WO2015136312A1

WO2015136312A1 - Personnel tracking system

Info

Publication number: WO2015136312A1
Application number: PCT/GB2015/050751
Authority: WO
Inventors: Charlotte RICHARDSON
Original assignee: S3 Id Limited
Priority date: 2014-03-13
Filing date: 2015-03-13
Publication date: 2015-09-17
Also published as: GB2524071A; GB201404519D0

Abstract

A personnel tracking system (10) for tracking personnel in a predefined area (56) includes a controller (12), a plurality of base stations (14), and a signal arrangement (40) which permits signal communication between each of the base stations (14) and the controller (12). Each base station (14) includes a base station transceiver (16) for transmitting base station signals (30) to a cell area (18) associated with the respective base station (14). The system (10) includes a plurality of identity tags (20), each of which includes a unique tag identification code (22);a tag satellite receiver (24) for receiving location signals (36) from a satellite system (26) and generating a tag location message (38);and a tag transceiver (28) for receiving the base station signals (30) and transmitting tag signals (32) to the base stations (14). The tag signals (32) include the tag identification code (22) and the tag location message (38).

Description

Personnel Tracking System

The present invention relates to personnel tracking systems, particularly but not exclusively personnel tracking systems for tracking personnel in a predefined area.

90% of the world's oil/gas fields are onshore and mostly in remote locations. Typically each one spreads over a large area e.g. 100km by 50 km. The wells need regular maintenance and service personnel are at risk in these locations due to possible explosions, toxic gas, equipment failures, vehicle breakdown and getting lost. The safety and security of the workforce could be greatly improved by having the facility to locate and communicate with all in-zone personnel quickly and simultaneously. Conventionally, personnel tracking systems comprise identity tags which are carried by personnel which communicate the location of each person carrying one of the tags to a central controller. In some systems, the tag is an RFID tag, and the system tracks the movement of personnel via detectors located around a predefined area, for example, in doorways around a building. While such systems are appropriate for relatively small predefined areas, the hardware required to provide accurate location precludes the use of such systems for relatively large remote areas such as those mentioned above. Another disadvantage of conventional tracking systems is that the tags are limited in functionality and provide little more than the transmission of an identity code to the system.

Other conventional tracking systems use tracking devices based on GPS, which report via mobile phone networks. However in the large remote areas mentioned above, frequently, mobile phone network infrastructure does not exist or is unsatisfactory, precluding the use of such systems.

Other systems utilise radio communication networks, but commonly suffer capacity shortfalls - they cannot provide a sufficiently fast update rate for a sufficiently large number of people to be effective in enabling real-time tracking of large numbers (eg thousands) of people. Fast update rates are required for effective tracking but place high loads on tracking device batteries. A large user base places a high load on the communication channels available.

Tag battery life is a major problem for users in remote areas. Any tracking system in which tag batteries have to be recharged frequently or in which the tag battery life is uncertain because the occurrence of transmission events is not controlled carries the risk of battery drain leading to system failure as users cannot be tracked.

Utilising public infrastructure, for example, existing communication networks such as mobile phone networks, also increases the security risk from unauthorised users and the risk of hacking or terrorist attack.

Other constraints are that any system must be capable of safe operation in hazardous environments eg spark hazard in explosive environments, and of simultaneous multi-way communication eg notification of an impending emergency.

According to a first aspect of the present invention, there is provided a personnel tracking system for tracking personnel in a predefined area, the system including a controller, a plurality of base stations, a signal arrangement which permits signal communication between each of the base stations and the controller, each base station including a base station transceiver for transmitting base station signals to a cell area associated with the respective base station, the system including a plurality of identity tags, each of which includes:

· a unique tag identification code;

• a tag satellite receiver for receiving location signals from a satellite system and generating a tag location message; • a tag transceiver for receiving one or more of the base station signals and transmitting tag signals to one of the base stations;

• the tag signals including the tag identification code and the tag location message.

Possibly, each base station transmits the base station signals on one predefined frequency, which may be a different frequency to the transmission frequency of the base stations of any adjoining cell areas. Possibly, the one predefined frequency is one of N predefined base station frequencies, where N is sufficiently large that no neighbouring base stations utilise the same frequency. Possibly, N is greater than 1 and may be between 2 and 4. Optimally, N may be 4. Possibly, adjoining cell areas overlap, and may overlap to provide substantially continuous base station radio signal coverage over the predefined area.

Possibly, the tag transceiver transmits the tag signals on a tag frequency. Possibly, each tag includes a frequency selector for selecting the tag frequency from a plurality of available tag frequencies, each of which relate to one of the base station frequencies. Possibly, the frequency selector selects the tag frequency in response to the base station signals received. Possibly, the frequency selector assesses one or more criteria relating to the base station signals received from the base stations and selects the tag frequency in accordance with the one or more criteria.

Possibly, the one or more criteria include a first criterion, which may relate to the signal strength of the base station signals. Possibly, the system includes a base station register associated with each base station. Possibly, each tag is moveable between an unregistered condition in which the respective tag is not registered to any base station, and a registered condition, in which the respective tag is registered to one of the base stations.

Possibly, the one or more criteria include a second criterion, which may relate to the number of tags already registered to each of the base stations. Possibly, each base station signal includes a capacity message, which may indicate the number of tags registered to the respective base station.

Possibly, the one or more selection criteria include a third criterion, which may relate to the time period which has elapsed since the respective tag was last registered.

Possibly, the controller includes a master register. The system may include an output for displaying the master register, which may include display means. The master register may be output in the form of a master register representation, such as a map and/or a table.

The controller may include a processor, which may include one or more sets of instructions, which may derive location coordinates from the location messages, and may derive personal detail items such as names from the identification codes. The controller may include a database of identification codes with associated personnel detail items. The master register representation may include one or some of the personal detail items associated with each tag, along with the latest location co-ordinates for the respective tag. Possibly, the system includes a registration routine in which one tag is registered to a new base station the first time the new base station receives a first tag signal from the one tag. Possibly, when the new base station receives the first tag signal from the one tag, the registration routine deregisters the one tag from the register of the old base station.

The base stations may transmit the base station signals on a predetermined time frequency, which may be at least once per minute, more desirably may be at least once every 30 seconds, and optimally is substantially every ten seconds.

Possibly, the tag identification code and the tag location message are transmitted onward from the one base station to the controller via the signal arrangement.

Each tag may be certified as being intrinsically safe (IS). The maximum transmission power of each tag may be 500 mW. Each cell area may be at most 5 km in diameter, and may be no less than 1 km in diameter.

Possibly, the system is separate from any global communication network, and in particular is separate from global cellular communication (mobile telephone) networks. Possibly, the base station signals and the tag signals are transmitted at frequencies in the range 420-470 MHz, with each frequency having a band width of nominally 25 KHz.

Possibly, the base station signals and the tags conform to a predefined signal protocol. The protocol may include the base station signals and the tag signals. The tag signals may include request signals and response signals.

According to a second aspect of the present invention, there is provided a signal protocol for a personnel tracking system for tracking personnel in a predefined area, the protocol including base station signals transmitted by base stations and tag signals transmitted by tags, the tag signals including request signals and response signals. Possibly, the personnel tracking system includes any of the features described above.

Possibly, the response signals include the tag location messages. Possibly, the response signals are sent at an occurrence rate, which may be determined by the occurrence of the base station signals. Possibly, the occurrence rate is substantially the same for all the tags in the system and may be the same over a whole operating period of the system. Possibly, the occurrence rate is between 1 second and 1 minute, more desirably is between 5 seconds and 30 seconds, and optimally is ten seconds.

Possibly, the request signals include the identification codes. Possibly, the request signals are only sent when the tag is to be registered to a new base station.

Possibly, each base station transmits one base station signal which may be substantially continuous, and may comprise a sequence of sets. Possibly, each set includes a plurality of frames. Possibly, the frames are of different types which are fixed in order in each of the sets. Possibly, all of the base station signals of all of the base stations comprise similar sets which comprise the different frame types in the same fixed order.

Possibly, each set includes a poll frame, which may poll all of the tags registered to the respective base station to determine their location, and which may be the first frame of each of the sets. Each poll frame defines a response signal slot, in which the response signal from a tag registered to the respective base station may be receivable.

Possibly, each poll frame defines a plurality of response signal slots, and may define up to 2047 response signal slots. Possibly, the maximum number of tags which can be registered in the cell area of each base station is equal to the number of response signal slots in the poll frame of the base station signal transmitted by the respective base station. Possibly, each poll frame includes one or more response synchronisation messages which may precede the response signal slots and may permit the registered tags to synchronise the transmission of the response signals to the correct response signal slots.

Each synchronisation message may include the capacity message for the respective base station, indicating the number of tags registered to the respective base station.

Possibly, each set includes an invite frame, which may invite new tags to register with the respective base station, and which may immediately follow the poll frame. Possibly, each invite frame defines a registration request slot, in which the request signal from an unregistered tag may be receivable.

Possibly, each invite frame includes a request signal slot allocation message which precedes the registration request slot and relates to the next available registration request slot. Possibly, each set includes a registration frame, which may issue registration information to newly registered tags, and which may immediately follow the invite frame. Possibly, the registration information includes a response signal slot allocation message which relates to the response signal slot which has been allocated to a newly registered tag in the poll frame of the next set.

Possibly, each set has a set duration time which has a predefined value. The value may be between 1 second and 1 minute, more desirably of between 5 seconds and 30 seconds, and optimally has a duration of substantially ten seconds.

Possibly, the set duration time of each of the sets for each base station is substantially the same. Possibly, when the cumulative duration time of the poll frame, the invite frame and the registration frame is less than the set duration time, the set includes a null frame to maintain the set duration time at the correct predefined value.

Possibly, the set duration time determines the occurrence rate.

According to a third aspect of the present invention, there is provided a method of tracking personnel in a predefined area, the method including providing a personnel tracking system, the system including a controller, a plurality of base stations, a signal arrangement which permits signal communication between each of the base stations and the controller, each base station including a base station transceiver for transmitting base station signals to a cell area associated with the respective base station, the system including a plurality of identity tags, each of which includes:

• a unique tag identification code;

• a tag satellite receiver for receiving location signals from a satellite system and generating a tag location message;

· a tag transceiver for receiving one or more of the base station signals and transmitting tag signals to one of the base stations;

• the tag signals including the tag identification code and the tag location message. According to a fourth aspect of the present invention, there is provided a method of tracking personnel in a predefined area, the method including providing a signal protocol for a personnel tracking system, the protocol including base station signals transmitted by base stations and tag signals transmitted by tags, the tag signals including request signals and response signals. Possibly, the personnel tracking system includes any of the features described in the preceding paragraphs or later within this document.

Possibly, the method includes any of the steps described in the preceding paragraphs or later within this document.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view of a personnel tracking system, with Fig. 1 A being a relatively enlarged detail view of a tag ;

Fig. 2 is a schematic view of cell areas of a personnel tracking system; Fig. 3 is a schematic block diagram of the personnel tracking system of

Fig. 1 ;

Figs. 4 to 9 show a signal protocol of the personnel tracking system:

Fig. 4 is a schematic view of a base station signal at full capacity;

Fig. 5 is a schematic view of a base station signal at below full capacity;

Fig. 6 is a relatively enlarged schematic view of a registration frame of the base station signal shown in Fig. 5 indicated by the reference VI;

Fig. 7 is a relatively enlarged schematic view of an invite frame of the base station signal shown in Fig. 5 indicated by the reference VII, with tag signals;

Fig. 8 is a relatively enlarged schematic view of a poll frame of the base station signal shown in Fig. 5 indicated by the reference VIII, with tag signals; and

Fig. 9 is a schematic view of the protocol during registration of a tag to a new base station. Fig. 1 shows a personnel tracking system 10 for tracking personnel in a predefined area 56, the system 10 including a controller 12, a plurality of base stations 14, a signal arrangement 40 which permits signal communication between each of the base stations 14 and the controller 12, each base station 14 including a base station transceiver 16 for transmitting base station signals 30 to a cell area 18 associated with the respective base station 14, the system 10 including a plurality of identity tags 20, each of which includes:

• a unique tag identification code 22;

• a tag satellite receiver 24 for receiving location signals 36 from a satellite system 26 and generating a tag location message 38;

• a tag transceiver 28 for receiving the base station signals 30 and transmitting tag signals 32 to the base stations 14;

• the tag signals 32 including the tag identification code 22 and the tag location message 38.

The tag identification code 22 and the tag location message 38 are transmitted onward from the one base station 14 to the controller 12 via the signal arrangement 40. The signal arrangement 40 could include cables or wires 42 connecting the base stations 14 to the controller 12. In one example, the signal arrangement 40 includes a wired Ethernet backbone.

Each base station 14 transmits the base station signals 30 on one predefined frequency, which is a different frequency to the transmission frequency of the base stations 14 of any adjoining cell areas 18.

The one predefined frequency could be one of N predefined base station frequencies, where N is sufficiently large that no neighbouring base stations 14 utilise the same frequency. N is greater than 1 and could be between 2 and 4, and optimally, N is 4. For example, Fig. 2 shows an arrangement of cell areas 18 in which the cell areas 18 are hexagonal in plan shape, so that each internal cell area 18 is surrounded by six adjoining cell areas 18. In this figure, each cell area 18 is identified by a smaller numeric cell area identifier 44 and the base station signal frequency for each base station 14 is indicated by a larger numeric frequency identifier 46, which is either 1 , 2, 3 or 4. Thus, in this example, four different frequencies are employed and N equals 4. In another example (not shown), a plurality of cell areas 18 could be arranged in a single sequential series and N could equal 2. In another example (not shown), the cell areas 18 could be arranged in blocks at least two cell areas wide and at least two cell areas long, and N could be three.

In practice and/or desirably, adjoining cell areas 18 overlap to provide substantially continuous base station radio signal coverage over the predefined area 56. At any time, the tags 20 within the predefined area 56 will usually be receiving a plurality of base station signals 30 of different strength and frequency.

The tag transceiver 28 transmits the tag signals 32 on a tag frequency 34 which is selected from one of the base station frequencies. Referring to Fig. 3, each tag 20 includes a frequency selector 52 which assesses one or more selection criteria 54 relating to the base station signals 30 received from the base stations 14 and selects the tag frequency 34 in accordance with the one or more selection criteria 54. The one or more selection criteria 54 include a first criterion 54A, which relates to the signal strength of the base station signals 30.

The system 10 includes a base station register 48 associated with each base station 14. Each tag 20 is moveable between an unregistered condition in which the respective tag 20 is not registered to any base station 14, and a registered condition, in which the respective tag 20 is registered to one of the base stations 14.

The one or more selection criteria 54 include a second criterion 54B, which relates to the number of tags 20 already registered to each of the base stations 14. Each base station signal 30 includes a capacity message 78, which indicates the number of tags 20 registered to the respective base station 14. In one example, if more than 500 tags 20 are already registered to the base station 14 with the strongest signal, the tag 20 will look for another base station 14.

The one or more selection criteria 54 include a third criterion 54C, which relates to the time period which has elapsed since the respective tag 20 was last registered. This avoids flip flopping of tags 20 between base stations 14, where the respective tag 20 is moving along the boundaries of cell areas 18. In one example, the tag 20 could employ a "leaky bucket" algorithm to provide received signal strength indication (RSSI) and prevent flip flopping, thus providing the first, the second and the third criteria 54A, 54B, 54C.

The controller 12 includes a master register 58. The system 10 includes output 60 for displaying the master register 58, which includes display means 62 such as a screen. The master register 58 could be output in the form of a master register representation 72, such as a map and/or a table. The controller 12 includes a processor 64, which includes one or more sets of instructions 66, which derive location coordinates 74 from the location messages 38, and derive personal detail items 70 such as names from the identification codes 22. The controller 12 could include a database 68 of identification codes 22 with associated personnel detail items 70. The master register representation 72 could include one or some of the personal detail items 70 for each tag 20, along with the latest location co-ordinates 74 for the respective tag 20.

The controller 12 includes a registration routine 76 in which one tag 20 is registered to a new base station 14 the first time the respective new base station 14 receives a first tag signal 32 from the one tag 20. When the new base station 14 receives the first tag signal 32 from the one tag 20, the registration routine 76 deregisters the one tag 20 from the register 48 of the old base station 14. In this way, battery usage of the tag 20 is minimised.

The base stations 14 transmit the base station signals 30 on a predetermined time frequency, which, in one example could be at least once per minute, more desirably could be at least once every 30 seconds, and optimally could be substantially every ten seconds.

Each tag 20 is certified as being intrinsically safe (IS) to permit use in hazardous areas such as oil and gas fields. The maximum transmission power of each tag 20 could be 500 mW to meet the IS certification requirement and give good battery life. The maximum power requirement for the tags 20 means that each cell area 18 could be at most 5 km in diameter. For practical and economic purposes the cell areas 18 are usually no less than 1 km in diameter.

Advantageously, the system 10 is separate from any global communication network, and in particular is separate from global cellular communication (mobile telephone) networks. This provides increased security. In another example, the system could be connected to the internet, for example, by means of a firewall or other security device (not shown).

The base station signals operate at frequencies in the range 420-470 MHz, with each frequency having a band width of nominally 25 KHz.

In use, referring to Figs. 1 and 3, a user carrying the tag 20M with the identification code 22 is initially within the cell area 18N and registered to the base station 14N, which transmits the base station signal 30N at a frequency N. The tag 20M includes the satellite receiver 24 which receives the location signals 36 from the satellite system 26 and generates the location message 38. The tag transceiver 28 transmits the tag signals 32 to the base station 14N, the tag signals 32 including the tag identification code 22 and the tag location message 38. The tag identification code 22 and the tag location message 38 are transmitted onward from the base station 14N to the controller 12 via the signal arrangement 40.

At the controller 12, the master register representation 72 is updated to show the latest location coordinates 74 of the user carrying the tag 20M. These coordinates are updated in accordance with the predetermined time frequency of transmission of base station signals 30.

In this initial position, the tag 20M is also receiving base station signals 30P, 30Q from neighbouring base stations 14P, 14Q respectively, but as the signal strengths of the base station signals 30P, 30Q are low relative to the signal strength of the base station signal 30N, the selection criteria 54 are not met for changing the frequency of transmission of the tag signal 32, so the tag 20M continues to transmit the tag signal 32 on frequency N to the base station 14N.

As the user moves out of the cell area 18N as shown by arrow A and into the cell area 18P, the relative signal strengths of the base station signals 30N, 30P change until the selection criteria 54 are met for the tag 20M to begin transmitting its tag signals 32 on frequency P. When this occurs, the tag 20M is deregistered from base station 14N and registered to base station 14P.

Advantageously, the system 10 is capable of handling large numbers of users (up to approximately 2000 users) within a relatively large predefined and remote area 56 with a high update rate of every 10 seconds, enabling substantially continuous real time tracking of this large number of people.

It will be noted that the tag 20 is passive in the sense that it responds to the base station signals received (rather than, for example, transmitting interrogatory signals) and intelligent in the sense that it makes a selection decision on the basis of the received signals (rather than, for example, an allocation made by the controller 12). In general the system is arranged so that the tags 20 only transmit tag signals 32 to the closest available base station 14, thus preserving battery power.

In one example, the base station signals 30 and the tag signals 32 could conform to a predefined signal protocol 50, as shown in Figs. 4 to 9 and described below. The signal protocol 50 includes the base station signals 30 transmitted by the base stations 14 and the tag signals 32 transmitted by the tags 20, the tag signals 32 including request signals 96 and response signals 98.

The request signal 96 includes the identification code 22. The response signal 98 includes the tag location message 38.

Each base station 14 transmits its respective base station signals 30 substantially continuously and each signal 30 comprises a sequence of frame sets 82. Each set 82 includes a plurality of frames 84, which are of different types which are fixed in order in each of the sets 82. All of the base station signals 30 of all of the base stations 14 comprise similar sets 82, which comprise the different frame types in the same fixed order.

Each set 82 includes a poll frame 84A, which polls all of the tags 20 registered to the respective base station 14 to determine their location; an invite frame 84B which invites new tags 20 to register with the respective base station 14; and a registration frame 84C which issues registration information to newly registered tags 20. In this example, the poll frame 84A is the first frame 84 of each set 82.

Each poll frame 84A defines a plurality of response signal slots 86, in which response signals 98 from tags 20 registered to the respective base station are receivable. In one example, each poll frame 84A defines up to 2047 response signal slots 86. The maximum number of tags 20 which can be registered to each base station 14 is equal to the number of response signal slots 86 in the poll frame 84A. Each poll frame 84A includes one or more response synchronisation messages 92 which, in this example, precede the response signal slots 86. The synchronisation messages 92 permit the registered tags 20 to synchronise the transmission of the response signals 98 to the correct response signal slots 86.

In one example, each synchronisation message 92 also includes the capacity message 78 for the respective base station 14, indicating the number of tags 20 registered to the respective base station 14. In this example, the invite frame 84B immediately follows the poll frame

84A. Each invite frame 84B defines one or more registration request slots 88.

Each invite frame 84B includes one or more request signal slot allocation messages 90 which in this example precede the registration request slots 88. Each request signal slot allocation message 90 relates to the next available registration request slot 88.

In this example, the registration frame 84C immediately follows the invite frame 84B. The registration frame 84C includes registration information 100 relating to each newly registered tag 20. The registration information 100 includes a response signal slot allocation message 102, which relates to the response signal slot 86 which has been allocated to the respective tag 20 in the poll frame 84A of the next set 82. Referring to Figs. 4 and 5, each set 82 has a set duration time 94 which has a predefined value. In one example, the duration time 94 could have a value of between 1 second and 1 minute, more desirably of between 5 seconds and 30 seconds, and optimally could have a value of ten seconds. The set duration time 94 of each of the sets 82 for each base station 14 is substantially the same. When the cumulative duration time of the poll frame 84A, the invite frame 84B and the registration frame 84C is less than the set duration time 94, the set 82 includes a null frame 84D to maintain the set duration time 94 at the correct predefined value. This ensures that the frequency of transmission of the tags 20 is controlled so that tag battery life is preserved. Fig. 4 shows a base station signal 30 at maximum capacity with the whole set duration time 94 filled. Fig. 5 shows a base station signal 30 at below maximum capacity including null frames 84D.

The set duration time 94 is determined by the maximum number of tags 20 in the system 10 and a judgement as to how many of these tags 20 could be registered to a base station 14 at any one time, which dictates the number of response signal slots 86 which must potentially be provided. The cell areas 18 of different base stations 14 could be of different sizes to cater for high and low concentrations of users in different parts of the predefined area.

The response signals 98 are sent at an occurrence rate, which is determined by the occurrence of the base station signals 30, which is determined by the set duration time 94. The occurrence rate is substantially the same for all the tags 20 in the system 10 and is the same over a whole operating period of the system. Thus in accordance with the set duration time 94, the occurrence rate is between 1 second and 1 minute, more desirably is between 5 seconds and 30 seconds, and optimally is ten seconds.

In one example, the poll frame 84A could include a maximum number of 2047 response signal slots 86. The invite frame 84B could include a maximum number of 2047 registration request slots 88.

In use, Fig. 9 shows a normal registration sequence. In the first set shown, referred to with the reference numeral 82n, the base station 14 receives a request signal 96 from an unregistered tag 20 in one of the registration request slots 88 of the invite frame 84B. The base station 14 responds in the registration frame 84C of the same set 82n by issuing the respective tag 20 with registration information 100 including a response signal slot allocation message 102.

In the next set, referred to with the reference numeral 82n+i , the base station 14 sends the synchronisation message 92 in the poll frame 84A and the tag 20 responds by transmitting the response signal 98 into the allocated slot of the same poll frame 84A.

When registered, each tag 20 will continue to respond in subsequent sets 82 by transmitting the response signals 98 into the allocated response signal slots 86 of the poll frames 84A. Thus, once registered, the only battery consuming transmission required by the tags 20 is to transmit the response signals 98 once every 10 seconds, or in accordance with the set duration time.

When the tag 20 moves away from its registered base station 14 towards a new base station 14, as described above, the relative signal strengths of the base station signals 30 change until the selection criteria 54 are met for the tag 20 to begin transmitting its tag signals 32 on the frequency of the new base station 20. When this occurs, the tag 20 sends the request signal 96 on the new frequency in one of the registration request slots 88 of the invite frame 84B of the base station signal 30 of the new base station 14. The controller 12 deregisters the tag 20 from the register 48 of the old base station 14 and registers the tag 20 to the register 48 of the new base station 14.

Thus on registration, the only additional battery consuming transmission required by the tags 20 is to transmit the request signals 96.

The registration frame 84C could include information 100 which is transmitted to all registered tags 20, to groups of registered tags 20 or to a single registered tag 20. The information could include a forced deregistration message, which forces a receiving tag 20 to select a new frequency and send a request signal 96 to another base station 14. Each tag 20 could include an alarm initiator 80, which for example could be in the form of a button on the tag 20. When the alarm initiator is initiated, the tag 20 sends an emergency message via the base stations 14 to the controller 12 in the first available slots in any available base station signal 30.

The alarm initiator 80 could include a "person down" alarm and/or a

"large scale" alarm.

Each tag 20 could include one or more indicators 104 to indicate messages received from the controller 12 comprising the information 100. The indicators 104 could include audible indicators or visual indicators, for example, lights, LEDs, screens, buzzers etc. The messages could include low battery warning, return to base, general alarm, contact base, etc.

The response signals 98 could include a tag battery message to enable the controller 12 to monitor tag battery states.

Each tag 20 could include one or user input devices 106 to send messages to the controller 12 via the response signals 98. The devices 106 could include for example, buttons, keypad, touch screens etc. The messages could include message acknowledged, leaving area/system, help required, returning to base, etc.

The tags 20 of the system 10 are relatively low in power and can be certified ATEX zone 1 , approved for use in hazardous gas environments. The system is standalone and separate from global telephone networks for security. The system permits a relatively large number of users to be tracked over a relatively large predefined remote area (eg desert locations of 5000 square kilometres) with a high update rate permitting substantially continuous real time personnel tracking. The system does not rely on user proximity to sensors or detectors, or on public infrastructure.

The Applicant has found that having a fixed set duration time and hence a fixed occurrence rate provides surprising advantages in terms of preserving tag battery life. The Applicant has found that there is a balance to be struck between battery life and tracking continuity. The Applicant has found that by employing the invention herein described, set duration time periods of 10s (giving location message occurrence rates of 6 times per minute) have been achievable with a tag battery life of 48 hours operation.

The system provides relatively precise control of the occurrence of transmission events by the tags 20 so that tag battery usage is consistent. The system 10 conserves battery usage by careful design of the signal protocol 50 to minimise and regularise the occurrence of the tag transmission events.

Various other modifications could be made without departing from the scope of the invention. The system and its component parts (including the tags and the base stations) could be of any suitable size, shape and design and utilise any suitable frequency. The sets could have a different duration time. For example, with a longer set duration time, the system could handle higher numbers of users. The frames could be in a different order in the sets. There is thus provided a personnel tracking system which provides substantially continuous tracking of personnel over large remote areas. The system is accurate, robust and reliable and is designed to provide extended and consistent tag battery life to provide ease of use and user confidence.

Claims

1 . A personnel tracking system for tracking personnel in a predefined area, the system including a controller, a plurality of base stations, a signal arrangement which permits signal communication between each of the base stations and the controller, each base station including a base station transceiver for transmitting base station signals to a cell area associated with the respective base station, the system including a plurality of identity tags, each of which includes:

• a unique tag identification code;

• a tag transceiver for receiving one or more of the base station signals and transmitting tag signals to one of the base stations;

2. A system according to claim 1 , in which each base station transmits the base station signals on one predefined frequency, which is a different frequency to the transmission frequency of the base stations of any adjoining cell areas.

3. A system according to claim 2, in which the one predefined frequency is one of N predefined base station frequencies, where N is sufficiently large that no neighbouring base stations utilise the same frequency.

4. A system according to claim 3, in which N is greater than 1 , and may be between 2 and 4, and, optimally, N is 4.

5. A system according to any of the preceding claims, in which adjoining cell areas overlap to provide substantially continuous base station radio signal coverage over the predefined area.

6. A system according to any of the preceding claims, in which the tag transceiver transmits the tag signals on a tag frequency, and each tag includes a frequency selector for selecting the tag frequency from a plurality of available tag frequencies, each of which relate to one of the base station frequencies.

7. A system according to claim 6, in which the frequency selector assesses one or more criteria relating to the base station signals received from the base stations and selects the tag frequency in accordance with the one or more criteria.

8. A system according to claim 7, in which the one or more criteria include a first criterion, which relates to the signal strength of the base station signals.

9. A system according to any of the preceding claims, in which each tag is moveable between an unregistered condition in which the respective tag is not registered to any base station, and a registered condition, in which the respective tag is registered to one of the base stations.

10. A system according to claim 9 when dependent on claim 7 or claim 8, in which the system includes a base station register associated with each base station, each base station signal includes a capacity message, which indicates the number of tags registered to the respective base station and the one or more criteria include a second criterion, which relates to the number of tags already registered to each of the base stations.

1 1 . A system according to claims 9 or 10 when dependent on claims 7 or 8, in which the one or more selection criteria include a third criterion, which relates to the time period which has elapsed since the respective tag was last registered.

12. A system according to any of the preceding claims, in which the controller includes a master register.

13. A system according to any of the preceding claims, in which the controller includes a processor, which includes one or more sets of instructions, which derives location coordinates from the location messages, and derives personal detail items such as names from the identification codes.

14. A system according to claim 13, in which the controller includes a database of identification codes with associated personnel detail items.

15. A system according to claims 13 or 14 when dependent on claim 12, in which the master register is output in the form of a master register representation, which includes one or some of the personal detail items associated with each tag, along with the latest location co-ordinates for the respective tag.

16. A system according to claim 9 or any claim dependent thereon, in which the system includes a registration routine in which one tag is registered to a new base station the first time the new base station receives a first tag signal from the one tag.

17. A system according to claim 16, in which when the new base station receives the first tag signal from the one tag, the registration routine deregisters the one tag from the register of the old base station.

18. A system according to any of the preceding claims, in which the base stations transmit the base station signals on a predetermined time frequency, which is at least once per minute, more desirably is at least once every 30 seconds, and optimally is substantially every ten seconds.

19. A system according to any of the preceding claims, in which the tag identification code and the tag location message are transmitted onward from the one base station to the controller via the signal arrangement.

20. A system according to any of the preceding claims, in which each tag is certified as being intrinsically safe (IS), the maximum transmission power of each tag being 500 mW.

21 . A system according to any of the preceding claims, in which each cell area is at most 5 km in diameter, and is no less than 1 km in diameter.

22. A system according to any of the preceding claims, in which the system is separate from any global communication network, and in particular is separate from global cellular communication (mobile telephone) networks.

23. A system according to any of the preceding claims, in which the base station signals and the tag signals are transmitted at frequencies in the range 420-470 MHz, with each frequency having a band width of nominally 25 KHz.

24. A system according to any of the preceding claims, in which the base station signals and the tags conform to a predefined signal protocol.

25. A system according to claim 24, in which the protocol includes the base station signals and the tag signals.

26. A system according to claim 25, in which the tag signals include request signals and response signals.

27. A personnel tracking system for tracking personnel in a predefined area, the system including a signal protocol, the protocol including base station signals transmitted by base stations and tag signals transmitted by tags, the tag signals including request signals and response signals.

28. A system according to claims 26 or 27, in which the response signals include the tag location messages.

29. A system according to any of claims 26 to 28, in which the response signals are sent at an occurrence rate, which is determined by the occurrence of the base station signals.

30. A system according to claim 29, in which the occurrence rate is substantially the same for all the tags in the system and is the same over a whole operating period of the system.

31 . A system according to claims 29 or 30, in which the occurrence rate is between 1 second and 1 minute, more desirably is between 5 seconds and 30 seconds, and optimally is ten seconds.

32. A system according to claims 26 or 27 or any claim dependent thereon, in which the request signals include the identification codes.

33. A system according to claims 26 or 27 or any claim dependent thereon, in which the request signals are only sent when the tag is to be registered to a new base station.

34. A system according to any of the preceding claims, in which each base station transmits one base station signal which comprises a sequence of sets and may be substantially continuous.

35. A system according to claim 34, in which each set includes a plurality of frames, which are of different types and may be fixed in order in each of the sets.

36. A system according to claim 35, in which all of the base station signals of all of the base stations comprise similar sets which comprise the different frame types in the same fixed order.

37. A system according to claims 35 or 36, in which each set includes a poll frame, which polls all of the tags registered to the respective base station to determine their location, and which is the first frame of each of the sets.

38. A system according to claim 37, in which each poll frame defines a response signal slot, in which the response signal from a tag registered to the respective base station is receivable.

39. A system according to claim 38, in which each poll frame defines a plurality of response signal slots, and may define up to 2047 response signal slots.

40. A system according to claim 39, in which the maximum number of tags which can be registered in the cell area of each base station is equal to the number of response signal slots in the poll frame of the base station signal transmitted by the respective base station.

41 . A system according to any of claims 38 to 40, in which each poll frame includes one or more response synchronisation messages which precede(s) the response signal slots and permit(s) the registered tags to synchronise the transmission of the response signals to the correct response signal slots.

42. A system according to claim 41 , in which each synchronisation message includes a capacity message for the respective base station, indicating the number of tags registered to the respective base station.

43. A system according to any of claims 35 to 42, in which each set includes an invite frame, which invites new tags to register with the respective base station.

44. A system according to claim 43, in which the invite frame immediately follows the poll frame.

45. A system according to claims 43 or 44, in which each invite frame defines a registration request slot, in which the request signal from an unregistered tag is receivable.

46. A system according to any of claims 43 to 45, in which each invite frame includes a request signal slot allocation message which precedes the registration request slot and relates to the next available registration request slot.

47. A system according to any of claims 35 to 46, in which each set includes a registration frame, which issues registration information to newly registered tags.

48. A system according to claim 47, in which the registration frame immediately follows the invite frame.

49. A system according to claims 47 or 48, in which the registration information includes a response signal slot allocation message which relates to the response signal slot which has been allocated to a newly registered tag in the poll frame of the next set.

50. A system according to any of claims 34 to 49, in which each set has a set duration time which has a predefined value.

51 . A system according to claim 50, in which the value is between 1 second and 1 minute, more desirably of between 5 seconds and 30 seconds, and optimally has a duration of substantially ten seconds.

52. A system according to claims 50 or 51 , in which the set duration time of each of the sets for each base station is substantially the same.

53. A system according to any of claims 50 to 52, in which when the cumulative duration time of the poll frame, the invite frame and the registration frame is less than the set duration time, the set includes a null frame to maintain the set duration time at the correct predefined value.

54. A system according to any of claims 50 to 53, in which the set duration time determines the occurrence rate.

55. A method of tracking personnel in a predefined area, the method including providing a personnel tracking system in accordance with any of the preceding claims.

56. A signal protocol for a personnel tracking system for tracking personnel in a predefined area, the protocol including base station signals transmitted by base stations and tag signals transmitted by tags, the tag signals including request signals and response signals.

57. A personnel tracking system for tracking personnel in a predefined area substantially as hereinbefore described and/or with reference to any of the accompanying drawings.

58. A method of tracking personnel in a predefined area substantially as hereinbefore described and/or with reference to any of the accompanying drawings

59. A signal protocol for a personnel tracking system for tracking personnel in a predefined area.