Abstract:
A ferromagnetic threat warning system utilizing two or more radio frequency transmitters or transceivers. A remote transmitter or transceiver is attached to each ferromagnetic threat that may enter the area to be protected. A master receiver or transceiver is mounted at each door controlling entry into the area to be protected. Communication between any master unit and one or more remote units generates an alarm when one or more threat objects approaches within a preselected distance of any master unit. Approach of a threat object within the preselected distance from a door can be detected when the strength of a radio signal between the remote unit associated with the threat object and the master unit near the door reaches a predetermined level. Generation of an alarm can be initiated at either the affected remote unit or the affected master unit. Generation of an alarm at either the affected remote unit or the affected master unit can also result in generation of an alarm at the other of the two units.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is in the field of methods and apparatus used in pre-screening to prevent entry of ferromagnetic threat objects into the vicinity of a magnetic resonance imaging (MRI) magnet. 
     2. Background Art 
     Even small ferromagnetic objects that are inadvertently carried into a magnetic resonance imaging examination room can become potentially lethal projectiles in the very high field and high field gradient surrounding the MRI magnet. Large ferromagnetic objects, such as oxygen tanks, floor scrubbers, tables, and pipe wrenches pose threat of great harm to patients undergoing MRI, as well as damage to the MRI instrument itself. Such objects are called “ferromagnetic threat objects” herein. A terrible accident occurred to a small boy when an oxygen tank was inadvertently brought into the magnet room, causing his death. Many “near-misses” have occurred, and most MRI centers can relate such potentially-dangerous incidents. 
     Oxygen tanks are painted green by convention, thus heightening the illusion that these are safe in a magnetic resonance environment, which is absolutely untrue. What is needed, then, is a way of ensuring that oxygen tanks, floor scrubbers, and other ferromagnetic threats are not brought into the magnet room under any circumstances. The present invention provides a method and apparatus for this purpose. 
     BRIEF SUMMARY OF THE INVENTION 
     In the preferred embodiment, the system of the present invention is comprised of a master transceiver module or radio unit, and at least one remote transceiver module or radio unit. In the following, the term “master gender” module or radio unit identifies the type of radio unit that is used as a master module, while the term “remote gender” module or radio unit identifies the type of radio unit that is used as a remote module. This terminology is used strictly as an aid in differentiating between the two different types of radio units. 
     The radio units periodically transmit radio frequency signals. Each radio signal has a predetermined transmission strength. Each signal can be encoded to identify the threat warning system. In some embodiments, as explained further below, each signal can be encoded to identify whether the gender of the transmitting unit is a master module or a remote module. Also, as explained further below, in some embodiments, each signal can be encoded to identify exactly which module is transmitting the signal. The master radio unit is mounted next to a door which provides entry into an area to be protected, such as an MRI magnet room. Each ferromagnetic threat object which may be brought near the MRI suite of rooms has a remote radio unit mounted thereon. An alarm condition is activated when one of the remote modules encroaches on an area within a preselected distance of the master module. 
     In the preferred embodiment, these master and remote radio units operate in a two-way communications methodology. In this embodiment, local alarms in either or both the master module and the encroaching remote module can be triggered. The triggering in either gender of module can be initiated when a signal received by the module reaches a predetermined signal strength level, if that signal was transmitted by a module of opposite gender from the receiving module. That is, a master module will only alarm upon receiving a signal from a remote module, and a remote module will only alarm upon receiving a signal from a master module. Also, when a non-alarming module receives a message encoded to indicate that another module has alarmed as a result of receiving a signal at the predetermined strength level from the non-alarming module, the non-alarming module will then activate its local alarm. This ensures that not only will the master module at the door alarm to indicate that a threat object is too close, but the particular encroaching threat object will also be identified by its local alarm. 
     Either the master unit or the encroaching remote unit might sense first that a received radio signal is above a predetermined signal strength, causing an alarm by that first unit. Eventually, the second of the two modules would also sense that a received signal has exceeded the predetermined signal strength, indicating that the predetermined safe separation distance has been encroached upon. The second module would then also activate its local alarm based on the received signal strength. This can be thought of as a “signal strength” dual alarming mode. However, as a rule, before this happens, the first alarming module will transmit a return signal encoded to signify that the first module is alarming, and further encoded to identify the second (non-alarming) module from which the predetermined level radio signal was received, resulting in the immediate generation of an alarm by the second module based upon the signal message contents, rather than upon signal strength. This can be thought of as a “mutual reinforcement” dual alarming mode. 
     In an alternative second embodiment, the remote modules can be only capable of transmitting, and the master module can be only capable of receiving. In this second embodiment, only the master module may alarm, and triggering of both modules may not be accomplished. 
     In an alternative third embodiment, the master module transmits on a first frequency and listens on a second frequency. The remote module transmits on the master module&#39;s listening frequency and listens on the master module&#39;s transmitting frequency. There is no need for a module gender ID. Gender identification for a module is accomplished by determining whether the module is transmitting on the first frequency or on the second frequency. In this third embodiment, either or both modules may alarm in a “signal strength” dual alarming mode. 
     In an alternative fourth embodiment, the master module transmits on one frequency and listens on a second frequency, as in the third embodiment. The remote module transmits on the master module&#39;s listening frequency and listens on the master module&#39;s transmitting frequency. Gender identification for a module is accomplished by determining whether the module is transmitting on the first frequency or on the second frequency. In this fourth embodiment, each signal is encoded with a unique module ID, so that both modules may alarm in a “mutual reinforcement” dual alarming mode. Both modules may also alarm in a “signal strength” dual alarming mode. 
     The operational radio frequency or frequencies of the system are selected to avoid interference with the MRI radio frequency or its harmonics. 
     The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a sketch of a suggested positioning of the system components; 
         FIG. 2  illustrates the triggering relationship between modules in a first embodiment; 
         FIG. 3  is a block diagram of a remote module capable of both transmitting and receiving; 
         FIG. 4  is a block diagram of a master module capable of both transmitting and receiving; 
         FIG. 5  illustrates the basic radio digital message format; 
         FIG. 6  shows the radio digital message format which would be transmitted from a master module with the alarm inactive; 
         FIG. 7  shows the radio digital message format which would be transmitted from a master module with the alarm active; 
         FIG. 8  shows the radio digital message format which would be transmitted from a remote module with the alarm inactive; 
         FIG. 9  shows the radio digital message format which would be transmitted from a remote module with the alarm active; 
         FIG. 10  is a block diagram of a remote module capable only of transmitting, in a second embodiment; 
         FIG. 11  is a block diagram of a master module capable only of receiving; 
         FIG. 12  illustrates the triggering relationship between modules in a third embodiment; 
         FIG. 13  is a block diagram of a remote module capable of both transmitting and receiving, in the third embodiment; 
         FIG. 14  is a block diagram of a master module capable of both transmitting and receiving, in the third embodiment; 
         FIG. 15  illustrates the basic radio digital message format, in the third embodiment; 
         FIG. 16  is a block diagram of a remote module in a fourth embodiment; 
         FIG. 17  is a block diagram of a master module in the fourth embodiment; and 
         FIG. 18  illustrates the basic radio digital message format, in the fourth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in  FIG. 1 , a master module or radio unit  12  can be located adjacent to the entry door of the MRI magnet room. It can have audible and visible alarm outputs, such as a siren and flashing lights. Remote modules or radio units  14 ,  16  can be attached to ferromagnetic threat objects that may be brought into the vicinity of the MRI magnet. The remote modules  14 ,  16  can be self-contained, battery operated modules that are attached to the ferromagnetic objects, such as oxygen tanks and floor polishers or scrubbers. 
     The master module  12  and the remote modules  14 ,  16  can all periodically transmit a radio frequency transmission, at a pre-set signal level. The transmission can include digital data to identify the system identification, the gender of the message source (whether it is a master or remote), and alarm status (active or inactive). The length of the transmission can be short, and the periodicity can be randomized. 
     The master module  12  and the remote modules  14 ,  16  all listen for valid radio messages and measure the corresponding received radio signal strength. The contents of the message can validate the fact that the transmitting module is in the same threat warning system as the receiving module, and that the transmitting module is the master radio unit  12  and the receiving module is a remote radio unit  14 ,  16 , or vice versa, while the signal strength is an indication of proximity. 
     There are two means by which an alarm can be initiated, via an indication that the signal strength measurement has reached the predetermined level, and via the indication that one of the modules  12 ,  14 ,  16  has initiated an alarm. The second means of initiating an alarm will only initiate an alarm in the module which transmitted the first radio signal which was received at the predetermined strength level. 
       FIG. 2  illustrates the first means of initiating an alarm, via the signal strength. If a radio signal is received from the same system identification, from an opposite gender of radio unit, and is above a pre-determined receive threshold, then a local alarm condition is activated at the receiving module. The circles represent the distance corresponding to a predetermined received signal strength at which an alarm is to be generated. The signal can also include an inactive alarm flag in the message. A local alarm condition can result in activating the local visible and audible alarm outputs associated with the receiving module. Further, the local alarm condition at the receiving module can result in the receiving module transmitting a radio signal message indicating that an alarm is active, including identifying the unique unit identifier of the radio unit which transmitted the original radio signal which was received at a strength which is over the predetermined threshold. 
     In the second means of triggering an alarm, if a radio signal is received from a radio unit having the same threat warning system identification, from an opposite gender radio unit, with the alarm condition set to active in the incoming message, and the incoming message contains a unique identifier that matches the unique identifier of the receiving module, then a local alarm condition is activated at the receiving unit, regardless of the received signal strength measurement. 
     The two means of generating an alarm provide redundancy in the preferred embodiment. The proximity of the two modules is monitored with independent signal strength circuits in each module, plus if one of the two triggers an alarm, then that in turn triggers the alarm at the paired module via the radio message contents. 
     The functional blocks of the two genders or types of modules or radio units are shown in the following Figures, using logic hardware. Alternatively, these functions could be performed using a micro-controller containing appropriate associated hardware and firmware. 
       FIG. 3  shows a block diagram of the functional system  20  of a typical remote module  14 ,  16  according to the preferred embodiment. The Tx control  22  initiates and controls the radio transmission. The period between transmissions is a pseudo-random time and is approximately one half second. When the module is transmitting, the T/R switch  24  is controlled to connect the transmitter  26  to the antenna  28 . This also isolates the radio frequency (RF) receiver  30  from the RF transmitter  26 . 
     The encoder  32  formats the digital contents of the radio message. The various message formats are disclosed below. The encoder  32  also controls the modulation of the RF transmitter  26 . The contents of the digital messages include the system identifier (ID), message source type, unique ID of the remote module, alarm status flag, and optionally the ID of a master module that is over the received signal strength threshold, and is causing the remote module to be in the alarm status. 
     The RF transmitter  26  converts the incoming digital stream to the appropriate radio signals and the antenna  28  radiates the RF signal in the air in an omni-directional pattern. 
     The T/R switch  24  connects the antenna  28  to either the RF transmitter  26  or the RF receiver  30  as controlled by the Tx controller  22 . 
     The RF receiver  30  converts the incoming radio signals received by the antenna  28  to digital baseband signals. 
     The decoder  34  performs functions of message synchronization and data decoding. Incoming messages are checked for system ID and source. The message contents are made available to the decision logic after the system ID and message source have been verified by the decoder  34 ; this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  36  measures the radio signal strength of the incoming message. 
     The threshold set point input  38  presents a predetermined level to the comparator  40 . This predetermined level is calibrated to coincide with a desired signal strength and in turn a desired minimum allowed proximity distance. 
     The comparator  40  generates an active output when the received signal strength is greater than the threshold. 
     The alarm decision logic processor  42  processes the incoming message contents and the signal strength comparator output, and determines if the alarm should be activated. An alarm is initiated in one of two ways. In the first way, the alarm decision logic processor  42  samples the comparator output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  44  and updating the alarm status flag to the encoder  32 . In the second way, if the alarm status flag from the message indicates an active alarm present at the master module  12 , and the ID of the remote module  14 ,  16  causing the alarm at the master module  12  matches the ID of the remote module  14 ,  16  receiving the message, then an alarm condition is also signaled. 
     The alarm indication output  44  turns on the audible and/or visible alarm indicators when triggered by the alarm decision logic processor  42 . The alarm will be active until the alarm condition goes away as determined by the alarm decision logic processor  42  and after a timeout period. 
     The remote ID unit  46  contains the unique digital address of the remote module  14 ,  16 . This ID is made available to the encoder  32  and to the alarm decision logic processor  42 . 
     The threat warning system ID unit  48  contains the unique digital address of the threat warning system. The remote gender ID unit  47  identifies this module as a remote gender unit. The remote gender ID unit  47  and the threat warning system ID unit  48  assist in validating the radio message contents. 
     The remote module is supplied electrical power from an integral battery pack  50 . 
       FIG. 3  also shows two signal protocols which will result in the generation of an alarm at the receiving remote module. In Protocol A, a local alarm is generated at the receiving remote module if the received system ID matches the system ID of the remote module, the signal is received from a master gender module, and the received signal strength is greater than a predetermined threshold. In Protocol B, a local alarm is generated if the received system ID matches the system ID of the receiving remote module, the signal is received from a master gender module, the transmitting master module is alarming, and the unique identifier of the remote module causing the master module to alarm matches the unique identifier of the receiving remote module. 
       FIG. 4  shows a block diagram of the functional system  60  of a typical master module  12  according to the preferred embodiment. The Tx control  62  functions the same as the Tx control  22  disclosed for the remote module in  FIG. 3 . 
     The encoder  72  formats the digital contents of the radio message. The various message formats are disclosed below. The encoder  72  also controls the modulation of the RF transmitter  66 . The contents of the digital messages include the threat warning system ID, the gender of the message source unit, the master ID of this module, an alarm status flag, and optionally the ID of a remote module which transmitted a signal that is over the received signal strength threshold, and is causing the master module to be in the alarm status. The remote module ID over the signal strength is taken from a circular remote ID alarm list  89 . 
     The RF transmitter  66 , T/R switch  64 , RF receiver  70  and the antenna  68  function the same as similar items disclosed for the remote module in  FIG. 3 . 
     The decoder  74  performs functions of message synchronization and data decoding. Incoming messages are checked for threat warning system ID and source gender. The message contents are made available to the decision logic processor  82  after the system ID and message source gender have been verified by the decoder  74 , and this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  76 , the threshold set point input  78 , and the comparator  80  function the same as similar units disclosed for the remote module in  FIG. 3 . 
     The alarm decision logic processor  82  processes the incoming message contents and the signal strength comparator  80  output, and determines if an alarm should be activated. An alarm is initiated in one of two ways. In one way, the alarm decision logic processor  82  samples the comparator  80  output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  84  and updating the alarm status flag to the encoder  72 . Second, if the alarm status flag from the message indicates an active alarm present at a remote module, and the ID of the master module causing the alarm at the remote matches the ID of the master module receiving the message, then an alarm condition is also signaled. 
     The alarm indication is the same as disclosed for the remote. However, the alarm indicators can be more elaborate since the master module power supply  90  can draw AC power from the electrical system in the building. 
     The master ID unit  86  contains the unique digital address of the master module  12 . This ID is made available to the encoder  72  and the alarm decision logic processor  82 . 
     The threat warning system ID unit  88  contains the unique digital address of the threat warning system. The master gender ID unit  87  identifies this module as a master gender unit. The threat warning system ID unit  88  and the master gender ID unit  87  assist in validating the radio message contents. 
     The remote ID alarm list  89  is a circular list of remote modules  14 ,  16  that are currently in the alarm mode. This list is updated by the alarm decision logic processor  82  and can be cleared out based on a set of appropriate rules. 
       FIG. 4  also shows two signal protocols which will result in the generation of an alarm at the receiving master module. In Protocol C, a local alarm is generated if the received system ID matches the system ID of the receiving master module, the signal is received from a remote gender module, and the received signal strength is greater than a predetermined threshold. In Protocol D, a local alarm is generated if the received system ID matches the system ID of the receiving master module, the signal is received from a remote gender module, the transmitting remote module is alarming, and the unique identifier of the master module causing the transmitting remote module to alarm matches the unique identifier of the receiving master module. 
       FIG. 5  shows a basic radio message format which contains a synchronization pattern, a threat warning system ID field, a message source gender field of either master or remote, a message source unique ID field, the alarm status of the transmitting module of either inactive or active, and optionally the unique source ID associated with an opposite gender module whose received signal strength has caused an alarm. Herein, a message source gender of 0 means a master gender unit transmitted the signal, and a message source gender of 1 means a remote gender unit transmitted the signal. Further, an alarm status of 0 means that the alarm is inactive, and an alarm status of 1 means that the alarm is active. 
       FIG. 6  shows a radio message format transmitted from a master module  12  with no alarm active. 
       FIG. 7  shows a radio message format transmitted from a master module  12  with an active alarm. 
       FIG. 8  shows a radio message format transmitted from a remote module  14 ,  16  with no alarm active. 
       FIG. 9  shows a radio message format transmitted from a remote module  14 ,  16  with an active alarm. 
     In an alternative second embodiment of the invention, the threat warning system can operate using a one-way radio link, where the master module contains only a receiver and each of the remote modules contains only a transmitter. In this embodiment, the master module initiates an alarm if it receives a radio signal from a remote module that is above the predetermined threshold signal strength. This embodiment does not provide the redundancy described above for the preferred embodiment. 
       FIG. 10  shows a block diagram of the functional system  200  of a typical remote module according to the aforementioned alternative second embodiment. This embodiment includes a Tx control  222 , an encoder  232 , a remote gender ID unit  247 , a system ID unit  248 , an RF transmitter  226 , and an antenna  228 . 
     The Tx control  222  initiates and controls the radio transmission. The period between transmissions is a pseudo-random time and is approximately one half second. 
     The encoder  232  formats the digital contents of the radio message. The encoder  232  also controls the modulation of the RF transmitter  226 . The contents of the digital messages include the system ID and the message source gender. 
     The RF transmitter  226  converts the incoming digital stream to the appropriate radio signals and the antenna  228  radiates the RF signal in the air in an omni-directional pattern. 
     The remote module is supplied electrical power from an integral battery pack  250 . 
       FIG. 11  shows a block diagram of the functional system  600  of a typical master module according to the aforementioned alternative second embodiment. An RF receiver  670 , converts the incoming radio signals received by the antenna  668  to digital baseband signals. 
     A decoder  674  performs functions of message synchronization and data decoding. Incoming messages are checked for system ID and source gender, and if verified correctly, this generates a valid message indicator. 
     The Rx signal strength measurement sensor  676  measures the radio signal strength of the incoming message. 
     The threshold set point input  678  presents a predetermined level to the comparator  680 . This predetermined level is calibrated to coincide with a desired signal strength and in turn a desired minimum allowed proximity distance. 
     The comparator  680  generates an active output when the received signal strength is greater than the threshold. 
     The alarm decision logic processor  682  samples the comparator  680  output during a valid message, if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  684 . 
     The alarm indication output  684  is the same as disclosed for the master module in  FIG. 4 . 
     The threat warning system ID unit  688  contains the unique digital address of the threat warning system. The master gender ID unit  687  identifies this module as a master gender unit. The threat warning system ID unit  688  and the master gender ID unit  687  assist in validating the radio message contents. 
       FIG. 12  illustrates the first means of initiating an alarm, via the signal strength, in an alternative third embodiment. If a radio signal is received from the same system identification, from an opposite gender of radio unit, and is above a pre-determined receive threshold, then a local alarm condition is activated at the receiving module. The circles represent the distance corresponding to a predetermined received signal strength at which an alarm is to be generated. A local alarm condition can result in activating the local visible and audible alarm outputs associated with the receiving module. In this embodiment, the master module  12 ′ transmits at a first frequency, denoted as “frequency  1 ”, and listens at a second frequency, denoted as “frequency  2 ”. Also, the remote modules  14 ′,  16 ′ transmit at frequency  2  and listen at frequency  1 . 
       FIG. 13  shows a block diagram of the functional system  700  of a typical remote module  14 ′,  16 ′ according to the alternative third embodiment. The Tx control  722  initiates and controls the radio transmission. The period between transmissions is a pseudo-random time and is approximately one half second. When the module is transmitting, the transmitter  726  is connected to the transmitting antenna  727 . 
     The encoder  732  formats the digital contents of the radio message. The encoder  732  also controls the modulation of the RF transmitter  726 . The contents of the digital messages include the system identifier (ID). 
     The RF transmitter  726  converts the incoming digital stream to the appropriate radio signals and the transmitting antenna  727  radiates the RF signal in the air in an omni-directional pattern. 
     The RF receiver  730  converts the incoming radio signals received by the receiving antenna  728  to digital baseband signals. 
     The decoder  734  performs functions of message synchronization and data decoding. Incoming messages are checked for system ID. The message contents are made available to the decision logic after the system ID has been verified by the decoder  734 ; this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  736  measures the radio signal strength of the incoming message. 
     The threshold set point input  738  presents a predetermined level to the comparator  740 . This predetermined level is calibrated to coincide with a desired signal strength and in turn a desired minimum allowed proximity distance. 
     The comparator  740  generates an active output when the received signal strength is greater than the threshold. 
     The alarm decision logic processor  742  processes the incoming message contents and the signal strength comparator output, and determines if the alarm should be activated. The alarm decision logic processor  742  samples the comparator output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  744 . 
     The alarm indication output  744  turns on the audible and/or visible alarm indicators when triggered by the alarm decision logic processor  742 . The alarm will be active until the alarm condition goes away as determined by the alarm decision logic processor  742  and after a timeout period. 
     The threat warning system ID unit  748  contains the unique digital address of the threat warning system. The threat warning system ID unit  748  assists in validating the radio message contents. The remote module is supplied electrical power from an integral battery pack  750 . 
       FIG. 13  also shows a signal protocol which will result in the generation of an alarm at the receiving remote module. In Protocol F, a local alarm is generated at the receiving remote module if the received system ID matches the system ID of the receiving remote module, and the received signal strength is greater than a predetermined threshold. 
       FIG. 14  shows a block diagram of the functional system  800  of a typical master module  12 ′ according to the alternative third embodiment. The Tx control  862  functions the same as the Tx control  722  disclosed for the remote module in  FIG. 13 . 
     The encoder  872  formats the digital contents of the radio message. The encoder  872  also controls the modulation of the RF transmitter  866 . The contents of the digital messages include the threat warning system ID. 
     The RF transmitter  866 , RF receiver  870 , the transmitting antenna  867 , and the receiving antenna  868  function the same as similar items disclosed for the remote module in  FIG. 13 . 
     The decoder  874  performs functions of message synchronization and data decoding. Incoming messages are checked for threat warning system ID. The message contents are made available to the decision logic processor  882  after the system ID has been verified by the decoder  874 , and this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  876 , the threshold set point input  878 , and the comparator  880  function the same as similar units disclosed for the remote module in  FIG. 13 . 
     The alarm decision logic processor  882  processes the incoming message contents and the signal strength comparator  880  output, and determines if an alarm should be activated. The alarm decision logic processor  882  samples the comparator  880  output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  884 . 
     The alarm indication is the same as disclosed for the remote. However, the alarm indicators can be more elaborate since the master module power supply  890  can draw AC power from the electrical system in the building. 
     The threat warning system ID unit  888  contains the unique digital address of the threat warning system. The threat warning system ID unit  888  assists in validating the radio message contents. 
       FIG. 14  also shows a signal protocol which will result in the generation of an alarm at the receiving master module. In Protocol G, a local alarm is generated if the received system ID matches the system ID of the master module, and the received signal strength is greater than a predetermined threshold. 
       FIG. 15  shows a basic radio message format for the alternative third embodiment, which contains a synchronization pattern and a threat warning system ID field. 
       FIG. 16  shows a flow diagram of the functional system  901  of a typical remote module according to an alternative fourth embodiment, which is similar to the third embodiment, except that it incorporates a “mutual reinforcement” dual alarming mode. The Tx control  922  initiates and controls the radio transmission. The period between transmissions is a pseudo-random time and is approximately one half second. When the module is transmitting, the transmitter  926  is connected to the transmitting antenna  927 . 
     The encoder  932  formats the digital contents of the radio message. The encoder  932  also controls the modulation of the RF transmitter  926 . The contents of the digital messages include the system identifier (ID), the unique remote module identifier, and the alarm status. 
     The RF transmitter  926  converts the incoming digital stream to the appropriate radio signals and the transmitting antenna  927  radiates the RF signal in the air in an omni-directional pattern. 
     The RF receiver  930  converts the incoming radio signals received by the receiving antenna  928  to digital baseband signals. 
     The decoder  934  performs functions of message synchronization and data decoding. Incoming messages are checked for system ID. The message contents are made available to the decision logic after the system ID has been verified by the decoder  934 ; this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  936  measures the radio signal strength of the incoming message. 
     The threshold set point input  938  presents a predetermined level to the comparator  940 . This predetermined level is calibrated to coincide with a desired signal strength and in turn a desired minimum allowed proximity distance. 
     The comparator  940  generates an active output when the received signal strength is greater than the threshold. 
     The alarm decision logic processor  942  processes the incoming message contents and the signal strength comparator output, and determines if the alarm should be activated, in either of two ways. In the first way, the alarm decision logic processor  942  samples the comparator output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  944 . In the second way, if the alarm status flag from the message indicates an active alarm present at the master module, and the ID of the remote module causing the alarm at the master module matches the ID of the remote module receiving the message, then an alarm condition is also signaled. 
     The alarm indication output  944  turns on the audible and/or visible alarm indicators when triggered by the alarm decision logic processor  942 . The alarm will be active until the alarm condition goes away as determined by the alarm decision logic processor  942  and after a timeout period. 
     The remote ID unit  946  contains the unique digital address of the remote module. This ID is made available to the encoder  932  and to the alarm decision logic processor  942 . 
     The threat warning system ID unit  948  contains the unique digital address of the threat warning system. The threat warning system ID unit  948  assists in validating the radio message contents. The remote module is supplied electrical power from an integral battery pack  950 . 
       FIG. 16  also shows two signal protocols which will result in the generation of an alarm at the receiving remote module. In Protocol H, a local alarm is generated at the receiving remote module if the received system ID matches the system ID of the receiving remote module, and the received signal strength is greater than a predetermined threshold. In Protocol I, a local alarm is generated at the receiving remote module if the received system ID matches the system ID of the remote module, if the transmitting master module is alarming, and if the ID of a remote module causing an alarm at the master module matches the ID of the receiving remote module. 
       FIG. 17  shows a flow diagram of the functional system  902  of a typical master module according to the alternative fourth embodiment, which is similar to the third embodiment, except that it incorporates a “mutual reinforcement” dual alarming mode. The Tx control  962  functions the same as the Tx control  922  disclosed for the remote module in  FIG. 16 . 
     The encoder  972  formats the digital contents of the radio message. The encoder  972  also controls the modulation of the RF transmitter  966 . The contents of the digital messages include the threat warning system ID, the alarm status, and optionally the ID of a remote module which transmitted a signal that is over the received signal strength threshold, and is causing the master module to be in the alarm status. The remote module ID over the signal strength is taken from a circular remote ID alarm list  989 . This embodiment illustrates the use of only one master module per system, so no master module unique identifier is necessary. Two or more master modules could be used in a single system, as illustrated in the preferred embodiment, in which case a master module unique identifier would be provided, as illustrated with the preferred embodiment. 
     The RF transmitter  966 , RF receiver  970 , the transmitting antenna  967 , and the receiving antenna  968  function the same as similar items disclosed for the remote module in  FIG. 16 . 
     The decoder  974  performs functions of message synchronization and data decoding. Incoming messages are checked for threat warning system ID. The message contents are made available to the decision logic processor  982  after the system ID has been verified by the decoder  974 , and this is signaled by a valid message indicator. 
     The Rx signal strength measurement sensor  976 , the threshold set point input  978 , and the comparator  980  function the same as similar units disclosed for the remote module in  FIG. 16 . 
     The alarm decision logic processor  982  processes the incoming message contents and the signal strength comparator  980  output, and determines if an alarm should be activated. Alarms can be activated in two ways. In one way, the alarm decision logic processor  982  samples the comparator  980  output during a valid message, and if the incoming radio message has a signal strength greater than the threshold set point, then an alarm is initiated, by signaling the alarm indication output  984 . Second, if the alarm status flag from the message indicates an active alarm present at a remote module, then an alarm condition is also signaled. If two or more master modules were used, as described above in the preferred embodiment, the receiving master module would also compare its own unique identifier with the unique identifier of the master module causing the remote module to go into alarm mode, before generating a “mutual reinforcement” alarm. The additional logic blocks required for the use of two or more master modules would be as shown in  FIGS. 3 and 4 . 
     The alarm indication is the same as disclosed for the remote. However, the alarm indicators can be more elaborate since the master module power supply  990  can draw AC power from the electrical system in the building. 
     The threat warning system ID unit  988  contains the unique digital address of the threat warning system. The threat warning system ID unit  988  assists in validating the radio message contents. 
       FIG. 17  also shows two signal protocols which will result in the generation of an alarm at the receiving master module. In Protocol J, a local alarm is generated if the received system ID matches the system ID of the master module, and the received signal strength is greater than a predetermined threshold. In Protocol K, a local alarm is generated if the received system ID matches the system ID of the master module, and if the transmitting remote module has an active alarm. If two or more master modules were to be used in a system in this embodiment, Protocol K would generate a local alarm at the receiving master module if the received system ID matches the system ID of the master module, if the transmitting remote module has an active alarm, and if the ID of the master module causing the remote module to alarm matches the ID of the receiving master module. 
       FIG. 18  shows basic radio message formats for the remote and master modules of the alternative fourth embodiment, which contains a synchronization pattern, a threat warning system ID field, an alarm status field, and a source ID field. In the signal from the remote unit to the master unit, the source ID field gives the unique identifier of the remote module transmitting the signal (the signal source ID), so that the master unit can identify any remote unit which approaches too close. In the signal from the master unit to the remote unit, the source ID field identifies any remote module which has approached too close and caused an alarm at the master module (the alarm source ID). If two or more master modules were to be used in a system in this embodiment, both the master module and the remote module would generate signals with both a signal source ID field and an alarm source ID field. 
     While the particular invention as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages hereinbefore stated, it is to be understood that this disclosure is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended other than as described in the appended claims.