Abstract:
Tracking of an article through geographic areas using electromagnetic signals, specifically radio frequency (RF) signals. The article contains a tag operating as a transmitter and receiver. Tag-readers in the defined geographic areas transmit RF signals and in response, the tag transmits RF signals received by the tag-reader. By knowing the areas in which the tag-readers are located, a system tracks the article by monitoring the tag-readers communicating with the tag.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus and method for tracking a moveable article, and in particular, to tracking a mail piece through a mail delivery system. 
     2. Description of the Related Art 
     Postal customers increasingly demand quick delivery times and expect that mail will be delivered on-time. As a result, overnight and similar forms of mail delivery are increasingly important. In order to meet these needs and expectations, postal services must provide internal systems that monitor operations to identify systemic and individual problems that can occur during mail delivery. 
     Traditionally, tracking mail involves recording a mail piece&#39;s mailing date and delivery date. The mailing date is compared to the delivery date to determine the amount of time the mail piece spent in transit. This method determines the overall time for delivery, but does not permit tracking of mail during the delivery process. If the mail piece does not reach its destination within a required number of days, this method cannot determine what cause the delay. 
     For instance, the traditional method cannot pin-point bottlenecks that slow down mail delivery. The traditional method cannot locate specific pieces of mail intended for a particular truck to ensure timely loading of the mail piece on the proper truck. The traditional method also cannot indicate when a mail piece is in an area where it should not be. Further, the traditional method cannot indicate when a mail piece is behind schedule or off a planned track. Lastly, the traditional method can not indicate when a mail piece has not moved for an extended period of time. 
     Accordingly, a need exists for tracking a mail piece as it travels through the mail delivery system. 
     SUMMARY OF THE INVENTION 
     The advantages and purpose of the invention are set forth in part in the description which follows, and in part are obvious from the description, or may be learned by practice of the invention. The advantages and purpose of the invention are realized and attained by means of the elements and combinations particularly pointed out in the appended claims. 
     To attain the advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention comprises a method and apparatus for tracking and locating a moveable article. Methods and apparatus consistent with this invention transmit an interrogation signal; receive the interrogation signal in the article; transmit a response signal from the article, the response signal providing a unique identification code; receive the response signal using a receiver having a unique receiver code; and generate an identification signal providing an indication of the identification code and the receiver code. 
     Methods and apparatus consistent with this invention may also provide a separate receiver within each of a plurality of sectors, provide a separate transmitter within each of the plurality of sectors, transmit a separate interrogation signal from each of the provided transmitters and receive the response signal using one or more of the provided receivers. 
     Methods and apparatus consistent with this invention may also repeat the interrogation signal transmitting step, the response signal transmitting step, the response signal receiving step, and generate a plurality of identification signals. The above method or apparatus may also record the identification signals in real-time over a period of time to track the location of the article through the sectors. 
     It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and together with the description, serve to explain the principles of the invention. In the specification, 
     FIG. 1 is a diagram of an apparatus, consistent with this invention, for tracking and locating a mail piece; 
     FIG. 2 is a diagram of a transmit and receive device (“tag”), consistent with this invention, that may be attached to a mail piece to aid in tracking the mail piece; 
     FIG. 3 is a flow chart of a tag process consistent with this invention; 
     FIG. 4 is a diagram of a transmit and receive device (“tag-reader”), consistent with this invention, that may be attached to a fixed wall or may be mobile; 
     FIG. 5 is a flow chart of an exemplary tag-reader process consistent with this invention; 
     FIG. 6 is a diagram of a data processing system, consistent with this invention, for use in tracking of a mail piece; 
     FIG. 7 is a flow chart of a process, consistent with this invention, for repeating the interrogation signal transmitting step with a plurality of tag-readers; 
     FIG. 8 is a flow chart of a parallel real-time process, consistent with this invention, for tracking and locating a mail piece; 
     FIG. 9 is a flow chart of an information signal collection and storing process, consistent with this invention; 
     FIGS. 10A-10C are flow charts of a process, consistent with this invention, for tracking a mail piece; 
     FIG. 11 is a flow chart of a process, consistent with this invention, for determining when a mail piece deviates from a predetermined route; 
     FIG. 12 is a flow chart of a process, consistent with this invention, for determining a location of a mail piece using multiple information signals; 
     FIG. 13 is a flow chart of a process, consistent with this invention, for generating an error signal when a mail piece is in a particular location for too long; 
     FIG. 14 is a flow chart of a process, consistent with this invention, for generating an error signal when a mail piece is in a particular location in which it should not be; and 
     FIG. 15 is a flow chart of a process, consistent with this invention, for generating an error signal when a mail piece leaves a particular location during a time period. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Reference is now made in detail to the present preferred exemplary embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the reference numbers are used throughout the drawings to refer to the same or like parts. 
     Apparatus and methods consistent with this invention track and determine a location of a mail piece using electromagnetic energy, such as radio frequency (RF) energy. The mail piece is fitted with a tag, which operates as a receiver and a transmitter. Within an area, devices known as tag-readers transmit interrogation signals, which are received by the tag affixed to the mail piece. Upon receiving the interrogation signal, the tag transmits a response signal containing a unique identification code. This identification code, in turn, identifies the mail piece affixed to the tag. Therefore, by using multiple tag-readers over an area, a system may track and determine the location of the mail piece affixed to the tag. This area may include the extensive areas of the mail delivery process, which may be any place in or around where the mail piece travels, such as a building, truck, delivery platform, or mail route. The area may even include places where mail should not be, such as an employee locker room, bathroom, etc. 
     FIG. 1 is a diagram of a system consistent with this invention for tracking and locating mail piece  106 . In this system, mail piece  106  is tracked through an area  101 . Area  101  is divided into four sectors, each sector having its own tag-reader, including a tag-reader  102  for sector  1 , a tag-reader  103  for sector  2 , a tag-reader  105  for sector  3 , and a tag-reader  104  for sector  4 . Each tag-reader  102 - 105  transmits an interrogation signal, within at least its sector. In particular, tag-reader  102  transmits a separate interrogation signal having an effective range  108 ; tag-reader  103  transmits an interrogation signal having an effective range  109 ; tag-reader  104  transmits an interrogation signal having an effective range  111 ; tag-reader  105  transmits an interrogation signal having an effective range  110 . Although sectors  1 - 4  do not overlap in FIG. 1, multiple tag-readers may be placed such that sectors are defined to overlap. Area  101  also includes the cameras  130 - 133 , which are explained in detail below. 
     In the preferred embodiment, each sector is covered by the effective transmission range of at least three tag-readers. For instance, sector  1  is covered by the transmission pattern  108  of tag-reader  102 , the transmission pattern  109  of tag-reader  103 , and the transmission pattern  110  of tag-reader  105 . In this fashion, the location of the mail piece may be determined using triangulation, as described below. Also in the preferred embodiment, the transmission patterns of tag-readers  102 - 105  is approximately 300 feet indoors. This distance may vary depending upon objects in the sector, or the user&#39;s preference. 
     FIG. 2 shows the components of a tag, such as tag  119 . Tag  119  contains a receiver  204 , a processor  206 , and a transmitter  208 . Tag  119  is connected to antennas  202  and  210 . The interrogation signal from the tag-reader  104  is received by tag receiver  204  from antenna  202  and processed by processor  206 . When processor  206  determines that it received an interrogation signal from a tag-reader, processor  206  instructs transmitter  208  to transmit a response signal via antenna  210 . The response signal contains the unique identification code (“tag code”) of tag  119 . Although the tag of FIG. 2 includes separate transmit and receive antennas, those skilled in the art recognize that a single antenna could be used with appropriate transmit/receive switching circuitry. In one embodiment, it is possible that tag  119  does not have a receiver. In this instance, tag  119  would transmit a response signal periodically. 
     FIG. 3 shows a process  300  that tag  119  performs in processor  206 . Tag  119  waits for an interrogation signal (step  302 ) to be received by receiver  204 . If tag  119  does do not receive an interrogation signal (step  304 ), tag  119  continues to wait (step  302 ). If tag  119  does receive an interrogation signal (step  304 ), then a response signal is transmitted by transmitter  208 . If a collision, that is, a simultaneous transmission by two or more tag transmitters, is detected (step  308 ), then tag  119  goes through the appropriate collision handling procedures (step  310 ) and retransmits the response signal (step  306 ). Collision procedures are discussed below in greater detail. If a collision is not detected (step  308 ), then the process  300  repeats and waits for an interrogation signal (step  302 ). 
     FIG. 4 shows the components of a tag-reader, such as tag-reader  102 . Tag-reader  102  contains a receiver  404 , a processor  406 , and a transmitter  408 . Transmitter  408  transmits an interrogation signal, which may be received by a tag, such as tag  119 . Receiver  404  receives response signals from tags. Processor  406  instructs transmitter  408  when to transmit an interrogation signal to the tags, and processes the response signal and any other signals from receiver  404 . Tag-reader  102  interacts with a network  117  and possibly other tag-readers, such as tag-readers  103 - 105 . Tag-reader  102  may communicate with other tag-readers by wireline or wireless communications. Tag-reader  102  sends an information signal to network  117 , which includes the tag code, a tag-reader code, and other information such as the date and time the tag-reader received the response signal. The tag-reader code uniquely identifies the tag-reader. 
     Alternatively, it is possible to put tag-reader transmitter  408  and tag-reader receiver  404  in different locations, i.e. not in the same casing. Further, it is possible to have multiple receivers for every transmitter  408 . Further still, it is possible to have multiple transmitters for every receiver  404 . 
     FIG. 5 shows a process  500 , which is executed in tag-reader processor  406 . Tag-reader  102  waits to be instructed to transmit an interrogation signal (step  502 ). This instruction may come from network  117  or may be generated by a clock in tag-reader  103 . If tag-reader  102  does not receive an instruction (step  504 ), then tag-reader  102  continues to wait (step  502 ). If tag-reader  102  receivers an instruction, then tag-reader  102  transmits an interrogation signal (step  506 ) through transmitter  408 . Tag-reader  102  then waits for a response signal (step  508 ) from tag  119 . If a response signal is received, or if there is a time-out (step  510 ), then tag-reader  102  runs collision procedures, if necessary (step  512 ). Tag-reader  102  transmits an information signal to computer  118  through network  117  if response signal is received. Then the process  500  may start again. Alternatively, processor  406  is programmed to periodically transmit an interrogation signal without waiting for an instruction. 
     Tag-readers  102 - 105  can be placed on a wall, mounted in a vehicle, or held in hand. The link between the tag-reader  102  and network  117  may be a wireline or wireless link. In the case where the tag-reader is hand held or otherwise portable, the link is wireless. The transmission pattern of communication signals between tag-reader  102  and tag-readers  103 - 105  is not shown, and is not necessarily the same as transmit patterns  108 - 111  of interrogation signals. 
     In the preferred embodiment, tag-readers  102 - 105  and tag  119  are devices manufactured by ID Systems, Inc, such as the FLEXTAG™ system. Preferably, RF signals between a tag and tag-reader are at a frequency of 902-928 MHz, but other frequencies are possible. 
     If other tags transmit response signals at the same time as tag  119 , then it is possible that there may be radio interference, also known as a collision, such that tag-reader  102  cannot discern and receive the response signals. The problem of collision can be resolved, however, by well known protocols. 
     The ALOHA protocol is an example of one possible protocol. Using this protocol, the tag  119  waits for an acknowledgment signal from a tag-reader, such as tag-reader  102 , indicating that the response signal was accurately received. If tag  119  does not receive an acknowledgment signal within a specific timeout period, tag processor  406  assumes that the response signal collided with another tag&#39;s response signal. In this case, processor  406  schedules a retransmission of the response signal after a random delay time. This process continues until tag  119  receives an acknowledgment signal. 
     In the preferred embodiment, an algorithm similar to ALOHA is used but with “frequency hoping” in the 902-908 MHz frequencies. In effect, this multiplies the ALOHA one-channel method by a factor equal to the number of channels used. In this preferred algorithm, each tag does not immediately transmit a response signal. Instead, each tag chooses a random time slot to transmit. For instance, if the response signal is 50 bytes and the tag transmission rate is 1 Mbyte per second, then there are 20,000 time slots (1 Mbyte divided by 50 bytes) in a one second period. 
     Each tag randomly selects one of the 20,000 time slots and transmits during its chosen time slot. If the tag does not receive an acknowledgment then it selects another time slot and retransmits. In the preferred embodiment, one second (corresponding to 20,000 time slots) is sufficient to avoid collision between tags in the mail system and enough time to allow a response when mail is moving in and out of sectors. 
     In accordance with this invention, locating the mail piece in a defined area may include repeating the interrogation signal transmitting step, the response signal transmitting step, the response signal receiving step, and generating a plurality of information signals. In FIG. 1, mail piece  106  is affixed to tag  119 . As mail piece  106  moves throughout the sectors as shown by line  107 , tag  119  communicates with various tag-readers  102 - 105 . Each tag-reader, such as tag-reader  102 , transmits multiple interrogation signals over a period of time. Thus, tag  119  transmits a response signal after it receives each interrogation signal. Tag-reader  102 , or tag-readers  103 - 105 , depending on where tag  119  is, receives the periodic response signal and creates multiple information signals as time progresses. 
     FIG. 6 is a diagram of a data processing system, consistent with this invention, for use in tracking and locating the mail piece. Computer  118  includes a memory  602 , a secondary storage device  604 , a processor  605  such as a central processing unit, an input device  606 , a display device  603 , and an output device  608  such as a printer or CRT. Memory  602  and secondary storage  604  may store application programs and data for execution and use by processor  605 . In particular, memory  602  stores an application  609  used to track mail piece  106 . 
     Computer  118  instructs tag-readers  102 - 105  when to transmit an interrogation signal. Preferably, computer  118  cycles through tag-readers in order to reduce interference between interrogation signals and response signals. FIG. 7 is a flow chart of a process, consistent with this invention, for repeating the interrogation signal transmitting step with a plurality of tag-readers. In process  700 , which is part of application  609 , the system logs into a database (step  702 ), such as an Oracle 7  database, which may be stored in memory  602  or secondary storage  604 . The system opens a tag-reader data file (step  704 ), and reads a record from the data file (step  706 ). If it is not at the end of a file (step  708 ), the system continues to read the next record (step  706 ). 
     If the system is at the end of the data file (step  708 ), it selects the current record&#39;s tag-reader code (step  710 ) and instructs that tag-reader to transmit an interrogation signal (step  712 ). The system then waits (for a period tau) for that tag-reader to return information signals (step  714 ). This delay allows each tag-reader to transmit without interference from other tag-readers, and gives tags a chance to respond to the tag-reader. If all records are not processed (step  716 ), then the system moves to the next tag-reader code, which becomes the current tag-reader code (step  710 ). If all the records are processed (step  716 ), then the system logs out of the database (step  718 ), and the system determines if it wants to cycle through all the tag-readers again (step  720 ). If the system does cycle through the all the tag-readers again, then it delays (step  627 ) for the same reasons it delayed previously in step  716 . 
     Methods and apparatus consistent with this invention may transmit the information signal through a network. The information signal contains the tag code of the tag that transmitted a response signal and the tag-reader code of the tag-reader that received the response signal. The information signal may also contain the date and time the tag-reader received the response signal. 
     In the illustrated embodiment, tag-readers typically communicate with each other as shown by lines  112 - 115  and  120 - 121  (FIG. 1) representing communication links between tag-readers  102 - 105 . If a tag-reader is portable, it can either have a wireless link to network  117 , or it can up-link to network  117  periodically. One of the tag-readers, in this example tag-reader  105 , is connected via a link  116  through network  117  to a computer  118 . Tag-reader  105  thus transmits to computer  118  information signals from tag-readers  102 - 105  concerning mail piece  106 . Alternatively, each tag-reader  102 - 105  may connect directly or indirectly to computer  118 . Network  117  may comprise a wired network such as the Internet or an intranet, or any equivalent network such as a wide area network, a local area network, or a public or private wireless network. Also, network  117  may represent, for example, a wireline, telephone line, or an intranet. Many buildings are already wired for intranets and have convenient connection locations. Therefore, it may be convenient to connect a tag-reader through an intranet to computer  118  rather than using separate wireline connections. Tag-readers  102 - 105  may be connected to the network in many different ways. For instance, tag-readers  102 - 105  may each have an Ethernet interface by which they connect directly to network  117 . 
     Methods and apparatus consistent with this invention for locating the mail piece may record the information signals in real-time over a period of time to track the location of the mail piece through the sectors in real-time. In the illustrated embodiment, as tag-readers transmit interrogation signals and collect response signals, information signals are immediately forwarded through network  117  to computer  118 . Computer  118  stores these information signal in a database, such as Oracle 7 . The database also contains a look-up table that associates each tag code with a particular mail piece, and the database also contains a look-up table that associates each tag-reader code with a physical location. Using the database, a user can view the location of any mail piece at any time, or replay the location history of the mail piece. Computer  118  tracks multiple mail pieces by assigning unique tag code to each tag and unique tag-reader code to each tag-reader. 
     FIG. 9 is a flow chart of an information signal collection and storing process  900 , consistent with this invention. Process  900  is implemented by a system including application  609  controlling operation of processor  605  in computer  118 . In process  900 , the system logs into a data base (step  902 ), such as Oracle 7 , and opens a tag data file (step  904 ). Memory  602  or secondary storage  604  may store the database. The system then waits for information signals (step  906 ) from tag-readers, such as tag-readers  102 - 105  that are delivered through network  117 . 
     If an information signal is not received (step  908 ), then the system continues to wait. If an information signal is received (step  908 ), then the system writes a record to the data base (step  910 ). The record contains three fields, possibly more: the tag code, the tag-reader code, and the time the response signal was received by the tag-reader. A fourth field could contain information about the mail piece associated with the tag code. 
     FIGS. 10A-10C are flow charts of a process for processing information collected in process  900 . Process  1000  provides for real-time tag database viewing, and may be implemented by a system including application  609  controlling operation of processor  605  in computer  118 . In process  1000 , the system logs into a database (step  1001 ), such as an Oracle 7  database, which is stored in memory  602  or secondary storage  604 . 
     In the preferred embodiment, the computer that executes process  1000  is in a mail “facility.” For example, in the United States Postal Service (USPS), there are three types of facilities that process mail pieces. First, in any geographical area there are “plants,” also known as Processing &amp; Distribution Centers (PDCs). All mail is sent to a plant where stamps are canceled and the mail is aggregated for distribution. Second, there are “stations,” which are commonly known as local post offices. There may be multiple stations in one geographical area assigned to a plant. Third, the facility could be an airport mail center (AMC), which distributes mail by airplane. 
     In process  1000 , the system opens a tag data file (step  1002 ), and reads a record from the data file (step  1003 ). It determines if it is at the end of the data file (step  1004 ), and if so, it executes step  1012 . If it is not at the end of a file, the system determines if the record is a duplicate (step  1005 ). If so, it returns to step  1003  to read another record. 
     If the system is not at the end of the file, it determines if the record has erroneous data (step  1006 ). If so, the system sets an error code (step  1007 ) and stores the tag data in the database (step  1008 ). If the record does not have erroneous data, the system computes a delivery date (step  1009 ). The delivery date is the date the mail piece should be delivered to the customer. For instance, the USPS should deliver all mail pieces within one, two, or three days after it is placed in the Postal Service. 
     In step  1010 , the system determines the “range of destination” of the current facility. The “range of destination” is a list of zip codes serviced by the facility. The system then stores the delivery data and range of destination in the database (step  1011 ). 
     If the system is at the end of the file (step  1004 ), the system selects tag identifications with current records to process (step  1012 ). If a record remains to be processed (step  1013 ), the system determines if the mail piece is “originating” (step  1015 ). The mail piece is originating when two conditions are met. First, the mail piece must have been introduced in the mail system the same day. Second, the mail piece&#39;s originating zip code must be a zip code that the facility serves, i.e. in the “range of destination.” 
     If the mail piece is originating (step  1015 ) and the facility is a station (step  1016 ), then the system determines an “on-time” code (step  1018 ). The on-time code is either (1) “on-time,” (2) “late,” or (3) “dead.” The system determines this code by comparing the current status of the mail piece against a predetermined schedule. The predetermined schedule is a look up table that contains the critical times of events that are supposed to happen in order for the mail piece to be delivered on-time. If the mail piece meets all the critical times, it obtains an on-time code. The schedule is dependent upon a number of factors. First, the schedule depends on whether the mail piece is originating and whether the facility is a station. It may also depend on whether the mail piece is first-class, priority, or overnight. It may also depend on other factors. 
     For example, if the mail piece is first-class and originating at a station, then according to a predetermined schedule, the mail piece may need to be on a 6:00 p.m. truck headed for a plant. 
     If it is any time before 5:45 p.m. the mail piece is “on-time.” If it is after 5:45 p.m., but before 6:00 p.m., then the mail piece is “late.” This indicates that a specific course of action must occur within the next 15 minutes or the mail piece may miss the scheduled truck. A real-time operator may correct a “late” code. If the time is after 6:00 p.m., then truck left the station and the mail piece is “dead.” No matter where the mail piece is, if the current day is after the delivery date (calculated at step  1009 ), then the mail piece is “dead.” 
     If the mail piece is originating (step  1015 ) and the facility is an AMC (i.e., not a station and not a plant) (step  1019 ), the system also determines an “on-time” code (step  1021 ). The on-time codes in an AMC may be are the same as in a station. Because an AMC has different operations than a station or a plant, the predetermined schedules are naturally different. In an AMC, it is important that mail pieces are on the correct departing airplane. 
     If the mail piece is originating (step  1015 ) and the facility is a plant (step  1019 ), the system determines if the mail piece is entering the plant (step  1022 ). The system can tell if the mail piece is entering a plant in a number of ways. For instance, if it is the first time there is data concerning the mail piece at this plant, then it may be assumed that the mail piece just arrived. If the mail piece is entering the plant, the system determines the on-time code using a different predetermined schedule (step  1024 ). Again, the predetermined schedules at a plant are different than at a station or an AMC because plants perform different functions. If the mail piece did not just enter the plant (step  1022 ), the system sets a different on-time code using a predetermined schedule (step  1026 ). 
     If the mail piece is not originating (step  1015 ) and if the facility is a plant (step  1027 ), the system determines whether the mail piece is an overnight mail piece (step  1028 ). If the mail piece is an overnight mail piece, the system determines an on-time code using a different predetermined schedule (step  1030 ). Otherwise, if the mail piece is not an overnight mail piece, the system uses a different predetermined schedule (step  1032 ). If the facility is a station (step  1033 ), the system determines an on-time code using a different predetermined schedule (step  1035 ). If the facility is not a station, then it must be an AMC, and the system determines an on-time code using a different predetermined schedule (step  1037 ). 
     The system then stores the on-time code in the database (step  1038 ). The system determines if the mail piece is leaving the facility (step  1039 ). The system may determine this by the location of the mail piece. If the mail piece is leaving the facility, the system computes the time in the facility (step  1040 ) and stores the computed time in the database (step  1041 ). If, instead, the mail piece is just entering the facility (step  1042 ), the system computes the trip time from the previous facility (step  1043 ) and stores the computed time in the database (step  1041 ). Next, the system returns to step  1013  for additional processing. 
     FIG. 8 is a flow chart of a parallel real-time process, consistent with this invention, for tracking and locating the mail piece. In FIG. 8, the interrogation signal process  700 , the tag-reader process  500 , the tag process  300 , the information collection and storage process  900 , and the real-time database viewing process  1000  all operate in parallel. The database viewing process  804  can also be a post processing event. 
     Methods and apparatus consistent with this invention for locating the mail piece may compare the tracked location of the mail piece with a predetermined route, and generate an error signal when the tracked location deviates from the predetermined route. FIG. 11 is a flow chart of a process  1100  for generating an error signal when the tracked location deviates from the predetermined route. Process  1100  is implemented by the system including application  609  controlling operation of processor  605  in computer  118 . In process  1100 , the system logs into a data base (step  1102 ), such as Oracle7, and opens the tag data file (step  1104 ). Memory  602  or secondary storage  604  may store the database. The system then reads tag records (step  1106 ) until it reaches the end of file (step  1108 ). The system then searches its memory for a predetermined tag (tag X) (step  1110 ), and calculates its location (step  1118 ). The system compares the current location and time to a predetermined route (step  1121 ) and determines whether tag X is in a correct location (step  1124 ). If tag X is not in the correct location, the system generates an error signal (step  1127 ). Then, the system logs out of the database (step  1130 ) and process  1100  may repeat (step  1122 ). 
     Methods and apparatus consistent with this invention for locating the mail piece may receive the response signal using a plurality of receivers, and locate the mail piece based upon the different signals received by each of the plurality of receivers. In the illustrated embodiment, to the extent that the transmission patterns  108 - 111  of the tag-readers overlap, multiple tag-readers may receive signals from mail piece  106 . This allows the use of triangulation or other techniques to approximate the location of the mail piece within a particular sector. Other techniques are well known in the art and include calculations based on signal strength, signal direction, signal delay, or transmit pattern overlap. 
     FIG. 12 is a flow chart of a process  1200 , consistent with this invention, for locating the mail piece using multiple information signals. Process  1200  is implemented by the system including application  609  controlling operation of processor  605  in computer  118 . In process  1200 , the system logs into a data base (step  1202 ), such as Oracle7, and opens the tag data file (step  1204 ). Memory  602  or secondary storage  604  may store the database. The system then reads tag records (step  1206 ) until it reaches the end of file (step  1208 ). The system then selects a current tag record (step  1210 ) and searches its memory for a duplicate record (step  1212 ). If there are multiple records, the system calculates the location of the tag-reader by triangulation or another location calculation method based on multiple records (step  1216 ). If there are no duplicate records (step  1214 ) then the system calculates the tag location based on the information in one record (step  1218 ). The system then sets the tag location field in the database to the calculated location (step  1220 ). If there are no more records to process (step  1224 ), then the system logs out of the data base (step  1226 ). The process  1200  may repeat (step  1228 ). If there are more records to process, then the system selects a new current tag record ( 1210 ) and repeats the process by searching for duplicate records (step  1212 ). 
     Methods and apparatus consistent with this invention for locating the mail piece may determine a time period when the mail piece should leave a particular location, and generate an error signal when the mail piece does not leave the particular location during the time period. In the illustrated embodiment, when mail piece  106  enters a sector, computer  118  can store the mail piece identification code and entry time in memory and then start a timer. If mail piece  106  has not moved after a predetermined amount of time, the system generates an error signal. A look-up table can be used to determine the maximum time the mail piece should remain within particular sectors. 
     For example, at a predetermined amount of time before a mail truck leaves a building, the system can “poll” an entire floor, that is, have all the tag-readers on that floor transmit a signal to all tags that may be on that floor. Computer  118  attached to network  117  stores a list of mail pieces that must be on the outgoing truck. If a particular mail piece should be on a truck but is not, those mail pieces may be loaded onto the truck. The invention could also be used in this manner to ensure that all airline baggage is on the proper airplane and is not left in the airport. 
     FIG. 13 is a flow chart of a process  1300  for generating an error signal when the article does not leave the particular location during a predetermined time period. Process  1300  is implemented by the system including application  609  controlling operation of processor  605  in computer  118 . In process  1300 , the system logs into a data base (step  1302 ), such as Oracle7, and opens a tag data file (step  1304 ). Memory  602  or secondary storage  604  may store the database. The system then reads tag records (step  1306 ) until it reaches the end of file (step  1308 ). The system then searches its memory and creates a current list of what tags are in a particular location X (step  1310 ). The system then compares this current list with a previous list of tags in location X. If there are tags that are in location X in the current list that are not in the previous list (step  1314 ), then the system stores the approximate time these new tags must have entered location X (step  1316 ). If there is a tag that has been in a particular location too long (step  1318 ), then the system generates an error signal (step  1322 ). Then, the system sets the previous list to the current list (step  1320 ) and logs out of the database (step  1324 ). The process then may repeat itself (step  1326 ). 
     Methods and apparatus consistent with this invention for locating the mail piece may determine a time period when the mail piece should not be in a particular location, and generate an error signal if the mail piece is in the particular location during the time period. In the illustrated embodiment, it is possible to monitor sectors in which the mail piece should never reside. If the mail piece is detected in a sector where it should not be, an error signal is generated. For example, the mail piece should never be in the locker room. In another scenario it is possible that the mail piece is not allowed in certain sectors during certain times of the day. For instance, if the mail piece is destined for Chicago, it should not be on flight to New York. 
     FIG. 14 is a flow chart of a process  1400 , consistent with this invention, for generating an error signal when the mail piece should not be in a particular location and it is in that particular location. Process  1400  is implemented by the system including application  609  controlling operation of processor  605  in computer  118 . In process  1400 , the system logs into a data base (step  1402 ), such as Oracle7, and opens a tag data file (step  1404 ). Memory  602  or secondary storage  604  may store the database. The system then reads tag records (step  1406 ) until it reaches the end of file (step  1408 ). The system then searches its memory for records and creates a list of all tags in a particular location Y (step  1410 ). The system compares this list to a list of all tags that should not appear at that time in location Y (step  1412 ). If there are tags that are in location Y that should not be there at that time (step  1414 ), then the system generates an error signal (step  1420 ). Then, the system logs out of the database (step  1416 ) and the process may repeat itself (step  1418 ). 
     Methods and apparatus consistent with this invention for locating the mail piece may determine a time period when the mail piece should not leave a particular location, and generate an error signal when the mail piece leaves the particular location during a time period. In another embodiment, if a tag is hidden in a piece of framed art in a museum, the system would detect when that piece of art left a particular room and would generating an error signal. 
     FIG. 15 is a flow chart of a process  1500 , consistent with this invention for generating an error signal when the mail piece leaves a particular location during a time period. Process  1500  is implemented by a system including application  609  controlling operation of processor  605  in computer  118 . In process  1500 , the system logs into a data base (step  1502 ), such as Oracle7, and opens a tag data file (step  1504 ). Memory  602  or secondary storage  604  may store the database. The system then reads tag records (step  1506 ) until it reaches the end of file (step  1507 ). The system then searches its memory for records and creates a list of all tags in a particular location Z (step  1508 ). The system compares this list to a list of all tags that should appear at that time in location Z (step  1510 ). If there is a tag that is not in location Z that should be there at that time (step  1512 ), then the system generates an error signal (step  1514 ). Then, the system logs out of the database (step  1516 ) and the process then may repeat itself (step  1518 ). 
     Methods and apparatus consistent with this invention for locating the mail piece may turn on a camera after receiving a response signal. The method or apparatus may also move a camera in response to the tracked location of the mail piece. In the illustrated embodiment, mail piece  106  may travel from sector  1  to sector  2  to sector  3  to sector  4 . The system could provide the means to activate each security camera  133 ,  130 ,  132 , and  131 , respectively, as the mail piece travels through each sector. The invention could also provide the servo motors attached to a camera with information to follow mail piece  106 . 
     A camera that could be used for this purpose is AUTODOME™, which is available from Burle Technologies, Inc. This camera is a radio controlled pan, tilt, and zoom camera that uses RF signals for control of the movement. Pointing and focusing can be done manually, using a joy stick, or automatically, using preprogrammed instructions. In the prior art, the cameras only respond to a preprogrammed routine. In accordance with this invention, this routine can be dynamic. 
     It is apparent to those skilled in the art that various modifications and variations can be made in the embodiment of this invention and in the construction of this invention without departing from the scope or spirit of the invention. For instance, any type of moveable article can be tracked, not just mail pieces. Types of moveable articles may include, but are not limited to, luggage, framed-art, vehicles, or work pieces in a building. Further, various types of tags and tag-readers, supplied by different vendors, may be used without departing from the scope of the invention. 
     As another example, although computer  118  is depicted with various components, one skilled in the art appreciates that computer  118  can contain additional or different components. Additionally, although computer  118  is shown connected to network  117 , computer  118  may be connected to other networks, including other wide area networks or local area networks. Furthermore, although aspects of this invention are described as being stored in memory, one skilled in the art appreciates that these aspects can also be stored on or read from other types of computer program products or computer-readable media, such as secondary storage devices, including hard disks, floppy disks, or CD-ROM; a carrier wave from a network such as the Internet; or other forms of RAM or ROM. These aspects of this invention may also include modules, implemented in software, hardware, or a combination, configured to perform a particular method implementing an embodiment consistent with this invention. In addition, the computer-readable media may include instructions for controlling a computer system, such as computer  118 , to perform a particular method. 
     As another example, transmission patterns  108 - 111  displayed in FIG. 1 could be larger or smaller. Transmission patterns  108 - 111  could also be small to prevent overlapping at all. Further, transmission patterns do not have to be symmetrical in any way. For instance, the transmission patterns  108 - 111  could be directional, as to only cover only a portion of three hundred sixty degrees. Further, in the discussion above it is assumed that tag-reader&#39;s  102 - 105  receive patterns are symmetrical. They too could be directional receivers. 
     Other embodiments of the invention are apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.