an embodiment of the present invention will now be described with reference to fig1 . in fig1 there is shown a first transmitter / receiver module 1 , which is arranged to be carried by a person . the transmitter / receiver module may also be arranged to be fitted to a heavy vehicle such as a forklift or a truck , or to a large stationary object , such as a steel cutting guillotine or a brake press . it will be understood that the transmitter / receiver module may be stand - alone ( i . e . it may be merely placed on or adjacent to the object ), it may be arranged to be fitted permanently or temporarily to the object ( by means of a mounting bracket , or other suitable attachment means ) or it may be incorporated into the vehicle or machine during the manufacture of the machine or vehicle . the transmitter / receiver module provides a coded and cycling transmission signal . it is to be understood that the term “ coded ” within the present context is taken to mean any suitable type of radio signal encoding , such as pulse coding . the coded signal may or may not be encrypted , depending on the needs of the user . the term “ cycling ” will be understood to mean that the transmitter / receiver module cycles periodically through “ send ” and “ receive ” functions . that is , the transmitter / receiver module alternates between a first mode during which the transmitter / receiver module transmits a signal and a second mode during which the transmitter / receiver module receives a signal transmitted by another transmitter / receiver module . in the present embodiment , the coding is provided by a pulse technology modulator , which is controllable in both direction and distance mediums . this allows the transmitter and receiver to be optimised depending on the size and the layout of a site such as a factory , a warehouse , or a wharf . for example , in a warehouse environment , forklifts may be fitted with one transmitter / receiver module , and persons proceeding on foot may be fitted with another transmitter / receiver module . in another example , dangerous machinery in a sheet metal fabrication factory may be fitted with one transmitter / receiver module , and persons working in the vicinity of the machine could be supplied with another transmitter / receiver module , so that a person will be made aware of a possible threat to their safety if they happen to come within a predefined zone surrounding the machine . each site will have different distance and direction needs , depending on the layout and topography of the site . a first transmitter / receiver module detects the proximity of another transmitter / receiver module by determining whether a signal from another transmitter / receiver module is received . in other words , the signal strength of all transmitter / receiver modules are purposely limited by their antenna design and power output to extend only to a predefined zone , as required by the topology and layout of a site , and by the safety zone that is required around a particular vehicle or piece of machinery . for most applications , an embodiment of the present invention provides an average of 30 to 40 meters coverage in a forward / backward direction with 10 to 18 meters side coverage . this defines a rectangular zone which is adequate for most applications . however , it is possible to increase or decrease this coverage zone by varying the output power and or antenna size and design , and such modifications are within the contemplation of a person skilled in the art . in an embodiment of the present invention , the transmitter / receiver module have incorporated several features to monitor signal integrity for situations where there are momentary dropouts or loss of signal . this is shown in fig2 which provides a flow chart in accordance with an embodiment of the present invention . in this simplified embodiment , it is assumed that there are only two types of objects that carry , or are fitted with , the transmitter / receiver modules , namely forklifts and persons proceeding on foot . the transmitter / receiver module in this embodiment is arranged to be aware of whether it is attached to a person or to a forklift . this may be done by any suitable method . for example , the transmitter / receiver module “ type ” ( i . e . whether the transmitter / receiver module is attached to a forklift or carried by a person ) may be hard - wired into the transmitter / receiver module , or it may be changeable by a built - in switching device ( e . g . via software , or via a mechanical switch ) or by any other suitable means . in another example , the transmitter / receiver module may be associated with a “ base station ”, which may act as both a battery re - charger and as a data entry station . when a user collects their transmitter / receiver module after recharging the battery , they may be able to assign their “ type ” to the module via in - built software . moreover , the transmitter / receiver module may be arranged to carry more than rudimentary information . for example , a user may also be able to input a unique identifier , such as their name , allowing them to be uniquely identified by other transmitter / receiver modules . this may be useful in a large number of situations . in one example , the proximity warning system may be arranged to track the whereabouts of all users and carriers of the transmitter / receiver modules . a first user involved in an accident may be able to send a “ panic ” signal to a second user , and the signal received from the first user may be used to determine the first users identity and location . in another example , if an urgent message must be relayed to a first user , a second user may be able to determine where the first user is situated within a building , and thereby be able to relay the message to the first user . at the beginning of a cycle , the first transmitter / receiver module determines ( 10 ) whether it has received a signal from a second transmitter / receiver module . if a signal has been received from a second transmitter / receiver module , and the first transmitter / receiver module recognises the signal as an appropriately coded signal , then the first transmitter / receiver module performs an internal check ( 11 ) to determine whether it is attached to a forklift or carried by a person . if the first transmitter / receiver module is attached to a forklift then the first transmitter / receiver module further checks to see whether the forklift is reversing ( 12 ). if the forklift is reversing then the first transmitter / receiver module enables a random timer ( 13 ), to create a time gap before returning to step 10 . if the first transmitter / receiver module receives a signal ( 10 ) and the first transmitter / receiver module is not attached to a forklift , then the first transmitter / receiver module engages a built - in warning light and buzzer ( 14 ) to alert the person holding or wearing the first transmitter / receiver module to the close proximity of a forklift . the first transmitter / receiver module then immediately re - checks the proximity of the forklift by returning to step 10 to determine whether the forklift remains within the zone defined by the strength of the transmitted signal . if the first transmitter / receiver module does not receive a signal ( 10 ), it then checks to determine whether it is required to transmit a signal ( 15 ) according to an in - built timer . if it is not the correct time to transmit a signal , the first transmitter / receiver module returns to the signal checking mode ( 10 ). if it is the correct time to transmit a signal , then the first transmitter / receiver module checks to determine whether it is attached to a forklift ( 16 ). if transmitter / receiver module is attached to a forklift , then the transmitter / receiver module checks to determine whether the forklift is reversing ( 19 ). if the forklift is reversing , then the transmitter / receiver * module transmits a signal with information to suggest that it is a forklift ( 20 ). the transmitter / receiver module then sets the random timer ( 18 ) and returns to check whether a signal has been received ( 10 ). if the transmitter / receiver module is attached to a person ( 16 ), then the transmitter / receiver module transmits a signal encoded with information to suggest that the transmitter / receiver module is attached to a person ( 17 ). the transmitter / receiver module then sets the random timer ( 18 ) and returns to the step of checking whether a signal has been received ( 10 ). the transmitter / receiver module is arranged so as to minimise interference from known and unknown sources . this is achieved by the use of coding in the signal . a receiver processing an electromagnetic signal will first determine whether the signal received contains the correct encoding . that is , the transmitter / receiver module has either an electronic hardware or software means for determining whether the format of the signal received by the transmitter / receiver module is in a recognisable format . if the signal received does not match an accepted format or standard of encoding used by the module , the signal will be rejected as a spurious or background noise signal . it is to be understood that the coding may also be changed to suit the site on which an embodiment of this invention is to be applied . for example , if another form of wireless communication is already employed on a site ( say , for example , a radio frequency network used for speech communication between workers ), then an embodiment of the present invention may be arranged to minimise interference with the radio frequency network already in use at the industrial site . this would be achieved by choosing a coding format which preferably bears little or no resemblance to the format of the radio signals used by the speech communication radio frequency network , or by operating in a frequency range that is different from the radio frequency range . it will be understood that it is within the scope of a person skilled in the art to choose an appropriate coding format , taking into account the special needs of a particular area or industry , as well as local laws and communications standards . in an embodiment of the present invention , the range of the transmitted signal produced by the transmitter / receiver module would be within the range of 10 metres to 100 metres , although it will be understood that the construction of a transmitter / receiver module with a range of less than 10 metres or greater than 100 metres is within the scope of a person skilled in the art . moreover , it is within the scope of an embodiment of the present invention for the transmitter / receiver module is fitted with more than one transmitting or receiving antenna . in some circumstances , it may be necessary to provide more than one antenna for the transmitter / receiver module . for example , the machine may be very large , such as a large ore carrying truck used in open cut mines , such that one antenna would not be capable of defining an adequate zone around the vehicle . alternatively , the layout or geography of a site may be such that multiple antennas are required to provide full coverage of the site . for example , at a wharf , stacked containers may define narrow corridors that preclude a single antenna from operating effectively . furthermore , the height and shape of these container “ corridors ” may change over time , thereby changing the fundamental layout of the site . in such a situation , transmitter / receiver modules would preferably be fitted with multiple antennas , so that a signal may be propagated efficiently and effectively despite the changing topography of the site . moreover , it is within the scope of the present invention to provide a circuit whereby a multi - antenna transmitter / receiver module is aware of which particular antenna is receiving a transmitted signal . this is preferable because at some frequencies , electromagnetic transmissions may be highly directional . in such a situation , the ability to determine which antenna has received the signal will allow an operator and / or the internal electronic circuit is arranged , to determine approximately where the other object or person is located . in a further embodiment of the present invention , multiple antennes may be installed at fixed locations around a site . when combined with a transmitter / receiver module which is arranged to transmit a unique coded signature , an embodiment of the present invention is able to monitor the location of a number of persons and / or objects within the zones defined by the antennae , and such information may then be conveniently displayed on a computing system or other appropriate hardware at a central location . such a system may have many uses , such as tracking whether a person or object is within the defined zone or outside a defined zone . moreover , the transmitter / receiver module may be combined with a “ panic ” button , so that a person or machine may be able to alert a central authority of their location and their involvement in a possible hazardous or dangerous incident . it will be understood that such modifications fall within the scope of the present invention . in another example , this embodiment of the present invention could be used to carry out fire or evacuation checks , to ensure that all persons within a defined site have safely left the site . preferably , the transmitter and receiver operate on a frequency range that is “ free ” ( i . e . unrestricted by law or practice ) to all users , thereby negating the need for special licences and / or permits to operate the transmitter , as may be required by law . it will be understood that whilst a particular embodiment of the invention may work in the “ free ” frequency range , it is within the scope of a person skilled in the art to modify the present invention to change the operational frequency of the transmitter / receiver module , so that the transmitter and receiver may operate at any convenient electromagnetic frequency . for example , the circuit may be easily modified to allow the transmitter / receiver module to be used in countries where a different operating frequency range is desired . it will be understood that the second transmitter / receiver may be adapted to be suitable for any moving object , whether it is a person , another vehicle , or any other object . the transmitter / receiver module 1 incorporates a warning device , which in the present embodiment takes the form of a visual and audible warning device , namely , a flashing light coupled to a warning buzzer . a first warning light 2 will indicate when the system is out of range . that is , when the transmitter / receiver module is not able to receive a signal from another transmitter / receiver module . a second light 3 is coupled to a warning buzzer 4 . the second light and the warning buzzer will be activated when a predetermined minimum safe distance has been breached . this will warn the user of an impeding danger or collision . as the transmitter / receiver module is physically worn or held by a person or placed prominently within the cabin of a vehicle or near a machine , the likelihood of not becoming aware of the warning ( due to a lack of line of sight in the case of visible warnings , or background noise in the case of audible warnings ) is greatly reduced . in an embodiment of the present invention , the casing which houses the transmitter / receiver module is designed and manufactured to withstand harsh environmental conditions as normally found within heavy industry sites . for example , the housings are waterproof to the extent that they will withstand damage from water , rain , hail or snow , as well as other fluids , such as common solvents including petrol , diesel , and other volatile organic chemicals . moreover , the transmitter / receiver modules are manufactured to withstand temperature extremes , with an effective operating range of − 30 ° c . to + 50 ° c . it is also within the scope of the present invention to operate the transmitter / receiver module using any suitable power supply technology . where necessary , the transmitter / receiver module may incorporate an in - built power supply such as a rechargeable battery 6 , to allow the transmitter / receiver module to be portable , and allow their use in locations where conventional power supplies are not available , such as in open cut mines , tunnels , etc . in another example , where the transmitter is arranged to be attached to a stationary machine , such as a steel cutting guillotine , the transmitter / receiver module may be arranged to operate from a suitable power outlet , such as a household or industry standard ( 240v or 415v ) power supply . any such modification is considered to be within the scope of a person skilled in the art . further modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention .