GB2508468A

GB2508468A - Tracking system where components communicate at two different frequencies

Info

Publication number: GB2508468A
Application number: GB1315244.2A
Authority: GB
Inventors: Derek Gennard
Original assignee: S3 ID Ltd
Current assignee: S3 ID Ltd
Priority date: 2012-08-27
Filing date: 2013-08-27
Publication date: 2014-06-04
Anticipated expiration: 2033-08-27
Also published as: GB2508468B; GB201315244D0; WO2014033443A1; GB201215213D0

Abstract

A tracking system 10 for tracking the movement of personnel around a predetermined location such as a work place, a hospital or a secure facility (e.g. a prison), includes a base station 12. The base station 12 includes a transmitter 14 for transmitting first signals 16 at a first frequency and a transceiver 18 for transmitting and receiving second signals 20 at a second, different frequency. The system 10 includes one or more tags 22, each tag in use being worn by a user. Each tag includes a receiver 24 for receiving the first signals 16 from the base station 12 at the first frequency and a transceiver 26 for transmitting and receiving the second signals 20 at the second frequency from and to the base station. The base station could include a two-axis loop aerial (42) or a tri-axial ferrite rod antenna (38). The first signals might include a system code. The tags may reside in out-of-range, in-range-active, and in-range-passive states, wherein they behave differently. The tags may include visual and/or audible alarm means such as buzzers and LEDs (78A, 78B).

Description

Tracking Systems and Components Therefor The present invention relates to tracking systems and components therefor, particularly but not exclusively tracking systems for tracking the movement of personnel around a predetermined location.

Conventionally, it is known to provide tracking systems for tracking the movement of personnel around a predetermined location such as a work place, a hospital or a secure facility such as a prison. Such tracking systems often include detectors which can contactlessly detect and communicate with radio frequency identification (REID) tags which are carried by users, the detectors being located at predetermined positions around the location.

However conventional tracking systems suffer a number of disadvantages.

The RFID tags only have a short range, usually up to 2m, and the detectors detect within a direction specific, relatively small field, so that for long periods the personnel carrying the tags are invisible to the system. The detectors can be misaligned. The systems are unable to actively monitor more than a relatively small number of tags (for example, one) at a time in each detector field. Furthermore, conventionally, the tags carried by the user are inert to the user, meaning that they do not communicate with the user.

In this specification the following abbreviations and terms will be used: RFID -Radio frequency identification Transceiver -Transmitter and receiver FSK -Frequency shift key modulation According to a first aspect of the present invention, there is provided a tracking system for tracking the movement of personnel around a predetermined location, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the system including one or more tags, each tag in use being worn by a user, each tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.

Possibly, the base station transmits the first signals in an omni-directional first signal field. Possibly, the first signal field has a radius of at least 2m, and may have a radius of no more than 25 m, and possibly no more than 10 m.

Possibly, the base station includes a base antenna arrangement including either a two axis loop antenna, or a tn-axial ferrite rod antenna.

According to a second aspect of the present invention, there is provided a tracking system base antenna arrangement including either a two axis loop antenna, or a tn-axial ferrite rod antenna.

Possibly, the ferrite rod antenna includes three ferrite core rod antenna parts, which may be at arranged substantially orthogonally to each other.

Possibly, the loop antenna includes a pair of loop parts. Each loop part may be substantially planar. Possibly, each loop part is orientated substantially vertically, and may be orientated at an angle to the plane of the other loop part. The angle of orientation may be substantially 900.

Possibly, the loop parts may be arranged substantially in parallel, and may be substantially similar.

Possibly, each loop part is elongate in the vertical direction, and may have an aspect ratio of vertical height to horizontal width of at least 5:1, more possibly at least 8:1, and optimally at least 9:1. Possibly, each loop part has an aspect ratio of vertical height to horizontal width of no more than 14:1, more possibly no more than 11:1, and optimally no more than 9.75:1.

Possibly, the horizontal width of each loop pad is at least 80mm, and may be at least 120mm. Possibly, the horizontal width of each loop part is no more than 240 mm, and may be no more than 200 mm. Optimally, the width of each loop pad is substantially 160 mm.

Possibly, the vertical height of each loop part is at least 1000mm, and may be at least 1300mm. Possibly, the vertical height of each loop pad is no more than 2000 mm, and may be no more than 1700 mm. Optimally, the vertical height of each loop pad is substantially 1500 mm.

Possibly, the loop antenna is formed of wire having a length of at least 3000 mm and optimally of substantially 3300 mm and having an area of at least 0.2 m2, and optimally of substantially 0.24 m2.

Possibly, the loop antenna includes a support, which supports the loop pads. The support may be circular in plan cross section.

Possibly, the base antenna arrangement is arranged to transmit signals in the range 120 -130 kHz, and may transmit signals of substantially 124 -126 kHz.

Possibly, the signals transmitted by the base antenna arrangement comprise the first signals.

According to a third aspect of the present invention there is provided a tag for a tracking system for tracking the movement of personnel around a predetermined location, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the tag in use being worn by a user, the tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.

Possibly, the first frequency is in the range 120 -130 kHz, and may be substantially 124-126 kHz.

Possibly, the second frequency is in an ISM frequency band, and may be the range 863 -872 kHz, and may be substantially 868 kHz.

Possibly the base station includes an encoder for encoding the first signals. Possibly, the first signals include a predetermined system code.

Possibly, the base station includes a modulator for modulating the first signals. Possibly, the modulator is an ESK modulator.

Possibly, the base station includes a switching device which formats the first signal in a polarisation sequence. The polarisation sequence may comprise a plurality of parts which correspond in number to the number of antenna parts.

Possibly, in use, the base station continuously transmits the first signal.

Possibly, in use, when the tag is in an out-of-range condition outside the first signal field, the base station transmits the first signals, but not the second signals, the first signals are not strong enough to activate the tag, and the tag does not transmit or receive the second signals.

Possibly, in use, in an in-range active condition in which the tag is inside the first signal field, or when the user carrying the tag moves into the first signal field, the base station continues to transmit the first signals which are now strong enough to activate the tag, causing the second signals to be transmitted and received between the tag and the base station.

Possibly, in use, in an in-range passive condition in which the tag is inside the first signal field, the base station continues to transmit the first signals which are strong enough to activate the tag, the base station does not transmit the second signals, and the tag does not transmit or receive the second signals.

Possibly, the receiver is a super-heterodyne receiver. The receiver may include a first signal processor arrangement which may provide substantially 90dB gain for a pulsed supply current of substantially 20 pA.

The first signal processor arrangement may include a plurality of first signal filters, and may include three first signal filters. The first signal filters may comprise any or all of the group containing a low noise amplifier, a bypass filter, a local oscillator.

Possibly, the tag includes a controller, which may include memory storage for a set of instructions, and may include a processor for processing the instructions. The processor may process instructions in response to the first and/or second signals.

The tag may include one or more indicators or alarms, which may be visual and/or audible, and may be activated by the controller in response to the first and/or second signals.

The tag may include a power supply, which may include a power store such as a battery.

The controller may include a clock/timer, and may move the tag between the in-range passive condition and the in-range active condition at predetermined intervals.

Possibly, the tag is arranged to require two activation signals before moving to the in-range active condition. Possibly, the two activation signals comprise a threshold activation signal and a validated activation signal.

Possibly, the receiver includes a threshold detector which may provide the threshold activation signal, which may be provided on receipt of a first signal of sufficient strength.

Possibly, the tag includes a demodulator, which demodulates the first signals. Possibly, the tag includes a decoder which decodes the first signals, and which may be in signal sequence downstream from the demodulator.

Possibly, the tag includes a validator, which may compare the decoded first signal to a stored code and provide a validated activation signal if the decoded first signal matches the stored code.

According to a fourth aspect of the present invention, there is provided a method of tracking the movement of personnel around a predetermined location, the method including providing a tracking system, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the system including one or more tags, each tag in use being worn by a user, each tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.

Other features of the tracking system, the tracking system base antenna arrangement and/or the tag are as described in any of the statements above.

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 perspective diagrammatic view of a tracking system for tracking the movement of personnel around a predetermined location; Fig. 2 is a perspective view of a tn-axial ferrite rod antenna of a tracking system base antenna arrangement; Figs. 3A and 3B are perspective diagrammatic views of a two axis loop antenna of a tracking system base antenna arrangement without and with a support respectively; Figs. 3C and 3D are perspective diagrammatic views of alternative configurations of the two axis loop antenna of the tracking system base antenna arrangement without supports; Figs. 3E, 3F and 3G are plan views of the loop antennas of Figs. 3A, 30 and 3D respectively; Fig 4 is a schematic view of a base station; and Fig 5 is a schematic view of a tag.

Figs. 1 to 5 show a tracking system 10 for tracking the movement of personnel around a predetermined location such as a work place, a hospital or a secure facility such as a prison. The tracking system 10 includes a base station 12, the base station 12 including a transmitter 14 for transmitting first signals 16 at a first frequency and a transceiver 18 for transmitting and receiving second signals 20 at a second, different frequency. The system 10 includes one or more tags 22, each tag 22 in use being worn by a user. Each tag 22 includes a receiver 24 for receiving the first signals 16 from the base station 12 at the first frequency and a transceiver 26 for transmitting and receiving the second signals 20 at the second frequency from and to the base station 12.

The base station 12 includes a controller 90, a Wi-Fi transceiver 32, a clock 92 and a power supply unit 88. The transmitter 14 includes a base antenna arrangement 36. The applicant has found that there are two base antenna arrangements 36 which provide the required field 108. In one embodiment the base antenna arrangement 36 includes a tn-axial ferrite rod antenna 38. The ferrite rod antenna 38 includes three ferrite core rod antenna pants 40, which are arranged substantially orthogonally to each other, as indicated by the labels X, Y and Z which designate orthogonal axes in Fig 2.

In this case, the rod antenna parts 40 are relatively short in length and the base antenna arrangement 36 is located internally to a housing 116 of the base station 12.

In another embodiment the base antenna arrangement 36 includes a two axis loop antenna 42, which is located externally to the housing 116. This embodiment is preferred because it provides a larger field 108. The loop antenna 42 includes a pair of substantially planar, substantially similar loop parts 44. Each loop part 44 is orientated substantially vertically and is orientated at an angle 46 to the other loop part 44. The angle of orientation 44 is substantially 90°.

The loop parts 42 are electrically connected substantially in parallel.

Each loop part 44 is elongate in the vertical direction. In one example, each loop part 44 could have an aspect ratio of vertical height 50 to horizontal width 48 between Sand 14 to 1; more preferably could be between 8 and 11 to 1; and optimally could be between 9 and 9.75 to 1.

In one example, the horizontal width 48 of each loop part 44 could be between 80mm and 240mm and could more preferably be between 120mm and 200mm.

In one example, the vertical height 50 of each loop part 44 could be between 1000mm and 2000mm and could more preferably be between 1300mm and 1700mm.

In one example, the horizontal width 48 of each loop part 44 could be substantially 160mm. The vertical height of each loop part 44 could be substantially 1500mm. The aspect ratio could be substantially 9.375.

In one example, the loop antenna 42 could be formed of wire having a length of at least 3000 mm and optimally of substantially 3300 mm and having an area of at least 0.2 m2, and optimally of substantially 0.24 m2.

The loop antenna 42 includes a support 52, which supports the loop parts 44. In one example, as shown in Fig. 3B, the support 52 could be circular in plan cross section, and as shown in Fig. 3E., in plan the loop parts 44 could be in form of a cross. In other examples (not shown), the support 52 could be square in plan cross section, or any other suitable polygonal plan cross sectional shape.

Figs. 3C and 3D show two alternative configurations of the loop antenna. In Figs. 3C and 3F, loop antenna 142 comprises a pair of loop parts 44 which are arranged in plan in an L shape, separated by a relatively small gap 51. In Figs. 3D and 3G, loop antenna 242 comprises a pair of loop parts 44 which are arranged in plan in a T shape, again separated by a relatively small gap 51. Substantially planar supports (not shown) could be provided in each of these configurations to support each of the loop parts 44. The Applicant has found that the latter two designs provide the required functionality with improved ease of compliance with ATEX certification requirements.

The base antenna arrangement 36 is arranged to transmit the first signals 16 at the first frequency, which in one example, could be in the range -130 kHz, more preferably could be in the range 124 -126 kHz, and optimally could be substantially 125 kHz.

The base station controller 90 includes an encoder 104 for encoding the first signals 16 and an FSK modulator 102 for modulating the first signals 16.

The base station 12 includes a switching device 94 which formats the first signal 16 in a polarisation sequence by switching an input signal between the antenna parts 40, 44. The polarisation sequence comprises a plurality of pads which correspond in number to the number of antenna parts 40, 44.

The second frequency could be an ISM frequency band. In one example, the second frequency is in the range 863 -872 kHz, and could be substantially 868 kHz.

Due to the larger and less isotropic transmission range of the second frequency, allowance is made for the base station 12 to coexist with other base stations 12 by the allocation of up to 15 sub-channels. The transmitter 14 uses Listen before Talk (LBT) to detect whether other base stations 12 are transmitting before starting any transmission. If there is a clash then the system administrator needs to configure the base station 12 to use another sub channel.

The WiFi transceiver 32 is capable of transmitting up to 400m in open field conditions and 40m in normal in-door conditions. The transceiver 32 communicates with the controller 90 via a serial SPI bus 114. TCP/IP protocol is used to exchange data between the controller 90 with a server 28 via WiFi signals 34 and the internet 30.

Tag IDs are stored in a database on the server 28, along with other information such as the battery status, person ID, timestamp, etc. The database can be queried remotely and the information displayed via a web browser on client machines remote from the base stations 12.

The real time clock 92 ensures that accurate time stamps are available.

The base station 12 will always attempt to keep in sync with the server 28, which is configured as a time server.

In one example, the base stations 12 could be DC powered by a dual 24 VDC / mains 110230 VAC source. In one example, the components within the base stations 12 might not be Intrinsically Safe (IS) and could be AlEX certified by enclosure. The base stations 12 could be enclosed by a carbon loaded GRP box, full of glass beads for ATEX approval (exq). The operating temperature range of the system 10 is -20 to --55°.

The receiver 24 is a super-heterodyne receiver. The receiver 24 includes a rod antenna 64 and a first signal processor arrangement 54. The first signal processor arrangement 54 includes three first signal filters 56, comprising a low noise amplifier (LNA) 58, a bypass filter 60 and a local oscillator 62.

In one example, a tuned miniature ferrite rod antenna 64 combined with a tuned low-power LNA, a 100 KHz crystal local oscillator 62 and a tuned 25 KHz bypass filter gives 90 dB of gain for just 20 pA supply current (pulsed).

The tag 22 includes a controller 66, which includes memory storage 68 for a set of instructions 70, and includes a processor 72 for processing the instructions 70. The processor 70 processes the instructions 70 in response to the first and the second signals 16, 20.

The tag 22 includes a pair of indicators 78 comprising an audible alarm in the form of a buzzer 78A and a visual alarm in the form of an LED 78B.

The indicators 78 are activated by the controller 66 in response to the first and the second signals.

The tag 22 includes a power supply 80 which includes a power store such as a battery 82. In one example, the tag power store 82 could comprise disposable Li-M02 long-life cells, with a minimum battery life of 2 years.

The controller 66 includes a clock/timer 84.

The receiver 24 includes a threshold detector 76 in the form of an op-amp threshold detector which in use provides a threshold activation signal 98 to the tag controller 66, which is provided on receipt of a first signal 16 of sufficient strength.

The receiver 24 includes a zero crossing detector 74 which in use provides a second activation signal 100 to the tag controller 66. The controller 66 includes a demodulator 110, a decoder 86 and a validator 106 which in sequence demodulate, decode and validate the second activation signal 100 to provide a validated activation signal 112.

In use, the base station 12 continuously transmits the first signal 16 in an omni-directional first signal field 108. In one example, the first signal field 108 has a radius of between 2 and lOm, but in other examples the field 108 could have a radius of up to 25m. As shown in Fig 1, where the system 10 includes a plurality of base stations 12, the fields 108 of adjacent base stations do not overlap.

The first signals 16 are encoded by the encoder 104, and include a predetermined system code. The first signals are then ESK modulated by the modulator 102. The modulated first signals 16 include a carrier part and a modulated part. The modulated part comprises the system code, which could be a word.

The modulated part is substantially 600 baud, and has a deviation substantially of ±600 Hz. Thus, in one example, the carrier part could have a frequency of 125.003 kHz, with the modulation resulting in frequencies of (0) 124.402 kHz and (1) 125.604 kHz.

The switching device 94 then formats the modulated first signal 16 in a polarisation sequence which comprises a plurality of parts which correspond in number to the number of antenna parts 40, 44. Thus in the case of the three axis ferrite rod antenna 38, the system code word is repeated continuously in a polarisation sequence X, Y, Z, X, Y, Z, ... In the case of the two loop antenna 42, the system code word is repeated continuously in a polarisation sequence X, Y, X, Y, When the tag 22 is in an out-of-range condition, the base station 12 transmits the first signals 16, but not the second signals 20. The first signals 16 are not strong enough to activate the tag 22, and the tag 22 does not transmit or receive the second signals 20.

The tag 22 is relatively low power, such that it is able to remain in a permanently listening condition.

When the user carrying the tag 22 moves into an in-range active condition in the first signal field 108, the base station 12 continues to transmit the first signals 16 which are now strong enough to activate the tag 22, causing the second signals 20 to be transmitted and received between the tag 22 and the base station 12.

The first signals 16 are processed by the first signal processor arrangement 54. The threshold detector 76 provides the threshold activation signal 98 in the form of a wake up interrupt signal to the tag controller 66. The zero crossing detector 74 provides the second activation signal 100 to the tag controller 66. The second activation signal 100 is demodulated by the demodulator 110 and then decoded by the decoder 86 which retrieves the system code word transmitted by the base station 12, which is then compared by the validator 106 against a stored code word. If the code words match, the validator 106 provides a validated activation signal 112 and the controller 66 causes the second signals 20 to be transmitted and received between the tag 22 and the base station 12. Thus the tag 22 is arranged to require two activation signals, a threshold activation signal 98 and a validated activation signal 112, before moving to the in-range active condition.

On entering the field 108 the second signal 20 sent by the tag 22 is a registration signal. The information carried by the second signals 20 could include time of entry into the field 108, and user identity information.

After a period of time in the field 108, the tag 22 moves to an in-range passive condition, in which the base station 12 continues to transmit the first signals 16 which are strong enough to activate the tag 22, the base station 12 does not transmit the second signals 20, and the tag 22 does not transmit or receive the second signals 20. The controller 66 could move the tag 22 between the in-range passive condition and the in-range active condition at predetermined time intervals.

In another embodiment, the controller 66 could move the tag 22 between the in-range passive condition and the in-range active condition on receipt of commands received from the server 28 and/or base station 12 via the second signals 20.

On leaving the field 108, the controller 66 detects the lessening or loss of the first signal 16 and sends a second signal 20 comprising a deregistration signal to the base stationl2. Built in hysteresis prevents a tag 22 on the edge of the field 108 from constantly registering and deregistering.

The buzzer and LED indicators 78 could be used to inform the user of alarm conditions, and could be activated in response to second signals 20 from the base station.

Advantageously, the system 10 provides a larger field 108 than hitherto, so that fewer base stations 12 are required to provide coverage of an area. This is made possible because of the improved base antenna arrangement 36, the improved tag receiver 24 and the use of the first and second frequencies. The first frequency acts as a locate and wake frequency, the second frequency acts as a communication frequency. Using two frequencies also provides greater bandwidth for communication.

The first frequency of 125 kHz is conventionally only used for relatively small fields. Conventional systems using this frequency have poor adjacent channel rejection. In addition, there is a lot of environmental background noise at this frequency, for example, from energy saving light bulbs. The super-heterodyne receiver 24 and the first signal processing arrangement 54 have been found to provide excellent adjacent channel and noise rejection, and the use of FSK signalling further enhances decode reliability. Typically most 125 kHz systems will fringe at 8-lOm under low noise conditions.

However the system of the present invention fringes at around 30m, giving a high level of noise immunity at a field 108 of radius lOm. Advantageously, the larger field 108 means that the field 108 can be tuned to suit the local environmental conditions.

The use of a second, different frequency for communication permits the system to handle a plurality of tags 22 within the same field 108, while optimising the power requirements of the tag 22. In one example, the base station 12 could handle up to 50 tags 22, and the system 10 could handle up to 255 base stations 12.

Various other modifications could be made without departing from the scope of the invention. The various components could be of any suitable size and shape, and could be formed of any suitable material.

The tags 22 could include other indicators or alarms which activate under predetermined conditions. For example, one buzzer and/or LED could indicate that the tag is in a registered condition, or of an emergency situation, or other predefined messages.

Any of the features or steps of any of the embodiments shown or described could be combined in any suitable way, within the scope of the

overall disclosure of this document.

There is thus provided a tracking system which utilises fewer base stations having a larger, omni-directional field than previous systems, giving more complete area coverage of a location. The tags are intelligent and can transmit alarm indications or other messages from the base station to the user.

Claims

CLAIMS1. A tracking system for tracking the movement of personnel around a predetermined location, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the system including one or more tags, each tag in use being worn by a user, each tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.
2. A system according to claim 1, in which the base station transmits the first signals in an omni-directional first signal field.
3. A system according to claim 2, in which the first signal field has a radius of at least 2m, and may have a radius of no more than 25 m, and possibly no more than 10 m.
4. A system according to any of claims 1 to 3, in which the base station includes a base antenna arrangement including either a two axis loop antenna, or a tn-axial ferrite rod antenna.
5. A tracking system base antenna arrangement for a tracking system for tracking the movement of personnel around a predetermined location, the arrangement including either a two axis loop antenna, or a tn-axial ferrite rod antenna.
6. A system according to claim 4 or an arrangement according to claim 5, in which the ferrite rod antenna includes three ferrite core rod antenna pads, which may be arranged substantially orthogonally to each other.
7. A system according to claim 4 or an arrangement according to claim 5, in which the loop antenna includes a pair of loop parts, in which each loop part is substantially planar, orientated substantially vertically, and orientated at an angle to the plane of the other loop part.
8. A system or an arrangement according to claim 7, in which the angle of orientation is substantially 9Q0
9. A system or an arrangement according to claims 7 or 8, in which the loop parts are arranged substantially in parallel, and may be substantially similar.
10.A system or an arrangement according to any of claims 7 to 9, in which each loop part is elongate in the vertical direction, and may have an aspect ratio of vertical height to horizontal width of at least 5:1, more possibly at least 8:1, and optimally at least 9:1, and may have an aspect ratio of vertical height to horizontal width of no more than 14:1, more possibly no more than 11:1, and optimally no more than 9.75:1.
11.A system or an arrangement according to any of claims 7 to 10, in which the horizontal width of each loop part is at least 80mm, and may be at least 120mm, and may be no more than 240 mm, and may be no more than 200 mm, and may be substantially 160 mm.
12.A system or an arrangement according to any of claims 7 to 11, in which the vertical height of each loop part is at least 1000mm, and may be at least 1300mm, and may be no more than 2000 mm, and may be no more than 1700 mm and may be substantially 1500 mm.
13.A system according to claim 4 or any claim dependent thereon, or an arrangement according to claim 5 or any claim dependent thereon, in which the loop antenna is formed of wire having a length of at least 3000 mm and optimally of substantially 3300 mm and having an area of at least 0.2 m2, and optimally of substantially 0.24 m2.
14.A system or an arrangement according to claim 7 or any claim dependent thereon, in which the loop antenna includes a support, which supports the loop parts.
15.A system according to claim 4 or any claim dependent thereon, or an arrangement according to claim 5 or any claim dependent thereon, in which the base antenna arrangement is arranged to transmit signals in the range 120 -130 kHz, and may transmit signals of substantially 124 -126 kHz.
16.A system according to claim 4 or any claim dependent thereon, in which the signals transmitted by the base antenna arrangement comprise the first signals.
17.A tag for a tracking system for tracking the movement of personnel around a predetermined location, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the tag in use being worn by a user, the tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.
18.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17, in which the first frequency is in the range 120 -kHz, and may be substantially 124-126 kHz.
19.A system according to claim 1 or any claim dependent thereon, or a tag according to claims 17 or 18, in which the second frequency is in an ISM frequency band, and may be the range 863 -872 kHz, and may be substantially 868 kHz.
20.A system according to claim 1 or any claim dependent thereon, in which the base station includes an encoder for encoding the first signals.
21.A system according to claim 1 or any claim dependent thereon, in which the first signals include a predetermined system code.
22.A system according to claim 1 or any claim dependent thereon, in which the base station includes a modulator for modulating the first signals, which may be an ESK modulator.
23.A system according to claim 1 or any claim dependent thereon, in which the base station includes a switching device which formats the first signal in a polarisation sequence.
24.A system according to claim 23 when dependent on claim 7 or any claim dependent thereon, in which the polarisation sequence comprises a plurality of parts which correspond in number to the number of antenna parts.
25.A system according to claim 1 or any claim dependent thereon, in which, in use, the base station continuously transmits the first signal.
26.A system according to claim 1 or any claim dependent thereon, in which, in use, when the tag is in an out-of-range condition outside the first signal field, the base station transmits the first signals, but not the second signals, the first signals are not strong enough to activate the tag, and the tag does not transmit or receive the second signals.
27.A system according to claim 1 or any claim dependent thereon, in which, in use, in an in-range active condition in which the tag is inside the first signal field, or when the user carrying the tag moves into the first signal field, the base station continues to transmit the first signals which are now strong enough to activate the tag, causing the second signals to be transmitted and received between the tag and the base station.
28.A system according to claim 1 or any claim dependent thereon, in which, in use, in an in-range passive condition in which the tag is inside the first signal field, the base station continues to transmit the first signals which are strong enough to activate the tag, the base station does not transmit the second signals, and the tag does not transmit or receive the second signals.
29.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the receiver is a super-heterodyne receiver.
30.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the receiver includes a first signal processor arrangement, which includes a plurality of first signal filters, and may include three first signal filters, which may provide substantially 90dB gain for a pulsed supply current of substantially 20 pA.
31.A system or a tag according to claim 30, in which the first signal filters comprise any or all of the items in the group containing a low noise amplifier, a bypass filter, a local oscillator.
32.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag includes a controller, which may include memory storage for a set of instructions, and may include a processor for processing the instructions, and which may process the instructions in response to the first and/or second signals.
33.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag includes one or more indicators or alarms, which may be visual and/or audible, and may be activated by the controller in response to the first and/or second signals.
34.A system according to claim 1 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag includes a power supply, which may include a power store such as a battery.
35.A system according to claims 27 and 28 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the controller includes a clock/timer, which may move the tag between an in-range passive condition and an in-range active condition at predetermined intervals.
36.A system according to claims 27 and 28 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag is arranged to require two activation signals before moving to an in-range active condition.
37.A system or a tag according to claim 36, in which the two activation signals comprise a threshold activation signal and a validated activation signal.
38.A system or a tag according to claim 37, in which the receiver includes a threshold detector which may provide the threshold activation signal, which may be provided on receipt of a first signal of sufficient strength.
39.A system according to claim 22 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag includes a demodulator, which demodulates the first signals.
40.A system according to claim 20 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tag includes a decoder which decodes the first signals.
41.A system or a tag according to claim 40, in which the tag includes a validator, which compares the decoded first signal to a stored code and provides a validated activation signal if the decoded first signal matches the stored code.
42.A method of tracking the movement of personnel around a predetermined location, the method including providing a tracking system, the tracking system including a base station, the base station including a transmitter for transmitting first signals at a first frequency and a transceiver for transmitting and receiving second signals at a second, different frequency, the system including one or more tags, each tag in use being worn by a user, each tag including a receiver for receiving the first signals from the base station at the first frequency and a transceiver for transmitting and receiving the second signals at the second frequency from and to the base station.
43. A method according to claim 42, a base antenna arrangement according to claim 5 or any claim dependent thereon, or a tag according to claim 17 or any claim dependent thereon, in which the tracking system includes any of the features defined in any of claims 1 to 4, or claims 6 to 16, or claims 18 to 41.
44.A tracking system for tracking the movement of personnel around a predetermined location substantially as hereinbefore described and/or with reference to any of the accompanying drawings.
45.A tracking system base antenna arrangement substantially as hereinbefore described and/or with reference to any of the accompanying drawings.
46.A tag for a tracking system for tracking the movement of personnel around a predetermined location substantially as hereinbefore described and/or with reference to any of the accompanying drawings.
47.A method of tracking the movement of personnel around a predetermined location, substantially as hereinbefore described and/or with reference to any of the accompanying drawings