Abstract:
A method for monitoring an area that involves transmitting a first electromagnetic wave signal from a mobile platform moving over a ground surface, toward the ground surface. A receiver is used that is located remote from the mobile platform to receive the first electromagnetic wave signal after the signal is reflected from the ground surface. The first electromagnetic wave signal is processed to form a first synthetic aperture radar (SAR) image. Subsequently the receiver is used to receive a second electromagnetic wave signal transmitted from the mobile platform at a time subsequent to transmission of the first electromagnetic wave signal. The second electromagnetic wave signal is then processed to obtain a second SAR image. The first and second SAR images are then coherently analyzed to determine areas of non-correlation between the images.

Description:
FIELD 
       [0001]    The present disclosure relates to border and perimeter monitoring systems and applications, and more particularly to a system and method that use electromagnetic wave signals to generate images of a border or perimeter area that can be compared to detect the presence and movement of individuals or objects in the geographic area being monitored. 
       BACKGROUND 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    Presently there is a growing interest in monitoring border areas, for example, the geographic border between the United States and Mexico. Cost effective monitoring of the perimeter of an important structure, for example a military facility, a bridge, power generating station, water treatment station, etc., is also increasing in interest. 
         [0004]    Presently available monitoring systems have typically been somewhat costly to implement and/or to operate, or have suffered from one or more other drawbacks. For example, monitoring operations performed by individuals traversing a region by ground vehicles or by airborne vehicles can be quite costly. Attempts at reducing the operating or implementation costs for a monitoring station have sometimes involved the use of a plurality of terrestrially mounted cameras. However, such cameras often need to be mounted on towers or elevated platforms, and once mounted, may be difficult to inspect and/or repair. For monitoring large geographic areas, sometimes hundreds or more cameras are required to fully image the area in question. And the optical image provided by a camera can be significantly adversely affected by environmental conditions such as rain, fog, snow, etc. 
         [0005]    Other attempts at implementing monitoring systems have involved obtaining image data from one or more cameras or optical sensors located on a flying airborne platform. The electronic image data obtained by equipment on the airborne mobile platform is relayed via RF signals, or via a transponded satellite link, to electronic equipment at a ground based control station. The image data is processed at the ground station and used by individuals charged with managing the monitoring operation. As will be appreciated, this type of system involves a very significant cost, both in its initial implementation and in its on-going operations. Typically large amounts of data need to be up-linked to the selected satellite and then down-linked from the satellite to the ground station. The use of a satellite link adds significant cost and complexity to such a system, not to mention the cost of the complex electronics that must be carried on board the airborne platform. 
       SUMMARY 
       [0006]    In one aspect the present disclosure relates to a method for monitoring an area. The method may comprise: transmitting a first electromagnetic wave signal from a mobile platform moving over a ground surface, toward the ground surface; using a receiver located remote from the mobile platform to receive the first electromagnetic wave signal after the first electromagnetic wave signal is reflected from the ground surface; processing the first electromagnetic wave signal to form a first synthetic aperture radar (SAR) image; subsequently using the receiver to receive a second electromagnetic wave signal transmitted from the mobile platform and reflected from the ground surface, at a time subsequent to transmission of the first electromagnetic wave signal; processing the second electromagnetic wave signal to obtain a second SAR image; and analyzing the first and second SAR images to determine areas of non-correlation between the images. 
         [0007]    In another aspect the present disclosure relates to a method for monitoring a geographic area. The method may comprise: transmitting a first electromagnetic wave signal from an airborne mobile platform flying over a ground surface, toward the ground surface; using a receiver located remote from the airborne mobile platform to receive the first electromagnetic wave signal after the first electromagnetic wave signal is reflected from the ground surface; processing the first electromagnetic wave signal to form a first synthetic aperture radar (SAR) image; subsequently using the receiver to receive a second electromagnetic wave signal transmitted from the airborne mobile platform at a time subsequent to transmission of the first electromagnetic wave signal, and after the second electromagnetic wave signal has been reflected from the ground surface; processing the second electromagnetic wave signal to obtain a second SAR image; and analyzing the first and second SAR images to determine areas of non-correlation between the SAR images; and using the areas of non-correlation to form a two dimensional change map of a predetermined geographic region, the two dimensional change map highlighting differences between the two SAR images to make the differences visually perceptible to an individual. 
         [0008]    In another aspect the present disclosure relates to a system for monitoring an area. The system may comprise: a mobile platform; an electromagnetic wave signal transmitter supported on the mobile platform for transmitting electromagnetic wave signals toward a ground surface over which the mobile platform is traversing; a receiver located remote from said mobile platform to sequentially receive first and second ones of said electromagnetic wave signals transmitted from said transmitter after said first and second electromagnetic wave signals have reflected from said ground surface within a predetermined geographic region over which said mobile platform is traversing; and a processor adapted to process the first and second electromagnetic wave signals to form first and second synthetic aperture radar (SAR) images, and to analyze the images to determine areas of non-correlation between the images within the predetermined region of the ground surface. 
         [0009]    It will be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
           [0011]      FIG. 1  is a block diagram of a system in accordance with one embodiment of the present disclosure; 
           [0012]      FIG. 2  is a block diagram of the processor shown in  FIG. 1 ; and 
           [0013]      FIG. 3  is a flowchart of operations performed by the system in carrying out a monitoring operation. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
         [0015]    Referring to  FIG. 1 , there is shown a bistatic monitoring system  10  in accordance with one embodiment of the present disclosure. The system  10  includes a mobile platform  12 , which in this example is shown as an unmanned air vehicle (hereinafter “UAV”  12 ) having an on-board electronics system  14  that includes a radio frequency (RF) transmitter  16  and an antenna  18 . The RF transmitter  16  generates electronic signals that are transmitted as electromagnetic wave (hereinafter simply “RF”) signals  20   a,    20   b  and  20   c  towards a ground surface  22  as the UAV flies over a predetermined geographic are or region  23 . The RF signals  20   a - 20   c  are transmitted sequentially as the UAV  12  makes a plurality of passes over the same predetermined geographic area  23 . Thus, there will be some tangible degree of time separation between when the signals  20   a - 20   c  are generated. For example, the signals  20   a,    20   b  and  20   c  may be separated in time by minutes, hours, days, weeks or even months. It is expected that in most applications, the UAV  12  will pass over a predetermined region a plurality of times, and that successive passes will be separated most typically by hours or days. The RF signals  20   a - 20   c  generated by the RF transmitter  16  of the UAV  12  are typically selected to be within the frequency band of about 200 MHz-30 GHz. 
         [0016]    While the UAV  12  is shown as forming the mobile platform, it be appreciated that a land vehicle may potentially also be used for transmitting the RF signals  20   a - 20   c.  For example, a land vehicle could be driven along an elevated ridge adjacent to a canyon or valley, with an on-board antenna directing RF signals towards the floor of the canyon or valley. The reflected signals could then be received by a terrestrial based receiver located near the canyon or valley floor, or possibly on a ridge on the opposite side of the canyon or valley. 
         [0017]    The system  10  could also be implemented in a marine application. For example, a ship could be used to transmit electromagnetic wave signals that are reflected off of water, picked up by a different ship or land-based monitoring station, and used to look for other vessels in a predetermined portion of an ocean or sea. Still further, potentially a space application could be implemented where a transmitter is located on a space vehicle, for example on a satellite. 
         [0018]    Referring further to  FIG. 1 , the system  10  further includes a terrestrial based monitoring station  24 , which will be referred to for convenience simply as the “ground station”  24 . The ground station  24  includes an antenna  26 , an electromagnetic wave signal receiver  28  (hereinafter simply “RF receiver”  28 ), a processor  30 , a data storage system  32  that forms an archive for synthetic aperture radar (SAR) images generated by the processor  30  (hereinafter simply the “SAR image archive”  32 ), and a display system  34 . The antenna  26  preferably is mounted on a tower  26   a  and receives the RF signals  20   a - 20   c  after they have been reflected from the ground surface  22 . The signals output from the antenna  26  are input to the RF receiver  28  which generates electrical signals corresponding to the received RF signals, which are input to the processor  30 . The electrical signals are used by the processor  30  to generate a synthetic aperture radar image of the predetermined geographic area  23  that is traversed by the UAV  12  which is stored in the SAR image archive  32 . It will also be appreciated that the components of the ground station  24  need not be co-located at a common location, as long as they are able to communicate (either via wired or wireless links). For example, there could be several towers with antennas monitoring different regions of a border, and a common ground station processing all of the data received from all of the antennas. 
         [0019]    Each SAR image stored in the SAR image archive  32  represents a complex-valued image made up of a large plurality of pixels, typically on the order of millions pixels. Each pixel will have an associated magnitude and phase. On each pass by the UAV  12  over the designated geographic area  23 , the processor  30  uses the reflected RF signals received by the antenna  26  to generate an SAR image of the predetermined geographic area  23  that is traversed during that particular pass, that is then stored in the SAR image archive  32 . Thus, after two passes by the UAV  12  over the area  23 , the processor  30  will have created and stored two SAR images, after three passes the processor will have created and stored three SAR images, and so forth. The geometry of the two flight paths is chosen to ensure coherency between the two data collections. It will be appreciated, however, that one could create more than one SAR image per pass. Multiple SAR images per pass might improve the quality of the SAR images obtained. 
         [0020]    Referring to  FIG. 2 , the processor  30  includes an image registration subsystem  36  that is used to “register” any two images, and most typically two successively created SAR images. By “register” it is meant that the two images are mathematically warped to correct for residual geometric differences and to align surface features of the two SAR images. A correlation analysis subsystem  38  coherently analyzes the two selected images, pixel-by-pixel, in phase and in magnitude, to determine regions of pixels that do not correlate, and to identify those regions that do correlate. The correlation regions are chosen to be small enough to provide sufficient spatial resolution of the changes, but large enough to contain enough pixels to reduce the measurement noise. Typically, a correlation region may by 3×3 to 8×8 pixels, depending on the sensor and data characteristics. The correlating and non-correlating pixel regions are then used to form a high resolution, two-dimensional “change” map in which the non-correlating regions are highlighted, for example darkened, to make them more easily visually perceptible. By “change” map, it is meant a map of the predetermined geographic area  23 , created from two SAR images, that has any changes between the two images, such as the presence or absence of vehicles, individuals, or geographic features, highlighted to make them easily visually perceptible. The resulting two-dimensional change map is then displayed on the display system  34 . The display system  34  may be a CRT or LCD display, or any other form of display suitable for displaying graphical images. 
         [0021]    Referring now to  FIG. 3 , a flowchart  100  is shown to illustrate various operations that are performed by the system  10  shown in  FIG. 1 . Initially, at operation  102  the UAV  12  makes two or more passes over the predetermined geographic region  23  and generates RF signals (such as signals  20   a - 20   c ) that are directed toward the ground surface  22 , and reflected from the ground surface. At operation  104  the ground station  24  receives the reflected RF signals using antenna  26 . At operation  106  the RF receiver  28  typically captures the RF signals provided by the antenna  26  and encodes them in a digital format, and provides its output to the processor  30 . At operation  108  the processor creates a plurality of SAR images, one associated with each pass of the UAV  12  over the predetermined geographic region  23 . At operation  110  the processor  30  stores each SAR image in the SAR image archive  32 . At operation  112  the processor  30  accesses the SAR image archive  32  and obtains two (or potentially more) of the SAR images At operation  114  the processor  30  registers the two SAR images. At operation  116  the processor  30  performs a pixel-by-pixel comparison of the two SAR images to determine corresponding pixel regions from the two SAR images that do not correlate. At operation  118  the processor  30  uses the information obtained from its analysis to construct the high resolution, two-dimensional change map of the predetermined geographic region  23 . At operation  120  the processor  30  transmits the two-dimensional change map to the display system  34  for display and subsequent analysis. The subsequent analysis may be performed by an individual or by an automated algorithm adapted to digitally analyze the change map. 
         [0022]    The system  10  and method of the present disclosure provides the significant advantage that no separate communication signal is needed from the UAV  12  to any external subsystem or transponder in order to pass large amounts of data. This significantly simplifies the acquisition of information from the UAV  12  and can significantly reduce the overall cost of implementing and operating a monitoring system. In particular, since the system  10  is a bistatic system, it does not require a receiver or mission data communication system to be located on-board the UAV  12 , which reduces cost associated with outfitting the UAV for use with the system  10 . This also reduces the weight of the UAV  12  and can contribute to longer in-flight operational times for the UAV. Unlike a ground-based only system (e.g., using fixed, ground-based cameras or sensors), the high resolution, two-dimensional change map produced by the system  10  is much better able to resolve false alarms such as blowing vegetation and animal activity. 
         [0023]    The system  10  and method of the present disclosure is especially well suited for monitoring perimeter areas, such as geographic borders between countries or the perimeter of a facility (e.g., power station, water treatment facility, etc.). 
         [0024]    While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art.