiments, the RF energy can be utilized to form Synthetic Aperture Radar (SAR) to provide imagery at night or when the earth is obscured by clouds, fog, smoke, or dust etc. In addition, the RF energy can be used as a communication link for high rate data transfer. The high data rate is achieved by using the same large parabolic aperture  202  that is used to receive the radiation. In this case the FZPA also adds to the overall gain during data transmission to remote locations. Further in embodiments, the entire architecture is easily made of parts of the satellite bus to deliver an integrated system suitable for launches of multiple units on various launch vehicles. Thus, the baffle, which is essentially a cavity to stop stray radiation from hitting the measuring device, is an integral part of the bus. The baffle in this case becomes an integral part of the bus and is situated behind the parabolic aperture. Alternatively it is easily conceivable to be situated in front of the parabolic aperture. 
         [0025]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.