Abstract:
A position location system adapted for Military Operations in Urbanized Terrain (MOUT) training exercises that equips a player with a magnetic sensor for detecting a low frequency (LF) magnetic field from a plurality of stationary antennas. The distance between the player-borne sensor and a stationary antenna is proportional to the logarithm of the magnetic field intensity because the player remains within the “near field” of the stationary antenna. With scheduled transmissions from six stationary antennas, the position and angular orientation of a player bearing a three-axis magnetic field sensor may be resolved in three-dimensions to within one foot (30 cm). The LF magnetic field intensity is generally unaffected by structural obstructions or multipath distortion. The stationary antennas and transmitters may be permanently installed at little expense in any urban structure so that sensor-equipped emergency workers may be located and tracked within the structure during emergency fire and rescue operations.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a personnel position location and tracking system and more particularly to an in-structure three-dimensional high-accuracy position monitoring system employing low frequency radio waves. 
     2. Description of the Related Art 
     The United States Army places high importance on training for urban warfare such as Military Operations in Urbanized Terrain (MOUT). Interest in training technology for MOUT has matured over the past decade because of the accumulation of experiences in Somalia, Serbia and Afghanistan. Based on evaluations by the United States Army and other military forces, specifications were promulgated for MOUT combat training systems that include means for locating and tracking trainees (“players”) inside buildings and tunnels during simulated MOUT combat exercises. With experience, it was found that location accuracy to within one meter and tracking updating within one second satisfied the MOUT training requirements both inside and outside of structures. 
     The Global Positioning System (GPS) is a widely-used and very useful system for position location and tracking but the GPS relies on high-frequency radio signals from orbiting satellites that cannot penetrate structures generally. Moreover, the five meter location accuracy of the GPS is not entirely sufficient for MOUT training purposes. Alternative position tracking technologies known in the art are also generally unable to deliver the performance features required for MOUT training exercises inside structures such as rooms, tunnels and bunkers. These include ultrasonic echo-location, inertial navigation systems (INS), position sensor grids, radio frequency (VHF/UF) echo-location, and passive/active infrared (IR) detection. 
     Generally, these prior art systems monitor the location of a trainee or “player” by measuring the time-of-arrival (TOA) of energy transferred between the player and a plurality of synchronized emitters in the room. The player position is computed by simple trilateration using the TOA data, the propagation velocity of the energy, and the known emitter locations. Moreover, each emitter must be uniquely identified by some signal characteristic. For example, ultrasonic energy propagates at about one foot per millisecond through air at sea level and radio frequency (RF) energy propagates through the same medium at about one foot per nanosecond. Because TOA measurements made in milliseconds are inherently simpler and more precise than those made in nanoseconds, ultrasonic trilateration is simpler and cheaper than RF trilateration, for example. Of course, these prior art systems may also monitor the TOA of energy emitted by the player at a plurality of sensors stationed about the room to similar effect, relying on the reciprocity principle. 
     Another approach known in the art is to instrument the training facility or “room” with a grid of uniquely-coded sensors spaced appropriately for the required positioning precision. Player position is monitored directly by signaling with the sensor most proximate the player. Energy broadcasts, mechanical pressure, local capacitance or any other well-known and useful method may be used to trigger the proximate sensor. Disadvantageously, such a system requires the pre-installation of a large plurality of sensors (versus a few for the TOA approach) and the accurate resolution of player positions in three dimensions may impose excessive complexity on the system. 
     Using such systems requires regular recomputation of the player position. This may occur at the player or at the sensor/emitter stations. Ideally, computation load is places at the sensor/emitter stations to minimize the electronic power consumption aboard the trainee player. Substantial power and signal wiring may be required to interconnect all sensor/emitter stations and any related processing systems. 
     The INS is well-known for aircraft and missile guidance systems. The typical INS employs a gyroscope and accelerometers oriented to detect acceleration in three dimensions. Position translation may be computed by integrating the accelerations over time. Drift of INS position may be reset using the GPS when available but otherwise, position error from drift is a major disadvantage of the INS. 
     Table 1 compares the performance features of these prior art systems in conditions expected during MOUT training exercises: 
     
       
         
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Prior Art Position Monitoring Technology 
               
             
          
           
               
                 Performance Feature 
                 Ultrasound 
                 Sensor Grid 
                 VHF 
                 UHF 
                 Infrared 
                 Inertial 
               
               
                   
               
               
                 Freq (MHz) 
                 0.05 
                 N/A 
                 900 
                 30,000 
                 10 10    
                 N/A 
               
               
                 Wavelength (cm) 
                 0.7 
                 N/A 
                  33 
                    1 
                  0.0001 
                 N/A 
               
               
                 Accuracy 
                 Excellent 
                 Depends 
                 Good 
                 Good 
                 Good 
                 Fair 
               
               
                 Stability 
                 Fair 
                 Excellent 
                 Excellent 
                 Excellent 
                 Excellent 
                 Poor 
               
               
                 Measures Orientation? 
                 No 
                 No 
                 No 
                 No 
                 No 
                 Depends 
               
               
                 Position Resolved at? 
                 Either 
                 Building 
                 Either 
                 Either 
                 Either 
                 Player 
               
               
                 Multipath Resistance 
                 Excellent 
                 Robust 
                 Poor 
                 Fair 
                 Good 
                 Robust 
               
               
                 Room Ambiguity Resistance 
                 Poor 
                 Robust 
                 Poor 
                 Fair 
                 Good 
                 Good 
               
               
                 Gunfire/Noise Resistance 
                 Poor 
                 Depends 
                 Excellent 
                 Excellent 
                 Excellent 
                 Fair 
               
               
                 Smoke/Fog Resistance 
                 Good 
                 Depends 
                 Excellent 
                 Excellent 
                 Poor 
                 Excellent 
               
               
                 Resistance to Obstructions 
                 Fair 
                 Depends 
                 Fair 
                 Poor 
                 Poor 
                 Excellent 
               
               
                 Thermal Imager Compatibility 
                 Excellent 
                 Depends 
                 Excellent 
                 Excellent 
                 Poor 
                 Excellent 
               
               
                 Live Fire Damage Resistance 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Poor 
                 Excellent 
               
               
                 Player Unit Complexity/Cost 
                 Low 
                 Low 
                 High 
                 High 
                 Moderate 
                 Very High 
               
               
                 Building Site Complexity/Cost 
                 High 
                 Very High 
                 High 
                 High 
                 Moderate 
                 Low 
               
               
                   
               
             
          
         
       
     
     Close examination of Table 1 demonstrates that none of the prior art technologies offers the performance features necessary for MOUT training exercises with reasonable complexity and cost. For example, the ultrasound techniques known in the art are vulnerable to inaccuracies arising from multipath interference, building obstructions and weapons noise and do not detect orientation in three-dimensions. The UHF and VHF systems are generally quite expensive and robust but their performance is vulnerable to obstructions and room ambiguity. The INS is generally robust but is very expensive and has poor stability from long term drift, for example. 
     There is still a strong need in the art for a player locator system adapted for MOUT training exercises that can provide the necessary performance features with reasonable complexity and cost. There is also a need for such a system for use in tracking the positions of emergency workers during fire and rescue operations in an urban structure, where conditions may be similar to those expected during MOUT training exercises. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below. 
     SUMMARY OF THE INVENTION 
     This invention solves the above described problems by introducing for the first time a position location system that relies on detection by a magnetic sensor of a low frequency (LF) magnetic field from a plurality of stationary antennas. The distance between a stationary antenna and the player-borne sensor is proportional to the logarithm of the magnetic field intensity because the player remains within the “near field” of the stationary antenna. With scheduled transmissions from six stationary antennas, the position of a player equipped with a three-axis magnetic sensor may be resolved in three-dimensions to within one foot (30 cm). Player orientation (angular position) may also be resolved in three dimensions. The LF magnetic field intensity is generally unaffected by structural obstructions, multipath distortion or any of the other performance-degrading problems discussed above in connection with Table 1. 
     It is a purpose of this invention to provide a player locator system adapted for MOUT training exercises that can provide the necessary performance features with reasonable complexity and cost. The performance features of this invention are summarized in Table 2: 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Performance Feature 
                 LF System of This Invention 
               
               
                   
                   
               
             
             
               
                   
                 Freq (MHz) 
                     0.1 
               
               
                   
                 Wavelength (cm) 
                 300,000 
               
               
                   
                 Accuracy 
                 Excellent 
               
               
                   
                 Stability 
                 Excellent 
               
               
                   
                 Measures Orientation? 
                 Yes 
               
               
                   
                 Position Resolved at? 
                 Building or Player 
               
               
                   
                 Multipath Resistance 
                 Excellent 
               
               
                   
                 Room Ambiguity Resistance 
                 Excellent 
               
               
                   
                 Gunfire/Noise Resistance 
                 Excellent 
               
               
                   
                 Smoke/Fog Resistance 
                 Excellent 
               
               
                   
                 Resistance to Obstructions 
                 Excellent 
               
               
                   
                 Thermal Imager Compatibility 
                 Excellent 
               
               
                   
                 Live Fire Damage Resistance 
                 Good 
               
               
                   
                 Player Unit Complexity/Cost 
                 Low 
               
               
                   
                 Building Site Complexity/Cost 
                 Low 
               
               
                   
                   
               
             
          
         
       
     
     It is another purpose of this invention to provide a position location and tracking system adaptable for use in tracking the positions of emergency workers during fire and rescue operations in an urban structure, where conditions may be similar to those expected during MOUT training exercises. It is an advantage of the system of this invention that the stationary antennas and transmitters may be permanently installed at little expense in any urban structure so that sensor-equipped emergency workers may be located and tracked within the structure during emergency fire and rescue operations. 
     In one aspect, he invention is a method for reporting the position of a player unit in a position locator system including a controller and one or more instrumented zones each bounded by two antennas on two generally opposite sides, including the steps of radiating a first magnetic signal from the antenna on a first side of a first instrumented zone, receiving the first magnetic signal at the player unit and generating a first sensor signal representing the first magnetic signal received at the player unit; radiating a second magnetic signal from the antenna on a second side of the first instrumented zone generally opposite the first side thereof, receiving the second magnetic signal at the player unit and generating a second sensor signal representing the second magnetic signal received at the player unit; broadcasting a player position data signal corresponding to a combination of all of the sensor signals and receiving the player position data signal at the controller. 
     In an exemplary embodiment, the invention is a position locator system including one or more instrumented zones, a plurality of antennas each having an orientation axis and a predetermined location on the periphery of at least one of the instrumented zones, a transmitter coupled to each antenna for producing therein a signal current, whereby a magnetic signal is radiated therefrom, and one or more player units each having a field sensor for producing a sensor signal representing the magnetic signals radiating from the antennas and a signal processor for generating player position data responsive to the sensor signal. 
     The foregoing, together with other objects, features and advantages of this invention, can be better appreciated with reference to the following specification, claims and the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of this invention, reference is now made to the following detailed description of the embodiments as illustrated in the accompanying drawing, in which like reference designations represent like features throughout the several views and wherein: 
     FIG. 1 illustrates a perspective view of an instrumented zone of this invention as defined by the disposition of several antennas on the periphery of the instrumented zone; 
     FIG. 2 illustrates a two-dimensional view of the ambiguity resolution feature of this invention in the instrumented zone of FIG. 1; 
     FIG. 3 illustrates a two-dimensional view of the relationships among a plurality of instrumented zones organized into several instrumented regions in accordance with this invention; 
     FIG. 4 illustrates the root-mean-square (RMS) magnetic field signal strength in decibels (dB) as a function of orthogonal distance in feet from the center of a ten-foot loop antenna in which a 128 kHz sinusoidal current is flowing; 
     FIG. 5 illustrates several variations in root-mean-square (RMS) magnetic field signal strength in decibels (dB) as a function of orthogonal distance in feet from the antenna of FIG. 4; 
     FIG. 6 illustrates an exemplary embodiment of a three-axis magnetic field sensor suitable for use in the system of this invention; 
     FIG. 7 is a functional block diagram illustrating the player unit of this invention; 
     FIG. 8 is a functional block diagram illustrating the stationery position locator system elements of this invention; 
     FIG. 9 is a waveform diagram illustrating the TDMA schedules of this invention; and 
     FIG. 10 is a block diagram of a flow chart illustrating the position monitoring method of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The position location and tracking system of this invention provides the means for monitoring the location of a player unit to within one foot in three dimensions within an instrumented zone such as the instrumented zone  20  shown in FIG.  1 . Instrumented zone  20  is defined by the three pairs of loop antennas L 1 /L 2 , L 3 /L 4  and L 5 /L 6  disposed at the periphery thereof. Each loop antenna is disposed symmetrically about an orientation axis exemplified by the orientation axis  22  of antennas L 1 /L 2  and the orientation axis  24  of antennas L 3 /L 4 . Each loop antenna pair is disposed with one element on opposite sides of instrumented zone  20  with the corresponding orientation axis orthogonal to the orientation axes of the other two loop antenna pairs, substantially as shown. Thus, loop antennas L 1  defines a first edge of instrumented zone  20  and loop antennas L 1  defines a second edge substantially opposite the first edge. Loop antenna pair L 3 /L 4  define another two edges substantially orthogonal to the first and second edges defined by loop antenna pair L 1 /L 2 . The loop antennas may each consist of a simple loop of conductive wire suitable for carrying an electrical current at any operating frequency in the low frequency (LF) region from 50-500 kHz, for example, and may be disposed about a loop diameter of 0.001 wavelengths at the operating frequency, for example. 
     Conventional radio systems use the radiated electromagnetic field, which is observed only at some distance from the antenna, and is therefore denominated the far field. Closer to the antenna, the so-called near field includes the electric electrostatic field and the magnetic induction field. Unlike the radiated field, which can be transmitted for some considerable distance, the induction field decays with the cube of distance and is therefore a very local phenomenon. However the induction field may be generated using a relatively small antenna. Because the operating wavelength of each loop antenna L 1 -L 6  is several orders of magnitude larger than any dimension of instrumented zone  20 , all points within instrumented zone  20  are well within the near field of each loop antenna L 1 -L 6 . This means that the magnetic field radiated from each loop antenna L 1 -L 6  varies in magnitude according to a predictable inverse cube law with distance. Waveguide and multipath effects are negligible in the near field. Moreover, any metal objects or structures within instrumented zone  20  are so small in terms of wavelength that they do not appreciably affect the magnetic field strength radiated by any of loop antennas L 1 -L 6  except at the surface of the metal. Each loop antenna L 1 -L 6  may be expanded to instrument a multistory building without materially changing the near-field characteristics in the LF frequency band. 
     Each loop antenna L 1 -L 6  in instrumented zone  20  is pulsed according to a predetermined LF time-division multiple access (TDMA) schedule so that only one loop antenna is transmitting at any one instant. For example, a single pulse may include one or more cycles of current at the loop antenna operating frequency. One of many useful ways to schedule the six loop antennas L 1 -L 6  is to alternate in numerical sequence from L 1  to L 2  to L 3  to L 4  to L 5  to L 6  and finally back to L 1  again to repeat the LF TDMA scheduling cycle. Referring to FIG. 2, when loop antenna L 1  is pulsed, a player unit  26  detects the resulting magnetic field and determines a distance from loop antenna L 1  related to the strength of the magnetic field detected at the operating frequency. This distance D 1  is ambiguous because it does not locate player unit  26  on a particular side of loop antenna L 1 . Based on distance D 1  alone, player unit  26  may be located anywhere along the interior line  28  or the exterior line  30  (FIG.  2 ). After loop antenna L 1  stops transmitting, loop antenna L 2  may then transmit a pulse, which is detected at player unit  26  and interpreted as a distance D 2  based on the magnetic field strength at the operating frequency. This distance D 2  may be combined with distance D 1  derived from the earlier L 1  pulse to refine the location of interior line  28 , removing the ambiguity and canceling a portion of any errors in the two measurements. This illustrates the position monitoring performance of pulses from any single loop antenna pair such as loop antennas L 1 /L 2 . Each additional loop antenna pair L 3 /L 4  and/or L 5 /L 6  may be similarly pulsed on a LF TDMA schedule to resolve another (orthogonal) interior line on which player unit  26  is located. For example, pulsing loop antenna L 3  provides a measure of the distance D 3  and pulsing loop antenna L 4  provides a measure of the distance D 4 , thereby resolving the interior line  32 . The intersection of lines  32  and  28  provide the location of player unit  26  in two dimensions. If desired, the third loop antenna pair L 5 /L 6  may then be pulsed to resolve a third interior line (perpendicular to the page) at player unit  26 . Any separation of the intersections of the three lines may be allocated to cancel additional detection errors, thereby refining the accuracy of the detected location. If instrumented zone  20  encompasses a multistory building, the location of player unit  26  may be compared to blueprint data to identify the location in terms of a floor and a room, for example. 
     In extensive MOUT training scenarios or in complexes of very large multistory buildings, the system of this invention may be configured as a plurality of instrumented zones making up one or more “regions” such as shown in FIG.  3 . Each region, exemplified by the region  34 , encompasses a fixed number (for example, nine) of instrumented zones, exemplified by the instrumented zones Z 1  through Z 9 . ALF TDMA schedule may be established for each of the  54  individual loop antennas within region  34  so that no two loop antennas are transmitting simultaneously. This eliminates interference throughout the entire system because there is always a separation of two to three zones between any two transmitting loop antennas, which substantially attenuates the neighboring magnetic field signal. For example, when the loop antenna L 1  is pulsed in the instrumented zone Z 5 , there is a three-zone attenuation of the magnetic field radiated by any active neighboring loop antenna, which introduces negligible amplitude error at the player unit. 
     In FIG. 3, a position between two adjacent instrumented zones, such as the position  36  between zones Z 8  and Z 9 , may be determined during the ambiguity resolution interval when the second loop antenna within a loop antenna pair resolves the distance ambiguity to the exterior line instead of the interior line discussed above in connection with FIG.  2 . This capability permits player position monitoring in the streets between buildings, for example. 
     FIG. 4 illustrates the root-mean-square (RMS) magnetic field signal strength in decibels (dB) as a function of orthogonal distance in feet from the center of a ten-foot loop antenna in which a 128 kHZ sinusoidal current is flowing. The variation is nearly linear in decibels (dB), which suggests that, for example, a simple logarithmic amplifier may be used as an analog signal processor to generate a player position signal representing the logarithm of the magnetic field intensity. Such a player position signal would vary generally linearly with distance from the transmitting loop antenna and any second-order nonlinearity may be calibrated as a compensation in a microprocessor look-up table, for example. FIG. 5 illustrates the disadvantageous variations in magnetic field strength as the player position approaches the edges of the loop antennas, which may introduce inaccuracies within several feet of the loop antenna. The chart line  38  shows the root-mean-square (RMS) magnetic field signal strength in decibels (dB) as a function of orthogonal distance in feet from the center of the loop antenna. The chart line  40  shows the same variable measured orthogonally with a point midway between the center and the edge of the loop antenna. The chart line  42  shows the same variable measured orthogonally to a point midway between the point of line  40  and the edge of the loop antenna. Most of the variation occurs within the first several feet of the loop antenna and may be compensated out by comparing the results from the opposing antenna in the loop antenna pair. 
     FIG. 6 shows a three-axis magnetic field sensor  44  including a sensor element  46  oriented along an X-axis, a sensor element  48  oriented along a Y-axis, and a sensor element  50  oriented along a Z-axis such that the orientations of sensor elements  46 - 50  are mutually orthogonal. By using the three mutually-orthogonal sensor elements  46 - 50  with each coupled to a separate log amplifier (FIG.  7 ), the orientation of the player unit (additional to the position) may be resolved in three dimensions. 
     FIG. 7 shows an exemplary embodiment of the player unit  52  of this invention including three-axis magnetic sensor  44  from FIG.  6 . Each sensor element  46 ,  48  and  50  is coupled to a separate log amplifier  52 ,  54  and  56 . Each log amplifier  52 - 56  is coupled to a separate peak sample-and-hold (SH) circuit  58 ,  60  and  62 , respectively. SH circuits  58 ≧ 62  operate to capture the peak value of the logarithm of the magnetic field amplitude from a corresponding one of sensor elements  46 - 50 , and produce a separate sensor signal  64 ,  66  and  68 , respectively. The algebraic sum of sensor signals  64 - 68  represents the total magnetic field amplitude and hence the distance from the active loop antenna. The relative amplitudes of sensor signals  64 - 68  each represent a trigonometric function of the orientation angle of the sensor with respect to the active loop antenna. The player microcontroller  70  accepts the fixed analog sensor signals  64 - 68 , converts them to digital data, and saves them as player position data in a local memory. This sampling and saving process recycles on the LF TDMA schedule (FIG. 9) so that player position data are produced and saved for every loop antenna pulse scheduled in the system. Microcontroller  70  may also include some look-up table means for adjusting sensor signals  64 - 68  to compensate for magnetic field variations in certain portions of instrumented zone  20 , for example. 
     Once player unit  52  has recorded player location data for each of the loop antennas in a LF TDMA schedule, the player position data must be processed to derive player position and orientation. This more demanding data processing is preferably performed in the stationary system elements having access to fixed electric power and computing hardware. So player position data must be regularly transmitted from player unit  52  to a local zone microcontroller (FIG. 8) and from there to a central controller, where they may be processed and displayed centrally together with position location data from other player units engaged in the system. This is accomplished with the low-power data transmitter  72  (FIG.  7 ), which broadcasts a high-frequency (HF) radio signal (400 MHz, for example) containing player position data from player microcontroller  70  according to a HF TDMA schedule managed by a central controller. Microcontroller  70  also broadcasts player unit identification (ID) data established by means of the DIP switch  74 . As soon as player unit  52  enters an instrumented zone, position location system transmissions are detected at the synch pulse receiver  76  and the low-power receiver  78 . Because both LF and HF TDMA schedules (FIG. 9) are managed externally, player unit  52  must immediately request a HF TDMA slot in which to transmit player position data. Low-power receiver  78  is provided to accept this HF TDMA schedule information and other communications from the instrumented zone. Synch pulse receiver  76  is provided to inform player microcontroller  70  of the beginning of each TDMA interval. This synchronizes the storage of player position data from sensor signals  64 - 68  and the player position data broadcasts in the appropriate HF TDMA intervals. Low-power data transmitter  72 , synch pulse receiver  76  and low-power receiver  78  each are coupled to an antenna  80 ,  82  and  84 , respectively. 
     FIG. 8 shows a stationery instrumented zone transmitter assembly that includes a zone microcontroller  86  coupled to a fiber optic backbone  88  with which data may be exchanged with the central controller  90 . Central controller  90  may be coupled to a number of instrumented zones (see FIG. 3, for example) and manages the LF and HF TDMA scheduling for all loop antenna pulsing and player position data broadcasts as described below in connection with FIG.  9 . In FIG. 8, a DIP switch  92  is provided to impose a zone ID code on zone microcontroller  86 , which is needed to synchronize the operation of the loop antennas  94 ,  96 ,  98 ,  100 ,  102  and  104  with all other instrumented zones in the region. Central controller  90  assigns TDMA operating intervals based on the instrumented zone ID code, which varies from zone to zone within a region (FIG.  3 ). Each loop antenna  94 - 104  is powered by a transmitter  106 ,  108 ,  110 ,  112 ,  114  and  116 , respectively. The state machine  118  provides the requisite synchronization of transmitters  106 - 116  under the control of zone microcontroller  86 . The low-power transmitter  120  and the low-power receiver  122  are provided to communicate with the corresponding low-power data transmitter  72  and low-power receiver  78  in player unit  52  (FIG.  7 ). The synch pulse receiver  124  is provided to inform zone microcontroller  86  of the beginning of each TDMA interval. Player position data broadcasts from low-power data transmitter  72  (FIG. 7) are received by low-power receiver  122  and transmitted directly to zone microcontroller  86 , where the data are time-stamped and forwarded to fiber optic backbone  88 , from which the player position data are received at central controller  90  for analysis. 
     FIG. 9 is a waveform diagram illustrating the operation of the LF and HF TDMA schedules. Two TDMA schedules are required; the LF TDMA schedule  126  and the HF TDMA schedule  128 . These two schedules may overlap in time because the LF transmissions do not interfere with the HF transmissions and vice-versa. It is convenient to synchronize the intervals of both LF and HF TDMA schedules to the same sync pulse  130  from the receiver  76  and  124 . 
     Beginning at the synch pulse  130 , a single LF TDMA epoch is divided equally into nine zone intervals exemplified by the zone interval  132 . Each zone interval is subdivided into six loop antenna intervals, exemplified by the loop antenna interval  134 . Each loop antenna in the region of nine instrumented zones fires in a 54-pulse sequence (18.5 msec pulses, for example) over the epoch of, for example, one second. The inventors believe that this one-second epoch interval may be reduced by up to a factor of 8 depending on the damping characteristics of the particular loop antenna and transmitter embodiments selected for the system. Each loop antenna current pulse, exemplified by the pulse  136 , generates a magnetic field that is sensed and measured at every player unit in the instrumented zone. With each loop antenna pulse, three orthogonal player position data are stored in the player unit and broadcast to the nearest instrumented zone microcontroller at the next appropriate HF TDMA interval. 
     Beginning at the synch pulse  138  (which may be coincident with synch pulse  130 , for example), a single HF TDMA epoch is divided equally into nine zone intervals exemplified by the zone interval  140 . Each zone interval is subdivided into 16 player broadcast intervals, exemplified by the player broadcast interval  142 . The first of the 16 player broadcast intervals is reserved as a player unit request (PUR) interval for use by new player units to request a player reporting interval assignment. The remaining 15 player broadcast intervals are sufficient to handle up to 15 player units within a single instrumented zone. 
     FIG. 10 is a block diagram of a flow chart illustrating the position monitoring method of this invention. Considering a single pair of loop antennas L 1 /L 2 , the position location method starts at step  144  with the scheduled current pulse in loop antenna L 1 . At the next step  146 , the player unit receives the L 1  magnetic signal and produces a sensor signal S 1  representing the L 1  magnetic field amplitude. At the step  148 , player position data D 1  representing sensor signal S 1  are stored at the player unit. In the next step  150 , the scheduled current pulse is generated in loop antenna L 2 . At step  152 , the player unit receives the L 2  magnetic signal and produces a sensor signal S 2  representing the L 2  magnetic field amplitude. At the step  154 , player position data D 2  representing sensor signal S 2  are stored at the player unit. In the next step  156 , a player position data signal representing the player position data D 1  and D 2  is broadcast by the player unit. In the final step  158 , the player position data signal is received at a controller, and the player position is then calculated. 
     Clearly, other embodiments and modifications of this invention may occur readily to those of ordinary skill in the art in view of these teachings. Therefore, this invention is to be limited only by the following claims, which include all such embodiments and modifications when viewed in conjunction with the above specification and accompanying drawing.