Patent Publication Number: US-6661335-B1

Title: System and method for locating radio frequency identification tags

Description:
RELATED APPLICATIONS 
     This patent application is related to U.S. application Ser. No. 09/405,358, entitled “SYSTEM AND METHOD FOR COMMUNICATION WITH RADIO FREQUENCY IDENTIFICATION TAGS USING TWO MESSAGE DFM PROTOCOL”; U.S. application Ser. No. 09/405,364, entitled “SYSTEM AND METHOD FOR COMMUNICATING WITH DORMANT RADIO FREQUENCY IDENTIFICATION TAGS”, and U.S. application Ser. No. 09/406,091, entitled “SYSTEM AND METHOD FOR LOCATING RADIO FREQUENCY IDENTIFICATION TAGS USING THREE-PHASE ANTENNA”, which are being filed concurrently herewith on Sep. 24, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radio frequency communication, and specifically to the use of radio frequency identification (RFID) tags in conjunction with one or more radio frequency sensors to determine the position of the tag or an asset to which the tag is attached within three dimensional space. 
     2. Description of the Related Technology 
     The field of RFID (Radio Frequency Identification) is growing rapidly. Applications of RFID technology are wide ranging and include counting objects as they pass near to a sensor, uniquely identifying a specific tag (hereinafter “transponder”) and associated asset, and placing data within the RFID transponder for later recovery. The process of “reading” and communicating with an RFID transponder generally comprises bringing the transponder in proximity to an RFID sensor which emanates a radio frequency wake-up field having a limited range. The RFID transponder detects the presence of the wakeup field of the sensor, and subsequently various forms or protocols of handshake occur between the transponder and the sensor in order to exchange data. All of this communication between the transponder and the sensor is performed using radio frequency carriers of some kind. When multiple transponders are involved, anti-clash protocols of the type well understood in the data processing arts are employed in order to multiplex or provide multiple access to the sensor by the multiple transponders. 
     The main advantages of an RFID sensor and transponder system over other forms of ID tagging include (i) the orientation of the transponder with respect to the sensor is not critical for a correct read of the transponder information; (ii) communication can occur within comparatively harsh operating environments including dirt, grease, opaque gasses, etc.; and (iii) the communication range between the sensor and transponder can be significant (in excess of 100 feet in certain cases) even when the RF frequencies used are within the power limitations of Federal Communications Commission (FCC) rules concerning unlicensed transmitters. Accordingly, RFID technology is useful for several applications, especially those relating to security and asset management. 
     For example, in applications where enhanced security is desired, RFID systems using electromagnetic energy with very low frequency are attractive since the low frequency energy tends to suffer low losses from shielding materials such as metal boxes, aluminum foil, and the like. Those who would surreptitiously remove the tagged assets from a building usually try to use such shielding techniques. However, these low frequencies typically require large antennas within the transponder in order to achieve reasonable levels of RF coupling between the sensor and the transponder. It is impractical to place large wire antennas within small transponders; accordingly, comparatively small magnetic loop antennas are the coupling methods of choice for such small transponders. These magnetic loop antennas exhibit a serious drawback, however, in that they have a characteristic “figure-8” sensitivity pattern and, in certain positions and/or orientations, can reject or otherwise not detect the fields generated from the sensor. Stated differently, the magnetic loop antenna of the transponder can only receive energy from the sensor antenna coils only when the orientation of the sensor and transponder coils is similar. Specifically, the “rejection” solid angle for a loop antenna can be thought of as a band of a certain solid angle measured from the center and oriented 360 degrees around the circumference of the loop (see FIG.  1 ). When such rejection occurs, the transponder may be well within the sensor&#39;s intended wake up field, but fails to detect the sensor&#39;s emissions, and therefore also fails to communicate therewith. A related problem is when the position and/or orientation of the transponder within the field is varied, thereby taking the sensor(s) out of the “figure-8” pattern of the transponder antenna, and interrupting communication between the transponder and sensor. 
     Additionally, many existing RFID transponder/sensor systems do not have the ability to locate the transponder in spatial space. Those which do have this ability suffer from significant drawbacks since none of them will function using the low frequency signals needed to pass through foil and other shielding. The added capability of spatial positioning, however, allows the sensor to gather more information about the transponder, i.e., its relative location in space with respect to the sensor or some other reference point. This capability provides a very significant advantage over other asset management systems (RFID or otherwise) which can not determine the position of the assets. 
     Furthermore, existing RFID systems in which the transponder includes a motion sensor or other device which activates or otherwise permits the waking up of the transponder do not have provision for the transponder to communicate with the system sensor (reader) during periods when the transponder is not in motion, such as during installation or maintenance. Accordingly, such prior art transponders must be physically moved or agitated during these periods in order to enable the transponder to communicate with the sensor. This approach is cumbersome and inefficient. 
     Based on the foregoing, an improved apparatus and method for spatially locating an RFID transponder having a magnetic loop antenna within one or more sensor fields is needed. Furthermore, an improved apparatus and method for maintaining effectively constant and uninterrupted communication with the aforementioned RFID transponder regardless of physical position or orientation is needed. Lastly, an improved apparatus for interrogating and communicating with the RFID transponder when the transponder is not in motion or otherwise dormant is needed. 
     SUMMARY OF THE INVENTION 
     The foregoing needs are addressed by the invention disclosed herein. 
     In a first aspect of the invention, an improved system for and method of determining the position of one or more radio frequency identification (RFID) transponders with respect to one or more sensors is disclosed. In a first embodiment, the system comprises a plurality of stationary sensor arrays located within certain physical areas. Each sensor array comprises a plurality of antenna coils arranged in unique physical orientations with respect to each other and capable of transmitting radio frequency signals of differing phase. The RFID transponder includes a magnetic loop antenna which receives the plurality of signals generated by the antenna coils, and compares the phase of at least two of the received signals in order to determine the relative position of the transponder(s) with respect to the sensors. 
     In a second aspect of the invention, an improved system for and method of maintaining constant communication between one or more RFID transponders and their associated sensor(s) using a multi-message protocol is disclosed. In one embodiment, the system comprises sensor arrays having a plurality of antenna coils in a predetermined physical relationship which emit two direction finding mode (DFM) signals in succession. For the emission of the first DFM signal, each of the plurality of antenna coils is energized so as to emit a signal in its given orientation. For the emission of the second DFM signal, one of the plurality of coils is turned off such that no radio frequency signal is emitted from that coil. The spatial relationship of the transponder and individual antenna coils precludes all of the signals from each sensor array from being rejected by the transponder during the emission of both the first and second DFM signals. In this fashion, the transponder coil can be kept in constant communication with the sensor, regardless of its orientation with respect to the sensors. This feature effectively eliminates the communication problems associated with the typical “figure-8” pattern associated with the transponder&#39;s antenna coil. 
     In a third aspect of the invention, an improved method of determining the location of the transponder with respect to two or more sensor arrays through elimination of sensor rejection is disclosed. In one embodiment, the method comprises positioning the transponder with an internal antenna coil within the field generated by the coils of the individual sensor arrays, transmitting a first signal from each of the antenna coils of the two or more arrays, transmitting a second signal from a subset of the antenna coils of the same arrays (i.e., with one or more coils turned off), and determining the position of the transponder relative to the two or more sensor arrays based on the intensity of the first and second signals received by the antenna coil of the transponder. 
     In a fourth aspect of the invention, a system for and method of transmitting data between a sensor having a transmit coil and a RFID transponder having a receiving coil is disclosed. In one embodiment, the system comprises a hand-held sensor probe or wand which emits a highly intense and localized wake-up field at a predetermined frequency. This field is sensed by the receiving coil of the transponder, and its physical parameters (such as intensity and/or frequency) compared to predetermined values present within the transponder. If the sensed parameters of the wake-up field meet certain predetermined criteria, the transponder generates an internal wake-up signal, and begins communicating with the sensor. This system and method are particularly useful when using transponders having an internal motion detector, thereby allowing communication with the dormant (e.g., motionless) transponder without the need to physically move the transponder. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of the magnetic field sensitivity pattern of a typical prior art loop antenna having a circular shape, based on 10 db downpoints of field strength. 
     FIG. 2 is diagrammatic representation of the three carrier wave phase vectors associated with the present invention. 
     FIG. 3 is a plot illustrating the positional angle measured by the transponder of the present invention as a function of the intensity of the signals received by the transponder from two three phase sensor arrays. 
     FIG. 4 is a plan view of an exemplary arrangement of the three phase sensor array of the present invention, placed with respect to a door. 
     FIG. 5 is a logical block diagram showing a portion of the communications between an RFID sensor and transponder of the present invention. 
     FIG. 6 is a block diagram of one embodiment of a stand-alone PC-based asset management system with network link according to the present invention. 
     FIG. 7 is a block diagram of one embodiment of a security system with network link according to the present invention. 
     FIG. 8 is a block diagram of one embodiment of the sensor hardware and arrays shown in FIGS. 4,  5 , and  6 . 
     FIG. 9 is a block diagram of one embodiment of the sensors used in the sensor arrays of FIG.  8 . 
     FIG. 10 is a schematic diagram of the three phase transmitter and antenna array portions of the sensor shown in FIG.  9 . 
     FIG. 11 is a physical assembly diagram showing one embodiment of the antenna coil configuration of the sensor arrays shown in FIGS. 8 and 9. 
     FIGS. 12A and 12B are diagrams of one embodiment of the end coils (left end and right end, respectively) for the Z-direction of the antenna coil configuration shown in FIG.  11 . 
     FIG. 13A is a diagram of one embodiment of the X-direction coil of the antenna coil configuration shown in FIG.  11 . 
     FIG. 13B is a diagram of one embodiment of the Y-direction coil of the antenna coil configuration shown in FIG.  11 . 
     FIG. 14 is a block diagram of one embodiment of the hardware configuration for the RFID transponder shown in FIG.  5 . 
     FIG. 15 is a plan view of the sensor array portion of the asset management and security system of the present invention in an exemplary installation, illustrating characteristic patterns of the transponder wake up range and sensor read range. 
     FIG. 16 is a logical state chart for the internal states of one embodiment of the transponder of FIG.  14 . 
     FIG. 17 is a schematic diagram of a portion of one embodiment of the transponder wake up circuit of the transponder of FIG.  14 . 
     FIG. 18 is a logical flow chart illustrating one embodiment of the two message direction finding mode (DFM) process performed by the asset management system of the present invention. 
     FIG. 19 is a logical flow chart of one embodiment of the position location technique utilized by the asset management system of the present invention. 
     FIG. 20 is a block diagram of one embodiment of the portable wake up sensor probe and system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is now made to the drawings wherein like numerals refer to like parts throughout. 
     As used herein, the terms “tag” and “transponder” are used interchangeably, and are meant to include any mobile or semi-mobile radio-frequency device capable of transmitting, receiving, or both transmitting and receiving radio frequency energy in one or more frequency bands. Similarly, the terms “sensor”, “transceiver”, and “reader” are used interchangeably, and are meant to include any fixed or semi-mobile radio frequency devices capable of receiving, transmitting, or both receiving and transmitting radio frequency energy in one or more frequency bands. 
     Fundamental Operating Principles 
     The fundamental operating principles of the present invention are now described with reference to FIGS. 2 through 4. The present invention broadly comprises a system by which an RFID transponder may communicate with an array of RF sensors, the signals emitted by the sensors aiding the transponder in locating its position relative to the sensors. As shown in FIG. 2, the present invention utilizes sensors (antenna coils) which emit three carrier waves each separated by a phase angle θ, the phase angle in the present embodiment being equal to 120 degrees. In another embodiment, other phase angles are envisioned. The three carrier waves are described by vectors where the magnitude of the magnetic field intensity H is described by: 
       H =sin(ωt+θ)  Equation 1 
     Where: 
     θ=0, 120, and 240 degrees (0π, 2/3π, and 4/3π radians), respectively, and 
     ω=angular frequency (radians/sec). 
     Each of the three phase-shifted carrier signals is transmitted by a corresponding loop antenna coil (see discussion of FIGS. 11-13 below), each antenna coil having its own “figure-8” pattern. Each of the three antenna coils is arranged on one of three axes of a Cartesian (i.e., X, Y, Z) coordinate system. Hence, the magnetic field intensity around the coils varies both as a function of position and time. Accordingly, the foregoing antenna coil and three phase carrier arrangement can be used to both transmit data to the transponder, and also provide information to the transponder regarding its position relative to the coils. 
     As shown in FIG. 3, the system of the present invention utilizes the relative strength or intensity of the electromagnetic three phase signals radiated by two (or more) sensor arrays, herein designated as “A” and “B”, in order to provide position information to the transponder. Specifically, the positional angle (relative to the antenna coils) is shown to be a function of the relative signals strengths present from the A and B arrays. The ratio of the signal strengths is defined as: 
     
       
         Ratio AB =20*log 10 ( I   A   /I   B )  Equation 2 
       
     
     Where: 
     I A =Intensity of received signal from “A” sensor array, and 
     I B =Intensity of received signal from “B” sensor array. 
     Hence, by knowing the intensity of the signals received from two antenna arrays, the tag may compute its positional angle relative to the known position of the arrays. 
     Referring now to FIG. 4, one exemplary embodiment of the three phase sensor array of the present invention is described in order to further illustrate the foregoing principles. As shown in FIG. 4, the antenna arrays of the exemplary embodiment are arranged in pairs  404 ,  406 ,  408 , where one antenna coil assembly  402  of each pair is defined as being in one location relative to a door  410  (i.e., “inside” a building or an area in the present illustration, designated by the letter “A) and the other coil assembly of each pair is defined as being in a second location relative to the door  410  (i.e., “outside” of the building or an area, designated by the letter “B”). The coil spatial and phase orientation is shown for an “N” pair system. Note that each of the coil assemblies contains three antenna phase coils having a differing electrical phase as previously described. The physical orientation of the coils may be as defined in FIG.  1 . It will be appreciated that the illustrated system of sensor arrays may be set up over an infinitely long barrier if desired, or configured in other arrangements. 
     RFID Asset Management System 
     Referring now to FIG. 5, the operation of one embodiment of the RFID asset management system of the present invention is described in detail. As shown in FIG. 5, the system is comprised generally of a radio frequency reader or sensor system  510  and a corresponding transponder  520  which are each in communication with one another. Initially, the transponder  520  is dormant or in an inactive state when outside the field generated by the sensor(s)  510 . This dormant state reduces the power consumption of the transponder, since its internal processor  1406  (FIG. 14) and other components are inactive. The sensor system  510  generates an electromagnetic “wake up” field  130  using an alternating current (AC) signal as is well known in the art; see the discussion of FIG. 15 below for more details regarding the sensor field pattern in various applications. This wake up field is generated by the sensor system  510  in order to toggle the transponder  520  into the active state when the transponder  520  enters the wake up field (i.e., when the field detected by the internal magnetic loop antenna  1402  of the transponder as illustrated in FIG. 14 is of sufficient intensity), and the transponder is in motion. In one embodiment, the sensor system  510  communicates with the transponder  520  using an 8.13 kHz phase modulated signal, although it will be appreciated that other frequencies and/or modulation schemes may be used. It will further be recognized that broadband (i.e., “spread spectrum”) radio frequency techniques such as DSSS or FHSS may also be used, although the descriptions of the exemplary embodiments contained herein are in terms of a non-spread spectrum system. The transponder  520  then enters an “active” state and continues the communications protocol as generally illustrated in FIG.  5 . Specifically, the transponder  520  next initiates an anti-clash routine  532  with the sensor system  510 , after which the sensor acknowledges the transponder ID (including a cyclic redundancy code or CRC) and issues a “rename” command  534  to the transponder  520  in order to assign it to one of a plurality of time slots for the direction finding mode (DFM), explained in greater detail below with respect to FIG.  18 . At this point, the sensor  510  requests information  536  as to the location of the transponder in the sensor field(s) from the transponder  520 . The transponder  520  then responds with the requested location information  538 , which is derived using one of a variety of techniques such as the relative signal strength method described with respect to FIG. 19 herein. Hence, the transponder  520  is used in the present invention to supply, inter alia, information regarding its position to the sensor(s)  510 . 
     Referring now to FIG. 6, one embodiment of a stand-alone PC-based asset management system with network link incorporating the three phase principles of the present invention is described. As shown in FIG. 6, the asset management system  650  generally comprises a personal computer (PC) based sensor processing system  600  coupled to a host computer system  620  via one or more data links  612 ,  614 . In the present embodiment, the data links  612 ,  614  comprise network interfaces such as those commonly associated with a local area network (LAN) or wide area network (WAN) of the type well known in the art, although it will be appreciated that many other different types of data links such as SONET or wireless interfaces such as those compliant with IEEE Standard 802.11 may be used. Quite literally any interface capable of transferring information from one component to another can be substituted. The sensor processing system  600  comprises one or more sensor processing subsystems  601 ,  630  which each include a PC  606 ,  636  coupled to a plurality of individual daughter boards  602 ,  608 ,  610  and  632 ,  638 ,  640 , respectively, each board having one or more radio frequency sensor coils (not shown) disposed thereon. In the illustrated embodiment, the daughter boards are connected to their respective PCs  606 ,  636  via frequency, coherent phase (F/2F) format data links, such as links  604 ,  634 , of the type well known in the magnetic recording arts, although it will be appreciated that other types of data links may be used. 
     Referring now to FIG. 7, one embodiment of a security system with network link incorporating the present invention is described. As shown in FIG. 7, the system  700  comprises a microcontroller  702  having an asset management sensor antenna array  703 , a plurality of access sensors/card readers  704 ,  706 , and a door strike  708  coupled thereto. The microcontroller  702  includes an asset management sensor daughter board (not shown), the latter which permits the microcontroller to interface with the asset management sensor array  703 . The asset manager sensor array  703  includes one or more sensor arrays  402  of the type previously described for communication with one or more transponders  520 . The microcontroller  702  is further coupled to a host computer  620  such as a personal computer or minicomputer via an interface processor (not shown) and a data link  710  of the type well know in the data processing arts. The microcontroller  702  in the illustrated embodiment is a Model No. Micro-5 microcontroller manufactured by the Casi-Rusco Corporation, although other devices may be substituted. The microcontroller  702  receives an asset management database from the host computer  620  during operation, and all assets entering the sensor system&#39;s range are stored and forwarded to the host computer  620 . Conventional card readers  704 ,  706  are coupled to the microcontroller  702  to allow personal access and asset management using the same system. 
     Referring to FIG. 8, one embodiment of a sensor hardware configuration of the antenna coil assemblies  402  and sensor system  510  of FIGS. 4 and 5, respectively, is now described. In the illustrated embodiment, the sensor, system  510  is configured such that the antenna coil arrays  402  are disposed in pairs  404 ,  408  in relation to a door, an opening, a virtual barrier, or other location (shown generically as a door  410  in FIG.  4 ). As in FIG. 4, a set of sensor arrays  402  are partitioned on opposite sides, shown as side “A” and side “B” of the door  410 . The arrays  402  may be factory set (for example, set for field strength, location, orientation, etc.), or set by an installer or the end user. The sensor arrays  402  are each connected to a sensor bus  832 . In the illustrated embodiment, the sensor bus is configured according to the RS-485 format, although other arrangements may be used. The sensor bus  832  connects to a sensor interface  840 , which is further connected to the host computer  620  (FIG.  6 ). The sensor interface  840  includes an industry standard  8051  type microprocessor, and random access memory (RAM) and read-only memory (ROM) circuits (not shown) to track the locations of the transponders and to process communications with the sensor arrays and with the host computer. It will be recognized, however, that other components and configurations may be used in place of the microprocessor/ROM/RAM combination described herein. 
     The sensor arrays  402  of the present embodiment may be configured as bar-shaped units located on the top or side of a door, or above a drop ceiling, although other configurations are possible. The antenna coils of the sensor arrays  402  emit an interrogating or wake up field to the transponders, and deliver the transponder identification data received from the transponder(s)  520  to the host computer  620 . The location of the transponder with respect to the sensor arrays  402  (i.e., “A” side or “B” side) is also provided to the host computer  620 , using the method described with respect to FIG. 16 below. Additionally, the sensor system  510  establishes whether the transponder  520  has passed through the virtual barrier. This feature aids in entry control to restricted areas, as well as associating an asset with a person. The host computer  620  is also warned of any condition that may compromise the sensor system  510  or transponder  520  capabilities (e.g., jamming) by decoding the status byte sent by the tag during the anti-clash process. If the tag determines that there is interference in the band where the tag is listening for communication from the sensor, the tag emits the jamming alert in an un-synchronized manner. 
     Referring to FIG. 9, one embodiment of a sensor array, such as sensor array  402  shown in FIGS. 4 and 8, will be described in detail. In the illustrated embodiment, the sensor arrays  402  receive an ultra high frequency (UHF) response from a transponder  520  (FIG. 5) at a set of UHF antennas (“ 1  of N” antenna  902  through “N of N” antenna  904 ). Each of these antennas  902 ,  904  sends the received signal to a set of corresponding UHF receivers (“1 of N” receiver  912  through “N of N” receiver  914 ). Each of the UHF receivers  912 ,  914  processes the received UHF signal and sends its output to a message decoder and encoder (MDE) circuit  920 . The MDE  920  includes an the input and output (I/O) interface  921  for the sensor array with the sensor interface  840  shown in FIG.  8 . Master timing, transmit signals, and receive signals are all communicated on the I/O interface of the MDE  920 . 
     The UHF receivers ( 902 - 904 ) operate on a multichannel frequency and are configured so as to be capable of continuous reception. A UHF antenna  1414  (FIG. 14) of the transponder  520  is configured to transmit data to the sensor system  510  using a 434 MHz nominal amplitude modulated (AM) carrier. The data bits are biphase coded using half bits of  330  microseconds in the illustrated embodiment. Biphase coding is well known in the art, and accordingly will not be discussed further herein. Signal strength information from the UHF receivers informs the message encoder  920  of incoming RF energy from the transponders  520 , such as a burst of data. The UHF receivers send the signal levels and any received data to the message encoder  920  for communication with the host computer  620  via the sensor interface  840 . 
     As previously noted, the sensor array  402  also generates a “wake up” field  530  directed to the transponder(s)  520 . The door sensor interface  840  sends master timing and other control signals to the MDE  920 . The MDE  920  decodes the message from the sensor interface  840  and sends the signals to the master timing decoder  922 . The output of the master timing decoder  922  is connected to a three phase modulator/frequency generator  930 . The output of the three phase modulator/frequency generator  930  is input to a three phase amplifier  932  which then supplies the three phase antenna array coils  940 . The three phase antenna array coils  940  generate the wake up field  530  which is transmitted to the transponders  520 . The three phase amplifier  932  and the three phase antenna array coils  940  comprise a three phase transmitter circuit and are described in greater detail in conjunction with FIG.  10 . 
     FIG. 10 illustrates one exemplary embodiment of the three phase amplifier  932  and antenna array coils  940  of the door sensor shown in FIG.  9 . As shown in FIG. 10, the amplifier  932  and antenna array  940  comprise three discrete circuit phases  1000 ,  1030 ,  1060  which are electrically coupled to corresponding antenna elements  1002 ,  1032 ,  1062 . The three phases  1000 ,  1030 ,  1060  each take a respective input signal  1004 ,  1034 ,  1064  from the three phase modulator/frequency generator  930  (FIG.  9 ), amplify the signal, and output the amplified alternating current signal to the respective antenna element  1002 ,  1032 ,  1062 . As described with reference to FIG. 2 above, the signals for each phase  1000 ,  1030 ,  1060  of the illustrated embodiment are shifted by 120 degrees of electrical phase as is well known in the electrical arts. Accordingly, the electromagnetic radiation emitted from the respective antenna elements  1002 ,  1032 ,  1062  is correspondingly shifted in phase such that magnetic and electric field intensity at a given point in space attributable to each antenna element is also shifted in phase. While the illustrated circuit phases  1000 ,  1030 ,  1060  employ, inter alia, operational amplifiers  1006 ,  1036 ,  1066  to perform the amplification function, it will be recognized that a variety of other components and arrangements well know in the art may be used to amplify the input signals  1004 ,  1034 ,  1064 . 
     Referring now to FIG. 11, one embodiment of the physical configuration of the antenna transmit/receive coils  940  within the sensor arrays  402  of the present invention is described. Each sensor array  402  comprises generally three antenna elements  1002 ,  1032 ,  1062  which are electrically coupled to the output of the circuit phases  1000 ,  1030 ,  1060  as shown in FIG.  10 . The antenna elements  1002 ,  1032 ,  1062  are physically comprised of loops of conductive material which are arranged so as to generate magnetic flux in three orthogonal axes (e.g., the X, Y, and Z axes in a Cartesian system) when alternating electrical current is passed there through. Specifically, the sensor array illustrated in FIG. 11 is comprised of a substantially rectangular “X” antenna coil  1002 , a substantially rectangular “Y” antenna coil  1032 , and a pair of semi-circular “Z” end coils  1061 ,  1063  electrically arranged in a Helmholz circuit of the type well known in the antenna arts. The Helmholz arrangement of the end coils  1061 ,  1063  provides the Z-direction with an antenna aperture similar to that of one of the larger X or Y coils  1002 ,  1032 , thereby providing a more uniform field intensity in each of the three axes. 
     In a second embodiment, the antenna coils  1002 ,  1032 ,  1062  of the sensor array  402  are comprised of a ferrite loaded material wound with a plurality of turns of thin (i.e., 20 ga.) copper wire. Other antenna coil configurations may be used as well. 
     The four antenna coil elements  1002 ,  1032 ,  1061 ,  1063  are physically connected to their respective circuit phases (and to each other in the case of the end coils  1061 ,  1063 ) via plug-in connectors  1120 ,  1122 ,  1124 ,  1126  mounted on a printed circuit board  1100  located within the sensor array housing cover  1106 . The cover  1106  forms a portion of the exterior housing of the sensor array  402 , which is also comprised of two end covers  1102 ,  1103  and a base plate  1104 . The PCB  1100  is affixed to the base plate  1104 , as are the end covers  1102 ,  1103  and the housing cover  1106 . The end covers  1102 ,  1103 , base plate  1104 , PCB  1100 , and housing cover  1106  cooperate to maintain the antenna coils  1002 ,  1032 ,  1061 ,  1063  in the desired physical alignment (i.e., the X and Y coils orthogonal to each other as well as the two end coils) when the sensor array  402  is assembled. The housing cover, end covers, and base plate are constructed of plastic, plastic, and metal, respectively. These materials are chosen for their magnetic permeability with respect to the RF energy generated or received by the antenna coils. It will be recognized by those of ordinary skill in the mechanical arts, however, that a wide variety of housings or structures useful for maintaining the antenna coils in the desired positions, as well as materials of construction thereof, may be substituted for the arrangement of FIG.  11 . 
     FIGS. 12A through 13B further illustrate the sensor antenna coil arrangement and direction of electrical current flow of each coil in the sensor array of FIG.  11 . FIGS. 12A and 12B are diagrams of the end coils  1061 ,  1063 , respectively, for the Z axis. FIGS. 13A and 13B illustrate the X-direction coil  1002  and Y-direction coil  1032  of the sensor array, respectively. 
     Referring now to FIG. 14, one embodiment of the internal hardware configuration for the asset transponder  520  of FIG. 5 is described. As previously described, a low frequency tuned loop antenna  1402  receives transmissions from the sensor system  510  (FIG.  5 ). In one embodiment, the low frequency used is approximately 8 kHz, although frequencies in the range of 0 to 1 GHz may also be utilized. The transmissions received by the transponder include, inter alia, the wake up field  530  generated by the sensor system  510  (FIG. 5) as well as other data such as the “rename” command  534  and request for location information (DFM)  536 . The output of the loop antenna  1402  is fed to a wake up detector and receiver circuit  1404 . The wake up detector portion of the circuit  1404  detects a wake up field received by the transponder loop antenna  1402  by measuring the intensity of the received signal, and the receiver portion of the circuit  1404  receives and processes the low frequency data signals received by the loop  1402 . The wake up detector and receiver circuit  1404  is further described in conjunction with FIG. 17 below. 
     As shown in FIG. 14, the output of the circuit  1404  feeds a communications processor  1406  disposed within the transponder  520 . The processor  1406  is responsible for the processing of, inter alia, portions of the anti-clash, CRC, and ID code functionality described below. The processor of the present embodiment is a Model No. 161V58 8-bit processor manufactured by Microchip, although other types of processor may be used. The system can also be encoded into a state machine and developed as an application specific integrated circuit (ASIC). A tamper detector  1410  and/or motion sensor  1411  are also interconnected with the processor  1406 . The tamper detector  1410  helps determine if attempts are being made to tamper with the transponder  520 , while the motion sensor  1411  assists the processor in determining if the transponder  520  is in motion. The tamper detector  1410  of the illustrated embodiment comprises a switch-type arrangement using a normally closed switch of the type well known in the electrical arts, although other types of detectors may be used. The motion sensor  1411  is, in the present embodiment, a bi-morphous accelerometer of the type well known in the art, although it will again be recognized that other types may be substituted. 
     The processor  1406  of the transponder  520  further connects to a transmitter circuit  1412  which in turn supplies a tuned ultra-high frequency (UHF) loop antenna  1414 . The UHF loop  1414  is used for sending messages, such as the ID code, to the sensor system  510 . The UHF loop of the illustrated embodiment operates at a frequency of 434 MHz, although other frequencies may be used. In one embodiment, the UHF loop does not receive any signal from the sensors. 
     In the present embodiment, the transponder  520  remains dormant until either: (i) the motion sensor  1411  is activated by motion of the transponder  520  and a low frequency, e.g., 8 kHz, RF signal is detected; (ii) the tamper detector  1410  is activated (such as by someone trying to open the transponder  520  or remove it from the asset to which it is attached); or (iii) a high-intensity localized wake up field is received as described with respect to FIG. 20 herein. 
     When the 8 kHz wake up field is detected, the transponder emits its ID code at the respond command from the sensor system  510 . The 8 kHz field is modulated, thereby permitting data transfer from the sensor system  510  to the tag  520  via the signal. When the sensor system detects the ID code of the transponder (which is transmitted back to the sensor system by the transponder loop antenna), the sensor system  510  records the transponder ID and instructs the transponder  520  to stop transmitting. Subsequently, the sensor system  510  transmits a direction finding command to check for the position of the transponder  520 . When the transponder  520  is in the control of the sensor system  510 , the rate of interrogations by the sensor system varies according to loading. Specifically, as more transponders enter the RF field, the number of times per second that a given transponder is located is reduced. This reduces the speed of the radio location system as a whole, but advantageously does not impede the ability of the system to detect or communicate with additional transponders  520 . 
     FIG. 15 is a plan view of a barrier (e.g., door  410 ) illustrating the characteristic sensitivity patterns  1500  of one exemplary installation of a sensor array  402  and sensor system  510  according to the present invention. Shown in FIG. 15 are (i) the maximum sensor read range  1502  (dependent on radiated frequency and power) for a three phase emitter; (ii) the maximum wake up range  1510  of the transponder  520  when the transponder antenna is parallel to the X axis (FIG. 1) of one emitter coil of the antenna array for the illustrated embodiment; and (iii) the maximum wake up range  1512  of the transponder  520  when the transponder antenna is perpendicular to the X axis (FIG. 1) of one emitter coil of antenna array  402  and only one coil is energized. The direction of movement of a hypothetical tagged asset moving through the door  410  is also shown by arrow  1508 . Vector  1504  illustrates the maximum useful range achievable by the system  510 , based on the overlap of the maximum wake up range and sensor array range. In the present embodiment, this range is approximately 6 m, although other distances may be used depending on the needs of the user and the specific application. Furthermore, it is contemplated that other sensitivity patterns may be created depending on such considerations. 
     FIG. 16 illustrates the various states of the transponder  520  of an exemplary embodiment of the present invention during operation of the system. As illustrated in FIG. 16, the transponder(s)  520  generally operates in one of seven states, namely: (i) the “stand by” state  1602 ; (ii) the “reset” state  1606 ; (iii) the “respond” state  1610 ; (iv) the “active” state  1614 ; (v) the “inhibit” state  1618 ; (vi) the “renamed” state  1624 ; and (vii) the direction finding mode (DFM) tracking pulse state  1630 . These states and their inter-relationships are described below. It is noted that while discussed in terms of logical states, the operation of the transponder  520  as described herein is achieved using one or computer algorithms running on the microprocessor  1406  of the transponder, as well as the host computer processor or other intelligence associated with the sensor system  510 . Such computer algorithms are ideally stored in the volatile and non-volatile memory devices (i.e., ROM, RAM, and magnetic storage media) incorporated within the transponder and sensor system, although other arrangements and storage schemes may be used. 
     In the stand by state  1602 , electrical current consumption is reduced to minimum levels, and the transponder  520  waits for wake up signal. During this state, the microprocessor  1406  in the transponder is inactive. The transponder enters the reset state  1606  when the wake up signal  532  is received. 
     In the reset state  1606 , the transponder is waiting for a transmission command to be issued by the sensor system  510 . The wake up receiver circuit (FIG. 17) has seen a continuous signal for the minimum predetermined wake up time and activates the microprocessor  1406 . The transponder waits for a “respond”, “retry” or “reset” command before it sends its ID information to the sensor system  510  and enters the respond state  1610 . 
     In the respond state  1610 , the transponder sends its ID to the system  510 . First, in one embodiment, a preamble (tag activity pulse, or TAP) is sent. The TAP informs the sensor system  510  that at least one transponder  520  is responding. A random number generator within the transponder processor  1406 ; selects one of eight time slots for transmission of the ID. The transmission of the actual ID starts at this randomly selected time slot. While waiting for this time slot, the transponder observes the signals emitted by the sensor system  510 ; as soon as a “channel free” signal disappears, the transponder stops to wait for transmission back to the sensor system and enters inhibit state  1618 . 
     The ID message sent from the transponder  520  (i) informs the sensor system  510  the ID code of the tag which responded; (ii)and the status of the alarm bits in the tag. 
     In the inhibit state  1618 , the transponder  520  waits for a “retry” command that offers the next chance to transmit, since the free 434 MHz channel was acquired by another transponder. The retry command returns the transponder to the respond state  1610  where the tag waits for another respond command. 
     A transponder that has sent its ID on the available 434 MHz channel without being inhibited enters the active state  1614 . All commands issued by the sensor system  510  that are appropriate and should only be executed by a single, isolated transponder, are accepted in the active state  1614  only. In the illustrated embodiment, the only such command is the “rename” command (described below), although the use of other such commands is contemplated herein. While in active state  1614 , the transponder  520  also receives an acknowledgement to its ID transmission. This acknowledgement contains, inter alia, a cyclic redundancy code (CRC) of the type well known in the signal processing arts, as well as the number of a time slot it has been allocated to use in direction finding mode (DFM) by the sensor system  510 . The CRC is checked, and if valid, the transponder is commanded to enter the renamed state  1624 . If the CRC is not valid, the transponder enters the inhibit state  1618  and awaits a “retry” command. 
     The Rename command send a message to only the recently responding tag. The tag is identified by using CRC code as a unique identifier. The command instructs the tag to use a predetermined time slot for all successive DFM commands. 
     In the renamed state  1624 , the transponder  520  waits for the DFM initiation command from the sensor system  510  or, alternatively, a “reset” command. If the DFM initiation command is received, the transponder enters the DFM tracking pulse state  1630 . If the “reset” command is received, the transponder is returned to the respond state  1610  as shown in FIG.  16 . 
     In the DFM tracking pulse state  1630 , the transponder first waits for the next DFM tracking command header, the header being used to synchronize all the tags being in DFM to the same time base. The DFM tracking command of the present embodiment comprises generally a DFM header field, tracking opcode (four bits), and a DFM group identification code (also four bits). The header synchronizes the transponder with the sensor system&#39;s bit clock (not shown), and provides a secure indication to the transponder that a new message is commencing. The opcode provides the transponder with instructions for the direction finding mode of operation. The DFM group is used to identify the transponder in the event that a large number of transponders are in use simultaneously. 
     When this next DFM command header is detected, the transponder waits for its DFM time slot (assigned via the “rename” command), and subsequently sends the DFM directional information to the sensor system  510 . The transponder then returns to the renamed state  1624  and awaits further commands form the sensor system  510 . 
     It is noted that the embodiment of the invention illustrated in FIG. 16 also includes an “emergency call” state  1646 . In this state, the transponder  520  issues an emergency message to the sensor system  510  using the 434 MHz channel to alert the sensor system  510  of tampering or other predefined external digital input. The external input causes an interrupt into the tag which starts a predetermined message to be emitted at 434 MHz. This message follows the protocol used by all tags during the respond command. 
     Referring now to FIG. 17, a schematic of one exemplary embodiment of the wake up circuitry  1404  of the transponder  1400  (FIG. 14) will be described. In one embodiment, the transponder  520  receives a signal at the receive antenna  1402  which is propagated to an 8 kHz input section  1710 . A pair of diodes D 1   1703  and D 2   1705 , acting as a symmetrical diode limiter, are added to stop overload of transistor Q 5   1707  of the 8 kHz section  1710  due to the stronger input level of the wake up signal from a Portable Transponder Sensor. The anode of D 2   1705  connects to point  1702  and the cathode connects to ground, while the cathode of D 1   1703  connects to point  1702  and the anode connects to ground. In one embodiment of the transponder, the voltage induced by the wake up signal at point  1702  is about one volt. The anode of a diode D 3   1704  connects to point  1702  and is used to generate a transponder wake up signal at point  1706 . A capacitor C 10   1711  connects between the cathode of D 3  and ground  1715 . In one embodiment, the voltage level of the transponder wake up signal at point  1706  is about 0.3 volts. A resistor R 32   1718 , having a value of 10,000 ohms in one embodiment, which is connected between the receive antenna  1402  and point  1702 , is optional. 
     Elimination of Sensor Rejection by the Transponder 
     As previously described, the loop antenna of the transponder  520  is made from a magnetic coil based primarily on space considerations. The field pattern of the transponder coil antenna has a classical “donut” shape well known in the art. This donut pattern exhibits a three dimensional vector in opposite directions, whereby the transponder antenna can reject the signal from a sensor if the transponder is held in the proper orientation (i.e., such that the sensor field is aligned along the longitudinal axis of the donut). In the situation where the sensor system  510  comprises a “near” sensor and a “far” sensor (such as the “A” and “B” side sensor arrays shown in FIG. 4 ) when the transponder  520  is located on one side of the door  410  or the other, if the transponder is oriented such that the signal from the near sensor array  402  falls within the solid angle of rejection of the transponder loop antenna, the signal from that sensor could be attenuated by up to 30 dB. Since the sensor system determines position based at least in part on the relative strength of the signals received from the sensor arrays  402 , the sensor system  510  may erroneously interpret the location information to indicate that the transponder is closer to the “far” sensor (i.e., the sensor on the opposite side of the door  410 ) rather than the near sensor. 
     To eliminate this type of error, the present invention creates conditions at the antenna of the transponder  520  such that the signal from the near sensor array cannot be rejected. Two techniques are utilized to achieve this result: (i) increasing the number of signals being coupled into the transponder; and (ii) turning off one of the orthogonal phases of the sensor array  402  in order to eliminate the “cancellation” effect. Each of these techniques is described in more detail below. 
     Increasing the number of signals being coupled to the transponder antenna is achieved by generating more than one signal at the sensor array  402 , as previously described with respect to FIGS. 2-4. Each signal generated by the sensor array must have a different polarization. Since there are only three spatial dimensions, a signal polarized to each of these three dimensions (i.e., X, Y and Z) will account for all spatial orientations. As a result of more signals (each of which are on different polarizations in the environment of the transponder), the solid angle where the transponder  520  could possibly reject any one sensor array phase is dramatically reduced. However, as will be recognized by those of skill in the art, the coupling efficiency for any of these signals to the transponder antenna depends also on the orientation of the transponder antenna. Accordingly, additional methods are necessary to ensure that the transponder antenna can not reject the signal from all of the sensor array phases regardless of the transponder&#39;s orientation. 
     The foregoing limitation is addressed by the second technique employed by the present invention; namely, electrically “turning off” one of the three sensor array phases. Specifically, it will be recognized that if the transponder  520  is held in a position in space where the three signals from the nearest sensor array  402  are attenuated, then these signals are necessarily attenuated through a cancellation effect. The cancellation effect is caused by two signals coupling into the transponder coil, and a third signal also coupling in to the transponder coil, but in the opposite direction. Specifically, referring to FIG. 1, the current in a coil is related to the phase of the magnetic field generated by the current in accordance to Lenz&#39;s law. This is also known as the “right hand rule”. Substituting the current in the wire with rotating vectors, then the vector sum of the currents induced into the loop in this example is as follows: when all three signals are turned on, then a current is induced into the coil from each of the three phases. The resultant vector is the vector sum of the three signals. It is possible for the vectors to combine such that the resultant vector is zero. For this to occur, the induced current must be equal for all three signals. If this is so, then when one signal is turned off, the vector sum can never be zero. 
     To eliminate this cancellation effect, one of the three loops of the rejected antenna array  402  is turned off using a conventional switching circuit (not shown) within the sensor system  510 . Advantageously, the choice of the phase to be turned off is not critical, since the absence of field emitted by any one phase will effectively eliminate the cancellation effect at the transponder under any orientation of the latter. Since the rejected array will no longer be rejected (i.e., its signal will no longer be reduced in intensity by the up to 30 dB previously noted), it will be properly recognized by the transponder loop and correlated to a correct angular position by the processing algorithms previously described according to the relationship illustrated in Equation 2 and FIG. 3 herein. 
     Additionally, the present invention employs a unique message transmission protocol between the sensor system  510  and the transponder(s)  520 . Specifically, the sensor system  510  transmits two messages from the antenna coils within its arrays  402 . The first message is transmitted from all three phases of each sensor array  402 , while the second is transmitted from only two of the three phases of each array. In this fashion, both of the messages emitted by the sensor arrays  402  can not be rejected by the transponder  520  regardless of its physical orientation with respect to the arrays, since the previously described “cancellation effect” is eliminated by turning one of the phases off. Stated differently, one of the two DFM messages transmitted by each sensor array  402  necessarily must be received by the transponder. Accordingly, each transponder  520  is kept in effectively constant communication with the sensor system  510 . If the two DFM messages are transmitted in temporal sequence, the transponders  520  are in communication with the sensor system during at least one of the messages, thereby maintaining continuity (albeit having “dead” time corresponding to the rejected message transmission interposed therein). Furthermore, the two messages of the present DFM protocol are identical in content, so even if a transponder rejects one of the two messages, no data or other message content is lost. 
     Referring to FIG. 18, the aforementioned two message DFM method is now described in greater detail in terms of a preferred algorithm  1800  running on the transponder and sensor system processors. Beginning at a start state  1802 , the algorithm  1800  moves to a transmit state  1804  where each of the sensors in the system transmits a first DFM message with all three antenna array phases being on. For example, in a system with an “A side” sensor array and a “B side” sensor array such as illustrated in FIG. 4 herein, both sensor arrays of each sensor pair would transmit the first DFM message. Proceeding next to a decision state  1806 , the algorithm determines if the transponder rejects the signal from one of the sensor arrays. If so, the algorithm continues to another state  1808  and, in one embodiment, turns off one of the three phases at each sensor. In the illustrated embodiment, a phase is turned off by sending a command to the generator circuit  930  (FIG.  9 ), although other ways of turning off one of the phases are contemplated. Proceeding next to a second transmit state  1810 , both of the sensor arrays of each pair transmit a second DFM message with one of the three phases turned off. The phase which is turned off can be any one of the three, as previously discussed. Moving to a decision state  1812 , the algorithm  1800  again determines if the transponder rejects the signal. If so, in one embodiment, there is a system error (based on the fact that both the three-phase and two-phase DFM messages can not physically be rejected by the transponder), and the algorithm  1800  advances to an error processing state  1814  for analysis of the transponder(s)  520  rejecting the second signal. This state compares the DFM position information returned from the tag from the two messages. If the position information is different, the data is discarded and two more DFM messages are sent. However, if a transponder does not reject the second “two-phase” DFM signal, as determined at the decision state  1812 , the algorithm  1800  processes the content of the second DFM message per function  1816 . 
     In one embodiment, the DFM message processing function  1816  is implemented as a truth table. The two responses by each transponder to the two transmissions from the sensors are compared in the following exemplary truth table: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 First Response 
                 Second Response 
                 Conclusion 
               
               
                   
               
             
            
               
                 A side 
                 A side 
                 transponder is in the A side 
               
               
                 A side 
                 B side 
                 inconclusive 
               
               
                 B side 
                 B side 
                 transponder is in the B side 
               
               
                 B side 
                 A side 
                 inconclusive 
               
               
                   
               
            
           
         
       
     
     After the message processing function  1816  completes, the algorithm  1800  advances to a decision state  1818  and determines if the function  1816  yielded valid position data. If so, the algorithm moves to state  1820  to report the position of the transponder to the sensor system  510  via the transponder-to-sensor system communication  538  illustrated in FIG.  5 . The algorithm  1800  then enters an end state  1822 . If, however, it is determined at decision state  1818  that the position data is not valid, the algorithm  1800  moves back to the first transmit state  1804  to begin the process again so as to try to obtain valid position data. 
     It is noted that if it is determined in decision state  1806  that none of the signals are rejected by the transponder, the algorithm  1800  advances to state  1824 . At state  1824 , the strongest signal of the signals received at the transponder antenna is determined, and the algorithm selects the strongest signal as indicative of the sensor that the transponder is closest to in location. Alternatively, the transponder measures the relative signal strengths of those signals received and develops an estimate of the relative angular position as previously described with respect to FIG.  3 . 
     Direction Finding Method 
     FIG. 19 illustrates the direction finding method of the present invention in greater detail. This method  1900  determines the position of the tag or transponder in relation to the multiple system sensor arrays  402  disposed around an access point such as a door  410  (FIG.  4 ). Beginning at a start state  1902 , the method employs a first decision state  1604  to determine if the transponder  520  is in motion. Recall that the transponder contains a motion sensor  1411  which enables the “wake up” of the transponder processor if the transponder is both in motion (over a given time interval) and within an RF field of a predetermined intensity. If the transponder is not in motion or the field not present, the method  1900  is terminated until it is determined that foregoing conditions are met. When these conditions are satisfied, the transponder attempts to synchronize with the commands being issued by the sensor arrays  402  in state  1906 . Eventually, each of the sensor arrays of the sensor system  510  issues a Direction Finding Mode (DFM) command at state  1908 . In one embodiment of the invention, after the DFM command is issued, each of the, sensors emits a plurality (e.g., thirty-two) cycles of an 8130 Hz signal, followed by a lesser number (e.g., nine) cycles of directional information at state  1910 . It will be recognized that the sensor may emit a signal at another frequency or frequencies if desired. During the nine cycles of directional information, the “A” sensor array of the sensor pair transmits the nine cycles with a 180 degree phase shift, while the “B” sensor does not phase shift the 8130 Hz signal. For the purposes of this discussion, a phase shift is accomplished by shifting a given number of cycles of an RF signal with respect to a center or “normal” frequency. Thus, the signals from the “A” and the “B” sensor arrays are phase shifted by 180 degrees from each other during the period of the nine cycles. 
     In state  1912  of the method  1900 , the receiving transponder waits until the aforementioned nine cycle period and compares the received signal from sensor “A” with the received signal from sensor “B”. This comparison yields an indication of which of the two sensor arrays  402  (i.e., either the “A” array or the “B” array) is closer to the transponder. In one embodiment, this comparison is performed by rationing the relative signal strengths of the signals from the two sensors arrays as previously described with respect to FIG.  3 . If the relative phase difference is 80 degrees or less, the transponder believes it is closer to the “B” sensor field; that is, on the “B” side of the door  410  of FIG.  4 . However, if it is determined at the decision state  1914  that the phase difference is greater than 100 degrees, then the transponder believes it is on the “A” side of the door. The transponder reports its position to the sensors system  510  accordingly in state  1916  (for the “B” side of the door  410 ) or state  1918  (for the “A” side of the door  410 ), and the process completes at an end state  1920 . 
     It will be recognized that while phase shifts of 80 and 100 degrees are used as decision criteria in the present embodiment, other decision criteria and in fact other approaches may be used to determine to which of the sensor arrays the transponder  520  is closer. 
     Dormant RFID Transponder Communication System 
     Referring now to FIG. 20, a system and method for communicating with a dormant RFID transponder is now described. In the embodiment of FIG. 20, the dormant transponder communication system  2000  comprises a portable transponder sensor (PTS)  2002  which is advantageously attached to a laptop or other portable computer  2004 . It will be recognized that other electronic computing devices, such as palmtop organizers, calculators, or even non-portable devices may be substituted for the illustrated laptop computer  2004  if desired. The system is used to associate the tagged asset to the transponder  520 , and once associated, the transponder  520  can be checked on a routine or maintenance schedule without having to activate the motion detector  1411  or other sensor within the transponder. 
     The system  2000  of the illustrated embodiment is capable of bi-directional communication with one or more transponders  520 , and allows the user to perform maintenance and inventory control functions associated therewith. The PTS  2002  includes a processor (not shown), an on-board phase shift keying (PSK) signal generator  2001 , and a 434 MHz receiver section  2003 , for generating commands and decoding F/2F data, such as non-return-to-zero coded data, which is received from the transponder(s)  520 . The processor may alternatively or simultaneously comprise the host processor of the computer  2004 . All of the transmit and receive antennas  2012 ,  2014  are mounted within the PTS  2002  for compactness and ease of use. The PTS  2002  is capable of generating a magnetic field of great enough intensity to allow for the dormant transponder  520  (such as a transponder having a motion sensor which is not in motion) to wake up when the PTS  2002  is placed in relative proximity to the transponder  520 . In the illustrated embodiment, the PTS  2002  generates a field with intensity on the order of one Gauss, although other field intensities may be used. 
     The acquisition portion  2017  of the PTS  520  receives commands, and downloads data to the laptop computer  2004  via a standard RS-232 serial data link of the type well know in the data processing arts, although others (such as USB, wireless, or infrared/optical coupling, for example) may be readily used. The laptop  2004  of the illustrated embodiment has a Windows®-based interface with appropriate screens and menu structures that direct or allow the operator to perform various desired functions relating to the transponder  520  or PTS  2002 . 
     The PTS  2002  of the embodiment of FIG. 20 comprises generally a hand held wand having an LCD screen  2019  (with back lighting) capable of displaying information relating to operation of the system  2000  and the transponder  2002 . The PTS  2002  further comprises an input device  2010  such as a series of keys or pushbuttons on its outer surface which permit the operator to accomplish a variety of data input and preprogrammed functions. A variable menu structure is also optionally used, whereby individual keys of the input device  2010  may be used to perform multiple functions. It will be recognized that while the illustrated embodiment uses a key/menu arrangement and LCD screen for information display and input, other configurations such as a touch-sensitive screen (with or without stylus), cathode ray tube, or thin film transformer (TFT) or plasma display may be used with equal success. 
     The PTS  2002  of FIG. 20 is further capable of changing the modes or states of the transponder  520  with which it is in communication, such as from “normal” to “transport” mode or vice versa. Normal mode is a state whereby the tag behaves as previously described. Note that the tag must be moved so that the accelerometer will wake up the microprocessor. However, this state is wasteful of battery life if the tag has not yet been installed, or is in transport and is constantly being shaken. “Transport mode” is a state where the tag does not use the signal from the accelerometer and so the tag enjoys a longer battery shelf life. 
     In another aspect of the invention, the PTS  2002  is able to receive tamper alert messages from the transponder  520  in an unsynchronized fashion; these alert messages are generated within the transponder by the processor  1406  when the transponder is tampered with. Specifically, the tamper detector  1410  provides a signal to the processor  1406  when the detector is activated (such as by someone trying to remove the transponder from the asset). These signals may be stored by the transponder for later retrieval by the PTS  2002 , or directly converted to a radio frequency message emitted by the transponder and received by the PTS when the transponder is tampered. Information received by the PTS  2002  further includes, inter alia, the ID of the tampered with transponder. 
     While the above detailed description has shown, described, and pointed out fundamental novel features of the invention as applied to various embodiments, it will be understood that various omissions, substitutions, permutations, and changes in the form and details of the apparatus and methods illustrated may be made by those skilled in the art without departing from the spirit of the invention. The foregoing description is not in any way meant to limit the scope of the invention; rather such scope is determined by the claims appended hereto.