Abstract:
A tag can receive first and second wireless signals respectively defining first and second transmission fields that overlap. When the tag moves from the first field to the second field through the region of overlap, the apparatus transitions only once from recognizing the tag is in the first field to recognizing the tag is in the second field. According to another aspect, first and second signposts are supported at spaced locations in the region of a crane, and respectively transmit first and second wireless signals that are different, and that respectively define first and second fields of transmission. According to yet another aspect, a signpost transmits wireless signals having a transmission field, and a system senses positional information regarding a tag movable relative to the signpost, the system using the positional information to determine a location of the tag in relation to the transmission field.

Description:
[0001]    This application claims the priority under 35 U.S.C. §119 of provisional application No. 60/837,467 filed Aug. 14, 2006. 
     
     FIELD OF THE INVENTION 
       [0002]    This invention relates in general to techniques for tracking containers or other objects and, more particularly, to tracking techniques that use radio frequency identification technology. 
       BACKGROUND 
       [0003]    It is desirable to be able to track containers or other objects, for example during shipment of a container. One known technique for tracking a container is to mount a radio frequency identification (RFID) tag on the container. The RFID tag can transmit wireless signals, and typically can also receive wireless signals. 
         [0004]    In more detail, there are RFID tags capable of receiving wireless signals transmitted by a signpost, where these wireless signals contain a digital code that uniquely identifies a particular signpost. These tags then transmit tag signals that contain a unique code identifying the tag as well as the unique code identifying the signpost. In circumstances where the transmission ranges or fields of two signposts effectively overlap, a tag moving through the overlap region is likely to receive signpost signals from each of the two tags in an alternating manner. It can be problematic if the tag repeatedly “flip-flops” back and forth between recognizing one signpost and recognizing the other signpost. A suitable technique to avoid this is desirable. 
         [0005]    A different consideration is that, in order to successively load or unload a plurality of containers to or from a location such as the cargo hold of a ship, a large crane is typically used. It would be advantageous to have a technique for tracking containers while they are being moved by such a crane. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    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: 
           [0007]      FIG. 1  is a diagrammatic side view of an apparatus that embodies aspects of the invention, and that includes a large crane, and a container supported by the crane. 
           [0008]      FIG. 2  is a diagrammatic front view of the crane and container of  FIG. 1 . 
           [0009]      FIG. 3  is a flowchart depicting a technique carried out by a radio frequency identification tag that is a component of the apparatus of  FIG. 1 . 
           [0010]      FIG. 4  is a flowchart depicting a technique that is an alternative embodiment of the technique shown in  FIG. 3 . 
           [0011]      FIG. 5  is a flowchart showing a technique that is a further alternative embodiment of the technique shown in  FIG. 3 . 
           [0012]      FIG. 6  is a flowchart depicting a technique that is still another alternative embodiment of the technique shown in  FIG. 3 , and that is carried out by a not-illustrated central system that cooperates with the apparatus of  FIG. 1 . 
           [0013]      FIG. 7  is a diagram showing the tag of  FIG. 1  moving through overlapping transmission fields of two signposts. 
           [0014]      FIG. 8  is a timing diagram showing aspects of a technique that is a further alternative embodiment of the technique shown in  FIG. 3 . 
           [0015]      FIGS. 9 and 10  are flowcharts showing an implementation of the technique of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a diagrammatic side view of an apparatus  10  that includes a large crane  11 , and a container  12  supported by the crane.  FIG. 2  is a diagrammatic front view of the crane and container of  FIG. 1 . In the disclosed embodiment, the container  12  is a conventional shipping container of a well-known type. For example, the container may conform to standards set by the International Organization for Standardization (ISO), and in particular may be a container that complies with an industry-standard specification known as an ISO 668:1995(E) Series 1 freight container. The vast majority of containers that are currently in commercial use conform to this ISO standard. 
         [0017]    The crane  11  is used for moving large objects, such as the shipping container  12 . As one particular example, the crane may be used to successively load or unload a plurality of containers to or from a location such as the cargo hold of a ship. The crane  11  includes an approximately rectangular frame that is made of steel, and that has vertical edges defined by four parallel, vertical posts  16 - 19 . The frame of the crane could alternatively be made of some other suitable material, such as aluminum. A respective wheel is rotatably supported at the lower end of each of the posts  16 - 19 , three of these wheels being visible at  21 - 23  in the drawings. The wheels facilitate transverse movement of the entire crane  11  in directions  26  and  27  ( FIG. 2 ). 
         [0018]    As shown in  FIG. 2 , the frame of the crane includes a horizontal lower beam  28  that extends between and is fixedly secured to the lower ends of the posts  16  and  17 . A similar horizontal lower beam  29  extends between and is secured to the lower ends of the posts  18  and  19 . With reference to  FIG. 1 , a middle beam  32  extends horizontally between and is secured to the posts  17  and  19 , at a location approximately halfway along the length of each post. A similar middle beam extends horizontally between and is secured to the posts  16  and  18 , but is not visible in the drawings. With reference to  FIG. 2 , an upper beam  33  extends horizontally between and is secured to the upper ends of the posts  16  and  17 . A similar upper beam extends horizontally between and is secured to the upper ends of the posts  18  and  19 , but is not visible in drawings. Yet another upper beam  34  extends horizontally between and is secured to the upper ends of the posts  17  and  19 . And a further upper beam extends horizontally between and is secured to the upper ends of the posts  16  and  18 , but is not visible in the drawings. With reference to  FIG. 1 , a diagonal beam  37  has one end fixedly secured to the intersection of post  17  and beam  34 , and has its other end fixedly secured to the intersection of post  19  and beam  32 . A similar diagonal beam is provided on the opposite side of the crane, but is not visible in the drawings. 
         [0019]    Two parallel, horizontal support beams  41  and  42  are fixedly supported beneath the upper beams  33  of the crane frame, and extend both forwardly and rearwardly beyond the frame of the crane. With reference to  FIG. 1 , a vertical post  46  and two diagonal struts  47  and  48  are provided over the support beam  42 . The post  46  has its lower end secured to the upper beam  33 . The diagonal struts  47  and  48  each have one end secured to the top of the post  46 , and the other end secured to a respective end of the support beam  42 . A similar vertical post with two struts is provided over the support beam  41 , one of these diagonal struts being visible at  51  in  FIG. 2 . A horizontal cross-beam  52  extends between and is secured to the upper ends of the vertical posts. A counterweight  54  is fixedly supported on the upper side of the support beams  41  and  42 , rearwardly of the frame of the crane. 
         [0020]    The crane  11  includes a cabin  61  that is disposed just below the support beams  41  and  42 . The cabin  61  is supported by the support beams  41  and  42  for horizontal movement therealong, as indicated diagrammatically by arrows  62  and  63  in  FIG. 1 . Inside the cabin  61  are not-illustrated controls that can be manually manipulated by a human operator, in order to control operation of the crane  11 . Several cables  64  extend vertically downwardly from the cabin  61 , and have their lower ends detachably coupled to the container  12 . The cabin  61  includes a not-illustrated hoist arrangement that can effect vertical movement of the cables  64 , in order to raise and lower the container  12 . A cable position sensor is shown diagrammatically at  68 , and produces an electrical signal that represents the vertical position of the cables  64 , and thus the vertical position of the container  12 . Although the sensor  68  in the disclosed embodiment is a cable position sensor, it could alternatively be any of a variety of other types of sensors, including but not limited to mechanical, optical electrical, magnetic, radio frequency, and infrared sensors or switches. 
         [0021]    A container tracking system includes a ground signpost  81  that is fixedly supported in approximately the center of the lower beam  29 , on the inner side of the beam  29 . The signpost  81  is a device of a type known in the art, and is therefore described only briefly here. The signpost  81  contains a not-illustrated antenna, and a not-illustrated transmitter circuit that uses the antenna to transmit a low-frequency signpost signal. The signpost signal is generated by amplitude modulating a carrier signal that, in the disclosed embodiment, has a carrier frequency of approximately 123 KHz. The antenna in the signpost is configured so that, in association with this carrier frequency, the low-frequency signpost signals exhibit near field characteristics of a primarily magnetic character. 
         [0022]    More specifically, electromagnetic signals can include both an electrical characteristic (the E field) and a magnetic characteristic (the H field). As noted above, the antenna in the signpost  81  is configured so that, at the specified carrier frequency, it generates signpost signals consisting primarily of the magnetic H field, with only a nominal E field. Moreover, this is a non-propagating signal, representing stored energy in the region around the signpost. Consequently, the signpost signals are relatively strong in the near field, but almost negligible in the far field. The localized nature of these signpost signals helps to facilitate compliance with governmental regulations regarding wireless transmissions, and also helps to minimize reception of these signals other than in the localized region around the signpost that transmits them. 
         [0023]    As mentioned earlier, the crane  11  has an approximately rectangular frame that is made of steel, and that is bounded on four sides by the vertical posts  16 - 19 . Since the wireless signpost signals transmitted by the signpost  81  are primarily magnetic in character, they can be affected by environmental factors. As one example, in the disclosed embodiment the frame of the crane is made of metal and is therefore electrically conductive. The electrically conductive metal frame tends to enhance these signals within the interior of the frame, while attenuating them in regions external to the frame. In  FIGS. 1 and 2 , reference numeral  82  designates a broken line that represents the transmission range or field of the signpost signals transmitted by the ground signpost  81 . The signpost signals transmitted by the signpost  81  include a digital code that is unique to the signpost  81 , so that a device receiving the signposts signals can tell they originated from the signpost  81 , rather than from some other signpost. 
         [0024]    A cabin signpost  86  is fixedly mounted to the underside of the cabin  61  of the crane. In the disclosed embodiment, the cabin signpost  86  is effectively identical to the ground signpost  81 , except that signpost signals transmitted by the cabin signpost  86  contain a unique digital code that is different from the unique digital code in the signpost signals transmitted by the ground signpost  81 . When the cabin  61  is disposed within the metal frame of the crane  11 , the magnetic signpost signals transmitted by the cabin signpost  86  are enhanced somewhat by the electrically conductive characteristics of the metal frame. 
         [0025]    Reference numeral  87  designates a broken line that represents the transmission range or field of the signpost signals transmitted by the cabin signpost  86 , when the cabin  61  is within the metal frame. In contrast, when the cabin  61  moves along the support beams  41  and  42  to a position in which it is outside the metal frame, as shown diagrammatically by broken lines in  FIG. 1 , the signpost signals from the cabin signpost  86  receive a reduced degree of enhancement from the metal frame. Reference numeral  87 A designates a broken line that represents the reduced transmission range or field of the signpost signals from the cabin signpost  86 , when the cabin  61  is outside the metal frame. The transmission power of the cabin signpost  86  may intentionally be set to a limited power level that ensures the wireless signals emitted by the cabin signpost do not trigger and wake up tags on containers within the cargo hold of a ship that is being loaded or unloaded. Under certain circumstances, especially when the cabin  61  is disposed within the metal frame of the crane, the fields  82  and  87  of the signposts  81  and  86  can overlap, as indicated diagrammatically at  88 . 
         [0026]    As a practical matter, the signpost signals transmitted by the signposts  81  and  86  can be affected not only by the metal frame of the crane, but to a lesser extent by a variety of other environmental factors, such as the metal material of a container  12  that is being moved by the crane. These other types of effects are not all discussed here in detail, but it should be recognized that they can be present. 
         [0027]    In addition to the signposts  81  and  86 , the container tracking system includes a radio frequency identification (RFID) tag  91 , which is fixedly mounted on the container  12 . Each container moved by the crane may have a similar tag mounted thereon. The tag  91  is a device of a type known in the art, except for some unique aspects that are discussed later. The tag  91  contains a not-illustrated antenna, and associated receiver circuitry that can receive the signpost signals transmitted by the signposts  81  and  86 . The tag  91  also contains a further antenna that is not illustrated, and associated ultra high frequency (UHF) transmitter circuitry. The tag  91  can use its UHF transmitter to transmit tag signals. The tag  91  generates the tag signals by effecting frequency shift keying (FSK) modulation of certain information onto a carrier signal that, in the disclosed embodiment, has a carrier frequency of 433.92 MHz. Other suitable carrier frequencies could alternatively be used. The tag signals are propagating signals that have both an E field and an H field, and the transmission range for the tag signals is substantially longer than that for the signpost signals. The tag signals include a digital code that uniquely identifies the tag  91  that transmitted the tag signals. In addition, when the tag  91  has received one or more signpost signals, the tag signals transmitted by the tag can include the unique digital signpost identification code from the received signpost signals. 
         [0028]    The container tracking system further includes a reader  94 , which is fixedly mounted on the inner side of the middle beam  32 . The reader  94  is a device of a known type, and can receive the UHF tag signals transmitted by the tag  91 , or by other similar tags. The container tracking system further includes a central system that is not separately illustrated, and that is operatively coupled to the signposts  81  and  86 , the reader  94 , and the sensor  68 , for example through not-illustrated cables. The reader  94  and the central system could share a common housing. 
         [0029]    With reference to  FIGS. 1 and 2 , assume that the container  12  is currently supported by the crane  11  at the position shown in  FIG. 1 . The container  12  with the tag  91  thereon are within the transmission range or field  87  of signpost signals transmitted by the signpost  86 . The tag  91  will thus be receiving these signposts signals. The tag  91  will be transmitting tag signals that include the unique digital tag code of the tag  91 , as well as the unique digital signpost code from the received signpost signals. In this regard, for the sake of simplicity here, it is assumed that the tag  91  transmits tag signals relatively frequently, at least when it is receiving signpost signals. Alternatively, however, the tag could transmit tag signals less frequently, on more of an “as needed” basis. The tag signals transmitted by the tag  91  will be received by the reader  94 , and will be forwarded to the not-illustrated central system, for example through a not-illustrated cable. Based on information from the tag signals, the central system will know that the tag  91  and thus the associated container  12  are somewhere in the vicinity of the cabin  61  with the cabin signpost  86  thereon. 
         [0030]    Still referring to  FIG. 1 , assume that the crane  11  lowers the container  12  vertically downwardly from the elevated position shown in  FIG. 1 , to a not-illustrated position in which the container is resting on the ground. The container  12  and the tag  91  will then be outside the transmission range or field  87  of the cabin signpost  86 , and within the transmission range or field  82  of the ground signpost  81 . The tag  91  will thus be receiving the signpost signals transmitted by the ground signpost  81 . Further, the tag will be transmitting tag signals that include the unique digital tag code of the tag  91 , as well as the unique digital signpost code of the signpost  81 . The reader  94  will be receiving these tag signals, and will be forwarding the information from them to the not-illustrated central system. The central system will thus know that the container  12  is somewhere close to the ground, in the general vicinity of the signpost  81 . 
         [0031]    As the crane lowers the container  12  from the elevated position shown in  FIG. 1  to the not-illustrated position where the container is resting on the ground, the container  12  will move from a position where the tag  91  is transmitting tag signals that include the digital signpost code of the signpost  86  to a position where the tag  91  is transmitting tag signals that include the digital signpost code of the signpost  81 . The not-illustrated central system can tell from this information that the container  12  has been moved downwardly by the crane, or in other words that the container  12  is being unloaded from something. Conversely, when the crane  11  lifts the container  12  from a position near the ground to the elevated position shown in  FIG. 1 , the tag  91  will move from a position where it is transmitting tag signals that include the unique digital signpost code of the signpost  81  to a position where it is transmitting tag signals that include the unique digital signpost code of the signpost  86 . The central system will thus know that the container  122  has been moved upwardly by the crane  11 , or in other words that the container  12  is being loaded onto or into something. 
         [0032]    As discussed earlier, it is possible for the transmission range or field  82  of the signpost signals from signpost  81  to overlap at  88  with the transmission range or field  87  of the signpost signals from signpost  86 . As the tag  91  moves vertically through this overlap region  88 , problems could potentially occur. First, if the signposts  81  and  86  are both transmitting signposts signals at exactly the same time, these signpost signals could possibly “collide” with each other in the overlap region  88 , as a result of which the tag  91  might not be able to make sense of either signpost signal. Accordingly, in the disclosed embodiment, the not-illustrated central system is coupled to and synchronizes operation of the signposts  81  and  86 , so that they transmit in an alternating manner. Consequently, signals from the two signposts will not collide with each other. 
         [0033]    A further potential problem is that, even when the signposts  81  and  86  are synchronized in this manner, when the tag  91  moves vertically through the overlap region  88 , the tag may receive signals from each of the signpost  81  and  86  in an alternating manner. This could potentially cause the tag  91  to repeatedly “flip-flop” between (1) transmitting tag signals that contain the unique signpost code of the signpost  81 , and (2) transmitting tag signals that contain the unique signpost code of the cabin signpost  86 . In order to avoid this potential flip-flopping problem, the tag  91  uses a novel technique that allows it to effect a single transition from recognizing one of the signposts to recognizing the other thereof. 
         [0034]    In more detail,  FIG. 3  is a flowchart depicting how the tag  91  carries out this technique. Beginning in block  101 , the tag  91  disables an internal timer. Control then proceeds to block  102 , where the tag checks to see whether it has received a signpost signal. Assuming that it has, control proceeds to block  103 , where the tag checks to see whether a predetermined amount of time “T” has elapsed since the tag last received a signpost signal. In the scenario under consideration, where the tag  91  is moving from one of the overlap regions  82  and  87  to the other thereof through the overlap region  88 , the tag will be receiving signpost signals almost continuously. Control will therefore proceed from block  103  to block  106 . 
         [0035]    In block  106 , the tag  91  checks to see whether the unique signpost identification code in the received signpost signal is the same as a current signpost identification code. If the tag is moving out of the field  82 , then the current identification code will be the identification code of the ground signpost  81 . Conversely, if the tag is moving out of the field  87 , then the current identification code will be the identification code of the cabin signpost  86 . The current identification code is the signpost identification code that the tag includes in the tag signals that it transmits. If the tag determines in block  106  that the newly-received signpost identification code is different from the current signpost identification code, then the tag has entered and is traveling through the overlap region  88 , and control proceeds to block  107 . 
         [0036]    In block  107 , the tag checks to see whether the timer is running. In the hypothetical scenario under discussion, the timer will not yet be running, and so control will proceed to block  108 . In block  108 , the tag starts the timer, and saves the unique signpost identification code from the signpost signal that it just received. The tag does not yet replace the current signpost identification code with this new signpost identification code. Control then returns to block  102 . 
         [0037]    In block  102 , the tag checks to see whether it has received yet another signpost signal. Assume that the tag finds it has just received another signpost signal, and proceeds to block  102  through block  103  to block  106 . If the signpost identification code in this latest signpost signal is the same as the current signpost identification code, control will proceed from block  106  to block  101 , where the tag will turn off the timer. By turning off the timer, the tag will effectively ignore the new signpost code that was previously received and stored at block  108 . Thus, as the tag is moving through the overlap region  88 , and is alternately receiving signpost signals from each of the signposts  81  and  86 , the tag will effectively ignore all signpost signals that contain signpost identification codes different from its current signpost identification code, until after the tag has passed through the overlap region  88 . 
         [0038]    In particular, when the tag exits the overlap region  88 , it will stop receiving signpost signals containing signpost identification codes that are identical to the tag&#39;s current identification code, and will thereafter receive only signpost signals containing the new signpost identification code. Consequently, in block  106 , the tag will not return to block  101  and disable the timer  101 . Instead, the tag will proceed each time to block  107 . In block  107 , the tag will find that the timer is already running, and then control will return directly to block  102  to await the receipt of a further signpost signal. 
         [0039]    In block  102 , if the tag finds that it has not received a signpost signal, the tag proceeds to block  112 , where it checks to see whether the timer has just expired. If not, then the tag returns to block  102  in order to continue to wait for another signpost signal. Otherwise, if the timer has just expired, then the tag has exited the overlap region  88 , and has received only signals from the new signpost for the time period measured by the timer  88 . This means that it is time for the tag to transition from recognizing the old signpost to recognizing the new signpost. Therefore, control proceeds from block  112  to block  113 , where the tag takes the new signpost identification code that it saved most recently at block  108 , and assigns this new signpost identification code to be the current identification code. 
         [0040]    As noted earlier, the current signpost identification code is the signpost identification code that the tag includes in its tag signals. From block  113 , control proceeds to block  114 , where the tag moves the current signpost identification code into the tag&#39;s transmission queue. When the tag completes any sequence of transmissions that may already be in progress and that use the prior value of the current identification code, the tag will find the new value of the current identification code in the transmission queue, and will then switch over to using the new value of the current identification code in the tag signals that it transmits. From block  114 , control returns to block  102 . 
         [0041]    In block  103 , and as discussed above, the tag checks to see whether a predetermined time “T” has elapsed since the tag last received a signpost signal. This will be the case, for example, where a container was stored at a ground level location outside the transmission field  82  of the ground signpost  81 , but has just been moved into the field  82 . Upon entering the field  82 , the tag will immediately receive a signpost signal transmitted by the signpost  81 , and will thus proceed from block  103  to block  116 , where it immediately assigns the signpost identification code from the received signpost signal to be the current identification code. Control then proceeds to block  114 , where this new current identification code is moved into the transmission queue. 
         [0042]    From the foregoing discussion, it should be evident that, as the container  12  is moved upwardly from the field  82  of ground signpost  81  through the overlap region  88  to the field  87  of cabin signpost  86 , the tag will wait until after it has exited the overlap region  88 , and then will make a single transition from recognizing the ground signpost  81  to recognizing the cabin signpost  86 . Similarly, when the container  12  is moved downwardly through the overlap region  88 , the tag  91  will make a single transition from recognizing the cabin signpost  86  to recognizing the ground signpost  81 . Thus, in a sense, the technique shown in  FIG. 3  introduces a degree of hysteresis into the tag&#39;s recognition of signposts, so that the tag does not repeatedly flip-flop between recognizing the signpost  81  and recognizing the signpost  86  while the tag is within the overlap region  88 . 
         [0043]      FIG. 4  is a flowchart depicting a technique that is an alternative embodiment of the technique shown in  FIG. 3 . In this technique, the ground signpost  81  is given priority over the cabin signpost  86 . In more detail, and beginning in block  151 , the tag disables the timer, and also resets a counter. Control then proceeds to block  152 , where the tag checks to see whether it has just received a signpost signal. If so, then control proceeds to block  153 , where the tag checks to see whether a predetermined time “T” has elapsed since the tag last received any signpost signal. In the scenario under discussion, the tag will be receiving signpost signals almost continuously, and so control will progress from block  153  to block  156 , where the tag will determine whether the signpost code in the most recently received signpost signal is from the ground signpost  81  or the cabin signpost  86 . If the received signpost signal is from the ground signpost  81 , then the tag immediately proceeds to block  157 , where it disables the timer and resets the counter. Then, in block  158 , the tag moves the signpost code from the received signpost signal into the transmission queue, so that tag signals transmitted by the tag will include the unique signpost code of the ground signpost  81 . Control then returns to block  152 . 
         [0044]    Referring again to block  156 , if the tag determines that the most recently received signpost signal is from the cabin signpost  86 , then the tag proceeds to block  161 , where the tag increments the counter. Then, in block  162 , the tag checks to see whether the timer is already running. If so, then the tag returns to block  152 . Otherwise, in block  163 , the tag starts the timer, and then returns to block  152 . 
         [0045]    In block  152 , if the tag finds that it has just received another signpost signal, it proceeds to block  166 , where it checks to see whether the count in the counter is greater than a predetermined count “C”. In effect, if the tag receives “C” successive signpost signals from the cabin signpost  86 , then control will proceed to block  167 , where the unique signpost identification code of the cabin signpost is placed in the transmission queue, so that this code will be used in tag signals transmitted by the tag. Control then proceeds to block  167  back to block  152 . 
         [0046]    In block  166 , if the tag determines that the count in the counter is below the predetermined count “C”, then control proceeds from block  166  to block  168 , where the tag checks to see whether the timer has just expired. If not, then control returns to block  152 . Otherwise, if the timer has just expired, then the tag has not received any signpost signal from the ground signpost  81  during the time interval measured by the timer, and the tag therefore proceeds to block  167  in order to transition over to use of the signpost identification code for the cabin signpost. 
         [0047]    In a situation where the tag has not been within the transmission field of any signpost for the time period “T”, and then enters a signpost field, the tag will soon find itself in block  153 , and will then proceed to block  171 . In block  171 , the tag immediately takes the unique signpost identification code from the received signal, and places this code in the transmission queue, so that this code will immediately be included in tag signals transmitted by the tag, regardless of whether the received signpost signal was from the ground signpost or the cabin signpost. 
         [0048]    Summarizing the technique shown in  FIG. 4 , the ground signpost  81  receives priority over the cabin signpost  86 . If the tag receives a signpost signal from the ground signpost  81 , then that signal takes precedence, and the tag will promptly include the unique signpost identification code of the ground signpost in its tag signals. On the other hand, whenever the tag receives a signpost signal from the cabin signpost  86 , the tag will not transition over to use of the identification code for the cabin signpost unless one of several conditions is met. First, if the signpost signal from the cabin signpost is received at the end of a predetermined time interval “T”, during which no signpost signals were received from any signpost, then the tag will immediately begin transmitting tag signals that contain the signpost code of the cabin signpost. Second, if the tag receives a signpost signal from the cabin signpost and then does not receive any signpost signal from the ground signpost during the time interval measured by the timer, then the tag will transition over to putting the signpost identification code of the cabin signpost into its tag signals. Third, if the tag receives “C” successive signpost signals from the cabin signpost without receiving any signpost signal from the ground signpost, then the tag will transition over to putting the signpost identification code of the cabin signpost into its tag signals. 
         [0049]      FIG. 5  is a flowchart depicting a technique that is a further alternative embodiment of the technique shown in  FIG. 3 . In block  181  of  FIG. 5 , the tag checks to see whether it has received a signpost signal. If not, then it waits for signpost signals in a loop that includes block  181 . On the other hand, when a signpost signal is received, control proceeds from block  181  to block  182 . One of the capabilities of the tag  91  is that it can determine the strength or amplitude of each signpost signal it receives. In block  182 , the tag checks to see whether the signal strength of the received signpost signal is greater than a predetermined threshold. In general, the thresholds are established so that, when the tag is in the overlap region  88 , the signpost signals from the ground signpost  81  will be below one threshold, and the signpost signals from the cabin signpost  86  will be below a different threshold. Consequently, if the tag is in the overlap region  88 , control will return from block  182  back to block  181 . On the other hand, if the received signpost signal is above its corresponding threshold, then the tag knows it is not within the overlap region  88 , and the tag proceeds to block  183 . In block  183 , the tag moves the unique signpost identification code from the received signpost signal into the transmission queue, so that this unique code will be promptly included in tag signals transmitted by the tag. From block  183 , control returns to block  181 . 
         [0050]      FIG. 6  is a flowchart depicting a technique that is still another alternative embodiment of the technique shown in  FIG. 3 . The technique of  FIG. 6  is implemented using the not-illustrated central system that is coupled to the reader  94 , the signposts  81  and  86 , and the sensor  68 . In block  191 , the central system reads information from the sensor  68  that represents the current vertical position of the container  12 . In block  192 , the central system uses this information from sensor  68  to determine whether the container  12  is currently closer to the cabin signpost  86  or the ground signpost  81 . If the container is closer to the cabin signpost  86 , then control proceeds to block  193 . In block  193 , the central system disables the ground signpost  81  so that the ground signpost  81  does not transmit any signpost signals, and enables the cabin signpost  86  so that it transmits signpost signals. Conversely, if it is determined in block  192  that the container  12  is closer to the ground signpost  81 , then control proceeds to block  194 , where the central system disables the cabin signpost  86  so that it does not transmit any signpost signals, and enables the ground signpost  81  to transmit signpost signals. Thus, through use of the technique shown in  FIG. 6 , only one of the two signposts  81  and  86  is enabled at any given point in time. Consequently, while the fields  82  and  87  of the two signposts  81  and  86  may overlap  88  in physical space, they will not overlap in time, because they will not both be transmitting at the same time in the overlap region  88 . In particular, any given point in time, one or the other of the two signposts  81  and  86  will be disabled. 
         [0051]      FIG. 7  is a diagram showing the transmission field  82  of the ground signpost  81 , and the transmission  87  of the cabin signpost  86 , as well as the overlap  88  therebetween. As the tag  91  moves upwardly through the transmission fields  81  and  82 , it passes successively through five regions R 1 , R 2 , R 3 , R 4  and R 5 . In region R 1 , the tag  91  receives signals from the ground signpost  81 , but not the cabin signpost  86 . In region R 2 , the tag is approaching the transmission field  87  of the cabin signpost  86 . Although reference numeral  87  designates the theoretical transmission range of the signals from cabin signpost  86 , as a practical matter these signals do not end precisely at any boundary line. Consequently, in the region R 2 , the tag  91  may begin to receive signals from the cabin signpost  86 , although these signals are likely to be relatively weak and possibly incomplete. For example, in the disclosed embodiment, signpost signals emitted by the signposts  81  and  86  each include a preamble that is followed by data and then error-checking information, such as a cyclic redundancy code (CRC). Tag  91  may receive all or part of the preamble, and thus recognize at a signpost signal is present, but the data and/or the CRC code may not be received, or may include errors. 
         [0052]    In due course, the tag will enter region R 3 , or in other words the overlap region  88 . In region R 3 , the tag should easily receive signals from both the ground signpost  81  and the cabin signpost  86 . Later, the tag will move into region R 4 . In theory, the tag should not be receiving signals from the ground signpost in region R 4 . However, with reference to the foregoing discussion of region R 2 , the tag may pick up relatively weak signals from the ground signpost in region R 4 , where the received information may be erroneous and/or incomplete. As one example, the tag may receive the preamble of a signal, but not the information that follows the preamble. The tag will eventually move from region R 4  into region R 5 , where it will receive signals from the cabin signpost  86 , but will not receive signals from, the ground signpost  81 . 
         [0053]      FIG. 8  is a timing diagram showing aspects of a technique that is a further alternative embodiment of the technique shown in  FIG. 3 . Reading from left to right in  FIG. 8  corresponds to upward movement of the tag  91  in  FIG. 7 . The top line in  FIG. 8  identifies the time periods that the tag successively spends in each of the regions R 1 , R 2 , R 3 , R 4  and R 5 , as the tag moves upwardly in  FIG. 7 . The next line in  FIG. 8  is a diagrammatic representation of the transmissions of signpost signals by the ground signpost  81  and the cabin signpost  86 , where the ground signpost  81  is represented by the letter A, and the cabin signpost is represented by the letter B. As explained earlier, these two signposts are synchronized, so that they transmit their signpost signals in an alternating manner. 
         [0054]    The next line in  FIG. 8  depicts the signpost signals that are received by the tag  91 . As discussed above, when the tag is in region R 1  it receives only the signpost signals A from the ground signpost. In region R 2 , the tag receives the signals A from the ground signpost, and possibly some incomplete or erroneous signals B from the cabin signpost. Then, in region R 3 , the tag reliably receives signpost signals A and B from both the ground signpost and the cabin signpost. Then, in region R 4 , the tag receives signpost signals B from the cabin signpost, and may possibly receive signals A from the ground signpost, where the signals A may possibly be incomplete or erroneous. Finally, in region R 5 , the tag receives only the signpost signals B from the cabin signpost. 
         [0055]      FIGS. 9 and 10  are flowcharts depicting an implementation of the technique of  FIG. 8 . In  FIGS. 9 and 10 , the ground signpost  81  is treated as the primary or major signpost, and the cabin signpost  86  is treated as the minor or secondary signpost. The major signpost (ground signpost) takes a precedence over the minor signpost (cabin signpost). Whenever the tag determines that it is receiving the preamble of a wireless signpost signal, the tag enters the flowchart of  FIG. 9A  at block  201 . Entry to block  201  may be an interrupt-driven event. 
         [0056]    Control progresses from block  201  to block  202 , where the tag checks to see whether a timer is running. The tag uses the timer to help determine when the tag should transition from recognizing one signpost to recognizing the other. If the tag determines that the timer is running, then the tag proceeds to block  203 , where it restarts the timer. In either case, control ultimately proceeds to block  206 , where the tag uses the CRC code in the received signpost signal to determine whether the received signpost signal is complete and accurate. If not, then the tag proceeds to block  207 , where it sets a variable LAST_CRC to indicate that the CRC check identified a problem. Control then proceeds to block  208 , where the tag exits the routine of  FIG. 9 . 
         [0057]    If it is determined in block  206  that the CRC code did not identify any errors, then control proceeds to block  211 , where the tag checks the unique signpost identification (SPID) code in the received signal, in order to see if it is within a valid range. In this regard, the SPID code in signals received from the ground signpost should be within one range, and the SPID code in signals received from the cabin signpost should be in a different range. If the received signal contains an SPID code that is not within either of these ranges, then control proceeds to block  207 . Otherwise, control proceeds to block  212 , where the tag starts the timer. 
         [0058]    Control then proceeds to block  213 , where the tag checks the SPID code from the received signpost signal in order to determine whether the signal was transmitted by the major signpost (ground signpost  81 ) or the minor signpost (cabin signpost  86 ). If the received signal is from the major signpost, then control proceeds to block  216 , where the tag takes the SPID code from the received signpost signal, and saves this code in a variable called LAST_MAJOR_SPID. Control then proceeds to block  217 , where the tag sets the variable LAST_CRC to indicate that the tag performed a CRC check on the received signal in block  206 , and did not identify any problems in the received signal. In addition, the tag takes the SPID code from the received signal and places it in a ring buffer. The ring buffer is an endless buffer having 20 locations or entries. Once the buffer becomes full, each subsequent new entry replaces the oldest existing entry in the buffer. The fourth line in  FIG. 8  shows a series of SPID codes that are successively inserted into the ring buffer. 
         [0059]    If the tag determines in block  213  that the received signal is from the minor signpost (cabin signpost), then control proceeds to block  218 , where the tag checks to see (1) whether the variable LAST_CRC is indicating the last CRC check failed and (2) whether the variable LAST_MAJOR_SPID has a value greater than zero. If either of these conditions is not met, then control proceeds directly to block  217 . Otherwise, if both conditions are met, then control proceeds to block  221 , where the tag takes the SPID code from the variable LAST_MAJOR_SPID, and places this SPID code in the ring buffer. The fourth line in  FIG. 8  shows the SPID codes that, in the example of  FIG. 8 , are successively inserted into the buffer. 
         [0060]    From block  217 , control proceeds to block  231 , where the tag checks to see whether the ring buffer has only a single entry. If so, then control proceeds to block  232 , where the tag takes the SPID code from the received signpost signal, and saves this code in a variable called THIS_SPID. In  FIG. 8 , the fifth line shows the sequence of SPID codes that, in the example of  FIG. 8 , are successively stored in the variable THIS_SPID. 
         [0061]    If it is determined in block  231  that the buffer currently contains more than one SPID code, then control proceeds to block  233 , where the tag checks to see whether any SPID codes from the minor signpost (cabin signpost) are present in a window in the buffer, where the window is the most recent “n” entries added to the buffer. In the disclosed embodiment, n=4. In other words, the size of the window is four entries. If the window contains something other than just SPID codes for the minor signpost, then control proceeds to block  236 . In block  236 , the tag takes the most recent SPID code that is within the window and that is from the major signpost, and stores this SPID code in the variable THIS_SPID. On the other hand, if it is determined in block  233  that the window contains only SPID codes from the minor signpost, then control proceeds to block  237 . In block  237 , the tag takes from the window the most recent SPID code from the minor signpost, and saves this in the variable THIS_SPID. From any of blocks  232 ,  236  and  237 , control proceeds to block  238 . 
         [0062]    In block  238 , the tag checks to see whether the value in the variable THIS_SPID is the same as the value in the variable LAST_SPID, where the variable LAST_SPID contains the most recent prior value from the variable THIS_SPID. If the values in these two variables are the same, then block  241  is skipped. Otherwise, control proceeds to block  241 , where the tag adds the SPID code from the variable THIS_SPID to the transmission queue, so that this SPID code will be transmitted in a wireless tag signal. Next, the tag takes the SPID code from the variable THIS_SPID, and saves it in the variable LAST_SPID. Control then proceeds to block  242 , where the tag exits the routine of  FIG. 9 . 
         [0063]    The flowchart of  FIG. 10  represents a portion of a main loop executed by a processor within the tag  91 . Beginning in block  261 , control eventually proceeds to block  262 , where the tag checks to see whether the timer has just expired. If not, then control returns to block  262 , where the tag waits for the timer expire, or for the receipt of a wireless signpost signal that initiates entry to the routine of  FIG. 9  at block  201 . If it is eventually determined at block  262  that the timer has just expired, then control proceeds to block  263 , where the tag disables the timer. 
         [0064]    From block  263 , control proceeds to block  266 , where the tag checks to see whether the SPID code that was most recently inserted into the buffer was for the minor signpost (cabin signpost). If not, then control returns to block  262 . Otherwise, control proceeds to block  267 , where the tag checks to see whether the SPID code transmitted most recently in a tag signal is for the major signpost (ground signpost). If not, then control returns to block  262 . Otherwise, control proceeds to block  268 , where the tag takes the SPID code most recently inserted into the buffer, and adds this SPID code to the transmission queue. Then, the tag clears the ring buffer, and sets each of THIS_SPID, LAST_SPID and LAST_MAJOR_SPID to 0. Control then returns from block  268  to block  262 . 
         [0065]    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.