Assessing distances

Abstract

Distances between a mobile transceiver (401) and a plurality of fixed transceivers (511) are assessed. Each fixed transceiver broadcasts beacon signals that identify the position of the fixed transceiver. At the mobile / tag transceiver, fixed / anchor transceivers forming a local set are identified and ranging signals (515) are transmitted to and ranging signals are received from the fixed transceivers in the local set. Distance data is assessed representing distances between the mobile transceiver and the fixed transceivers in the local set and this distance data is transferred to a gateway (523). The process preferably comprising receiving plural beacon signals, ordering the beacon signals in terms of signal strength and selecting a set of transmitted beacon signals with the largest signal strength.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This is the first application for a patent directed towards the invention and the subject matter. TECHNICAL FIELD The present invention relates to a method of assessing a distance between a mobile transceiver and a plurality of fixed transceivers in an environment. BACKGROUND OF THE INVENTION It is known to provide a method of assessing a distance between a first transceiver, having a transmitter and a receiver, and a second transceiver also having a transmitter and a receiver, by measuring a round-trip time-of-flight of a transmitted radio signal between the first transceiver and the second transceiver, as described in GB 2597674, assigned to the present applicant. This describes a method of improving accuracy by evaluating the intensity of a radio signal received by the first receiver from the second transmitter. Transmission power is than increased if the intensity is below a threshold and decreased if the intensity is above a second threshold. An embodiment of the present invention adopts this approach but the original disclosure shows ranging activities initiated by the fixed transceivers (anchors) with the mobile transceivers (tags) operating as slaves. To achieve this, sophisticated scheduling procedures are required, possibly of the type described in GB 2597728, assigned to the present applicant. A problem with this approach is that it is not possible for the tags to automatically locate themselves as being in the vicinity of a particular set of anchors, without requiring substantial server intervention via an appropriate gateway. BRIEF SUMMARY OF THE INVENTION According to a first aspect of the present invention, there is provided a method of assessing a distance between a mobile transceiver and a plurality of fixed transceivers in an environment, comprising the steps of: at each fixed transceiver: broadcasting beacon signals that identify the position of the fixed transceiver in the environment; and at the mobile transceiver: identifying a local set of fixed transceivers; transmitting ranging signals to and receiving ranging signals from the fixed transceivers in said local set; assessing distance data representing distances between the mobile transceiver and the fixed transceivers in the local set; and transferring said distance data to a gateway. In an embodiment, the location of a fixed transceiver is specified in terms of a group index and a unique index within the group. The identifying step may comprise the steps of: receiving a plurality of beacon signals; ordering the beacon signals in terms of signal strength; and selecting a set that transmitted beacons with the largest signal strength. The transmitting step may be instigated by transmitting virtual addresses for the fixed transceivers in the selected set, constructed from the group index and the unique index. In an embodiment, the transmitting step comprises the steps of: transmitting a plurality of ranging signals in a selected transmission slot to make a provisional assessment of distance; repeating the transmission step for a plurality of cycles, using the same slot, and making further provisional assessments of distance; and averaging said provisional assessments of distance to produce said distance data. In an embodiment, the mobile transceiver is attached to a mobile apparatus; radio frequency identification devices are attached to respective mobile items; the mobile transceiver includes a radio frequency identification device reader; and the method further comprises the steps of, at the mobile transceiver: energising the radio frequency identification device reader; reading radio frequency identification devices within range; and transmitting radio frequency identification device originating data to the gateway. According to a second aspect of the present invention, there is provided an apparatus for deployment as a fixed transceiver in an environment in which distances are assessed between the fixed transceiver and a mobile transceiver, comprising: radio communication devices for transmitting and receiving radio signals; and a processor, wherein said processor is configured to: broadcast beacon signals that identify the position of the fixed transceiver in the environment; receive ranging signals form a mobile transceiver and retransmit said ranging signals back to said mobile transceiver, thereby allowing said mobile transceiver to assess distance data representing distances between the fixed transceiver and the mobile transceiver, and transfer said distance data to a gateway. In an embodiment, the fixed transceiver is configured to include slot usage data in said beacon signals, wherein said slot usage data identifies slots that were used for ranging on the previous cycle. The fixed transceiver may also be configured to mask some of the available slots to conserve energy. According to a third aspect of the present invention, there is provided an apparatus for deployment as a mobile transceiver in an environment in which distances are assessed between the mobile transceiver and a plurality of fixed transceivers, comprising: radio communication devices for transmitting and receiving radio signals; and a processor, wherein said processor is configured to: identify a local set of fixed transceivers; transmit signals to and receive ranging signals from the fixed transceivers in the local group; assess distance data representing distances between the mobile transceiver and the fixed transceiver in the local set; and transfer said distance data to a gateway. A further advantage of this approach is that it facilitates the deployment of the system within existing radio networks that have duty cycle restrictions. In an embodiment, ranging is achieved using LoRa protocols operating at 2.4 GHz, as described in the aforesaid patents. However, in a present embodiment, it is possible to use existing sub-GHz LoRa networks to convey the distance data back to the server via the gateway, given that each tag within the environment may make use of its allocation of the available transmission bandwidth. Embodiments of the invention will be described, by way of example only, with reference to the accompanying drawings. The detailed embodiments show the best mode known to the inventor and provide support for the invention as claimed. However, they are only exemplary and should not be used to interpret or limit the scope of the claims. Their purpose is to provide a teaching to those skilled in the art. Components and processes distinguished by ordinal phrases such as “first” and “second” do not necessarily define an order or ranking of any sort. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS Figure 1 shows an environment containing items that need to be located; Figure 2 shows the environment of Figure 1 divided into an array of groups; Figure 3 shows an example of tag distribution; Figure 4 shows the deployment of a tag; Figure 5 shows the schematic representation of the tag identified in Figure 4 and a schematic representation of an anchor; Figure 6 shows an enlarged view of zones identified in Figure 2\ Figure 7 shows communication cycles for the group types identified in Figure 6; Figure 8 illustrates a communication cycle for an individual group of anchors; Figure 9 shows procedures performed by the network processor identified in Figure 5; Figure 10 shows operations performed by the microcontroller present within the anchor transceiver identified in Figure 5; Figure 11 shows procedures performed by the microcontroller present within the tag identified in Figure 5; Figure 12 shows the introduction of a tag into the environment; Figure 13 shows a table populated in the tag shown in Figure 12; Figure 14 shows the identification of a local set of anchors; Figure 15 shows a collection of tools having RFID devices; and Figure 16 shows how a tag communicates with the RFID devices identified in Figure 15. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION Figure 1 An environment is shown in Figure 1 that contains items that need to be located within the environment by the application of tags. To achieve this, distances are assessed between mobile transceivers (the tags) and a plurality of fixed transceivers, also known as anchors. In this example, the environment consists of an enclosed warehouse 101 approached by a driveway 102 from a main road 103. In addition to items being stored in the warehouse 101, items are also stored in a yard area 104 and vehicles are parked in a parking area 105. Fixed transceivers broadcast beacon signals that identify their position in the environment. At each mobile transceiver, a local set of fixed transceivers is identified and ranging signals are transmitted to and received from the fixed transceivers in the local set. Distance data is assessed representing distances between the mobile transceiver and the fixed transceivers in the local set. The distance data is then transferred to a gateway which in turn allows location information to be derived at a network processor. In this embodiment, and as described in US 11,665,663, assigned to the present applicant, distances are measured by measuring the duration of a round-trip time-of-flight of a transmitted (and re-transmitted) radio signal. Many signals of this type are transmitted such that, based on many range values, it is possible to put forward a best assessment as to what is considered to be the actual distance between a fixed transceiver (an anchor) and a mobile transceiver (a tag). In an embodiment, Semtech SX1280 devices are used that are capable of running a LoRa network at 2.4 GHz and which include procedures allowing time-of-flight measurements to be obtained. A ranging operation is instigated by a transceiver (designated as a master) which then ranges with another transceiver, designated as a slave. In this environment, ranging is initiated by a tag when it has been deployed within the environment, after it has moved within the environment and / or after a predetermined period of time has elapsed. The system is therefore configured such that it is possible for the tags to automatically locate themselves as being in the vicinity of a particular set of anchors. Figure 2 In an embodiment, the environment of Figure 1 is divided into an array of groups, that may also be identified as zones or tiles. As shown in Figure 2, there are a total of thirty-six groups labelled 201 to 236, with the total number of zones required to cover an environment depending upon the size of the environment and the maximum range over which valid transmissions may occur. The arrangement of groups in Figure 2 is idealized and most practical installations will have a much less symmetrical arrangement of groups, due to the physical constraints of the environment that will include walls and other obstacles. Each room will therefore tend to define a zone into which a group of anchors are deployed, although more than one group may be required in larger open spaces. Each group includes a plurality of anchors, which are sequentially numbered, such that each may be uniquely identified. Thus, each anchor may be identified by the combination of its group identification (201 to 236) followed by a unique index within the group which, for example, may range from 1 to 8. Thus, the second anchor in the second group would be identified as 202-2. The 2.4 GHz LoRa protocol has an advantage over similar protocols operating at frequencies below 2.4 GHz (sub-GHz LoRa), such as the licence free bands centred around 868 MHz in Europe and 915 MHz in the United States, in that it is not duty cycle limited. However, the sub-GHz protocols have two significant advantages in their favour. Firstly, transmission distances are longer, such that it is possible to establish a fully functional network over a relatively large environment with fewer gateways. If the network is to operate at 2.4 GHz, it may be necessary to install a substantial number of gateways, along with related infrastructure, compared to a sub-GHz system. Secondly, it is also appreciated that many sub-GHz systems are already in place; therefore, it would be preferable in many situations to deploy a tag location system over this existing infrastructure to minimise overall installation costs. In the present embodiment, ranging and distance measurement is achieved based on the return time-of-flight capabilities of a 2.4 GHz LoRa network. However, when distance data is transferred to a gateway, this is achieved using an existing sub-GHz network. However, it is appreciated that bandwidth restrictions exist when using these sub-GHz networks and that each device is only permitted to transmit data for a relatively short duty cycle. Consequently, it is desirable to transmit a relatively small quantity of high-quality data (which may be achieved by averaging many results) and by transmitting the data from the many tags present within the environment in preference to transmitting the data from the relatively fewer anchors. Figure 3 An example of a tag distribution is illustrated in Figure 3, in which six anchors A1 to A6 are attempting to locate the position of fourteen tags T1 to T14 within zone 216. As previously described, return time-of-flight ranging provides accurate distance assessment and is superior to using signal strength methods. However, errors do occur due to reflections, possibly from the floor, ceiling, external walls, a first internal wall 301 and a second internal wall 302. Figure 4 In an embodiment described in US 11,665,663 (assigned to the present applicant) the mobile tags were relatively modest in size, allowing them to be worn via wrist supports upon vulnerable patients within a healthcare setting. Tags of this size could also be attached to relatively small items of equipment and the power requirements of the tags were minimised by ensuring that the bulk of radio transmissions took place between anchors and gateways. When deploying a system making use of sub-GHz radio protocols, this approach is not possible due to bandwidth restrictions; therefore, transmissions to the gateways must take place from the mobile tags, in preference to these transmissions taking place from the fixed anchors. As a consequence of this, the tags do require greater levels of energy storage which in turn tends to increase their overall size. Furthermore, in addition to the 2.4 GHz radio transceiver, each tag must also include a sub-GHz transceiver and, in an embodiment, it is also preferable to include a GPS receiver to facilitate tag location when outside. Thus, referring to the environment of Figure 1, ranging in accordance with the present invention may be deployed within warehouse 101 but existing techniques using GPS receivers may be deployed within the yard area 104 and, if required, when outside of the environment altogether. An example of a tag 401 is shown in Figure 4. In this embodiment, screws are deployed to permanently attach the tag to movable objects such as a container 402. Other objects may include pallets, trolleys, stillages and crates etc. In this embodiment, the tag is the master for ranging activities and it initiates ranging activities with the anchors. The anchors are addressable via a virtual address (group id - index) and not their device IDs, which is used by the tags to instigate ranging. A database at the network processor knows all of these addresses and will have transmitted appropriate data to devices during a setup procedure. The anchors send out beacons to identify locality (effectively, their virtual addresses) at known times and these are specified in the local firmware: such that they do not need to be transmitted from the database. The virtual address of each anchor identifies which group it belongs to, along with its position or ranking (an index) within the group. When deployed, a tag listens for the beacons such that it will hear a number of identifications, allowing it to independently pick a set of anchors up to a maximum of forty in the present embodiment. Thus, the set of selected anchors can be selected from many groups and this selection will change as tags are moved in the environment. Thereafter, a tag can instigate ranging activities with the anchors in the selected set. In an embodiment, forty transmission frequencies are defined within the available bandwidth, giving a 1.6 MHz transmission bandwidth with gaps between each one. Many other divisions are possible, with the bandwidth of each channel being reduced if the number of frequency channels increases. To assess a distance, all forty of these frequencies are used. As used herein, each ranging operation uses a single frequency and the forty ranging operations are combined to make an individual provisional distance assessment. This is then repeated to provide a plurality of provisional distance assessments which are then averaged to produce a distance assessment that is uploaded from the tag to the gateway. In the present embodiment, with forty transmission frequencies, it is possible for forty anchors in close proximity to transmit simultaneously. Each cycle through the available forty frequencies (to produce forty ranges and one provisional distance) at mutually different frequencies. Furthermore, greater frequency separation may be selected for anchors in the same group. In an environment, such as that shown in Figure 2, where more than forty anchors are required, anchors of what may be identified as being of the same type do use the same transmission frequencies at the same time but are separated spatially so as not to interfere. Many embodiments are possible to achieve an appropriate distribution of anchors within the environment. However, most environments are divided by walls, therefore this does naturally lead to the establishment of groups; although ranging is not restricted to occurring within a single group and a set may be selected from many groups. The present embodiment establishes groups and places up to eight anchors in each group, although other allocations are possible in other embodiments. In a group, eight transmissions may occur simultaneously. A total of forty transmissions may occur simultaneously, so these may be allocated across a total of five groups. Thus, in this embodiment, there are a total of five different group types. These are arranged such that groups of the same type are not adjacent to each other and therefore do not interfere. Typically, ranging within a building may occur over distances of twenty-five metres to fifty metres, assuming that no walls are present. Other rooms are provided with their own group, and when separated by two or three rooms, the same frequencies will not interfere. Thus, an unlimited number of ranging operations may be performed simultaneously in the environment by a combination of frequency multiplexing and space multiplexing. However, before ranging operations can be instigated by a tag, the tag itself must locate a set of local beacons and identify them with their virtual addresses (group id - index). To achieve this, the tags listen for beacon signals (beacons) transmitted by the anchors on the same predetermined frequency in the 2.4Ghz band without interference. To achieve this, the beacon transmissions are time multiplexed. Figure 5 A schematic representation of tag 401 is shown in Figure 5. A microcontroller 501 communicates with a 2.4 GHz transceiver 502 and a sub-GHz transceiver 503. In this embodiment, the microcontroller 501 also receives data from an accelerometer 504 and from a global positioning system (such as GPS) receiver 505. An example of an anchor transceiver 511 is also shown in Figure 5. This also includes a microcontroller 512, a 2.4 GHz transceiver 513 and a sub-GHz transceiver 514. It is not necessary for anchor transceivers to include global positioning and accelerometers. In operation, ranging 515 is performed between the 2.4 GHz transceiver 502 and the 2.4 GHz transceiver 513. A network processor 522 is identified in Figure 5 and this may include separate systems for establishing a communication server and for running specific applications; including an application to determine tag positions from the distance data, possibly by deploying techniques as described in US 11,665,663; the whole contents of which is incorporated herein by reference. The network processor 522 communicates with a gateway 523 over a TCP / IP connection 524. The gateway in turn communicates with the sub-GHz transceivers 503, 514; possibly in an established LoRa radio network. Thus, ranging activities occur within a first radio environment in which activities may rely on mobile, battery-powered devices; thereby significantly enhancing the ability to introduce these devices into an established network. Thereafter, distance data is uploaded to the gateway 523 by the sub-GHz transceivers 503, 514; thereby making use of existing infrastructure or reducing the extent to which physical infrastructure needs to be introduced within the environment: that is to say, the sub-GHz network requires fewer gateways 523 compared to similar functionality established at 2.4 GHz. Thus, the apparatus 401 described with reference to Figure 5 may be deployed as a mobile transceiver in an environment in which distances are assessed between the mobile transceiver and a plurality of fixed transceivers. The apparatus includes radio communication devices 502, 503 for transmitting and receiving radio signals. In addition, a processor 501 is configured to identify a local set of fixed transceivers, transmit ranging signals to and receive ranging signals from the fixed transceivers in the local set, assess distance data representing distances between the mobile transceiver and the fixed transceivers in the local set, and transfer this distance data to the gateway 523. In addition, the anchor transceiver apparatus 511 is configured for deployment as a fixed transceiver in an environment in which distances are assessed between the fixed transceiver and a mobile transceiver. Radio communication devices 513, 514 are provided for transmitting and receiving radio signals in combination with a processor. In this deployment, the processor is configured to broadcast beacon signals that identify the position of the fixed transceiver in the environment. The anchor also receives ranging signals from a mobile transceiver 502 and retransmits these ranging signals back to the mobile transceiver, thereby allowing the mobile transceiver to assess distance data representing distances between the fixed transceiver and the mobile transceiver, and to transfer this distance data to the gateway 523 Figure 6 An enlarged view of zones 209 to 212, 215 to 218 and 221 to 224 is shown in Figure 6. In this embodiment, each zone, such as zone 209, has eight anchors identified as A1 to A8. This group of anchors may be identified as group 209 thus, for example, the second anchor in group 209 is identified by a virtual address 209-1. In each group, all eight anchors range simultaneously, therefore it is possible to allocate the available forty frequencies over five groups. This allocation is then repeated, such that each group is allocated to one of five group types, indicated as type A, type B, type C, type D or type E. As shown in Figure 6, these allocations are made such that no adjoining zones have the same group type. Figure 7 Communication cycles for each group type are shown in Figure 7. Each transmission cycle has a period of fifteen seconds consisting of a ranging portion R of twelve seconds (for performing ranging) followed by a beacon portion B of three seconds (for transmitting beacons). Within the fifteen second cycle, all forty frequencies are available. However, due to the presence of the beacon portions, only four of the five group types will actually be ranging at any specified time, therefore only thirty-two of the available frequencies are required for performing the ranging activities. The remaining frequencies are therefore available for the transmission of beacons. The same frequency or range of frequencies are used for the transmission of all of the beacon signals. These are therefore time multiplexed by the introduction of respective offsets 701. In this embodiment, group A has an offset of zero, group B has an offset of six-seconds, group C has an offset of twelve-seconds, group D has an offset of eighteen-seconds and group E has an offset of twenty-four seconds. After the introduction of these offsets, communication capabilities will continue to cycle for each group, consisting of a ranging portion of twelve seconds followed by a beacon period of three seconds. However, as shown in Figure 7, the beacon periods for each group type are mutually displaced in time, so that it is not possible for them to mutually interfere while operating at the same frequency. Figure 8 A communication cycle for an individual group of anchors A1 to A8 is shown in Figure 8. The twelve second ranging portion is divided into sixteen time slots identified as TO to T15. During each of these time slots, each anchor may range with a tag to produce data representing a provisional distance. In an embodiment, it is possible for each anchor to range with sixteen tags during each ranging portion. However, in the present embodiment, two slots are allocated to each tag (say, slot TO and slot T8). This allows the averaged distance data, for transmission to the gateway, to be calculated more quickly but only allows an anchor to range with eight tags during each ranging portion. In practice, an optimum level of tag communication may be determined based on the number of tags expected in the environment and the extent to which the tags move in the environment. The three second beacon portion is divided into eight beacon time slots. Within the group, the beacons are individually numbered to provide a virtual address and this number indexes the order in which the individual anchors transmit their beacon signal. Thus, within the communication range of a tag, each of a possible total of forty anchors have unique time slots for transmitting a beacon signal on the same predetermined frequency. Figure 9 Procedures performed by the network processor 522, (possibly representing a collection of communication and data processing systems), is illustrated in Figure 9. At step 901, the anchors are configured by transmitting configuration data to the microcontrollers 512 via the gateway 523 and the sub-GHz radio network. Typical gateways (523) have a limited number of slots and this is a one-to-many distribution process, therefore the duty cycle limitation becomes relevant. Referring to Figure 2, each group 201 to 236 may include up to eight anchors, therefore a total of two-hundred-and-eighty-eight anchors may be present within the environment shown in Figure 2, this being an example not a limitation. Thus, to perform configuration step 901, a significant amount of the available duty cycle is required and will take a significant amount of time (in the order of hours) to be completed. However, this configuration procedure is performed only once, unless the topology of the network as a whole is to be reconfigured. At step 902, distance data is received from tag transceivers 503. In terms of the duty cycle limitation, all of the data is now coming from the tag devices to the gateway 523 and there is a duty cycle allocation (typically of the order of one percent) for each individual device such that, overall, there is now significantly more bandwidth available. These restrictions do not exist in the 2.4 GHz network, where the ranging operations are performed. At step 903, tag locations are specified, as described in US 11,665,663. Thereafter, at step 904, graphical output interfaces are supported to display the location information specified at step 903. At step 905, a question is asked as to whether the network is to be reconfigured and if answered in the affirmative, configuration of the anchors is performed at step 901. A reconfiguration involves changing the topology described with reference to Figure 2; possibly, allowing the system to cover a larger area. Thus, on most iterations, the question asked at step 905 will be answered in the negative. A question is then asked at step 906 as to whether more data is available and when answered in the affirmative, distance data is again received at step 902. If answered in the negative, a question is asked at step 907 as to whether the process is to end. Figure 10 Operations performed by the microcontroller 512 within an anchor transceiver 511 are shown in Figure 10. Each individual transmission device (tags and anchors) deployed within the network has a unique address. The unique addresses for the anchors are identified at step 901, during the configuration process, such that each individual anchor will receive configuration data at step 1001. This comprises a group ID 1002, an index (1 to 8) within the group 1003 and a transmission offset 1004; based on the group type and as described with reference to Figure 7. When communicating with tags, individual tags are addressed using their unique (hard-coded) identification. However, when ranging operations are initiated by a tag, they will not make use of the anchor’s unique identification but will instead make use of the virtual address derived from the group ID 1002 and the index 1003 received by the anchor at step 1001. After completing the configuration of the overall network and providing the configuration data to each individual anchor, the system is now in a position to perform ranging operations to determine distances between tags and anchors. At each anchor within the network, ranging operations are initially delayed by the offset value 1004 at step 1005. As previously described, each group type will have a different offset to create temporal displacement of beacon transmissions in adjacent zones. A ranging window is opened at step 1006 during which the anchor, operating as a slave device, is available to be ranged in response to signals received from a tag; with the tag acting as ranging master. On a first iteration and possibly for some time thereafter, no successful ranging will take place because this is not possible until beacons, transmitted from the anchors, have been received and processed by the tags. If a ranging operation is performed, ranging activity is recorded at step 1007. Each anchor has sixteen ranging slots (0 to 15) and these ranging slots are identified in a bit mask 1008. In the bit mask, a 1 indicates that a particular slot was in use during the previous iteration, whereas a 0 indicates that the slot is available for allocation. In an embodiment and to conserve energy resources, slots 4, 5, 6and 7, and slots 12, 13, 14 and 15 are identified as being in use and no ranging activities are attempted within these slots until all of the remaining slots have been allocated, whereupon they are then released. In the example shown in Figure 10, a previous iteration has made use of slot 0 and slot 8. In an embodiment, this relates to the same pair of tag and anchor to speed up distance assessment. Thus, within each cycle, it is possible for ranging activities to be carried out twice, resulting in the assessment of two provisional distance values. Furthermore, again in an attempt to improve overall accuracy, four consecutive ranging windows are selected at step 1006, thereby producing a total of eight provisional distance values which are recorded and averaged locally at the tag. On each iteration, the anchor will listen in slots 0,1, 2, 3,, 8, 9, Wand 11, and record ranging activity as indicated at step 1007, until further allocations are made. Thereafter, at step 1009 a beacon signal is broadcast for reception by any tags within range looking to establish ranging operations. As illustrated at 1010, the beacon comprises an identification of the group, the index within the group and the offset, derived from configuration data 1002, 1003 and 1004 respectively. In addition, the bit mask 1008 is also added to the beacon. The use of beacons transmitted by the anchors provides the tag with information as to which slots are available for use. For a tag instructed to instigate ranging, it randomly selects one of the available time slots, thereby removing the need to perform a “clear channel” investigation on all of the slots. In addition, this reduces tag energy consumption. To ensure that the flags in the beacons are cleared, a tag identifies when it is performing its last use of a slot by setting an “end of ranging” flag in the last ranging transmission. At step 1011, a question is asked as to whether the processing is to stop and when answered in the negative, the next ranging window is opened at step 1006. Processing stops if the question asked at step 1011 is answered in the affirmative. Figure 11 Procedures performed by microcontroller 501 in each tag 401 are shown in Figure 11. The tags are the masters for ranging activities and must therefore initiate ranging activities with the anchors. The anchors are addressable via their virtual addresses, with the anchors sending out beacons to identify locality within windows specified by the local firmware. For the tags, energy is only available from a local battery supply, therefore this available energy must be conserved whenever possible. However, energy is required to initiate ranging operations and to upload data, using the sub-GHz network, to the gateway 523. Thus, when not actually required to perform transmissions, microcontroller 501 enters a sleep mode at step 1101. In this embodiment, two possibilities exist for interrupting the microcontroller 501 from its sleep mode. These comprise movement and elapsed time. In an embodiment, the accelerometer 504 generates an interrupt signal 1102 when movement occurs. When servicing this interrupt, the processor waits at step 1103 and then asks a question at step 1104 as to whether the movement has stopped. If this question is answered in the negative, the processor waits again at step 1103 and this procedure repeats until movement stops. This situation may be seen as the tag being located within the environment for the first time. However, when in the environment, further movements may take place effectively relocating the tag. Thus, under these circumstances, a movement interrupt 1102 would be generated again; and again, the procedure waits until the movement has stopped such that this may in turn be identified as a new location. In addition to interrupts occurring due to movement, interrupts are also generated after a predetermined time interval has elapsed. Thus, in an embodiment, periodically, ranging operations are performed to locate the tag irrespective of whether the tag has moved. Furthermore, if many assessments are made due to this type of interrupt, indicating that the tag has not moved, the increasing number of measurements may be averaged to enhance the accuracy of position evaluation. In response to being interrupted, the tag locates local anchors at step 1106 and then assesses distances at step 1107. The data is uploaded to the gateway at step 1108 and a question is then asked at step 1109 as to whether processing is to stop. When answered in the negative, the processor re-enters the sleep mode at step 1101. Figure 12 In an anchor group, all of the anchors (up to eight) are available for a period of twelve seconds to perform ranging activities with a total of eight tags, assuming each tag-anchor combination is ranged twice. There is then a period of three seconds when the beacon signals are sequentially transmitted, whereafter the cycle repeats. Each beacon signal is relatively short but it allows the anchor to identify itself to any tags in the vicinity that are looking to initiate ranging operations. Thus, when a beacon is detected by a tag, the tag is made aware of the group, the index within the group, the communication offset (representing the group type as A, B, C, D or E) and time slots available for ranging activities. When a new tag arrives within the environment, it must identify where it is; consequently, the tag turns on its ranging receiver 502 and listens for beacon signals broadcast by the anchors. In total, it could acquire the information of a maximum of forty anchors that are potentially in range. From this information, it then works out a set of anchors with which to range by making reference to received signal strength data. If an anchor can be ranged, it is accepted within the set given that time slots are available. A tag 1201 is shown in Figure 12 that has been introduced to the environment of Figure 1. In this example, it receives ten beacons and the numbers 1 to 10 in Figure 12 identify relative signal strengths, from the highest signal strength 10 to the lowest signal strength 1. The signal strengths will tend to be higher when the tag is closer to an anchor and when there are no obstacles between the tag and the anchor. The signal strengths may be used to provide a provisional (coarse) indication of position. However, these detected anchors are now identified as a set of ranging targets or slaves for ranging. The anchors are addressed with reference to their group id and index. The offset data identifies the group type and the mask identifies actual slots that can be used for ranging. In the example shown in Figure 12, a beacon is received from a first anchor 1211, followed by a beacon signal received from a second anchor 1212. The next beacon signal is received from a third anchor 1213, followed by a beacon signal received from a fourth anchor 1214. This process continues with a beacon received from a fifth anchor 1215, followed by receiving a beacon from a sixth anchor 1216. The next beacon is received from a seventh anchor 1217, followed by a beacon received from an eighth anchor 1218. A ninth beacon signal is received from a ninth anchor 1219 and a final beacon signal is then received from a tenth anchor 1220. Other anchors within the environment are not close enough to tag 1201 for their beacon signal to be received and are therefore not available for ranging activities to be performed. Figure 13 An example of a table populated by a tag following the activities described with reference to Figure 12 is illustrated in Figure 13. Beacon details are entered in the table as they arrive, as shown in a first column 1301 and received signal strength data is entered in a second column 1302. For the purposes of illustration, integer values are identified in Figure 13 but these values can include more significant figures and may represent various parameters. The table of Figure 13 is further populated with the virtual address data identified as the group number in a third column 1303 and the index number in a fourth column 1304. In this example, the additional beacon data is also stored in the table, such that the offsets (group types) are recorded in a fifth column 1305 and the bit mask is recorded in a sixth column 1306. Figure 14 After locating a set of local anchors, as described with reference to Figure 12 and Figure 13, the tag 1201 now includes information allowing it to make an assessment to the effect that it is likely to be in or around zone 216; whereas, previously, it could have been anywhere within the environment. The tag is now required to assess distances to all of the anchors within the identified set, which in this example is a total of ten located in a total of four groups. Each ranging activity involves the use of all forty available frequencies. In each cycle, during the ranging portion or window, two slots are used giving two provisional distance assessments. A further three cycles are used, such that an actual distance assessment is derived from the average of eight provisional assessments. Ranging activities continue, making use of the specified time slots, until for each anchor, a distance assessment has been made. Data representing distance assessments for all (ten) anchors are then uploaded to the gateway (using the sub-GHz network), whereafter the tag may return to its sleep mode. Figure 15 In some deployments, it is necessary to track the position of relatively small tools, as disclosed in GB 2619707 (assigned to the present applicant), possibly for calibration purposes. Although it may be desirable to display messages on the tools themselves, practical limitations exist in terms of attaching a relatively large tag to a relatively small tool. Furthermore, the cost of the tag may be prohibitive when compared to the value of the tool itself. Examples of tools of this type are shown in Figure 15. Referring to Figure 5, the tag 401 may include an RFID reader 1501 which may be energised by the microcontroller 501. As is known in the art, this results in an energisation signal being transmitted which energises RFID tags within range. RFID tags energised in this way then transmit a signal identifying a unique identification code which may be compared against identifications stored in a database held at the network processor 522. To make use of this functionality, referring to Figure 15, the tag 401 may be placed in a receptacle such as a tool bag 1502. In this example, a first RFID tag 1511 has been attached to the first tool 1521, a second RFID tag 1512 has been attached to a second tool 1522, a third RFID tag 1513 has been attached to a third tool 1523 and a fourth RFID tag 1514 has been attached to fourth tool 1524. A typical range for RFID communication is between one metre and two metres but after activation of the RFID reader 1501, it is possible for all of the tools contained within the bag to be scanned. Thus, by knowing the location of the tag transceiver 401, operable within the radio networks, it is possible to infer the location of the RFID tags 1511 to 1514 and hence the location of the tools 1521 to 1524. Thus, having located the tools, it is then possible for the network processor to produce appropriate output data, possibly indicating that calibration is required. Figure 16 As shown in Figure 16, periodically, the tag transceiver 401 will scan for RFID tags, resulting in data about the location of the tags being reported back to the gateway 523 via the sub-GHz transmitter 503. At step 1601, the RFID reader is energised and at step 1602 data is received back from the energised tags. This received data is stored at step 1603, whereafter a question is asked as to whether more data is available. When answered in the affirmative, more data is received at step 1602 and further storage occurs at step 1603. Eventually, the question asked at step 1604 will be answered in the negative and the data is then uploaded at step 1605. It is appreciated that many applications are possible for this technique. For example, in a hospital environment, a tag transceiver could be attached to a bed and arranged to scan to identify items within the local vicinity. This information could be used to determine whether any items have gone missing or whether any disposable items need replenishment. A yard may have shipping containers and it may be desirable to ascertain whether items remain in these containers or have been removed. Thus, the tag transceiver could be attached to the container and RFID tags could be attached to individual items.

Claims

The invention claimed is:

1. A method of assessing a distance between a mobile transceiver and a plurality of fixed transceivers in an environment, comprising the steps of: at each fixed transceiver:broadcasting beacon signals that identify the position of the fixed transceiver in the environment; and at the mobile transceiver:identifying a local set of fixed transceivers;transmitting ranging signals to and receiving ranging signals from the fixed transceivers in said local set;assessing distance data representing distances between the mobile transceiver and the fixed transceivers in the local set; andtransferring said distance data to a gateway.

2. The method of claim 1, wherein the location of a fixed transceiver is specified in terms of belonging to a group and a unique index within said group.

3. The method of claim 1 or claim 2, wherein said identifying step comprises the steps of:receiving a plurality of beacon signals;ordering said beacon signals in terms of signal strength; andselecting the set that transmitted beacon signals with the largest signal strength.

4. The method of claim 2, wherein said transmitting step is instigated by transmitting virtual addresses for the fixed transceivers in said selected set, constructed from a group identification and the unique index within the group.

5. The method of any of claims 1 to 4, wherein said transmitting step comprises the steps of:transmitting a plurality of ranging signals in a selected transmission slot to make a provisional assessment of distance;repeating said transmission step for a plurality of cycles, using a similar slot, and making further provisional assessments of distance; andaveraging said provisional assessments of distance to produce said distance data.

6. The method of any of claims 1 to 5, wherein said broadcasting step comprises the step of broadcasting slot usage data identifying similar slots that were used for ranging on the previous cycle.

7. The method of claim 6, comprising the step of masking some available slots to conserve energy.

8. The method of any of claims 1 to 7, wherein:said transferring step is performed using a radio transmission protocol having restricted duty cycle use; whereassaid transmitting step is not duty cycle restricted.

9. The method of any of claims 1 to 8, wherein said transmitting step is initiated:after detecting a movement of the mobile transceiver; and.after a predetermined period of time has elapsed.

10. The method of any of claims 1 to 9, wherein the mobile transceiver is attached to a mobile apparatus;radio frequency identification devices are attached to respective mobileitems;the mobile transceiver includes a radio frequency identification device reader; and the method further comprises the steps of, at the mobile transceiver:energising the radio frequency identification device reader;reading radio frequency identification devices within range; andtransmitting radio frequency identification device originating data to the gateway.

11. An apparatus for deployment as a fixed transceiver in an environment in which distances are assessed between the fixed transceiver and a mobile transceiver, comprising:radio communication devices for transmitting and receiving radio signals; anda processor, wherein said processor is configured on each cycle to:broadcast beacon signals that identify the position of the fixed transceiver in the environment;receive ranging signals form a mobile transceiver and re-transmit said ranging signals back to said mobile transceiver, thereby allowing said mobile transceiver to assess distance data representing distances between the fixed transceiver and the mobile transceiver, and transfer said distance data to a gateway.

12. The apparatus of claim 11, wherein the fixed transceiver is configured to store location data in terms of a group identification and a unique index within the identified group.

13. The apparatus of claim 11 or claim 12, wherein said fixed transceiver is configured to include slot usage data in said beacon signals, wherein said slot usage data identifies slots that were used for ranging on the previous cycle.

14. The apparatus of claim 13, wherein said fixed transceiver is configured to mask some of the available slots to conserve energy.

15. An apparatus for deployment as a mobile transceiver in an environment in which distances are assessed between the mobile transceiver and a plurality of fixed transceivers, comprising:radio communication devices for transmitting and receiving radio signals; anda processor, wherein said processor is configured to:identify a local set of fixed transceivers;transmit signals to and receive ranging signals from the fixed transceivers in said local set;assess distance data representing distances between the mobile transceiver and the fixed transceiver in the local set; andtransfer said distance data to a gateway.

16. The apparatus of claim 15, wherein said processor is configured to identify the local set of fixed transceivers by:receiving a plurality of beacon signals;ordering said beacon signals in terms of signal strength; andselecting the set of fixed transceivers that transmitted beacon signals with the largest signal strength.

17. The apparatus of claim 16, wherein said processor is configured to transmit to fixed transceivers in the selected local set by specifying virtual addresses based on a group identification and a unique index within the group.

18. The apparatus of any of claims 15 to 17, wherein said processor is configured to:transmit a plurality of ranging signals in a selected transmission slot to make a provisional assessment of distance;repeat said transmission step for a plurality of cycles, using an equivalent slot, and making further provisional assessments of distance; and average said provisional assessments of distance to produce said distance data.

519. The apparatus of any of claims 15 to 18, wherein said processor is configured to transfer the distance data to a gateway using a radio transmission protocol having a restricted duty cycle.10 20. The apparatus of any of claims 15 to 19, further comprising aplurality of radio frequency identification devices for attachment to respective mobile items, wherein the mobile transceiver is attached to a mobile apparatus and includes a radio frequency identification device reader, and said processor is configured to:15 energise said radio frequency device reader;read radio frequency identification devices within range; andtransmit radio frequency identification device originating data to the gateway.

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