Abstract:
A system and method are described for using RFID tags to track and monitor personnel and equipment in large environments and environments that are prone to multipath fading. The system scans the environment by selecting local interrogation zones where RFID tags may be located. Multiple antennae are used, each transmitting a portion of an activation signal, such that the activation signal will be formed in the selected local interrogation zone. Different subsets of the antennae are successively selected, each targeting the selected local interrogation zone, to repeat the activation signal for each subset of antenna. RFID tags in the local interrogation zone will receive the portions of the activation signals and process them to determine whether the full activation signal was destined for that local interrogation zone for each of the subsets of antennae. An activated RFID tag will transmit its tag information, including any data collected from sensors connected to the tag, back to the system. The systems and method will use the location information of the various RFID tags in the global environment and combine that with data received through cameras and other sensors to provide a display with the RFID tag location information superimposed. The data collected about various regions of the environment may be transmitted back to the RFID tags to provide the personnel with information about their surroundings.

Description:
This application claims the benefit under 35 USC 119(e) of U.S. Provisional Patent Application Ser. No. 61/112,950 filed Nov. 10, 2008 incorporated herein, in its entirety, by this reference to it. 
    
    
     FIELD 
     The described embodiments relate to systems for the tracking of objects and personnel using radio-frequency identification (RFID) devices and video cameras. 
     BACKGROUND 
     Objects within a localized area may be tracked using RFID devices that contain an antenna and a transmitter. These devices are commonly referred to as RFID tags or transponders. Typically, RFID tags remain in a passive state until a reader transmits a signal to the tag. Upon activation, the RFID tag will broadcast its response message. 
     Typical RFID-based location systems utilize signal strength and range and/or time difference of arrival of the signal at multiple antennae. However, these systems are prone to a number of problems. For example, relying on signal strength can be difficult as the signal strength can fluctuate due to multipath fading and other effects. It is labor intensive and time consuming to construct RF maps to account for these multipath effects, and the RF maps may need to be altered if the environment changes. Time difference of arrival systems are also prone to multipath errors and work better in line-of-sight environments. 
     Where there are a large number of tags to be identified, any of which may be transmitting at the same time, interference between transmitting tags also poses another problem. With a number of tags transmitting it takes longer to resolve the signal from each of the individual tags. 
     The use of time difference of arrival systems also requires tight synchronization among all of the antennae. Furthermore, the accuracy of these systems is limited by the accuracy of the measured distance between the antennae on which the time difference of arrival calculation relies on. Reliance on measured distances typically limits the application of these systems to a deployment with static antenna locations. Due to this limitation these systems could not be deployed in an ad hoc manner. 
     The multipath fading effects and line-of-sight requirements provide limitations on the type of environments in which these traditional systems may be deployed. While these systems may prove satisfactory in a warehouse, they are unable to function in environments that contain a number of surfaces that may reflect an RF signal. For example, the current RFID location systems would have great difficulty locating RFID tags contained within a building from a location outside of that building. 
     Overcoming the above problems of locating RFID tags opens up a number of new applications. When RFID tags can be located over a greater area and throughout building structures there is difficulty in conveying the RFID tag coordinates in a meaningful way to an end-user of the system. While the tags position can be precisely located relative to the antennae, in situations where there is no time to measure distances relative to the antennae some sort of visual indication is required so that an RFID tag can be located relative to a building structure or other elements in the environment. 
     SUMMARY 
     Accordingly, in one aspect of the invention, the problems of multipath fading and interference in locating an RFID tag are limited by activating a tag in a local interrogation zone using a number of subsets of the available antennae. 
     In another aspect of the invention, the RFID tag is configured to receive portions of the activation signal from a series of subsets of the available antennae. 
     In another aspect of the invention, the problem of deploying an ad hoc RFID location system is addressed by coupling a locating device with each of the antenna to determine the antenna&#39;s relative position to the other antenna. 
     In yet another aspect of the invention, the problem conveying a visual indication to an end-user of the RFID tag location is addressed by displaying a scaled video image with the RFID tag data superimposed thereon to the end-user. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A preferred embodiment of the present invention will now be described in detail with reference to the drawings, in which: 
         FIG. 1A  is a side view showing firefighters located throughout a structure that are tracked according to an embodiment of the invention; 
         FIG. 1B  is a block diagram showing one possible layout of detection antennae according to an embodiment of the invention; 
         FIG. 1C  is a side view of a vehicle equipped with a mobile platform according to another embodiment of the invention; 
         FIG. 1D  is a front view of the vehicle shown in  FIG. 1C , according to an embodiment of the invention; 
         FIG. 2  is a side view showing a display integrating multiple location data according to an embodiment of the invention; 
         FIG. 3  is a block diagram of the tracking system according to an embodiment of the invention; 
         FIG. 4  is a display view showing data that may be integrated in a single display according to an embodiment of the invention; 
         FIG. 5  is a diagram showing signal propagation characteristics according to an embodiment of the invention; 
         FIG. 6  is a diagram showing signal propagation characteristics over time according to the embodiment of the invention shown in  FIG. 5 . 
         FIG. 7  is a block diagram of a tag activation decoder in accordance with an embodiment of the invention; 
         FIG. 8  is a diagram showing an antenna layout according to another embodiment of the invention; 
         FIG. 9  is a block diagram of a tag activation decoder in accordance with another embodiment of the invention; 
         FIG. 10  is a block diagram of a tag activation decoder in accordance with another embodiment of the invention; 
         FIG. 11  is a diagram showing the signals transmitted by four antennae and received at a tag according to an embodiment of the invention; 
         FIG. 12  is a block diagram of a personnel module according to an embodiment of the invention; and 
         FIG. 13  is a perspective view of a mobile platform according to an alternate embodiment of the invention. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Reference is first made to  FIG. 1A , wherein a side view is shown of firefighters located throughout a structure being tracked according to an embodiment of the invention. An emergency fire scene is used as an example of an environment in which an embodiment of the invention may be deployed. Embodiments of the invention may be used that are applicable to different environments such as tracking animal livestock, airport security, or police and military environments. 
     RFID tracking system  100  is shown consisting of a plurality antennae. Antenna  1  may be mounted on a vehicle  3  or may also be a mobile antenna platform  6 . Mobile antenna platform  6  may be mounted on a standalone device, such as a tripod. Any of the antennae may further be equipped with a camera  2 . Camera  2  may be a video camera or a still image camera and may be capable of capturing both visible light and infrared images and video. Additional detection antennae  6  may be provided elsewhere, for example as a standalone detection antenna  106  or mounted on a first responder vehicle  108 . 
     In order to track personnel  4  they may be equipped with a personnel module  17  containing an RFID tag. Optionally, RFID tags may also be attached to equipment. Tracked personnel  4  may be located outside and throughout a building. Tracked personnel  4  may be firefighters, police, emergency medical personnel, soldiers or other personnel. The personnel module  17  may be programmed to contain data relevant to the wearer of the device such as a unique identification code, their name, title or any other information relevant to the personnel. Additionally, the personnel module  17  may be connected to a number of sensors to monitor environment variables, vital signs of the personnel, battery level, or any other data that may be relevant depending on the context. 
     Referring now to  FIG. 1B , there is shown a block diagram depicting one possible layout of detection antennae surrounding a global interrogation zone  90  according to an embodiment of the invention. Ideally, the antennae  6  should be spatially separated in a number of dimensions to provide the most accurate detection. To track personnel  4 , RFID subsystem  54  selects a local interrogation zone  55  within the global interrogation zone  90 . The size of the local interrogation  55  zone may be varied in size and may be selected anywhere within the global interrogation zone. The RFID subsystem  54  then chooses a series of subsets of the antennae to transmit an activation signal to RFID tags within the local interrogation zone  55 . The RFID subsytem then assigns a portion of the tag activation signal for each of the spatially separated antennae  6  in the subset. Antennae  6  transmit their respective signals, which are received by personnel modules  17  within the local interrogation zone. The personnel module determines whether it has received the proper tag activation signal from the proper number of subsets of the antennae. 
     The activated personnel module  17  then transmits a data signal in response to the activation signal. The data signal is preferably received by detection antennae  6  that then communicate the contents of the received data signals to RFID subsystem  54  through communication links  56 . Communication links  56  may be wired or wireless. The RFID subsystem  54  stores the coordinates of the local interrogation zone and the information from the received data signal. 
     Referring now to  FIGS. 1C and 1D , there are shown a side and front view of vehicle  3  equipped with a mobile antenna platform  71  according to another embodiment of the invention. Mobile platform  71  may support a plurality of antennae  1 , a camera  7 , an antenna location device  9 , and a GPS receiver  11 . Preferably, the plurality of antennae  1  supported by mobile platform  71  are spatially separated as exemplified in  FIG. 1D . 
     Referring now to  FIG. 2 , there is shown an interface terminal  5  of RFID tracking system  100 . RFID tracking system  100  preferably processes data received from personnel modules  17  and images and video received from cameras  2  to provide an integrated display  15 . Integrated display  15  may be configured to show external video, infrared video and sensor data received from personnel modules  17  to form a synthesized, real-time video view of the global interrogation zone. Alternatively, the display  15  may be updated periodically with still images and data. Accordingly, RFID tracking system  100  preferably provides context for location data received from personnel modules  17  by displaying an integrated three-dimensional view of the global interrogation zone. For example, a symbol for a first personnel module  17  located on the outside of a building may be displayed in a first color, while a symbol for a second personnel module  17  located inside a building may be displayed in a second color to indicate a different range. The symbols may be overlaid onto video of the global interrogation zone. Alternately, distinguishable symbols may be used to indicate relative range or occlusion. As another alternative, symbols may be scaled to indicate range. 
     To provide context for the integrated three-dimensional view, mobile antenna platforms  6  are preferably equipped with radar or laser ranging devices (not shown). Accordingly, the ranging devices scan the global interrogation zone and provide topographical data to RFID tracking system  100  for the purposes of video scaling and location plotting in a composite display. 
     Additional data channels may be provided to integrate relevant data into the display  15 . Such data may include information regarding high-temperature “traps” within a building, locations of hazardous materials, heat sensor data, smoke detector data and radiation sensor data. High-temperature traps, such as an area situated under a source of fire, may be identified through the use of infrared cameras. Accordingly, first responders may use this information to plan ingress and egress from a building in an emergency situation. 
     Referring now to  FIG. 3 , there is shown a block diagram of a display subsystem  200  of RFID tracking system  100  according to an embodiment of the invention. RFID subsystem  54  preferably transmits activation signals via antennae  6 , which may cause personnel modules  17  to transmit data signals. As exemplified, display subsystem  200  receives data signals with unique identifying information transmitted by personnel modules  17  via detection antennae  6  at RFID subsystem  54 . Received data is supplied to processor  13  by RFID subsystem  54 . Visible light and infrared video may be received from camera  7  at an image processor  8  and also at RFID subsystem  54  where it may be further supplied to processor  13 . An antenna location device  9  preferably performs radar or laser ranging to provide global interrogation zone topographical data to image processor  8 . Image processor  8  processes the video using topographical data received from antenna location device  9  to determine the correct image scaling parameters. A multi-sensor receiver  10  may be provided to receive additional information, such as information regarding high-temperature “traps” within a building, locations of hazardous materials, heat sensor data, smoke detector data and radiation sensor data. Data received by multi-sensor receiver  10  is provided to processor  13 . A GPS receiver  11  preferably provides location data to processor  13  that may be used to determine the coordinates of a local interrogation zone. Processor  13  operates upon the data received from multiple sources to integrate location data, personnel information and sensor data into a synthesized, composite display. Data may be stored in a database  14  or provided to a monitor  15  for display to a user. 
     As exemplified, RFID subsystem  54  calculates a local interrogation zone within a global interrogation zone and calculates a corresponding full activation signal and a portion of the full signal to be transmitted by each of a subset of antennae  6 , which are connected via a communications link. Antennae  6  may be directional or omni-directional. At least three antennae  6  are required to provide two-dimensional location data with reliable accuracy and recognition probability. At least four or more antennae  6  are required to provide three-dimensional location data with reliable accuracy and recognition probability. 
     The full activation signal preferably consists of a plurality of pulses, wherein the signal type may be selected from the group consisting of wide-band impulses, an RF carrier with amplitude shift keyed pulses, an RF carrier with amplitude and frequency shift keyed pulses, an RF carrier with phase shift keyed pulses and an RF carrier with a quasi-random signal envelope. 
     As indicated above, RFID subsystem  54  preferably receives data signals transmitted from individual personnel modules  17 , which may be attached to personnel or other objects in a global interrogation zone. Preferably, RFID subsystem  54  processes the received data signals to perform personnel module  17  recognition, calculate personnel module  17  location in the global interrogation zone, and determine ingress and egress times for specific personnel modules  17 . 
     To facilitate processing by RFID subsystem  54 , personnel modules  17  are preferably equipped to transmit a radio-frequency signal with a unique identifying code specific to each personnel module  17 . Personnel modules  17  may also be attached to other objects, such as equipment, to enable said objects to be tracked independently of personnel. In addition to a unique identifying code, personnel modules  17  may transmit other data, such as a personnel or object description. In some embodiments, data processed by RFID subsystem  54  may be transmitted to specific personnel modules  17 , for example to members of a rescue team. Information stored in database  14  may also be transmitted to the personal modules  17  through the RFID subsystem  54  in order to give direction to personnel, for example to avoid high temperature areas or direct personnel to an alternate egress route. 
     Referring now to  FIG. 4 , there is shown a display view showing data that may be integrated in a single display according to an embodiment of the invention. In raw image  18 , there is shown unscaled video received at camera  7 . The image exhibits skew due to the relative positioning of camera  7 . Using data from antenna location device  9 , image processor  8  may scale and deskew the video. Data from received from the multi-sensor receiver may also be used to adjust the image. In processed image  19 , there is shown a deskewed video image. Preferably processor  13  receives location data from RFID subsystem  54 , shown as personnel data image  20  and, together with processed image  19 , synthesizes a composite image  21 . 
     In a radio-frequency system there is some degree of interference from, for example, multipath signal propagation. Accordingly, to reliably activate personnel modules  17  using an activation signal, it is necessary to consider the effect of multipath interference. Referring now to  FIG. 5 , there is shown a diagram illustrating signal propagation characteristics according to an embodiment of the invention. Omni-directional antennae  27  and  28  transmit a signal in all directions. The signal transmitted from antenna  27  has a direct path  22  and a reflected path  24  to personnel module  25 . Reflected path  24  is reflected from object  76 . The signal transmitted from antenna  28  has a direct path  23  to personnel module  25 . It will be appreciated that the signal transmitted from antenna  28  may also be reflected from object  76  and other objects, however for the purposes of illustration this is not shown. 
     Radio-frequency waves propagating along signal path  22  travel the shortest distance. Radio-frequency waves propagating along signal path  23  travel the same distance plus an additional distance  26 . Additional distance  26  is equal to the difference between the distance of antenna  28  from personnel module  25  and the distance of antenna  27  from personnel module  25 . In the event of simultaneous transmissions from antennae  27  and  28 , the signal from antenna  27  will arrive at personnel module  25  first and the signal from antenna  28  will arrive at personnel module  25  delayed—relative to the signal from antenna  27 —by the time it takes to propagate additional distance  26 . Propagation along reflected path  24  may arrive at personnel module  25  at any time after the arrival of the signal along path  22 . The time taken for the reflected signal to arrive will vary according to the relative distance of object  76  from antenna  27  and personnel module  25 . 
     In the following example the activation signal may be defined as a pulse followed by another pulse exactly T* seconds later. In order for signals from antennae  27  and  28  to arrive sequentially at personnel module  25 , it is necessary for antenna  28  to compensate for additional distance  26  by broadcasting earlier. The time delta for the broadcast is selected so that signals from antennae  27  and  28  arrive sequentially, such as in this example, separated by time T*, in which case, the time delta is subtracted from the predetermined period of the sequential signal. 
     Referring now to  FIG. 6 , there is shown a diagram illustrating signal propagation characteristics over time according to the embodiment of the invention shown in  FIG. 5 . 
     A signal, comprising pulse  29 , is transmitted from antenna  27  at time t 0 . At time t 1 , a pulse  30  is transmitted from antenna  28 . Time t 1  is equal to T*−t prop , where t prop  is the additional propagation time  77  it takes a signal to propagate along additional distance  26 . Accordingly, pulse  30  arrives at personnel module  25  T* seconds after pulse  29 . A reflected pulse  31 , a reflection of pulse  30 , arrives after pulse  30  because of the additional propagation time. Reflected pulse  31  creates a problem in activating a specific local interrogation zone as there may be another location within the global interrogation zone where reflected pulse  31  is received exactly T* after another signal pulse thereby matching the defined activation signal. 
     Referring now to  FIG. 7 , there is shown a block diagram of a tag activation decoder circuit  85  that may be prone to errors from multipath effects. A personnel module  25  may be equipped with the tag activation decoder circuit  85  comprising an input line  80  from an antenna and receiver circuit. The signal received from input line  80  and passed to comparison circuit  50  and delay block  49 . As exemplified, delay block  49  is be configured to delay the input signal by T* seconds before outputting to comparison circuit  50 . Accordingly, if tag activation decoder circuit  85  receives a pulse signal with a period of T* at input line  80 , comparison circuit  50  will calculate a positive comparison and activate output line  84 . Accordingly, reflected pulse  31  will not interfere with direct pulses  29  and  30  unless it has exactly T* propagation delay. The comparison circuit  50  may be a Logic AND circuit or the personnel module  25  may employ digital signal processing logic to compare the signals and may not require delay block  49 . 
     The tag activation decoder circuit  85  shown in  FIG. 7  may be prone to erroneous activation from interfering signals or noise. In order to minimize the effects of multipath signals and noise the number of antennae may be increased where multiple subsets of the antennae are used to activate the personnel module  25 . By using multiple subsets of spatially separated antennae the effect of any single object reflecting signals is varied in each of the subsets due to the varying distances to the object from each antennae. Accordingly, the signal processing may be organized as follows:
         (a) defining local interrogation zone coordinates according to the relative location of antennae;   (b) choosing a first set of two antennae;   (c) defining a full activation signal;   (d) calculating the subset activation signal and transmit times for each of the antennae in the subset (i.e., the portion of the subset activation signal assigned to each antenna);   (e) transmitting the subset activation signals into the local interrogation zone;   (f) evaluating signals received by personnel modules:
           (i) if the signal consists of repeating pulses with period T*, recording a successful reception;   
           (g) calculating a full activation signal for a next combination of antennae; and repeating steps (d) through (f) until all combinations of antennae are used;   (h) comparing the number of successful time interval comparisons with the number of antennae combinations:
           (i) if the comparison is successful, transmitting a data signal from the personnel module; and   
           (j) defining next local interrogation zone coordinates and repeating the activation procedure with different groups of antennae.       

     It will be appreciated that the number of antennae in a set is not limited to two. Alternately, any number of antennae may be used in a set, subject to the limitation of the number of available antennae. 
     The maximum possible combinations of “k” antennae, chosen from a set of “n” antennae is given by: 
     
       
         
           
             
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     For example, if antennae A, B and C are grouped into subsets of 2 (k=2) and there are 3 antennae available (n=3), then there will be a total of 3 possible combinations of antennae (i.e., A-B, B-C and A-C). 
     Referring now to  FIG. 8  there is shown a diagram with an antenna layout according to another embodiment of the invention. There are shown 5 antennae (n=5). The subset of antennae can contain up to 4 antennae (k=4). As another example, there are 10 possible combinations of 2 antennae subsets that may be selected from the group. 
     Referring now to  FIG. 9 , there is shown a block diagram of a tag activation decoder circuit  57  in accordance with another embodiment of the invention. A signal is received via antenna  53  and may be amplified by amplifier  47  and demodulated in signal envelope detector  48  before being input to tag activation decoder circuit  57 . The input is passed to comparison circuit  50  and delay block  49 . Delay block  49  is configured to delay the input signal by T* seconds before outputting to comparison circuit  50 . Accordingly, if tag activation decoder circuit  57  receives a pulse signal with a period of T* at the input, comparison circuit  50  will calculate a positive comparison and increments counter  51 . If counter  51  reaches a predetermined count within a predetermined time frame, an output signal  34  is activated, triggering transmission of a data signal. The predetermined count does not necessarily have to equal the number of transmitting subsets of antennae. The predetermined count may be a safety threshold to avoid erroneous activation. The counter may be reset prior to sending the activation signal by transmitting a special reset signal from the RFID subsystem, or the counter may simply be reset if the counter is not incremented in a predetermined time. 
     If tag activation decoder circuit  57  is outside of the local interrogation zone and receives a multipath signal that is T* periodic with another portion of the activation signal, the counter  51  will prevent this multipath signal from activating the tag. 
     Referring now to  FIG. 10 , there is shown a block diagram of a tag activation decoder circuit  95  in accordance with another embodiment of the invention. A signal is received via antenna  53  and may be amplified by amplifier  47  and demodulated in signal envelope detector  48  before being input to tag activation decoder circuit  57 . The input is passed to comparison circuit  50  and delay block  49 . Delay block  49  is configured to delay the input signal by T* seconds before outputting to comparison circuit  50  and another delay block  49 , up to a predetermined number of delay blocks corresponding to the anticipated number of repeated subset activation signals to be received. Accordingly, if tag activation decoder circuit  57  receives a repeated pulse signal with a period of T* at the input, comparison circuit  50  will only calculate a positive comparison and activate counter  51  after n successive pulses have been received, corresponding to n−1 delay blocks. n is preferably equal to the number of antennae in each combination subset to be the most robust but fewer delay blocks may be used. If counter  51  reaches a predetermined count within a predetermined time frame, an output signal  34  is activated, triggering transmission of a data signal. 
     In some embodiments the tag activation decoder may be implemented by a digital signal processor that will delay, compare, and count the incoming signals similar to the components described above. 
     Referring now to  FIG. 11 , there is shown a diagram illustrating the signals transmitted by four antennae and received at a tag activation decoder circuit according to an embodiment of the invention. Signals  38 - 41  are ASK activation signal envelopes from each of four antennae. The superposition of these signals is shown without time delay at  42 , delayed by T* at  43 , delayed by 2T* at  44 , and delayed by 3T* at  45 . Pulse  34  indicates signal-in-time coincidence, corresponding to a successful comparison at comparison block  50 . 
     Referring now to  FIG. 12 , there is shown a block diagram of a personnel module  17  according to an embodiment of the invention. Personnel module  17  is preferably comprised of an RFID tag and a data module  79 . RFID tag is preferably provided with an antenna  53  coupled to an amplifier  47 . An amplified signal is demodulated by the demodulator  48  before being passed to tag activation decoder circuit  57  to determine if the received activation signal is intended for the current RFID tag. Tag activation decoder circuit  57  communicates with a tag controller  58 . Tag controller  58  is in communication with personnel monitoring system  59  and transmitter  60 . Personnel monitoring system  59  may be configured to receive and display data from the RFID subsystem. The data may include location information of the personnel module  17 , nearby colleagues, directions towards egress routes, hazardous areas to avoid, and communication messages to the personnel. 
     Tag controller  58  is preferably operatively connected to data module  79 , which may have auxiliary devices such as: a distress signal initiation device  63 , for example, a button to initiate a distress signal; a movement sensor  64  to detect when a wearer of personnel module  17  is inactive or incapacitated; a gyroscope  65 ; a heat sensor  66 ; a vital signs sensor  67  to monitor wearer health; an air quality sensor  68  to monitor environmental conditions for toxicity or insufficient oxygen levels; a battery level sensor  69  to warn if personnel module  17  has low remaining battery reserves; and extra channels for other useful auxiliary devices, for example, a metal detector. Data retrieved from auxiliary devices may form a dataset that is preferably stored in a memory (not shown). 
     A power supply  61  is provided to power transmissions and processing performed by personnel module  17 . 
     Personnel module  17  is preferably configured to receive a tag activation signal; recognize if the tag should be activated or not and transmit a data signal containing identifying information provided by data module  79 , such as environmental air quality data, heat sensor data, vital signs data; alerts, escape routes for a wearer of personnel module  17  and for team members. Personnel module  17  may also provide level information for batteries, oxygen and other supplies, and initiate a distress signal. 
     In one embodiment, personnel module  17  may be coupled to a firefighter helmet or a handheld device to provide location data, map obstacles and dangerous areas, indicate low battery or oxygen levels. A power initializer  62  may be provided to engage the power supply  61  of personnel module  17  before entering into a dangerous area. A gyroscope  65  or accelerometer may be used to detect whether the personnel is in movement or in a horizontal or vertical position. 
       FIG. 13  is a perspective view of a mobile antenna platform according to an alternate embodiment of the invention. In some embodiments, the mobile antenna platform  71  may be permanently mounted on a vehicle  3 . In an alternative embodiment, mobile platform  71  may be comprised of a portable platform having a lightweight support structure  74 , for example, a collapsible tripod wherein the antenna platform is height adjustable. In addition, the mobile antenna platform may be equipped with a locating device  73 , a data-power link  75  and a telecommunications link  72 . 
     The locating device  73 , depending on the environment and the required accuracy, may be implemented using GPS, radar, laser, sonar, or other optical devices, for example strobe lights and sensors. Other embodiments that are not required to be deployed quickly may not have a locating device and will rely on an operator to measure and enter coordinates manually. 
     In operation, greater spatial separation of detection antennae  6  may improve accuracy and reliability of personnel module location. Preferably, antennae are spaced around a local interrogation zone to improve performance. 
     The present invention has been described here by way of example only. Various modifications and variations may be made to these exemplary embodiments without departing from the spirit and scope of the invention, which is limited only by the appended claims.