Abstract:
Spaced first and second signposts transmit wireless signals containing different signpost identifications. A first path of travel passes through the transmission range of the first signpost, and a second path of travel passes through the transmission range of the second signpost but not the transmission range of the first signpost. A different configuration includes plural hallways extending away from a common intersection in respective directions, with a respective signpost in each hallway that transmits wireless signals containing a respective different signpost identification. Another configuration includes a hallway with first and second portions of different width, a first signpost in the first portion transmitting wireless signals containing a first signpost identification, and spaced second and third signposts in the second portion each transmitting wireless signals containing a second signpost identification different from the first signpost identification, and having a transmission range less than a width of the second portion.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to tracking techniques and, more particularly, to techniques for tracking items or vehicles using radio frequency identification technology. 
     BACKGROUND 
     According to an existing technique for tracking items or vehicles, a device known as a radio frequency identification (RFID) tag is mounted on each item or vehicle that is to be tracked. Signposts that transmit short-range signpost signals are provided near locations where tags are likely to pass, for example near a door through which tags routinely travel. The tags can receive the signpost signals from nearby signposts, and can also transmit wireless tag signals that include information from the signpost signals. The tag signals typically have an effective transmission range that is significantly longer than the effective transmission range of the signpost signals. Stationary devices commonly known as readers are provided to receive the tag signals. Existing systems of this type have been generally adequate for their intended purposes, but have not been satisfactory in all respects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A better understanding of the present invention will be realized from the detailed description that follows, taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of an apparatus that embodies aspects of the present invention, and that includes a signpost, a radio frequency identification tag, a reader, and a control system. 
         FIG. 2  is a diagrammatic view of a digital word that is embedded in signpost signals transmitted by the signpost of  FIG. 1 . 
         FIG. 3  is a diagrammatic view of a digital word that is transmitted in tag signals transmitted by the tag of  FIG. 1 . 
         FIG. 4  is a diagrammatic top view showing an arrangement that constitutes one possible application for a system of the type shown in  FIG. 1 . 
         FIG. 5  is a flowchart showing certain operations that are carried out by each of several tags in the embodiment of  FIG. 4 . 
         FIG. 6  is a diagrammatic top view of an arrangement that is an alternative embodiment of the arrangement shown in  FIG. 4 . 
         FIG. 7  is a diagrammatic top view of a further arrangement that represents yet another possible application for a system of the type shown in  FIG. 1 . 
         FIG. 8  is a diagrammatic top view of an arrangement that represents still another possible application for a system of the type shown in  FIG. 1 . 
         FIG. 9  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequence of operations shown in the flowchart of  FIG. 5 . 
         FIG. 10  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequences of operation shown in the flowcharts of  FIGS. 5 and 9 . 
         FIG. 11  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequences of operations shown in the flowcharts of  FIGS. 5 ,  9  and  10 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of an apparatus  10  that embodies aspects of the present invention. The apparatus  10  includes a signpost  11 , a radio frequency identification (RFID) tag  12 , a reader  13 , and a control system  14 . The apparatus  10  actually includes many signposts of the type shown at  11 , many tags of the type shown at  12 , and several readers of the type shown at  13 . However, for clarity in the discussion that follows,  FIG. 1  shows only one signpost  11 , one tag  12 , and one reader  13 . In the disclosed embodiment, the signpost  11  and the reader  13  are stationary, and the tag  12  can move relative to them. For example, the tag  12  may be mounted on a not-illustrated vehicle (such as a truck or forklift), or may be mounted on an item that is being transported (such as a box containing a television set). 
     The signpost  11  includes a microcontroller  21 . Persons skilled in the art are familiar with the fact that a microcontroller is an integrated circuit having a microprocessor, having a read-only memory (ROM) that contains a computer program and static data for the microprocessor, and having a random access memory (RAM) in which the microprocessor can store dynamic data during system operation. The signpost  11  also includes a low frequency transmitter  22  that is controlled by the microcontroller  21 , and that is coupled to an antenna  23 . The microcontroller  21  can use the transmitter  22  to transmit a low frequency signpost signal  24  through the antenna  23 . The transmitter  22  is a type of circuit known to those skilled in the art, and is therefore not illustrated and described here in detail. The antenna  23  can be a ferrite core antenna and/or a planar coil antenna of a known type, or any other suitable form of antenna. The antenna  23  is configured to transmit an omni-directional signal, but the antenna could alternatively be configured to transmit a signal that is to some extent directional. 
     In the embodiment shown in  FIG. 1 , the transmitter  22  generates the signpost signal  24  by effecting amplitude modulation of a carrier signal, where the carrier signal can have a frequency within a range of approximately 30 KHz to 30 MHz. Various countries have different governmental regulations regarding electromagnetic emissions. With due regard to these governmental regulations, the carrier frequency in the embodiment of  FIG. 1  is selected to be 123 KHz, but could alternatively be some other frequency, such as 125 KHz, 132 KHz or 13.56 MHz. A further consideration in the selection of a carrier frequency is that the signpost signals  24  are to exhibit near field characteristics of a primarily magnetic character. 
     In this regard, electromagnetic signals have both an electric component (the “E” field) and a magnetic component (the “H” field). The magnetic field (H field) has a significantly higher roll-off than the electric field (E field). Consequently, it is possible for the magnetic field to be significant in the near field, or in other words at locations near the transmitter. However, the electric field will always dominate in the far field, or in other words at locations remote from the transmitter. The low frequency transmitter  22  and the antenna  23  are configured so that the magnetic field (H field) dominates in the near field. Consequently, the transmission and reception of the signpost signals  24  may be viewed as more of a magnetic coupling between two antennas, rather than a radio frequency coupling. As a result, the signpost signals  24  intentionally have a relatively short transmission range. This transmission range is adjustable but, in the disclosed embodiment, is typically about four to twelve feet. The localized nature of the signals  24  helps to facilitate compliance with governmental regulations. It also helps to minimize reception of these signals by tags that are not in the general vicinity of the signpost  11 , but instead are beyond an intended transmission range of the signpost signals  24 . 
     The signpost  11  is operatively coupled to the control system  14  through an interface  27 . In the embodiment of  FIG. 1 , the interface  27  is a standard RS-232 serial interface. However, the interface  27  could alternatively be any other suitable type of interface, including but not limited to an Ethernet interface, an RS-485 interface, or a wireless interface. 
     The signpost  11  transmits the signpost signal  24  at periodic intervals. The time interval between successive transmissions may be configured to be relatively small, such as 100 msec, or may be configured to be relatively large, such as 24 hours, depending on the particular circumstances. The signpost signals  24  contain information that is discussed in more detail later. 
     The signpost signals  24  are often transmitted in a relatively noisy environment. In order to ensure reliable signal reception, known techniques may be used to improve the signal-to-noise ratio (SNR). In the embodiment of  FIG. 1 , the amplitude modulation of the 123 KHz carrier is effected using the well-known technique of amplitude shift keying (ASK), in order to improve the SNR. Alternatively, it would be possible to use frequency shift keying (FSK) or phase shift keying (PSK) to achieve an even higher SNR. However, FSK and PSK would typically require additional front-end analog circuitry in each of the tags  12 . Therefore, and since it is desirable to be able to implement both the signpost  11  and the tag  12  at a relatively low cost, the embodiment of  FIG. 1  uses ASK to achieve a reduced SNR. 
     Turning to the tag  12 , the tag  12  includes an antenna  41  that receives the signpost signals  24  transmitted by the signpost  11 . The antenna  41  is coupled to a low frequency receiver  42  of a known type. The receiver  42  is coupled to a microcontroller  43 . The receiver  42  receives the signpost signals  24 , extracts information from them, and then supplies this information to the microcontroller  43 . 
     The microcontroller includes a memory that is shown diagrammatically at  46 . Among other things, the microcontroller can store signpost identification information at  47  within the memory  46 , as discussed in more detail later. The microcontroller  43  also has a memory location  48  that it uses as a counter, for a purpose discussed in more detail later. The tag  12  includes a timer  49  that can be used by the microcontroller  43  to measure a time interval, as explained in more detail later. 
     In  FIG. 1 , the circuitry within the tag  12  is powered by a not-illustrated battery. The tag  12  has at least two different modes of operation, including a normal operational mode, and a sleep mode. In the sleep mode, some or all of the circuitry within the tag  12  is powered down, in order to conserve battery power. In other words, the sleep mode is a reduced power mode in comparison to the normal operational mode. 
     The microcontroller  43  controls an ultra high frequency (UHF) transceiver  51  of a known type. The transceiver  51  is coupled to a known type of antenna  52 . In the disclosed embodiment, the antenna  52  is omni-directional, but the antenna  52  could alternatively be configured to be directional. As is known in the art, it would be possible for the tag  12  to have two antennas at  56  that are perpendicular to each other, in order to facilitate more reliable reception of signpost signals  24 . However, for simplicity and clarity,  FIG. 1  shows only one antenna at  52 . 
     Using the transceiver  51  and the antenna  52 , the microcontroller  43  of the tag  12  can transmit tag signals at  56  to the reader  13 , and can receive reader signals transmitted at  56  by the reader  13 . In the embodiment of  FIG. 1 , the tag signals  56  are generated by FSK modulation of certain information onto a radio frequency (RF) carrier signal. This carrier signal has a frequency of 433.92 MHz, but it could alternatively have any other suitable frequency. One possible alternative frequency is 915 MHz. However, the embodiment of  FIG. 1  uses the frequency of 433.92 MHz, because it is available for use in a larger number of countries under current governmental regulations regarding the transmission of electromagnetic signals. 
     The transmission range for the UHF signals  56  is substantially longer than that for the signpost signals  24 . As discussed above, the transmission range of the signpost signals  24  is about 4 to 12 feet. In the disclosed embodiment, the transmission range for the UHF signals  56  can be up to about 300 feet. The signals  56  contain information that is explained in more detail later. 
     In  FIG. 1 , the reader  13  includes an antenna  71  that is coupled to a UHF transceiver  72 . As is known in the art, it would be possible for the reader  13  to have two antennas at  71  that are perpendicular to each other, in order to facilitate more reliable communication between the tag  12  and the reader  13 . However, for simplicity and clarity,  FIG. 1  shows only one antenna at  71 . 
     In the reader  13 , the transceiver  72  is coupled to a microcontroller  73 , and the microcontroller  73  is coupled to a network interface  76 . The network interface  76  is coupled through a network  77  to the control system  14 . In  FIG. 1 , the network  77  is a type of network that is commonly known in the art as an Ethernet network. However, the network  77  could alternatively be any other suitable type of network or communication system. 
       FIG. 2  is a diagrammatic view of a digital word  101  that is embedded in the signpost signals transmitted at  24 . The bits of the digital word  101  are incorporated into the signpost signal  24  by serially modulating the bits of the word  101  onto the 123 KHz carrier using amplitude modulation, as discussed above. The bits of the word  101  are transmitted serially from left to right in  FIG. 2 . 
     The digital word  101  includes several fields. The first field is a preamble  103 . The preamble  103  is a predefined pattern of bits that will allow a device receiving the signal  24  to recognize that the signpost signal is beginning, and to synchronize itself to the signpost signal. In the disclosed embodiment, the preamble  103  is approximately eight bits, but the specific number of bits can vary in dependence on factors such as characteristics of a particular receiver that is expected to receive the signpost signal. 
     The next field  104  in the word  101  is a signpost identification (ID)  104 . In the disclosed embodiment, the signpost ID  104  is a 12-bit integer value that uniquely identifies a particular signpost  11  that is transmitting the word  101 . As mentioned above, the system  10  may have a number of signposts  11 , and the use of a respective different signpost ID  104  by each signpost permits the system to distinguish signpost signals transmitted by one signpost from signpost signals transmitted by another signpost. This does not mean that the system could never have two signposts with exactly the same signpost code. For example, two signposts may be stationarily mounted in close proximity to each other, and may be configured to independently transmit signpost signals that contain the same signpost ID. 
     Another field in the word  101  is a group size value  106 . As discussed in more detail later, this value identifies how many signposts are members of a group of signposts, where the group includes the signpost that transmitted the received signpost signal containing the word  101 . 
     The next field in the word  101  of  FIG. 2  is an error control field  107 . Communications between the signpost  11  and other devices are essentially one-way transmissions. In addition, many applications for the apparatus  10  of  FIG. 1  involve environments that have relatively high noise levels. Accordingly, it is desirable for a receiving device to be able to evaluate whether a word  101  that it received in a signpost signal is correct, or has errors. Consequently, the error control field  107  is included in the word  101  in order to permit the receiving device to identify and/or correct errors. In the disclosed embodiment, the error control field  107  contains a cyclic redundancy code (CRC). However, it would alternatively be possible to use any other suitable error correction scheme, such as parity information, or a forward error correction (FEC) code. 
     The next field in the word  101  is a packet end field  108 . This field signals to a receiving device that the transmission is ending. In the disclosed embodiment, the packet end field  108  has eight bits that are all set to a binary zero. However, the packet end field  108  could alternatively have any other suitable configuration. 
     It would be possible for the word  101  to have one or more additional fields, for example as indicated diagrammatically at  111 . However, even assuming that additional fields were present, it is not necessary to specifically identify and explain them here in order to convey an understanding of the present invention. 
     As discussed above, the tag  12  has at least two operational modes, including a normal operational mode and a reduced-power sleep mode. When the tag  12  is in the sleep mode and receives a signpost signal  24 , the tag can switch from its sleep mode to its normal operational mode. Since the signpost  11  is normally near a reader  13 , the tag  12  will in due course respond to the signpost signal  24  by transmitting a type of tag signal  56  that is sometimes referred to as a beacon signal, in order to notify any nearby reader that the tag is present. 
       FIG. 3  is a diagrammatic view of a digital word  121  that the tag can include in its wireless tag signals. As shown in  FIG. 3 , the word  121  begins with a preamble  123 . The preamble  123  is functionally comparable to the preamble  103  in the word  101  of  FIG. 2 . In the disclosed embodiment, the preamble  123  lasts 1.296 msec, and has 20 cycles that each include a 30 msec logic high and 30 msec logic low, followed by one cycle that includes a 42 msec logic high and then a 54 msec logic low. However, any other suitable preamble could alternatively be used. The next field in the word  121  is a tag status field  124 . This field contains some current status information about the tag  12  that is making the transmission. 
     The next field is a message length field  126 , and defines the overall length of the word  121 . The message length field  126  is followed by a tag ID field  128 . The tag ID field  128  contains a binary code that uniquely identifies the particular tag  12  that transmitted the word  121 . Thus, when several tags  12  are present in the vicinity of a particular reader  13 , the reader can tell which tag  12  transmitted each signal that the reader receives. 
     The next field  129  in the word  121  is a data field. The data field  129  contains one or more items of data. In  FIG. 3 , the data field  129  contains several items of data at  132 - 134 , each of which is a signpost ID such as that shown at  104  in  FIG. 2 . The signpost IDs at  132 - 134  were each received in the signpost ID field  104  ( FIG. 2 ) of a respective signpost signal, as explained in more detail later. 
     The word  121  also includes an error control field  137 . In the disclosed embodiment, this is a CRC code, but it could alternatively be any other suitable information for detecting and/or correcting errors. The word  121  ends with a packet end field  138 . In the disclosed embodiment, the packet end field  138  is a string of binary zeros representing a logic low that lasts 36 msec. The packet end field  138  indicates to a receiving device that the transmission of the word  121  is ending. 
       FIG. 4  is a diagrammatic top view showing an arrangement that constitutes one possible application for a system of the type shown in  FIG. 1 . The arrangement  201  includes structure defining four spaced end parallel separators or islands  206 - 209 . Between each adjacent pair of the islands  206 - 209  is an elongate strip that serves as a lane for vehicles, such as a truck. In particular, the four islands  206 - 209  define three adjacent and parallel lanes  212 - 214 . Vehicles traveling within the lanes  212 - 214  move along respective paths of travel  216 - 218 . A vehicle may move in either direction along any of these paths of travel. 
     The arrangement  201  includes eight signposts  221 - 228 . The signposts  221 - 228  are each identical to the signpost shown at  11  in  FIG. 1 , but have been given respective different reference numerals in order to avoid confusion in the discussion that follows. The signposts  221  and  225  are stationarily mounted at spaced locations on the island  206 . Similarly, the signposts  222  and  226  are stationarily mounted at spaced locations on the island  207 , the signposts  223  and  227  are stationarily mounted at spaced locations on the island  208 , and the signposts  224  and  228  are stationarily mounted at spaced locations on the island  209 . Although  FIG. 4  shows the signposts  221 - 228  mounted on islands between the lanes, signposts could alternatively be supported at other locations. For example, signposts could be mounted at locations that are each centered above one of the lanes  212 - 214 . 
     The signposts  221 - 228  each emit wireless signpost signals containing information of the type discussed above in association with  FIG. 2 . As also discussed above, the signpost signals from each of the signposts  221 - 228  have an effective transmission range that is about 4 to 12 feet, and that is indicated diagrammatically in  FIG. 4  by a respective one of the broken-line circles  231 - 238 . In the arrangement  201  of  FIG. 4 , the effective transmission range of each signpost is approximately equal to the width of one of the lanes  212 - 214 . Where two signposts have overlapping transmission ranges, the signposts are synchronized and transmit their signpost signals in an alternating manner, so that the signpost signals do not interfere with each other. 
     A reader  13  is stationarily supported in approximately the center of the arrangement  201 , and in particular is supported on the island  208  at a location between the signposts  223  and  227 . The reader  13  in  FIG. 1  is identical to the reader  13  of  FIG. 1 .  FIG. 4  also shows three tags  241 - 243 . The tags  241 - 243  are each identical to the tag shown at  12  in  FIG. 1 , but have been given different reference numerals in  FIG. 4 , in order to avoid confusion in the discussion that follows. Each of the tags  241 - 243  may, for example, be mounted on a truck or other vehicle that is traveling in either direction along one of the lanes  212 - 214 . Thus, for example, if the tag  241  is on a vehicle that is traveling upwardly in  FIG. 4  within the lane  212  and along the path of travel  216 , the tag  241  will pass through the overlapping transmission ranges  235  and  236  of the signposts  225  and  226 , and then in due course will pass through the overlapping transmission ranges  231  and  232  of the signposts  221  and  222 . 
     Although  FIG. 5  shows the signposts  221 - 228  supported on the islands  206 - 209 , or in other words at the sides of the lanes  212 - 214 , it would alternatively be possible for some or all of the signposts  221 - 228  to be supported at other locations. For example, some or all of the signposts could be supported at respective locations that are each centered above one of the lanes  212 - 214 . As a practical matter, when a signpost is supported directly over a lane, it may be necessary to mount it at a relatively high position, so that there will be sufficient clearance for trucks or other tall vehicles to pass beneath it. However, as discussed above, the transmission range of the disclosed signposts is up to about 12 feet. Therefore, a signpost centered above a lane often needs to operate at substantially full power in order for its signal to reach tags supported on vehicles that are low the signpost. 
     In contrast, where the signpost is supported to the side of a lane, the transmission power is set so that the range is about three-quarters of the width of a lane. As an example, for a lane that is 8 feet wide, signpost power would be set at about half power, so that the range is about 6 to 7 feet. Where this power level is used, signposts would typically be provided on both sides of a lane, in the manner shown in  FIG. 4 . 
       FIG. 5  is a flowchart showing certain operations that are carried out by each of the tags  241 - 243  as they move in either direction along one of the paths of travel  216 - 218 . For simplicity, the flowchart of  FIG. 5  will be discussed with reference to the tag  241 . For the sake of discussion, it is assumed that the tag  241  is initially in the position shown in  FIG. 4 , and has not yet entered the transmission range or near field for any of the four tags  221 - 222  and  225 - 226 . In block  261  of  FIG. 5 , the tag  241  discards any signpost IDs that it may have previously stored at  47  in the memory  46  of its microcontroller  43  ( FIG. 1 ). The tag  241  disables its counter  48  ( FIG. 1 ) by setting the counter  48  to a value of zero. Further, the tag  241  disables its timer  49  ( FIG. 1 ). The tag  241  then proceeds from block  261  to block  262 . 
     In block  262 , the tag checks to see whether the timer  49  has just expired. If so, then the tag would proceed to block  263 , which will be discussed later. However, at this particular point, the tag has just disabled the timer in block  261 , and thus the tag  241  will determine in block  262  that the timer has not just expired. Consequently, the tag will proceed from block  262  to block  266 . In block  266 , the tag checks to see whether it has received a signpost signal from any signpost. If not, then the tag returns to block  262 , and essentially waits for a signpost signal by sitting in a loop that includes the blocks  262  and  266 . 
     If the tag eventually determines in block  266  that it has received a signpost signal, the tag proceeds to block  267 , where it starts the timer  49  (or restarts the timer  49  if the timer is already running). The tag then proceeds to block  268 , where it checks to see whether the signpost ID  104  ( FIG. 2 ) in the received signpost signal has already been stored at  47  in the memory  46  ( FIG. 1 ). If so, then the tag proceeds to block  271 , where it enters its reduced-power sleep mode, and then returns to block  262  in order to wait for another signpost signal. Blocks  268  and  271  represent one example of a condition that can cause the tag to enter the sleep mode, and is presented here purely by way of example. Any of a variety of conditions or events could alternatively be used to cause the tag to enter the sleep mode while the tag is waiting to receive signpost signals. 
     If the tag determines in block  268  that the signpost ID  104  in the received signpost signal has not yet been stored at  47 , then the tag proceeds to block  272 . In block  272 , the tag stores the received signpost ID  104  in section  47  of the memory  46 . Then, at block  273 , the tag checks to see whether the counter  48  ( FIG. 1 ) is currently zero, or in other words whether the counter  48  is currently disabled. If the counter is currently disabled, then the tag proceeds to block  276 , where it initializes the counter  48  with the group size value  106  ( FIG. 2 ) from the received signpost signal. 
     From block  276 , or from block  273  if the tag determined that the counter was not disabled, the tag proceeds to block  277 , where it decrements the counter  48 . Then, at block  278 , the tag checks again to see whether the counter  48  has reached zero. If the counter has not yet reached zero, then the tag is still waiting for signpost signals from additional signposts within a group of signposts. The tag therefore returns to block  262  in order to await signpost signals from other signposts in the group. On the other hand, if the tag determines at block  278  that the counter  48  has been decremented to zero, then the tag has received a signpost signal from each of the signposts in the group, and therefore proceeds to block  263 . 
     From the time when the tag detects receipt of a first signpost signal in block  266  until the tag reaches block  263 , the tag inhibits the transmission of tag signals at  56  using the UHF transceiver  51 . During this time interval, when UHF transmissions are being suppressed, the tag can also optionally conserve battery power by inhibiting reception of wireless signals through the receiver portion of its UHF transceiver  51 , or by turning off power to the receiver portion of its UHF transceiver  51 . 
     Referring again to  FIG. 4 , each of the signposts  221 - 228  will be transmitting a signpost signal in which the group size value  106  ( FIG. 2 ) is the number 4. This is because a tag traveling along any of the paths of travel  216 - 218  will pass through the fields or transmission ranges of four tags, and those four tags effectively constitute a group. Stated differently, the four tags  221 - 222  and  225 - 226  constitute a group with respect to lane  212 , the four tags  222 - 223  and  226 - 227  constitute a group with respect to lane  213 , and the four tags  223 - 224  and  227 - 228  constitute a group with respect to lane  214 . 
     When the tag reaches the point  286 , it enters the near fields or transmission ranges  235  and  236  of the tags  225  and  226 . Thus, the tag should promptly receive a signpost signal from one of the tags  225  and  226 , and then a signpost signal from the other thereof. For the sake of discussion, assume that the first signpost signal received by the tag is from the signpost  225 . In response to receipt of this signpost signal, the tag will start its timer  49 , and also initialize its counter  48  with the group size value  106  ( FIG. 2 ) from this received signpost signal. Thus, in this example, the counter  48  will be initialized to a value of 4, because the lane  212  is associated with a group of four signposts  221 - 222  and  225 - 226 . The tag will also take the signpost ID  104  ( FIG. 2 ) from the received signpost signal, and store this signpost ID at  47  ( FIG. 1 ). 
     Shortly thereafter, the tag should receive a signpost signal from the signpost  226 . The tag will restart the timer  49 , decrement the counter  48 , and then save at  47  the signpost ID  104  for the signpost  226 . As the tag continues to move along the path of travel  216 , it should receive additional signpost signals from each of the tags  225  and  226 . Each of these additional signpost signals will cause the tag to restart its timer  49 . Aside from this, however, the tag will essentially ignore these additional signpost signals. In due course, the tag will pass point  287 , and will stop receiving signpost signals from the signposts  225  and  226 . The time interval measured by the timer  49  is greater than the time needed for the tag to travel from point  287  to point  288  at normal operational speeds. Consequently, the timer  49  will not normally expire as the tag travels from  287  to  288 . 
     When the tag reaches the point  288 , it enters the near fields or transmission ranges  231  and  232  of the signposts  221  and  222 . The tag  241  will promptly receive a signpost signal from one of the signpost  221  and  222 , and then a signpost signal from the other thereof. For the sake of discussion, assume that the first signpost signal received by the tag is from the signpost  221 . The tag will store the signpost ID  104  from this signpost signal at  47  in the memory  43 . The tag will also restart the timer  49 , and decrement the counter  48 . Shortly after that, the tag will receive a signpost signal from the signpost  222 . The tag will store the signpost ID  104  from this signpost signal in the section  47  of the memory  43 , and will also restart the timer  49 . 
     The tag will then decrement the counter  48 , and will discover that the counter  48  has reached a value of zero. This tells the tag that a respective signpost signal has been received from each of the four signposts  221 - 222  and  225 - 226  in the signpost group that is associated with lane  212 . Therefore, as discussed above in association with  FIG. 5 , the tag will transmit one or more wireless tag signals that contain all of the signpost IDs stored at  47  in the memory  43 , in order to transfer this information to the reader  13 . In the disclosed embodiment, these signpost IDs are transmitted in the order in which they were successfully stored in the memory  46 . 
     The reader  13  will then forward this information to the control system  14  ( FIG. 1 ). The control system  14  can use this information to make two determinations. First, the control system  14  can determine which of the lanes  212 - 214  the tag  241  is currently traveling along. In particular, as discussed above, the tag will have received signpost IDs from each of the tags  221 - 222  and  225 - 226 , and this particular combination of signposts is associated with the lane  212  and the path of travel  216 . The second determination made by the control system  14  is the direction in which the tag  241  is currently moving along the path of travel  216 . In particular, if the signpost IDs for the signposts  225  and  226  were received before the signpost IDs for the signposts  221  and  222 , then the tag  241  is traveling upwardly in  FIG. 4  along the path of travel  216 . On the other hand, if the signpost IDs for the signposts  221  and  222  were received before the signpost IDs for the signposts  225  and  226 , then the tag  241  is traveling downwardly in  FIG. 4  along the path of travel  216 . 
     With respect to the example just discussed, and for the sake of explanation, assume that the tag  225  is not transmitting any signpost signals, for example due to a malfunction. As the tag  241  travels from the point  286  to the point  287 , it will receive signpost signals from the signpost  226 , containing a value in group size field  106  ( FIG. 2 ) that tells the tag to expect to receive signpost signals from each of four different signposts in a group. However, by the time the tag  241  reaches the point  289 , it will have received signposts signals from only three signposts, which are the signposts  221 - 222  and  226 . Consequently, the counter  48  will have been decremented to a value of 1, but not to a value of 0. However, after the tag has passed the point  289 , the tag will no longer be receiving signpost signals, and will not be repeatedly restarting the timer  49 . In due course therefore, the timer  49  will expire, and will cause the tag to transmit the signpost IDs stored at  47 . In this case, there will be three rather than four signpost IDs stored at  47 , corresponding to the three signposts  221 - 222  and  226 . 
       FIG. 6  is a diagrammatic top view showing an arrangement  296  that is an alternative embodiment of the arrangement  201  of  FIG. 4 . More specifically, the four signposts shown at  225 - 228  in  FIG. 4  have been omitted from the arrangement  296  of  FIG. 6 . In addition, the four signposts  221 - 224  in  FIG. 6  each transmit signpost signals in which the group size field  106  ( FIG. 2 ) contains a value of 2 rather than a value of 4. Aside from this, the arrangement  296  is generally equivalent to the arrangement  201 . 
     In the arrangement  296  of  FIG. 6 , the information provided from any of the tags  241 - 243  through the reader  13  to the control system  14  ( FIG. 1 ) is sufficient for the control system  14  to determine which lane that tag is currently traveling along. However, the control system  14  does not receive enough information to determine the direction in which the tag is traveling along the lane. 
       FIG. 7  is a diagrammatic top view of a further arrangement  301  that represents yet another possible application for a system of the type shown in  FIG. 1 . In  FIG. 7 , a hallway has a narrow portion  303  that opens into a wider portion  304 . The near field or transmission range of a typical signpost is not sufficient to cover the entire width of the wider portion  304  of the hallway. Therefore, two signposts are used for the wider portion  304 . In particular, as shown in  FIG. 7 , a single signpost  307  is stationarily mounted on the ceiling in the narrow portion  303  of the hallway, and two transversely spaced signposts  308  and  309  are stationarily mounted on the ceiling in the wider portion  304  of the hallway. The signposts  307 - 309  are each equivalent to the signpost shown at  11  in  FIG. 1 , but have been given different reference numerals in  FIG. 7  in order to avoid confusion in the discussion that follows. The signposts  307 - 309  have respective near fields or transmission ranges  311 - 313 , and the transmission ranges  312  and  313  of the two signposts  308  and  309  are together sufficient to cover the full width of the wider portion  304  of the hallway. 
     In  FIG. 7 , the signposts  308  and  309  transmit respective signpost signals that contain the same signpost ID  104  ( FIG. 2 ). The signpost  307  transmits signpost signals in which the signpost ID  104  is different from the signpost ID in the signpost signals of the signposts  308  and  309 . In the signpost signals transmitted by each of the signposts  307 - 309 , the group size field  106  ( FIG. 2 ) contains a value of 2. The signposts  308 - 309  are synchronized with each other, and transmit their signpost signals in an alternating manner, so that their signpost signals do not interfere with each other. 
     In  FIG. 7 , a reader  13  is stationarily mounted on the ceiling of the hallway, at a position that is disposed approximately centrally between the three tags  307 - 309 .  FIG. 7  shows two tags  318  and  319 , which are each equivalent to the tag  12  of  FIG. 1 , and which are each capable of moving within the illustrated hallway.  FIG. 7  shows exemplary paths of travel  321  and  322  for the two tags, but the tags are not restricted to these particular paths, and could follow any of a number of other paths as they move along the hallway in either direction. The tags  318  and  319  each operate in a manner similar to that discussed above in association with  FIG. 5 . Based on information that the tags  318  and  319  transmit through the reader  13  to the control system  14  ( FIG. 1 ), the control system  14  can determine the direction in which a given tag is traveling along the hallway. 
       FIG. 8  is a diagrammatic top view of an arrangement  331  that represents still another possible application for a system of the type shown in  FIG. 1 . In  FIG. 8 , four hallways  332 - 335  each extend away from a common intersection in a respective different direction. The hallway  335  is wider than each of the hallways  332 - 334 . The hallways  332 - 334  each have a respective signpost  341 - 343  stationarily mounted on the ceiling. The hallway  335  has two transversely spaced signposts  344  and  345  that are stationarily mounted on the ceiling. The signposts  341 - 345  have respective transmission ranges  347 - 351 . 
     The signposts  344  and  345  each transmit signpost signals having the same signpost ID  104  ( FIG. 2 ), and are synchronized to transmit their signpost signals in an alternating manner, in order to avoid interference. The signposts  341 - 343  each transmit signpost signals with respective signpost IDs  104  that are different from each other and from the signpost ID used by the two signposts  344 - 345 . The signpost signals transmitted by each of the signposts  341 - 345  have a group size field ( FIG. 2 ) that contains a value of 2. A reader  13  is stationarily supported on the ceiling above the common intersection of the four hallways  332 - 335 . 
       FIG. 8  shows three tags  356 - 358  that are capable of moving within the hallways  332 - 335 . The tags  356 - 358  are each equivalent to the tag shown at  12  in  FIG. 1 , but have been given different reference numerals in  FIG. 8  in order to avoid confusion in the discussion that follows.  FIG. 8  shows respective exemplary paths of travel  361 - 363  for the tags  356 - 358 , but the tags are not restricted to these particular paths of travel. The tags  356 - 358  each operate in a manner similar to that discussed above in association with  FIG. 5 . Each of the tags  356 - 358  can transmit information through the reader  13  to the control system  14  ( FIG. 1 ), including signpost IDs stored at  47  ( FIG. 1 ) within the tag. The control system  14  can use this information to determine a current path of travel of a given tag, for example from one of the four hallways  332 - 335  into another of these four hallways. In addition, the control system  14  can determine the direction in which a given tag is currently moving along its path of travel. 
       FIG. 9  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequence of operations shown in the flowchart of  FIG. 5 . With reference to  FIG. 1 , the receiver  42  within each tag is capable of detecting whether or not the tag is currently within the primarily magnetic near field of any signpost, and thus within the transmission range of a signpost.  FIG. 9  differs from  FIG. 5  primarily in that the tag does not use the timer  49  ( FIG. 1 ), but instead monitors whether or not the tag is currently within the magnetic near field of any signpost, or in other words within the transmission range of any signpost. 
     More specifically, in block  401  of  FIG. 9 , the tag discards any signpost IDs that the tag may have previously stored in  47  in the memory  46  ( FIG. 1 ). The tag also disables the counter  48  by setting it to zero. Then, at block  402 , the tag checks to see whether its receiver  42  is currently detecting the magnetic field of any signpost. If not, then the tag remains at block  402 , waiting to enter a signpost field. If the tag eventually does enter a signpost field, then it proceeds to block  403 , where it again checks for the presence of a signpost field. If the tag were to detect the absence of a signpost field, then the tag would proceed to block  406 , which is discussed later. But when the tag first encounters block  403 , the signpost field will still be present, and the tag will proceed to block  407 . 
     In block  407 , the tag checks to see whether it has actually received a signpost signal. If not, then it returns to block  403  to wait for a signpost signal. If it eventually determines in block  407  that is has received a signpost signal, the tag proceeds to block  408 , where it checks to see if the signpost ID  104  ( FIG. 2 ) in the received signpost signal has already been stored in its memory at  47  ( FIG. 1 ). If so, then the tag enters its sleep mode at block  411 , and returns to block  403 . Otherwise, the tag proceeds from block  408  to block  412 , where it stores the received signpost ID in its memory at  47 . 
     The tag then proceeds to block  413 , where it checks to see if the counter is currently zero. If so, then the counter has not been initialized, and the tag proceeds to block  416 , where it initializes the counter  48  with the value from the group size field  106  ( FIG. 2 ) in the received signpost signal. From  416 , or from block  413  if the tag determines that the counter is not zero, the tag proceeds to block  417 , where it decrements the counter. Then, at block  418 , the tag checks to see if the counter has reached zero, or in other words whether the tag has received a respective signpost signal from each signpost in the group. If not, then the tag returns to block  403  and waits to receive a signpost signal from another signpost. Otherwise, the tag proceeds from block  418  to block  406 . In block  406 , the tag switches to its normal operational mode (if it is not already in the normal mode). Then, the tag transmits all of the signpost IDs stored at  47  in its memory, using one or more tag signals of the type shown in  FIG. 3 . The stored signpost IDs would be inserted into respective fields, such as those shown at  132 - 134  in  FIG. 3 . 
     From the time when the tag first detects a signpost field in block  402  until the tag reaches block  406 , the tag inhibits the transmission of tag signals at  56  using the UHF transceiver  51 . During this time interval, when UHF transmissions are being suppressed, the tag can also optionally conserve battery power by inhibiting reception of wireless signals through the receiver portion of its UHF transceiver  51 , or by turning off power to the receiver portion of its UHF transceiver  51 . 
       FIG. 10  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequences of operation shown in the flowcharts of  FIGS. 5 and 9 . The flowchart of  FIG. 10  differs from the flowchart of  FIG. 9  primarily in that the counter  48  is not used. In other words, the tag does not look for signpost signals from a specific number of signposts that collectively form a group. In block  451  of  FIG. 10 , the tag discards any signposts IDs that it may have previously stored at  47  in its memory  46 . Then, at block  452 , the tag checks to see whether its receiver  42  is currently detecting the presence of a magnetic field from any signpost. If not, the tag waits at block  452  until a magnetic signpost field is detected. When a magnetic field is detected, the tag proceeds to block  453 , where it again checks for the presence of a magnetic signpost field. When the tag first moves from block  452  to block  453 , it will find that there is a magnetic signpost field, and it will therefore proceed from block  453  to block  457 . In block  457 , the tag checks to see whether it has received a signpost signal. If not, then it returns to block  453  in order to wait for a signpost signal. On the other hand, if it has received a signpost signal, then the tag proceeds to block  458 . 
     In block  458 , the tag checks to see whether the signpost ID  104  ( FIG. 2 ) in the received signpost signal is already stored in its memory at  47  ( FIG. 1 ). If so, the tag enters its sleep mode at block  461 , and returns to block  453  in order to wait for another signpost signal. Otherwise, the tag proceeds from block  458  to block  462 , where it stores the received signpost ID in its memory at  47 , and then returns to block  453 . 
     The tag may pass through overlapping fields of two or more signposts, but the tag will eventually move to a location where, in block  453 , it does not detect a magnetic field from any signpost. The tag will proceed to block  463 . In block  463 , the tag returns to its normal operational mode (if it is not already in the normal mode). Then, the tag transmits all signpost IDs that it has stored in  47 , using one or more tag signals of the type shown in  FIG. 3 . The respective signpost IDs will appear in respective fields, such as those shown at  132 - 134  in  FIG. 3 . 
     From the time when the tag detects a signpost field in block  452  until the tag reaches block  463 , the tag inhibits the transmission of tag signals at  56  using the UHF transceiver  51 . During this time interval, when UHF transmissions are being suppressed, the tag can also optionally conserve battery power by inhibiting reception of wireless signals through the receiver portion of its UHF transceiver  51 , or by turning off power to the receiver portion of its UHF transceiver  51 . 
       FIG. 11  is a flowchart showing a sequence of operations that can be carried out by a tag, and that is an alternative embodiment of the sequences of operations shown in the flowcharts of  FIGS. 5 ,  9  and  10 . The primary difference is that, in the flowchart of  FIG. 11 , the tag relies specifically on the timer  49  to determine when to transmit received signpost IDs. More specifically, in block  501  of  FIG. 11 , the tag discards any signpost IDs that it may have previously stored in its memory at  47  ( FIG. 1 ). The tag also disables the timer  49 . Then, in block  502 , the tag checks to see whether the timer has just expired. When the tag first encounters the block  502 , the tag will have just disabled the timer  49  in block  501 , and thus the tag will determine that the timer has not just expired. The tag will therefore proceed to block  503 , where it will check to see if it has actually received a signpost signal. If not, then it returns to block  502  to wait for a signpost signal. But if it has received a signpost signal, the tag will proceed from block  503  to block  506 , where it starts the timer  49 . 
     Then, in block  507 , the tag checks to see whether the signpost ID  104  ( FIG. 2 ) in the received signpost signal is already stored in its memory at  47  ( FIG. 1 ). If so, then the tag enters the sleep mode at  508 , and returns to block  502  in order to wait for another signpost signal. Otherwise, the tag proceeds from block  507  to block  511 , where it stores the received signpost ID in its memory at  47 . The tag then returns to block  502 , in order to wait for another signpost signal. 
     Each time the tag receives a signpost signal, it will restart its timer  49  in block  506 , such that the timer does not have an opportunity to expire. Eventually, however, the tag will travel to a location outside the transmission ranges of all signposts. As a result, the tag will not be receiving any signpost signals, and therefore will not be restarting the timer at block  506 . Consequently, the timer  49  will expire in due course, and the tag will detect this at block  502  and proceed to block  512 . 
     In block  512 , the tag enters its normal operational mode (if it is not already in the normal mode). The tag then transmits the signpost IDs that it stored at  47  in its memory, using one or more tag signals of the type shown in  FIG. 3 . The signpost IDs would appear in respective fields, such as those shown in at  132 - 134  in  FIG. 3 . 
     From the time when the tag detects receipt of a first signpost signal in block  503  until the tag reaches block  512 , the tag inhibits the transmission of tag signals at  56  using the UHF transceiver  51 . During this time interval, when UHF transmissions are being suppressed, the tag can also optionally conserve battery power by inhibiting reception of wireless signals through the receiver portion of its UHF transceiver  51 , or by turning off power to the receiver portion of its UHF transceiver  51 . 
     Although selected embodiments have been illustrated and described in detail, it should be understood that a variety of substitutions and alterations are possible without departing from the spirit and scope of the present invention, as defined by the claims that follow.