WO2005043480A1

WO2005043480A1 - A maritime safety system

Info

Publication number: WO2005043480A1
Application number: PCT/AU2004/000614
Authority: WO
Inventors: Mark James Pallister; Vanessa Joan Pallister; David Ward
Original assignee: Mobilarm Pty Ltd
Priority date: 2003-10-24
Filing date: 2004-05-10
Publication date: 2005-05-12
Also published as: US20050118905A1

Abstract

A maritime safety system comprising at least one transmitter unit (12, 14, 16, 18), each transmitter unit being arranged to transmit successive signals by wireless communication at pseudo random intervals and each transmitter unit (12, 14, 16, 18) having an associated transmitter range, a base station (10) arranged to receive said transmitted signals and an alarm device (20). The base station (10) is arranged to cause the alarm device (20) to generate an audible and/or visible alarm when a plurality of successive signals from at least one of the transmitter units (12, 14, 16, 18) are not detected, and the arrangement is such that when a transmitter unit (12, 14, 16, 18) is disposed a distance from the base station (10) which is greater than the transmission range associated with the transmitter unit and a plurality of successive signals are not detected, an audible and/or visible alarm is generated.

Description

A MARITIME SAFETY SYSTEM

Field of the Invention

The present invention relates to a maritime safety system and, in particular, to a maritime safety system of the type intended to provide a warning when a member of the crew of a vessel or an important article associated with the vessel is lost overboard. The present invention also relates to a base station for use with the maritime safety system.

Background of the Invention

When a member of the crew of a maritime vessel is lost overboard, the time elapsed before an alarm is raised is critical. This is particularly so in cold waters, where the chances of survival of the crew member diminish rapidly with time. However, even in warm waters, prompt recognition that the crew member is no longer on the vessel greatly enhances the chances of locating the crew member in a safe condition.

Existing systems for indicating that a crew member has fallen overboard require the overboard crew member to activate an alarm carried by the crew member, or another crew member on the vessel to activate an alarm disposed on the vessel.

However, both of these arrangements can result in appreciable delay, for example in a circumstance wherein the crew member lost overboard is unconscious and therefore incapable of activating the alarm, or in a circumstance wherein the absence of the overboard crew member is not immediately noted by another crew member. The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the present application.

In the following description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or *comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Summary of the Invention

In accordance with a first aspect of the present invention, there is provided a maritime safety system comprising: at least one transmitter unit, each transmitter unit being arranged to transmit successive signals by wireless communication at pseudo random intervals and each transmitter unit having an associated transmitter range; a base station arranged to receive said transmitted signals; and an alarm device; said base station being arranged to cause the alarm device to generate an audible and/or visible alarm when a plurality of successive signals from a transmitter unit are not detected; the arrangement being such that when a transmitter unit is disposed a distance from the base station which is greater than the transmission range associated with the transmitter unit and a plurality of successive signals are not detected from the transmitter unit, an audible and/or visible alarm is generated.

In this way, an indication is provided to crew members on the vessel that a crew member has potentially fallen overboard.

It will also be apparent that since an alarm is generated when signals from a transmitter unit are not detected, the alarm may indicate that the transmitter unit is damaged or that charging of a battery in the transmitter unit is necessary. In this sense, the system of the present invention is fail-safe in that an alarm is generated in the event of transmitter unit failure.

Preferably, the base station is arranged to cause the alarm to be generated when three successive signals are not detected.

In one arrangement, the transmitted signals are in the form of pulses periodically transmitted by the or each transmitter unit.

In order to distinguish signals transmitted from the or each transmitter unit from signals received from other sources, the base station may be arranged such that only signals of a predetermined frequency are detected.

In one embodiment, a plurality of transmitter units are provided and the system is arranged so as to distinguish the transmitter units from each other by allocating a unique code to each transmitter unit and incorporating a unique code into each transmitted signal.

In addition, or alternatively, the transmitter units may be arranged to transmit signals at different frequencies and the base station may be arranged to detect signals at the different frequencies and to distinguish detected signals on the basis of frequency.

In one embodiment, the base station further comprises a display for visibly displaying information associated with the safety system to a user. The information may be indicative of the status and/or operation of the transmitter units and may include information indicative of whether the or each transmitter unit is operating correctly, whether any of the transmitter units require charging, information indicative of the or each relevant identifier associated with the or each transmitter unit for which a signal has not been detected for a predetermined period of time, and so on. The display may be an LCD display.

The base station may also be arranged so as to interface with positioning means useable to determine the location of the base station. The positioning means may be a GPS- type positioning means arranged to generate GPS positioning data indicative of the location of the base station.

In one arrangement, the base station is arranged to interface with the positioning means so as to generate information indicative of the location of the base station when a plurality of successive signals from at least one of the transmitter units are not detected for a predetermined period of time.

The base station may also be arranged to interface with one or more external devices and to transfer an actuation signal to one or more of the external devices when a signal from at least one of the transmitter units is not detected for a predetermined period of time. The external devices may include a further alarm device, a dan buoy release, a self steering trip, an engine cut-out or a head-to-wind command to the autopilot. The external devices may also include an emergency position indicating radio beacon (EPIRB) assembly arranged to release an emergency position indicating radio beacon (EPIRB) when a signal from at least one of the transmitter units is not detected for a predetermined period of time.

In one arrangement, the system further includes a data storage device arranged to cooperate with the base station so as to store and selectively retrieve important system and transmitter unit information.

The or each transmitter unit may include a housing formed of at least partially transparent material so that any ingress of water into the housing is readily identifiable by a user, operative components of the transmitter unit being disposed in the housing.

The or each transmitter unit may include means for securing the transmitter unit to a person. The securing means may include a tab provided with an aperture for receiving a lanyard or a clip for attaching to an item of clothing.

The or each transmitter unit may include a status indicating device which may be in the form of an LED.

In one arrangement, the or each transmitter unit is arranged so that the LED is caused to indicate different status conditions depending on the current status and operation of the transmitter unit.

The or each transmitter unit may include a battery and a charging device which may be an inductive coupling type charging device for facilitating charging of the battery by interacting with an incident changing magnetic field so as to generate an induced voltage. The charging device may include a coil .

The or each transmitter unit may be arranged to transmit information indicative of the current status and/or operation of the transmitter unit.

In accordance with a second aspect of the present invention, there is provided a base station for a maritime safety system, the base station being arranged to receive signals transmitted from at least one transmitter unit by wireless communication, and the base station being arranged to generate an actuation signal when a plurality of successive signals from at least one of the transmitter units are not detected; the arrangement being such that when a transmitter unit is disposed a distance from the base station which is greater than a transmission range associated with the transmitter unit, the actuation signal is generated.

In accordance with a third aspect of the present invention, there is provided a transmitter unit for a maritime system, the transmitter unit being arranged to transmit successive signals by wireless communication, each signal being transmitted at a pseudo random offset relative to a predetermined time interval so as to thereby reduce the likelihood during use of signal transmissions from two transmitter units occurring simultaneously.

Description of the Drawings

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a maritime security system in accordance with an embodiment of the present invention with the system shown in a non- activated state; Figure 2 is a block diagram of a base station of the maritime safety system shown in Figure 1; Figure 3 is a diagrammatic representation of a transmitter unit of the maritime safety system shown in Figure 1; and Figure 4 is a diagrammatic representation of the maritime safety system shown in Figure 1 with the system shown in an activated state.

Description of an Embodiment of the Present Invention

Referring to the drawings, in Figure 1 there is shown a maritime safety system in accordance with an embodiment of the invention during use. The system comprises a base station 10, first, second, third and fourth transmitter units 12, 14, 16 and 18 respectively, an alarm device 20 and a positioning means, in this example in the form of a vessel global positioning system (GPS) 22. In the present example, the safety system also includes an emergency position indicating radio beacon (EPIRB) assembly 24 having an EPIRB 26 and an EPIRB releasing device 28.

During use, the base station 10, the alarm device 20, the vessel positioning means 22 and the EPIRB assembly 24 are disposed on a maritime vessel 30 and each of the transmitter units 12, 14, 16, 18 is secured to or otherwise carried by a crew member 32 of the vessel 30. Each of the transmitter units 12, 14, 16, 18 is of sufficient power that a signal transmitted by a transmitter unit 12, 14, 16, 18 is capable of being received by the base station 10 provided that the transmitter unit 12, 14, 16, 18 is located within a range 35 of the base station 10. The base station 10 is arranged to receive signals from the transmitter units 12, 14, 16, 18 and is arranged to distinguish the transmitter signals from signals received from other sources, for example by arranging the base station 10 such that only signals of a predetermined frequency are detected and by arranging each transmitter unit to periodically transmit a signal at the predetermined frequency.

The nature of the signal transmitted by each transmitter unit is such that the base station 10 is able to identify the transmitter unit 12, 14, 16, 18 and distinguish the transmitter units from each other. In this example, this is achieved by allocating a unique identification code to each transmitter unit 12, 14, 16, 18 and incorporating the relevant unique identification code into the signal transmitted by the relevant transmitter unit.

However, it will be understood that other arrangements for ensuring that the base station 10 is able to receive and distinguish signals from the transmitter units are possible. For example, the transmitter units may be arranged to transmit signals at different frequencies.

In order to reduce the likelihood that one or more transmitters will transmit signals simultaneously, the interval between successive transmissions for each transmitter unit may be different.

In this example, the transmitter units are arranged such that each transmitter unit 12, 14, 16, 18 transmits a signal to the base station 10 at pseudo random intervals, for example once every second and at a random offset within each one second interval. Such an asynchronous transmission protocol serves to reduce the likelihood of two signal transmissions occurring simultaneously. In order to further reduce the likelihood of simultaneous signal transmissions, the base station 10 is arranged so as to generate an audible alarm only when several expected transmissions are not received from a transmitter unit. For example, the base station 10 may be arranged so as to expect a signal from each transmitter unit every second and to generate an audible alarm when three successive signal transmissions are not received from a transmitter unit.

The base station 10 interfaces with the alarm device 20, the vessel global positioning system 22 and the EPIRB assembly 24 such that when three successive signals are not detected by the base station 10 from at least one of the transmitter units 12, 14, 16, 18, the base station 10 generates an actuation signal which causes the alarm device 20 to activate and generate an audible alarm. Non- detection of three successive signals also causes the vessel global positioning system 22 to log the location of the vessel 30 and the EPIRB releasing means 28 of the EPIRB assembly 24 to activate and release the EPIRB 26 into the surrounding water.

The base station 10 is shown in more detail in Figure 2. The base station 10 includes a control unit 33 for controlling and coordinating operations in the base station 10, one or more receivers 34 for receiving transmissions from the transmitter units 12, 14, 16, 18, an LCD display 36 for visibly displaying information associated with the safety system, and a data storage device 38 for storing important system and operational information.

The control unit 33 in this example includes a 16-bit microprocessor with 16 MHz clocking speed. The control unit 33 is arranged to interface with the alarm device 20 such that an audible alarm is generated in response to receipt of an actuation signal from the control unit 33 indicative that no signal has been detected from at least one of the transmitter units 12, 14, 16, 18 by the or at least one of the receivers 34 for three successive transmissions.

The control unit 33 may also be arranged to interface with positioning means other than the vessel global positioning means, such as an external active GPS antenna 40 arranged to supply GPS data in the absence of GPS data from the vessel global positioning means 22.

The control unit 33 is also arranged to interface with the LCD display 36 so that information relevant to the status and/or operation of the security system may be visibly communicated to a user. For example, information indicative of the status and/or operation of the transmitter units may be displayed, such as whether the transmitter units are operating correctly, or whether any of the transmitter units require charging. The display may also show the or each relevant identifier indicative of the or each transmitter unit 12, 14, 16, 18 for which a signal has not been detected for a predetermined period of time. In the present embodiment, the LCD display 36 is caused by the control unit 33 to display the following conditions where appropriate:

"Logged on-battery good"

This indicates normal operation of a transmitter unit 12, 14, 16, 18.

"Logged on-battery low"

This indicates that the transmitter unit is operating but the battery in the transmitter unit is low and needs recharging. "Not logged on"

This indicates that a transmitter unit is switched off.

"Charging"

This indicates that a transmitter unit is being recharged. "MOB"

This indicates that a signal has ceased being received from a transmitter unit and the transmitter unit appears to be lost. This condition causes an emergency response screen to be displayed. An MOB condition also causes the control unit 33 to display an identifier unique to the lost transmitter unit, the time that the transmitter unit was lost and the GPS location of the vessel on the LCD display 36. This information is also stored in the data storage device 38.

Although the display used in the present embodiment is an LCD display 36, it will be understood that other display devices are envisaged, such as an LED display device.

The control unit 33 is also arranged to interface with the data storage device 38 so as to save and selectively retrieve all important system and transmitter unit information, for example in the event that power to the base station 10 is lost. The data storage device 38 may be in the form of a non-voltile memory.

Optionally, the control unit 33 may also be arranged so as to interface with an external device 42 such that an appropriate actuation signal is forwarded to the external device 42 when a signal is not detected from one or more of the transmitter units for a predetermined period of time. For example, the external device 42 may be an additional alarm unit, a dan buoy release, a self steering trip, an engine cut-out, a head-to-wind command to the autopilot, and so on.

A transmitter unit 12 is shown in more detail in Figure 3.

The transmitter unit 12, 14, 16, 18 includes a housing 44, in this example formed of lightweight impact resistant polycarbonate material, the housing 44 being ultrasonically sealed so as to be waterproof to a rating of IP68. The housing 44 is also at least partially transparent so that any ingress of water into the housing 44 is readily identifiable by a user. Integral with the housing 44 is a tab 46 provided with an aperture 48 for receiving a lanyard.

However, although each transmitter unit 12, 14, 16, 18 of the present embodiment includes a tab 46 and an aperture 48 for receiving a lanyard, it will be understood that other arrangements for securing the transmitter unit to a person are envisaged. For example, the housing may include a clip for attaching to an item of clothing, a belt, and so on.

Inside the housing 44 is disposed a control unit 50 for controlling and coordinating operations of the transmitter unit, a transmitter 52 for periodically transmitting a signal under control of the control unit 50, an indicating device in this example in the form of an LED 54 for indicating transmitter unit status to a user, a battery 56 for supplying electrical power to components of the transmitter unit 12, 14, 16, 18, and a charging device 58 for facilitating charging of the battery 56.

In the present example, the LED 54 is caused by the control unit 50 to indicate different status conditions depending on the current status and operation of the transmitter unit. For example, the LED 54 may be caused to emit a short pulse of green light once every two seconds so as to indicate that the battery 56 is sufficiently charged and the transmitter unit is transmitting a signal, to emit a short pulse of red light once every two seconds so as to indicate that the battery charge is low and the transmitter unit is transmitting a signal, to emit a relatively long pulse of green light once every five seconds so as to indicate that the transmitter unit is disposed in a charger and the battery 56 is fully charged, to continuously emit red light so as to indicate that the transmitter unit is disposed in a charger and charging is in process but is not yet complete, and to emit no light so as to indicate that the transmitter unit is switched off.

However, it will be understood that other arrangements may be used so as to indicate transmitter unit status and/or operation.

The transmitter unit 12, 14, 16, 18 is also arranged to transmit a signal in the form of a data packet containing the unique identification code associated with the transmitter unit once per second.

In order to minimise the likelihood of simultaneous signal transmissions by two or more transmitter units 12, 14, 16, 18, each transmitter is provided with a pseudo random number generator 57 which is used to determine the offset time within each one second interval at which the transmitter unit transmits the signal. In the present example, the pseudo random number generator 57 is in the form of a linear feedback shift register (LFSR) . The output of an N bit LFSR ensures that 2^N unit outputs will be generated before the LFSR repeats its sequence. For example, if a 20 bit LFSR is shifted every second, 2²⁰ = 1048576 different outputs will be generated every twelve days before the LFSR repeats .

In order to further reduce the likelihood of multiple successive simultaneous transmissions of two transmitter units, each LFSR is seeded with the respective unique identification code.

The data packet consists of word aligned data which is organised in such a way as to allow a simple UART to be used to transmit data.

Prior to transmission of a data packet, the transmitter 52 must be enabled for a minimum period, in this example 2ms. During this time, alternating Is and 0s are transmitted at 20Kbit/s. This pattern allows the receiver 34 of the base station 10 to distinguish a binary 1 from a binary 0. A preamble precedes the remainder of the packet, the preamble containing a unique pattern that can not occur within the data, flags or error check portion of the data packet. The base station 10 uses the preamble to indicate the start of a new data packet.

The remainder of each data packet comprises the unique identification code, status flags, and an error check sequence, with each byte of data being scrambled using an exclusive OR process. The identification code and error check portions of the data packet make use of a data scrambling technique to ensure that a significant number of edge transitions occur on the transmitted data.

Each transmitter 52 is arranged so as to transfer data at a rate of 20 Kbit/s. With this transfer rate, in the present example, the length of a data packet is of the order of 4.4ms. Accordingly, the total transmit time for each signal including 2ms transmitter enable time is of the order of 6.4ms. The control unit 50 in this example includes a microprocessor which may be a 16-bit microprocessor with 16 MHz clocking speed.

The control unit 50 may also be arranged to cause the transmitter 52 to transmit information indicative of the current status and/or operation of the transmitter unit 12, 14, 16, 18. For example, the transmitter 52 may be caused to transmit information indicative of whether the respective transmitter unit is operating correctly, whether the respective transmitter unit requires charging, and so on.

The transmitter 52 in this example is arranged to transmit signals in the 433 MHz ISM band with FSK modulation to reduce interference.

The charging device 58 in this example is an inductive coupling type charging device and for this purpose may include a coil and associated circuitry, the coil interacting with an incident changing magnetic field so as to generate an induced voltage across the coil useable to charge the battery 56.

During use, in the event that a crew member 32 falls overboard as shown in Figure 4, the distance between the transmitter unit 14 associated with the overboard crew member 32 and the base station 10 will exceed the range 35. Accordingly, since the transmitter unit 14 is located outside of the range 35, even though a signal is transmitted by the transmitter unit 14, the base station 10 is incapable of detecting the signal. If three successive expected signals are not detected from the transmitter unit 14, the base station 10 sends an actuation signal to the alarm device 20 which causes the alarm device 20 to generate an audible alarm. The base station also causes the vessel positioning means 22 to log the position of the vessel 30 and the time that the crew member 32 was lost overboard. In the present example, the base station 10 also causes the EPIRB assembly 24 to release the EPIRB 26 into the surrounding water.

It will be appreciated that since immersion of a transmitter unit in water has the effect of reducing the intensity of the signal transmitted by the transmitter unit, during use, if the transmitter unit is immersed in water, the signal will not be detected by the base station and an alarm will be generated.

Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention.

Claims

CLAIMS :

1. A maritime safety system comprising: at least one transmitter unit, each transmitter unit being arranged to transmit successive signals by wireless communication at pseudo random intervals and each transmitter unit having an associated transmitter range; a base station arranged to receive said transmitted signals; and an alarm device; said base station being arranged to cause the alarm device to generate an audible and/or visible alarm when a plurality of successive signals from a transmitter unit are not detected; the arrangement being such that when a transmitter unit is disposed a distance from the base station which is greater than the transmission range associated with the transmitter unit and a plurality of successive signals are not detected from the transmitter unit, an audible and/or visible alarm is generated.

2. A maritime safety system as claimed in claim 1, wherein the base station is arranged to cause the alarm to be generated when three successive signals are not detected.

3. A maritime safety system as claimed in claim 1, wherein each transmitter unit is arranged to transmit a signal once every second at a pseudo random offset within each one second interval.

4. A maritime safety system as claimed in claim 1, wherein each transmitter unit further comprises a pseudo random number generator in the form of a linear feedback shift register (LFSR) to determine the time at which to transmit the signals.

5. A maritime safety system as claimed in claim 4, wherein each LFSR is seeded with a unique identification code.

6. A maritime safety system as claimed in claim 1, wherein the transmitted signals are in the form of periodically transmitted pulses .

7. A maritime safety system as claimed in claim 1, further comprising a plurality of transmitter units, and wherein the system is arranged so as to distinguish the transmitter units from each other by incorporating a unique code into each transmitted signal.

8. A maritime safety system as claimed in claim 7, wherein the transmitter units are arranged to transmit signals at substantially the same frequency and the base station is arranged such that only signals of a predetermined frequency are detected.

9. A maritime safety system as claimed in claim 1, wherein the transmitter units are arranged to transmit signals at different frequencies and the base station is arranged to detect signals at the different frequencies and to distinguish detected signals on the basis of frequency.

10. A maritime safety system as claimed in claim 1, wherein the base station further comprises a display for visibly displaying information associated with the safety system to a user.

11. A maritime safety system as claimed in claim 10, wherein the information is indicative of whether the or each transmitter unit is operating correctly, whether any of the transmitter units require charging, and/or of the or each relevant identifier associated with the or each transmitter unit for which a signal has not been detected for a predetermined period of time.

12. A maritime safety system as claimed in claim 1, wherein the base station is arranged to interface with positioning means so as to generate information indicative of the location of the base station when a plurality of successive signals from at least one of the transmitter units are not detected for a predetermined period of time.

13. A maritime safety system as claimed in claim 12, wherein the positioning means is a GPS-type positioning means arranged to generate GPS positioning data indicative of the location of the base station.

14. A maritime safety system as claimed in claim 1, wherein the base station is arranged to interface with one or more external devices and to transfer an actuation signal to one or more of the external devices when a signal from at least one of the transmitter units is not detected for a predetermined period of time, the external devices including a further alarm device, a dan buoy release, a self steering trip, an engine cut-out, a head- to-wind command to the autopilot, and/or an emergency position indicating radio beacon (EPIRB) assembly arranged to release an emergency position indicating radio beacon (EPIRB) .

15. A maritime safety system as claimed in claim 1, wherein the or each transmitter unit includes a status indicating device which may be in the form of an LED.

16. A maritime safety system as claimed in claim 15, wherein the or each transmitter unit is arranged so that the LED is caused to indicate different status conditions depending on the current status and operation of the transmitter unit.

17. A maritime safety system as claimed in claim 1, wherein the or each transmitter unit includes a rechargeable battery and a charging device arranged to facilitate inductive charging of the rechargeable batteries .

18. A maritime safety system as claimed in claim 1, wherein the or each transmitter unit is arranged to transmit information indicative of the current status and/or operation of the transmitter unit.

19. A base station for a maritime safety system, the base station being arranged to receive signals transmitted from at least one transmitter unit by wireless communication, and the base station being arranged to generate an actuation signal when a plurality of successive signals from at least one of the transmitter units are not detected; the arrangement being such that when a transmitter unit is disposed a distance from the base station which is greater than a transmission range associated with the transmitter unit, a plurality of successive signals are not detected and the actuation signal is generated.

20. A base station as claimed in claim 19, wherein the base station is arranged to cause the alarm to be generated when three successive signals are not detected.

21. A base station as claimed in claim 19, further comprising a display for visibly displaying information associated with the safety system to a user.

22. A base station as claimed in claim 21, wherein the information is indicative of whether the or each transmitter unit is operating correctly, whether any of the transmitter units require charging, and/or of the or each relevant identifier associated with the or each transmitter unit for which a signal has not been detected for a predetermined period of time.

23. A base station as claimed in claim 19, wherein the base station is arranged to interface with positioning means so as to generate information indicative of the location of the base station when a plurality of successive signals from at least one of the transmitter units are not detected for a predetermined period of time.

24. A base station as claimed in claim 23, wherein the positioning means is a GPS-type positioning means arranged to generate GPS positioning data indicative of the location of the base station.

25. A base station as claimed in claim 19, wherein the base station is arranged to interface with one or more external devices and to transfer an actuation signal to one or more of the external devices when a signal from at least one of the transmitter units is not detected for a predetermined period of time, the external devices including a further alarm device, a dan buoy release, a self steering trip, an engine cut-out, a head-to-wind command to the autopilot, and/or an emergency position indicating radio beacon (EPIRB) assembly arranged to release an emergency position indicating radio beacon (EPIRB) .

26. A transmitter unit for a maritime system, the transmitter unit being arranged to transmit successive signals by wireless communication, each signal being transmitted at a pseudo random offset relative to a predetermined time interval so as to thereby reduce the likelihood during use of signal transmissions from two transmitter units occurring simultaneously.

27. A transmitter unit as claimed in claim 26, wherein the transmitter unit is arranged to transmit a signal once every second at a pseudo random offset within each one second interval.

28. A transmitter unit as claimed in claim 26, wherein the transmitter unit further comprises a pseudo random number generator in the form of a linear feedback shift register (LFSR) to determine the time at which to transmit the signals.

29. A transmitter unit as claimed in claim 28, wherein the LFSR is seeded with a unique identification code.

30. A transmitter unit as claimed in claim 26, wherein the transmitted signals are in the form of periodically transmitted pulses.

31. A transmitter unit as claimed in claim 26, wherein the transmitter unit includes a status indicating device which may be in the form of an LED.

32. A transmitter unit as claimed in claim 31, wherein the transmitter unit is arranged so that the LED is caused to indicate different status conditions depending on the current status and operation of the transmitter unit.

33. A transmitter unit as claimed in claim 26, wherein the transmitter unit includes a rechargeable battery and a charging device arranged to facilitate inductive charging of the rechargeable battery.

34. A transmitter unit as claimed in claim 26, wherein the transmitter unit is arranged to transmit information indicative of the current status and/or operation of the transmitter unit.