Abstract:
A tracking system uses RFID (radio frequency identification) tag technology to facilitate the identification and tracking of items in an environment through a technique known as shadowing. As an object or target moves within a pre-described detection zone with communicating antenna and RFID sensors, the object or target blocks the line of sight between respective antenna and sensors, preventing electromagnetic coupling between the sensor and the antenna and thus casting an electromagnetic shadow along the line of sight. One approach of this invention uses this shadow technique to perform functions such as theft (shrink) deterrence/ detection; tracking the motion of objects through an environment by monitoring the shadow; and correlation analysis of people shadows to tagged items (e.g., merchandise, articles) to foster marketing and merchandising effectiveness.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a Nonprovisional of U.S. Provisional Application Ser. No. 60/471,885 filed May 19, 2003, and entitled “Article Identification and Tracking using Electronic Shadows Created by RFID Tags.” 
     
    
     FIELD OF INVENTION  
       [0002]     This invention relates to radio frequency identification (RFID) technology and, in particular, to RFID security systems having the capability to non-invasively detect and track an object within a security zone based on a loss of information.  
       BACKGROUND OF THE INVENTION  
       [0003]     Retailers focusing on loss prevention (e.g. theft) traditionally have three approaches to minimize loss: (1) anchoring the merchandise in place (e.g., attaching cables to the merchandise, putting articles in display cases and behind the counter); (2) using video surveillance systems and/or security guards to watch everyone in the store; and (3) using an alarm system with special tags attached to items that sounds off an alarm whenever a shoplifter tries to leave with a tagged item. Anchoring the merchandise in place makes it difficult for shoppers to examine items and try on garments. Customers have to wait for a clerk to release the item so the customer can try it on or look at it. This inconvenience motivates shoppers to shop at stores where the merchandise is more accessible. In addition, anchoring merchandise is costly as a store must provide labor just to release the merchandise. Video surveillance systems including monitoring devices, observation mirrors and security cards were among the earliest approaches to combat shoplifting. However, the cost of labor is expensive and the surveillance systems can be intrusive, especially in areas (e.g. dressing rooms) where shoppers would prefer some level of privacy.  
         [0004]     Radio frequency (RF) Tagging security systems are useful anywhere there is an opportunity for theft of items of any size. Using tagging technology (e.g., electronic article security (EAS), radio frequency identification (RFID)) enables a retailer to display popular items on the floor, where they can be seen, rather than putting them in locked cases or behind the counter. The use of EAS and RFID systems for detecting and preventing theft or unauthorized removal of articles or goods from facilities has become widespread. In general, such systems have RF antennas that detect resonant tags attached to articles in a security or detection zone of the antennas. Such systems are generally located at or around points of exit to detect the security tag, and thus the article, as it transits through the exit point.  
         [0005]     EAS systems are limited by the capabilities of their tags. Unfortunately, EAS tags contain no information. They are simply there or not there. Tag systems designed to add information to RF tags use radio frequency identification (RFID) technologies to read information from RFID tags. RFID tags can store information about the product as well as uniquely identify each of the products. Unfortunately, RFID systems encounter problems of obstruction or improper disorientation. In RFID technology, antennas and tags communicate with each other along a line of sight as antennas send integration signals and read response signals from the tags. RFID signals are blocked, absorbed or reflected or otherwise modified by conductive objects, including people. An RFID tag passing through a sensor net or detection zone of RFID antennas may not be detected by the antennas if the direct path between the tag and antennas is blocked. That is, a person can easily hide a tag so that it is not seen (e.g., read, detected) by the RFID system. The person hiding the tag can walk through the RFID system&#39;s detection zones without sounding an alarm and the system has no way to locate the loss tag or its associated article. If a RFID tag is hidden, it can not be identified or tracked with known tagging systems. A possible approach is combining the RFID system with a video surveillance system, which could follow a person associated with a hidden or lost tag in real-time. However, this approach is expensive and invasive. Accordingly, there is a need for systems which can non-invasively track tagged products regardless of whether the tag can be seen. Moreover, it would beneficial to provide a system that could non-invasively identify and track moving objects without video surveillance.  
         [0006]     Warehouses are using RF tagging technology with resonant tags located in the floor and matrix portable readers on forklifts for inventory purposes. Automotive vehicle systems have used RF tagging technology with resonant tags embedded in the road to help steer vehicles. As a vehicle travels down a road, an antenna on the vehicle identifies the embedded tags as markers to approach or stay to one side of to help the vehicle stay in a lane. However, neither of these approaches has solved the need to non-invasively identify and track moving objects.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     The preferred embodiments of the present invention specifically relate to the fields of security, RFID, marketing and retail. Other embodiments of the present invention may be applied to applications such as warehousing and distribution systems, manufacturing floor environments and people counting. The present invention uses passive RFID (radio frequency identification) tag technology to facilitate the identification and tracking of items in an environment through a technique known as shadowing. One approach of this invention uses this shadow technique to perform functions such as theft (shrink) deterrence/ detection; tracking the motion of objects through an environment by monitoring the shadow; and correlation analysis of people shadows to tagged items (e.g., merchandise, articles) to foster marketing and merchandising effectiveness.  
         [0008]     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the present invention.  
         [0009]     The term antenna described in the preferred embodiments herein generally refers to a circuit that couples from one device (e.g., an identification sensor or tag) to itself with the intent of transferring energy and receiving data back. Such antennas include RFID readers or interrogators that communicate with RFID sensors by interactive (inductive or radiated) coupling, as is well-known in the art. While not being limited to a particular theory, an RFID reader or interrogator generally includes a transmitter and a receiver that communicate with RFID sensors along a line of sight therebetween by sending interrogation signals to the sensors and reading response signals from the sensors. Of course, other antenna designs could also be used with the present invention, and the invention is not limited to the particular antenna described. In other words, the term antenna as used in this invention refers to interrogators and readers that communicate with responsive sensors or tags at frequencies in the MHz, GHz, THz, ionizing and non-ionizing radiation as readily understood by a skilled artisan.  
         [0010]     The term RFID sensor as described in the preferred embodiments herein includes a passive, semi active or active (battery powered) RFID tag. RFID tags are one type of well-known identification tags in the art and typically include a responsive circuit (e.g., a passive resonant radio frequency (RF) circuit, a dipole circuit, a patch circuit) for use in detecting when the tag is within a zone that is monitored by a reader or interrogator, as is well-known in the art. One well-known type of passive resonant RF circuit has a coil antenna and a capacitor which together form an LC circuit with a predetermined resonant frequency. Power for the sensor is typically derived in a conventional manner (e.g., from energy received at the coil antenna from an interrogation signal). Preferably, each sensor has a unique identification or serial number for identifying the individual sensor. This unique identification is sent within a response signal returned upon receipt of the interrogation signal to be read by a respective reader. Of course, other tag designs could also be used with the present invention, and the invention is not limited to the particular sensor described. For example, any identification (ID) sensor, defined as a responsive circuit, including RFID tags, dipole tags and patch antennas, could be included within the scope of the invention. According, RFID tags, RFID sensors, dipole tags and patch antennas are examples of ID sensors that are within the scope of the invention.  
         [0011]     The term shadow or electronic shadow as defined in the preferred embodiments herein refers to the absence of detection for a known RFID sensor. As an object or target moves within a pre-described sensor net environment of a volumetric security or detection zone with communicating antenna and RFID sensors, the object or target blocks the line of sight between respective antenna and sensors, preventing electromagnetic coupling between the sensor and the antenna and thus casting an electromagnetic shadow along the line of sight. For example, a person walking over a floor mounted sensor blocks the detection of the RFID sensor, thereby creating an electronic shadow. That is, the person standing above an RFID sensor will cause signal absorption, reflection and attenuation, which prevents a read of the RFID sensor by an associated antenna. The series of electronic shadows (the real time sequence of electronic shadows) can identify the vector movement of people within a volumetric detection zone.  
         [0012]     According to a preferred embodiment of the invention, a tracking system includes a RFID sensor, an antenna and a processing member. The RFID sensor has a resonant radio frequency (RF) circuit and is positioned at a first location. The antenna is positioned at a second location and is adapted to detect the RFID sensor by interactive coupling between the antenna and the RFID sensor via communication signals therebetween. The processing member is adapted to determine an absence of detection of the RFID sensor by the antenna, interpret the absence of detection as indicia of an electronic shadow between the first location and the second location, and infer a presence of an object as cause for the electronic shadow.  
         [0013]     As described by example below, a preferred embodiment of the tracking system includes an approach for communicating signals along a line of sight between a first location and a second location, for detecting the communicated signals at the second location, for determining an absence of detection of one of the communicated signals at the second location, for interpreting the absence of detection as indicia of a shadow between the first location and the second location, and for inferring a presence of an object as cause for the shadow.  
         [0014]     The preferred embodiment of the invention is also described as a method for targeting an object including the steps of communicating signals along a line of sight between a first location and a second location, detecting the communicated signals at the second location, determining an absence of detection of one of the communicated signals at the second location, interpreting the absence of detection as indicia of a shadow between the first location and the second location, and inferring a presence of the object as cause for the shadow.  
         [0015]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, and that the invention is not limited to the precise arrangements and instrumentalities shown, since the invention will become apparent to those skilled in the art from this detailed description. 
     
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0016]     The following detailed description of preferred embodiments of the invention will be better understood when read in conjunction with the following drawings, in which like-referenced numerals designate like elements, and wherein:  
         [0017]      FIG. 1  is a plan view of antenna coverage and overlapping detection regions that create a Sensor Net in accordance with a preferred embodiment of the present invention;  
         [0018]      FIG. 2  is an elevation view of a sensor net of antenna coverage similar to the sensor net shown in  FIG. 1 ; and  
         [0019]      FIG. 3  is an elevation view illustrating another sensor net in accordance with the preferred embodiments. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     While not being limited to a particular theory, the present invention is described in a system using Ultra High Frequency (UHF) RFID interrogators, antennas and tags (sensors), preferably operating at frequencies between 800 MHz and 1 GHz. However, the scope of the present invention includes interrogators, antennas and tags that operate at other frequencies (e.g., MHz, GHz, THz) that have similar electromagnetic properties in the area of signal absorption, reflection and attenuation with conductive, semi-conductive and high moisture content objects (including humans and animals) and including ionizing and non-ionizing radiation. In theory, the higher the frequency, the better the system works. As the frequency gets higher, the wavelength gets shorter as the shadows become more defined. Moreover the antennas and sensors can be made smaller for higher frequency bands.  
         [0021]     The electronic shadow tracking system of the present invention (hereinafter also referred to as tracking system) allows a computer program to map an area (e.g., a floor of a retail store, transportation center, convention center, warehouse, distribution center) by using RFID tags placed in a geometric pattern on the floor (fixed RFID sensors) and read by antennas placed in the ceiling, and/or on the walls or even in or on pedestals. Fixed RFID sensors may also be attached to store fixtures, racks or point of sale counters to identify and/or locate such objects. In addition to fixed RFID sensors, there may also be mobile RFID sensors attached to movable items within the environment that allow the tracking system to also identify articles, objects or persons attached to the mobile sensors. For example, the mobile RFID sensors may be attached to products being sold in a retail store or may be in the form of an employee badge to identify personnel working in the store. According to the preferred embodiments, a computer program can use RFID sensors and antennas to map an area, also referred to as a sensor net, detection zone or security zone at various locations, where it is desirable to track goods and/or people, such as, for example, a warehouse, airport, train station, subway station, bus station, stadium, convention center, and anywhere along a product distribution line.  
         [0022]     Generally, as communication signals (e.g., interrogation signals, response signals) are blocked by a shadow causing object, the blocked communication signals are not read by the respective antenna or RFID sensor, thereby indicating the presence of the shadow causing object in the line of sight between the antenna and RFID sensor. When an antenna does not receive an expected response signal from its interrogation signal, the tracking system infers the presence of the shadow causing object and maps an electronic shadow along the blocked line of sight.  
         [0023]     The computer program knows the location and geometry of the fixed RFID sensors and antennas. With a single antenna, and repeated continuous periodic communication between the antenna and sensors, the computer program can determine movement of a shadow causing object, based on the location of the resulting shadow. With multiple antennas, and knowledge of the location and geometry of the antennas and fixed RFID sensors, the computer program can estimate the size of the shadow causing object, as described in greater detail below. If the tracking system of the preferred embodiments determines that a tagged article is associated with a shadow (e.g., the tagged article moves with the shadow, the tagged article becomes blocked or disabled and an associated shadow remains) then the computer program can track the article with the shadow, and thus know where the article is located.  
         [0024]     Accordingly, this tracking system provides the benefit of gaining knowledge from the absence of knowledge. That is, before this invention, a security system would not know what happened or where a tagged item moved, especially if the tag was blocked or disabled, without the aid and extra cost of a visual monitoring system (i.e., cameras, monitors, personnel). However, in accordance with the preferred embodiments of this invention, the tracking system non-invasively tracks movement of a shadow causing object (e.g., person, cart, carrier, luggage, box, foil-lined bag, etc) as desired (e.g., associated with a tagged article, passing through a detection zone, getting on a train, moving on a conveyer belt, moving through distribution ports and channels). The tracking is non-invasive at least because the tracking system does not actually see or visually monitor people. Instead, the tracking system monitors the electronic shadow cast by the person or object. The tracking system does not discriminate, distinguish, nor profile according to race, religion, creed, national origin, gender or size independent criteria since it does not see external characteristics of any person. The tracking system can estimate the size of a shadow causing object (e.g., person) which could be helpful, for example, if trying to locate a small child.  
         [0025]     An example of a mobile RFID sensor is a plastic hard tag attached to a garment. Within the plastic hard tag is an RFID inlay (antenna and RFID read only or read write chip) and, optionally, an EAS sensor. Fixed RFID sensors are structurally the same or substantially the same as mobile RFID sensors well-known in the art, but are spread about an environment to be seen by the antenna on a regular and periodic basis. While not being limited to a particular theory, the fixed RFID sensors are preferably at least one wavelength apart (e.g., about one foot apart). While not being limited to a particular theory, one antenna located in a ceiling generally has an area of detection of about three meters squared on a floor below. If more than one antenna is desired, it is preferred to position additional antennas so that their areas of detection overlap. The amount of overlap should increase, that is, the antennas should be positioned closer together, if greater resolution of shadow causing objects is desired. In other words, by placing antennas so that each RFID sensor can be detected by more than one antenna, the shape of the shadow causing object can be estimated according to the shadows detected by each antenna from the particular object.  
         [0026]     The antennas are preferably placed in or on the ceiling, walls or pedestals to form a volumetric detection or security zone with the fixed RFID sensors placed in the floor, the walls or some other fixed structure. This volumetric detection zone, which is also referred to as a sensor net, may consist of an entire retail store, or may just be a controlled corral area within a store. For people counting, the volumetric detection zone may include the entrance and exit zones or regions.  
         [0027]     As a preferred approach for implementing the present invention, antennas are installed in a retail store such that either the entire contents, or merely the content that needs to be monitored, is within a substantially contiguous field of detection, herewith referred to as a sensor net. In order to construct a sensor net, RFID Tags are placed on/in enough surfaces (floors and/or walls) such that the antennas in the sensor net can see (e.g., read response signals from) the sensors. Within the sensor net, the tags and antennas may be positioned in a geometric matrix having a distance between the antennas (and between the sensors) in accordance with the amount of resolution desired; the greater the geometric density of the tags and antennas, the finer the resolution of the shadows. The position of the tags and antennas is known so the system has an initial map of the environment defined by the sensor net. It is understood that while the preferred embodiment of the invention is generally described in conjunction with a retail store, that the embodiments of the invention are not limited to a retail store environment, but are applicable to various environments where it is desirable to track goods and/or people, such as, for example, a warehouse, airport, train station, subway station, bus station, stadium, convention center, transportation center, museum, or anywhere along a product distribution line.  
         [0028]      FIGS. 1-3  illustrate Sensor Nets (e.g., detection zones) and how shadows form. Referring to  FIG. 1 , a tracking system in accordance with the preferred embodiments is illustrated by example at  10 . The tracking system  10  includes antennas  12 , RFID sensors  14  and a processing member (e.g., computer  16 , computer program  17 ). While not being limited to a particular theory, the antenna  12  and RFID sensors  14  are preferably the same structure as well-known antenna and RFID tags, but are spread about an environment to form a detection zone. That is, the antennas  12  (also known as interrogators) are preferably fixed and placed in or near a ceiling or another structure (e.g., wall, pedestal, columns, etc.) as is readily understood in the art. The RFID sensors  14  are preferably fixed under the floor within the detection zone to be seen by the antennas  12 . The RFID sensors  14  can also be placed in or on walls, columns, tables, or other structure within the detection zone. Both the antennas  12  and the sensors  14  are preferably fixed so that the tracking system  10  knows where the antenna and sensors are, which is critical for determining the location of articles and objects within the detection zone. In other words, the system  10  knows the map of the environment (e.g., sensor net detection zone) created by the antennas  12  and tags  14 . This mapping gives the system  10  knowledge of the environment so that upon the loss of information (e.g., the non-detection of an expected signal) the system can determine the location of a shadow and its associated shadow causing object.  
         [0029]     Regardless of their location in the detection zone, the antennas  12  and sensors  14  are positioned to detect electromagnetic shadow causing objects within the zone. The antennas  12  are also arranged to detect mobile RFID tags that are not magnetically blocked within the detection zone. As such, RFID tags may be attached to an article for sale, store personnel, or merchandise containers as readily understood by a skilled artisan.  
         [0030]     The computer  16  is electrically coupled to an output of each antenna  12  for interpreting and processing the response signals received or expected but not received from the RFID sensors  14  and other RFID tags in the detection zone. The results are interpreted by the computer program  17  (e.g., software, middleware, firmware, application) integrated with the computer  16  for determining the presence of objects within the detection zone. Each RFID sensor  14  is positioned so it generally communicates with at least one antenna  12 . As can be seen in  FIG. 1 , each antenna  12  is arranged to interrogate and read RF signals over a generally cone shaped volumetric area generally defined, for example, by field of view borders  18  associated with each respective antenna. Each antenna  12  is configured to see RFID sensors  14  located within the antenna&#39;s communication range, which can be identified in  FIG. 1  as the cone-shaped volumetric regions defined by the field of view borders  18 , assuming that the antenna can see the RFID sensor by an unobstructed line of sight therebetween.  
         [0031]     It is understood that the computer  16  and computer program  17  are shown as an example of a processing member and that other processing member designs, including computers and programs alone or in combination with other computers, networks, or programs, could also be used with the present invention, and the invention is not limited to the particular processing member described. It is also understood that the computer  16  and computer program  17  are coupled to the antennas  12  in any way that permits the computer and computer program to access the communication results of the antennas. Accordingly, the computer  16  and computer program  17  are adapted to access the readings of the antennas  12  and can be coupled to the system in a wired or wireless configuration as understood by as skilled artisan.  
         [0032]     According to the view shown in  FIG. 1 , each antenna  12  can see four of the RFID sensors  14  mounted under a floor  20 . It is understood that the invention is not limited to a particular number of sensors per antenna, as the number may be determined from many factors, including the level of resolution desired for the shadow causing object, the distance between the antennas  12  and RFID sensors  14 , and the wavelength of the interrogation signal. As long as the sensors  14  are working and no object is blocking the line of sight between a respective antenna  12  and sensor, then the antenna and sensor are able to communicate in a manner well know in the art. For example, an antenna  12  transmits an interrogation signal toward the RFID sensors  14  in the antenna&#39;s region of view. The sensors  14  that receive the interrogation signal resonate, returning a response signal with the sensor&#39;s identification back to the antenna  12 . Since the tracking system  10 , and in particular, the computer program  17  (e.g., software processing the communications) knows the detection zone environment, and thus knows the location of each antenna  12  and sensor  14 , then the tracking system can locate the line of sight between each antenna and sensor. If a sensor  14  in a respective antenna&#39;s detection zone returns a response signal that is received by the interrogating antenna  12 , then the tracking system  10  knows that no blocking object was present between the respective antenna and sensor during the communication period. However, if the antenna  12  does not receive a response signal from a sensor  14  in its detection zone, then the tracking system  10  derives that an object is blocking the line of sight between the respective antenna and sensor.  
         [0033]     Accordingly, from the loss of information (e.g., no response signal received by the antenna) the tracking system  10  gains information by recognizing that an object is blocking the communication path between the respective antenna  12  and sensor  14 . This loss of information creates an electronic shadow along the respective line of communication. Preferably, each RFID sensor  14  is positioned within the detection zone of more than one antenna  12 , which allows the tracking system  10  to estimate the size of the signal blocking object, by analyzing the shadows cast by the object. As the antennas  12  repeatedly and continuously send out interrogation signals and read corresponding response signals, the tracking system  10  can track movement of a signal blocking object in real time based on the RFID sensors  14  seen (e.g. response signal read) and not seen (e.g., signal blocked) by the antennas  12 .  
         [0034]      FIG. 1  illustrates an example of a shadow. If  FIG. 1 , three of the antennas are identified having reference numerals  22 ,  24  and  26 ; and four of the RFID sensors are identified having reference numerals  28 ,  30 ,  32 , and  34 , respectively. The antennas  22 ,  24  and  26  are identical to the antennas  12  discussed above, with each antenna having a unique identification. Likewise, the RFID sensors  28 ,  30 ,  32  and  34  are identical to the RFID sensors  14 , with each sensor having a unique identification. Still referring to  FIG. 1 , a shadow causing object  36  is shown on the floor  20  above sensors  30  and  32 . The shadow causing object  36  represents any object that blocks communication between the antennas  12 ,  22 ,  24 ,  26  and RFID sensors  14 ,  28 ,  30 ,  32 ,  34 .  
         [0035]     In the example embodiment illustrated in  FIG. 1 , antennas  12 ,  22 ,  24  and  26  transmit interrogation signals to alert the associated RFID sensors  14 ,  28 ,  30 ,  32 , and  34  in their field of view. If the interrogation signals and corresponding response signals are received as expected, then no shadow occurs. However, if an antenna  12 ,  22 ,  24 ,  26  does not receive a corresponding response signal to its transmitted interrogation signal, then the tracking system infers a shadow along the line of sight between the corresponding antenna and sensor. As can be seen in  FIG. 1 , the shadow causing object  36  casts an electronic shadow  38  over and between RFID sensors  28 ,  30 ,  32  and  34 . No shadow is formed over the RFID sensors  14  since there is no shadow causing object between any of the RFID sensors  14  and the antennas  12 . However, antenna  22  does not receive a response signal from sensor  30 , antenna  24  does not receive a response signal from sensors  28 ,  30 ,  32  and  34 , and antenna  26  does not receive a response signal from sensor  32 . Therefore the system  10  identifies the shadow  38  as being formed on the floor  20  over and between RFID sensors  28 ,  30 ,  32  and  34 .  
         [0036]     Preferably, the antenna and RFID sensors are positioned so that each sensor is in the field of view of more than one antenna. This allows the tracking system  10  to better estimate the size of the shadow causing object  36 . For example, still referring to  FIG. 1 , RFID sensor  28  is in the field of view of antennas  22  and  24 . With the shadow causing object  36  positioned as shown in  FIG. 1 , antenna  22  can see sensor  28  (e.g., antenna  22  receives a corresponding response signal from sensor  28 ). However, antenna  24  can not see sensor  28  because the shadow causing object  36  blocks communication therebetween. Therefore shadow  38  extends over RFID sensor  28  as the tracking system  10  gains the knowledge that the shadow causing object  36  is large enough to block line of sight communication between antenna  24  and sensor  28 . The tracking system  10  also gains information that the shadow causing object  36  does not cover the RFID sensor  28  and does not block the line of sight between the antenna  22  and the sensor  28 . Since the RFID sensor  28  is within the field of view of antennas  22  and  24 , but can only be seen by antenna  22 , the tracking system  10 , via the computer program  17 , is able to better estimate the size of the shadow causing object  36  as having a height sufficient to block communication between antenna  24  and sensor  28 , but not extending over sensor  28 . RFID sensor  30  can not been seen by its corresponding antennas  22  and  24 , thus allowing the tracking system  10  to presume that the shadow causing object  36  is located over the sensor  30 . Similarly, RFID sensor  32  can not be seen by its associated antennas  24  and  26 , thereby allowing the tracking system  10  to presume that the shadow causing object  36  is also located over the sensor  32 . RFID sensor  34  can not be seen by one of its corresponding antennas  24 , but can be seen by another of its corresponding antennas  26 . From this information, the tracking system  10  can determine that the shadow  38  extends over the sensor  34  but that the shadow causing object  36  does not extend over the sensor  34  or block the line of sight between the sensor  34  and the antenna  26 . Accordingly, the tracking system  10  via the computer program  17  can locate and estimate the size of the shadow causing object  36  as having a height sufficient to block the line of sight between antenna  24  and RFID sensors  28  and  34 , and also as having a footprint smaller than its shadow  38 .  
         [0037]     In a similar manner, the tracking system  10  can locate an unblocked mobile RFID sensor according to the antennas  12 ,  22 ,  24  and  26  that detect the unblocked sensor, as understood by a skilled artisan. For example, an unblocked mobile sensor detected by antennas  22  and  24  would be located within the detection zones of the detecting antennas  22  and  24 .  
         [0038]     Since the tracking system  10  is constructed to periodically communicate between the antennas and sensors, the tracking system can track movement of the shadow causing object  36  over time. In other words, the tracking system  10  can track people, vehicles, inventory, products, etc. as desired depending on the application. Likewise, the tracking system  10  can track tagged (e.g., having a mobile active or disabled sensor) merchandise and associate the merchandise with the shadow causing object  36  if appropriate. Based on the movement of the shadow causing object  36 , and the shadow&#39;s association with unpurchased merchandise, the tracking system  10  can be used to send alarms or otherwise notify personnel of floor activity as desired.  
         [0039]      FIG. 2  is a top view, partially in section, of the tracking system  10  shown in  FIG. 1 , in accordance with the preferred embodiments of the invention. In particular,  FIG. 2  illustrates an exemplary floor plan for placement of the RFID sensors  14  under a floor.  FIG. 2  also shows field of view circles  40 , with each field of view circle outlining an area of a floor seen by a respective antenna  12  placed above the floor and preferably adjacent a ceiling over the floor. For example, the field of view circles  40  indicate the intersection of the field of view boarders  18  and the floor  20  shown in  FIG. 1 . As can be seen in  FIG. 2 , each antenna  12  is preferably located and oriented to be able to see a plurality of RFID sensors  14 , and each sensor is arranged so that it can be seen or identified by more than one antenna.  
         [0040]     Still referring to  FIG. 2 , the shadow causing object  36  blocks communication between the antennas and sensors  28 ,  30 ,  32  and  34  to cause a shadow  38 . However, in a method similar to as described above for  FIG. 1 , the tracking system  10  can estimate the size of the shadow causing object  36  by triangulating on the RFID sensors that are not seen by their respective antennas. Using this approach, the computer program can determine that the shadow causing object has a footprint large enough to cover only RFID sensors  30  and  32 , and has a height that blocks communication between other antennas  28 ,  34 ,  42  and  44 . When used over time, the tracking system  10  can non-invasively track movement of the shadow causing object  36  within the sensor net of detection zones defined by the location and configuration of the antennas and sensors.  
         [0041]     It is understood that the antennas and sensors can be arranged further apart or closer together, depending on the level of resolution desired for the application of the tracking system  10 . If less resolution is desired, for example, where the tracking system  10  is able to identify and track a shadow causing object  36  but not overly concerned about the dimensions of the object, then the antenna  12  and sensors  14  can be spread farther apart. Further, if a greater level of resolution is desired to better identify each shadow causing object  36 , then the antenna  12  and sensors  14  may be placed closely together such that each sensor can be viewed by a plurality of antenna to precisely determine the size and shape of the shadow causing object.  
         [0042]      FIG. 3  is a top view similar to  FIG. 2  but showing a tracking system  10  in accordance with the preferred embodiments having its antennas and sensors spread farther apart than in the examples shown in  FIGS. 1 and 2 . As can be seen in  FIG. 3 , some of the RFID sensors  14  can be seen by a plurality of antennas while others may only be seen by one antenna. This spacing allows the tracking system  10  to identify and track a shadow causing object, but depending on the size of the shadow causing object, the tracking system  10  may not be able to identify the shadow causing object with the same resolution or as detailed as is determinable from the embodiment shown in  FIGS. 1 and 2 . For example, if a shadow causing object is small enough to block communication between only one sensor and antenna, then the tracking system can not estimate the size of the shadow causing object within the shadow. However, as the shadow causing object moves within the safety sensor net, it will cause a shadow over RFID sensors that can be seen by more than one antenna, allowing for a better estimate of the size of the shadow causing object. Accordingly, over time, as the shadow causing object moves within the security net, the tracking system  10  can still estimate the size and shape of the shadow causing object.  
         [0043]     The combination of electronic shadow vectors, along with the detection of mobile RFID sensors and their respective vector motion, creates an electronic mapping fingerprint. Electronic mapping fingerprints may also be the summation of electronic shadow vectors, with or without mobile RFID sensors, as is described in greater detail herein. A computer program analyzed the electronic mapping fingerprint as desired and provides security, marketing and logistics functions, sample uses of which include the following:  
       EXAMPLE 1  
       [0044]     Loss Prevention—Electronic mapping and electronic shadows may be used to determine the vector motion/direction (e.g. towards the exit of the store or towards the point of sale—cash register) and velocity (speed) of a customer in a retail store. By combining this information with the knowledge of a product in the possession of the customer (e.g., exact type, quantity, value), it may be determined if the product is likely being stolen. In particular, motion of a shadow with non-purchased goods towards an exit may indicate an attempted theft, especially if the shadow/tag does not go past a point-of-sale. This sequence of information and events could signal a store employee to approach the customer before exiting the store and thereby prevent a likely shoplifting event. The preferred tracking system of the invention can also detect theft upon a sudden loss of a tagged product (e.g., foil-lined bag, baby carriage, shopping bag, removed tag) in association with a shadow. In this situation, the tag disappears in relationship to a shadow, but the preferred tracking system can track the shadow associated with the loss tag. These examples of loss prevention are useful in dressing room areas, where tags are disabled and an associated customer leaves. The preferred tracking system of the invention is alerted by the sudden loss of the tag and can track the shadow caused by the associated customer. Detection zones in accordance with the preferred embodiments can also be placed, for example, at point-of-sale locations (e.g., cash or check-out registers) to detect swipe control and/or monitor traffic. One example of swipe control is where an employee at a point-of-sale does not scan all of the products into the register for sale. Even if the tags on the unscanned products are disabled, the tracking system can still track the shadow associated with the products. That is, when a tag disappears, the tracking system  10  can still track the shadow associated with the lost tag.  
       EXAMPLE 2  
       [0045]     Cross Merchandising (e.g., complementary product selections)—By using the same information as in Example 1 (e.g., vector/motion direction of customer and the knowledge of the products in hand by the customer), if the customer&#39;s vector direction is the point of sale cash register, this knowledge and information can be used to automatically display to the customer (via a well-known display system in the queue for checkout) or to prompt a sales employee to suggest an array of complimentary products that match the customer-selected items. As another demonstration of cross merchandising, when a customer, identified as a shadow, enters a dressing room, the tracking system in accordance with the preferred embodiments can advertise goods associated with the tagged merchandise brought into the dressing room with the customer on a well-known display device located in the dressing room. The computer program in accordance with the invention can match the merchandise brought into the dressing room with complementary products, and market the complementary products to the customer in the privacy of the dressing room. This marketing approach is more focused and personal than general advertisements elsewhere in the store, because the advertisement is shared only with the customer in the changing room, where the customer can personally consider the products advertised without public concerns or pressures. The advertisements are based on the tagged products that the customer has shown an interest in by taking the products into the dressing room. Accordingly, the tracking system provides a focused non-invasive marketing tool or personal shopper for a customer without people watching machines or store personnel.  
       EXAMPLE 3  
       [0046]     Customer flow/plan-a-gram/in store marketing—Based on the electronic shadow of customers and the reference map of fixed RFID sensors on stationary store fixtures and/or in the floor, a trend analysis can be established to understand customer flow. This trend analysis can allow merchandising to optimize customer flow within the store.  
         [0047]     This same scenario, along with the markdown racks and/or store marketing, can also establish trend analysis to optimize messaging and markdown locations within the retail environment. By combining this information with the knowledge of the products in the hand or before a customer, trend analysis can also be established based on, for example, the amount of time the customer considers a product and the percentage of times that the customer purchases considered products.  
         [0048]     The correct products in the correct locations can improve sales throughput. The ability to electronically verify exact product and type can be used to monitor inventory/location and take corrective action based on established sales rules.  
       EXAMPLE 4  
       [0049]     People Counter—The use of electronic shadows at the exit/entrance of a retail store can provide an accurate people count and real time history database. The value of people count and real time information can provide a measure of advertising campaigns. A tracking system in accordance with the preferred embodiments can also provide an accurate people count at other locations where it is desirable to know such information, such as, for example, transportation centers, convention centers, sporting venues, trade shows, etc.  
       EXAMPLE 5  
       [0050]     Sales Employee Efficiency—The tracking system can track a mobile RFID sensor on a sales clerk to monitor the employee&#39;s actions throughout a workday. For example, mobile RFID sensors on sales clerks in proximity to electronic shadows of customers can indicate interaction time between the sales employees and customers. Such interaction time can be logged by the tracking system as an aid to maximize salesperson performance. Also, the tracking system can track a mobile RFID sensor on a sales clerk and mobile RFID sensors on products to monitor the time spent and efficiency for transferring inventory onto the retail floor.  
       EXAMPLE 6  
       [0051]     Real-time inventory—The inherent electronic shadows can allow real-time inventory tracking, thereby providing cost savings in labor dollars. Real-time inventory also assures that the proper products are at the store in the proper location.  
         [0052]     As discussed above, the tracking system of the present invention is preferably implemented with real-time software (e.g., middleware, firmware, application instructions). The tracking system may detect motion by various methods. For example, the tracking system may detect motion by positive location, where a target (e.g., tagged item) is seen by an antenna and moves within the sensor net. The tracking system may also detect motion by permanent absence, where a target in the sensor net disappears, with no associated shadow. In this scenario, the tracking system can presume any of the following and notify a nearby sales clerk: a) the tag went bad; b) the tag is causing interference; or c) something fell. The tracking system can further detect motion by absence with a shadow, where the tag disappears and there is a shadow blocking the tag that is seen when the shadow moves away, or the tag remains gone and an associated shadow moves. The tracking system can also detect motion of a person shadow, where a person creates an electronic shadow that is seen by an antenna and the person moves within the sensor net. Motion detection can be hindered by obstructions, which can be caused by shopping carts getting pushed around, store columns, display tables, merchandising objects (e.g., shelves). These obstructions can be obviated with the use of multiple antennas.  
         [0053]     As described above, the tracking system in accordance with the preferred embodiments can detect theft. For example, theft may be detected by tracking a tag or a shadow motion vector towards an exit of non-purchased goods, especially if the tag/shadow does not first go through a point-of -sale. The tracking system can also detect theft by the sudden loss of tagged product (e.g., foil-lined booster bag, baby carriage, shopping bags), where the tag disappears in relationship to a shadow. The tracking system can non-invasively detect theft in a dressing room area, for example, where people disable tags in a dressing room, and an associated shadow leaves. Moreover, the tracking system can detect theft by swipe control at a point-of-sale. For example, where an employee does not scan all of the products into the register for sale, the tracking system can track the shadow associated with the products.  
         [0054]     Depending on its use, the tracking system provides numerous measures of efficiency. For example, the tracking system can monitor the performance of sales clerks wearing RFID badges, since the tracking system recognizes if and how often the clerks are greeting and serving customers by integrating the RFID badges of the sales clerks with the shadows of the customers. In addition, the tracking system helps merchandising by checking if the goods and products are at their proper location. Moreover, the tracking system can monitor customer interaction, since the tracking system can non-invasively determine where the customer is. Further, the tracking system can monitor customer path flow, traffic patterns, linger patterns and acceptance rates. Such information can guide merchants on drawing customers into the shopping experience.  
         [0055]     It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. For example, the embodiments could be modified to operate using other frequencies from the hertz band through the tera band to non-ionizing bands. Non-ionizing frequencies would work well as a coupling method differentiated by ionizing radiation as opposed to non-ionizing radiation. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention. Without further elaboration the foregoing will so fully illustrate my invention that others may, by applying current or future knowledge, readily adapt the same for use under various conditions of service.