Patent Publication Number: US-2013237182-A1

Title: Multi-hazard alarm system using selectable power-level transmission and localization

Description:
RELATED APPLICATIONS 
     This application is a continuation application that claims the benefit under 35 U.S.C. §120 of U.S. application Ser. No. 12/200,110 entitled “MULTI-HAZARD ALARM SYSTEM USING SELECTABLE POWER-LEVEL TRANSMISSION AND LOCALIZATION”, filed Aug. 28, 2008 which is a continuation that claims the benefit under 35 U.S.C. §120 of U.S. application Ser. No. 11/493,935 entitled “MULTI-HAZARD ALARM SYSTEM USING SELECTABLE POWER-LEVEL TRANSMISSION AND LOCALIZATION,” filed on Jul. 25, 2006, which is a continuation application that claims the benefit under 35 U.S.C. §120 of U.S. application Ser. No. 10/695,560, entitled “SELF-LOCATING ALARM SYSTEM EQUIPPED PARACHUTE,” filed on Oct. 27, 2003, which is a continuation-in-part of U.S. application Ser. No. 10/216,033, entitled “PORTABLE, SELF-LOCATING SMART DEFRIBILLATOR SYSTEM,” filed on Aug. 10, 2002, which is a continuation-in part of U.S. application Ser. No. 10/010,971, entitled “SELF-LOCATING ALARM SYSTEM EQUIPPED PARACHUTE,” filed on Dec. 4, 2001, which is a continuation-in-part of U.S. application Ser. No. 09/728,167, entitled “VOICE-ACTIVATED PERSONAL ALARM”, filed on Dec. 1, 2000, which is a continuation-in-part of U.S. application Ser. No. 09/325,030, entitled “SELF-LOCATING REMOTE MONITORING SYSTEMS,” filed on Jun. 3, 1999, which is a continuation of U.S. application Ser. No. 08/849,998, entitled “SELF-LOCATING REMOTE MONITORING SYSTEMS,” filed on Jul. 6, 1998, which is a U.S. National stage entry of PCT/US96/17473, filed on Oct. 28, 1996, which is a continuation-in-part of U.S. application Ser. No. 08/330,901, entitled “MULTI-HAZARD ALARM SYSTEM USING SELECTABLE POWER-LEVEL TRANSMISSION AND LOCALIZATION,” filed on Oct. 27, 1994, of which U.S. application Ser. No. 08/547,026, entitled “SELF-LOCATING REMOTE MONITORING SYSTEMS,” filed on Oct. 23, 1995, is a continuation-in-part. Each of these above-referenced patent applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates to personal alarm systems and in particular to such systems transmitting at a higher power level during emergencies. 
     2. Discussion of Related Art 
     Personal alarm systems are well known in the art (see for example U.S. Pat. Nos. 4,777,478, 5,025,247, 5,115,223, 4,952,928, 4,819,860, 4,899,135, 5,047,750, 4,785,291, 5,043,702, and 5,086,391). These systems are used to maintain surveillance of children. They are used to monitor the safety of employees involved in dangerous work at remote locations. They are even used to find lost or stolen vehicles and strayed pets. 
     These systems use radio technology to link a remote transmitting unit with a base receiving and monitoring station. The remote unit is usually equipped with one or more hazard sensors and is worn or attached to the person or thing to be monitored. When a hazard is detected, the remote unit transmits to the receiving base station where an operator can take appropriate action in responding to the hazard. 
     The use of personal alarm systems to monitor the activities of children has become increasingly popular. A caretaker attaches a small remote unit, no larger than a personal pager, to an outer garment of a small child. If the child wanders off or is confronted with a detectable hazard, the caretaker is immediately notified and can come to the child&#39;s aid. In at least one interesting application, a remote unit includes a receiver and an audible alarm which can be activated by a small hand-held transmitter. The alarm is attached to a small child. If the child wanders away in a large crowd, such as in a department store, the caretaker actives the audible alarm which then emits a sequence of “beeps” useful in locating the child in the same way one finds a car at a parking lot through the use of an auto alarm system. 
     A number of novel features have been included in personal alarm systems. Hirsh et al., U.S. Pat. No. 4,777,478, provide for a panic button to be activated by the child, or an alarm to be given if someone attempts to remove the remote unit from the child&#39;s clothing. Banks, U.S. Pat. No. 5,025,247 teaches a base station which latches an alarm condition so that failure of the remote unit, once having given the alarm, will not cause the alarm to turn off before help is summoned. Moody, U.S. Pat. No. 5,115,223 teaches use of orbiting satellites and triangulation to limit the area of a search for a remote unit which has initiated an alarm. In U.S. Pat. No. 4,952,928 to Carroll et al., and in U.S. Pat. No. 4,819,860 to Hargrove et al.; the apparatus provides for the remote monitoring of the vital signs of persons who are not confined to fixed locations. 
     Ghahariiran, U.S. Pat. No. 4,899,135, teaches a child monitoring device using radio or ultra-sonic frequency to give alarm if a child wanders out of range or falls into water. Hawthorne, U.S. Pat. No. 4,785,291, teaches a distance monitor for child surveillance in which a unit worn by the child includes a radio transmitter. As the child moves out of range, the received field strength, of a signal transmitted by the child&#39;s unit; falls below a limit and an alarm is given. 
     Clinical experience in the emergency rooms of our hospitals has taught that a limited number of common hazards account for a majority of the preventable injuries and deaths among our toddler age children. These hazards include the child&#39;s wandering away from a safe or supervised area, water emersion, fire, smoke inhalation, carbon monoxide poisoning and electrical shock. Child monitoring devices, such as those described above, have been effective in reducing the number of injuries and deaths related to these common preventable hazards. 
     However, considering the importance of our children&#39;s safety, there remains room for improvement of these systems. One such area for improvement relates to increasing the useful life of a battery used to power the remote unit of these toddler telemetry systems, as they have come to be called. 
     The remote unit is typically battery operated and, in the event of an emergency, continued and reliable transmission for use in status reporting and direction finding is of paramount importance. In other words, once the hazard is detected and the alarm given, it is essential that the remote unit continue to transmit so that direction finding devices can be used to locate the child. 
     The remote unit of most child monitoring systems is typically quite small and the available space for a battery is therefore quite limited. Despite recent advances in battery technology, the useful life of a battery is typically related to the battery size. For example, the larger “D” cell lasting considerably longer than the much smaller and lighter “AAA” cell. Though the use of very low power electronic circuits has made possible the use of smaller batteries, a battery&#39;s useful life is still very much a factor of its physical size, which, as stated above, is limited because of the small size of a typical remote unit. Therefore, additional efforts to reduce battery drain are important. 
     Given that much reliance is placed on the reliability of any child monitoring system, it would be desirable for the remote unit to transmit at a low power or not at all when no danger exists. In this way battery life is increased and system reliability is improved overall, since the hazards are usually the exception rather than the rule. 
     SUMMARY OF INVENTION 
     It is an object of the present invention to provide a personal alarm system in which the battery operated remote unit normally transmits at low power and switches to a higher power when the distance between the remote unit and base station exceeds a predetermined limit. 
     It is also an object of the present invention to provide such a system which includes sensors for the hazardous conditions typically confronting young children. 
     It is a further object of the present invention to provide such a personal alarm system which includes a periodic handshake exchange between the remote unit and base station to demonstrate that the system continues to be operational. 
     In accordance with the above objects and those that will become apparent below, a personal alarm system is provided, comprising: 
     a remote unit including radio transmitting means and radio receiving means;
 
the remote unit transmitting means being able to transmit at more than one power level and defining a higher power level;
 
a base station including radio transmitting means and radio receiving means;
 
the remote unit and the base station being in radio communication and defining a separation distance between the remote unit and the base station;
 
measuring means for determining whether the separation distance exceeds a predetermined limit;
 
means responsive to the measuring means for causing the remote unit transmitting means to transmit at the higher power level when the separation distance exceeds the limit; and alarm means for indicating when the separation distance exceeds the limit.
 
     In one embodiment of the invention, the base station transmits a periodic polling signal and the remote unit monitors the field strength of the received polling signal. If the received field strength falls below a limit, corresponding to some maximum distance between the two devices, the remote unit transmits at high power. The signal transmitted at high power includes an indication that transmission is at high power. When this signal is received by the base station, an alarm is given. The remote unit also is equipped to detect one or more hazards. 
     In another embodiment of the invention, there are multiple remote units each able to identify itself by including a unit identification number in its transmitted signal. The remote unit is equipped to detect one or more hazards and to identify detected hazards in its transmission. The base station is able to display the transmitting unit identification number and the type of any detected hazard. 
     In another embodiment, the base station, rather than the remote unit, measures the field strength of the received remote unit transmission and instructs the remote unit to transmit at high power when the received field strength falls below a preset limit. 
     In another embodiment) the remote unit includes both visual and audible beacons which can be activated by the base station for use in locating the child. 
     In another embodiment, the remote unit includes a panic button which the child or concerned person can use to summon help. 
     In another embodiment, the base station includes the ability to initiate a phone call via the public telephone system, for example by initiating a pager message to alert an absent caretaker. 
     In another embodiment, the remote unit includes a global positioning system (“GPS”) receiver which is activated if a hazard is detected or if the child wanders too far from the base station. The remote unit then transmits global positioning coordinates from the GPS receiver. These coordinates are received by the base station and used in locating the child. In an alternative embodiment, the remote unit is attached to a child, pet or vehicle and the GPS receiver is activated by command from the base station. The global positioning coordinates are then used by the base station operator to locate the remote unit. 
     In another embodiment, the remote unit is worn by an employee doing dangerous work at a remote location such as an electrical power lineman repairing a high voltage power line. The remote unit is equipped with a GPS receiver and an electrical shock hazard sensor and the remote unit will instantly transmit the workman&#39;s location in the event of electrical shock. The device will permit an emergency medical crew to rapidly find and give aid to the injured workman and possibly save a life. 
     It is an advantage of the present invention to periodically test system integrity by exchanging an electronic handshake and giving an alarm in the event of failure. 
     It is also an advantage of the present invention to prolong the remote unit battery life by transmission at low power in the absence of a defined emergency. 
     It is also an advantage of the present invention that the system is able to detect and give alarm for a number of common and dangerous hazards. 
     It is a further advantage of the present invention to permit rapid and precise location of the remote unit which is equipped with a GPS receiver. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       For a further understanding of the objects, features and advantages of the present invention, reference should be had to the following description of the preferred embodiment, taken in conjunction with the accompanying drawing, in which like parts are given like reference numerals and wherein: 
         FIG. 1  is a block diagram of a personal alarm system in accordance with one embodiment of the present invention and transmitting at selectable power levels. 
         FIG. 2  is a block diagram of another embodiment of the personal alarm system illustrated in  FIG. 1  including multiple remote units. 
         FIG. 3  is a block diagram illustrating another embodiment of the personal alarm system in accordance with the present invention. 
         FIG. 4  is a pictorial diagram illustrating a preferred message format used by the personal alarm system illustrated in  FIG. 2 . 
         FIG. 5  is a pictorial diagram illustrating another preferred message format used by the personal alarm system illustrated in  FIG. 2 . 
         FIG. 6  is a block diagram illustrating an embodiment of the personal alarm system of the present invention using the Global Positioning System to improve remote unit location finding. 
         FIG. 7  is a pictorial diagram illustrating a base station and remote unit of the personal alarm system of  FIG. 1 , in a typical child monitoring application. 
         FIG. 8  is a pictorial diagram illustrating a remote unit in accordance with the present invention being worn at the waist. 
         FIG. 9  is a pictorial diagram illustrating a mobile base station in accordance with the present invention for operation from a vehicle electrical system. 
         FIG. 10  is a pictorial diagram illustrating a base station in accordance with the present invention being operated from ordinary household power. 
         FIG. 11  is a block diagram illustrating a man-over-board alarm system in accordance with one aspect of the present invention. 
         FIG. 12  is a block diagram illustrating another embodiment of the man-over-board alarm system. 
         FIG. 13  is a block diagram illustrating an invisible fence monitoring system according to another aspect of the present invention. 
         FIG. 14  is a pictorial diagram illustrating a boundary defining a geographical region for use with the invisible fence system of  FIG. 13 . 
         FIG. 15  is another pictorial diagram illustrating a defined region having a closed boundary. 
         FIG. 16  is another pictorial diagram illustrating a defined region including defined subdivisions. 
         FIG. 17  is a block diagram illustrating another aspect of the invisible fence system. 
         FIG. 18  is a block diagram showing a fixed-location environmental sensing system according to another aspect of the present invention. 
         FIG. 19  is a block diagram of a personal alarm system including navigational location in which the geometric dilution of precision calculations are done at the base station. 
         FIG. 20  is a block diagram showing an invisible fence alarm system in which the fence is stored and compared at the base station. 
         FIG. 21  is a block diagram illustrating a man-over-board alarm system. 
         FIG. 22  is a partial block diagram illustrating a one-way voice channel on a man-over-board alarm system. 
         FIG. 23  is a partial block diagram illustrating a two-way voice channel on a man-over-board alarm system. 
         FIG. 24  is a block diagram illustrating an invisible fence system. 
         FIG. 25  is a pictorial diagram illustrating geographical regions for an invisible fence system. 
         FIG. 26  is a table defining a curfew for an invisible fence system. 
         FIG. 27  is a block diagram illustrating another embodiment of an invisible fence system. 
         FIG. 28  is a partial block diagram illustrating a base station connected to a communication channel via a modem. 
         FIG. 29  is a partial block diagram illustrating an alarm system including an oil/chemical sensor, and all sensors activating transmission at a higher power level. 
         FIG. 30  is a block diagram illustrating another embodiment of a personal alarm system. 
         FIG. 31  is a partial block diagram illustrating specific circuits used to select a transmission power level. 
         FIG. 32  is a partial block diagram illustrating other specific circuits used to select a transmission power level. 
         FIG. 33  is a block diagram illustrating a specific embodiment of a personal alarm system. 
         FIG. 34  is a block diagram illustrating a weather alarm system. 
         FIG. 35  is a pictorial diagram representing a specific embodiment of a weather region. 
         FIG. 36  is a pictorial diagram illustrating another specific embodiment of a weather region. 
         FIG. 37  is a partial block diagram illustrating a conditional activation of a navigational receiver for a weather alarm system. 
         FIG. 38  is a block diagram illustrating another specific embodiment of a weather alarm system. 
         FIG. 39  is a block diagram illustrating a specific embodiment of a remote monitoring unit. 
         FIG. 40  is a block diagram illustrating another specific embodiment of a remote monitoring unit. 
         FIG. 41  is a partial block diagram illustrating a plurality of sensors in a specific embodiment of a remote monitoring unit. 
         FIG. 42  is a partial pictorial diagram illustrating a typical status vector. 
         FIG. 43  is a partial block diagram illustrating an input device connected for providing the value of a second variable in a specific embodiment of the invention. 
         FIG. 44  is a block diagram illustrating a specific embodiment of a personal alarm system remote unit. 
         FIG. 45  is a block diagram illustrating a specific embodiment of a base station for use with a remote unit such as shown in  FIG. 44 . 
         FIG. 46  is a block diagram of a personal alarm system according to one aspect of the present invention. 
         FIG. 47  is a block diagram that illustrates another embodiment of a personal alarm system remote unit. 
         FIG. 48  is a partial block diagram that illustrates the use of a wireless phone within a personal alarm system remote unit according to a specific embodiment of the present invention. 
         FIG. 49  is a partial block diagram illustrating the wireless phone of  FIG. 48  and including a circuit that automatically dials “911” for transmitting the remote unit location. 
         FIG. 50  is a partial block diagram that illustrates the use of a cellular telephone for transmitting the remote unit location and for two-way radio communication. 
         FIG. 51  is a partial block diagram that illustrates the use of a PCS telephone for transmitting the remote unit location and for two-way radio communication. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , there is shown a block diagram of a personal alarm system according to one embodiment of the present invention and depicted generally by the numeral  10 . The personal alarm system  10  includes a remote unit  12  and a base station  14 . The remote unit  12  has a radio transmitter  16  and a receiver  18 , and the base station  14  has a radio transmitter  20  and a receiver  22 . The transmitters  16 ,  20  and receivers  18 ,  22  are compatible for two-way radio communication between the remote unit  12  and the base station  14 . 
     In a preferred embodiment, the base station  14  includes an interval timer  24  which causes the transmitter  20  to transmit at predetermined intervals. The receiver  18  of the remote unit  12  receives the signal transmitted by the base station  14  and causes the transmitter  16  to transmit a response to complete an electronic handshake. 
     The remote unit transmitter  16  is capable of transmitting at an energy conserving low-power level or at an emergency high-power level. When the distance between the remote unit  12  and the base station  14  exceeds a predetermined limit, the remote unit responds at the higher power level. 
     To accomplish the shift to the higher power level, the remote unit receiver  18  generates a signal  26  which is proportional to the field strength of the received signal, transmitted by the base station  14 . The remote unit  12  includes a comparator  28  which compares the magnitude of the field strength signal  26  with a predetermined limit value  30  and generates a control signal  32 . 
     The remote unit transmitter  16  is responsive to a circuit  34  for selecting transmission at either the low-power level or at the high-power level. The circuit  34  is connected to the control signal  32  and selects transmission at the low-power level when the received field strength equals or exceeds the limit value  30 , and at the higher power level when the received field strength is less than the limit value  30 . Alternatively, the remote unit transmitter  16  transmits at one of a selectable plurality of transmission power levels. In another alternative embodiment, transmission is selectable within a continuous range of transmission power levels. 
     Within an operating range of the personal alarm system  10 , the field strength of the base station  14  transmitted signal when received at the remote unit  12  is inversely proportional to the fourth power (approximately) of the distance between the two units. This distance defines a ‘separation distance,’ and the predetermined limit value  30  is selected to cause transmission at the higher power level at a desired separation distance within the operating range. 
     In another embodiment, the remote unit  12  includes a hazard sensor  36  which is connected to the transmitter  16 . The hazard sensor  36  is selected to detect one of the following common hazards, water immersion, fire, smoke, excessive carbon monoxide concentration, and electrical shock. In one embodiment, a detected hazard causes the remote unit  12  to transmit a signal reporting the existence of the hazardous condition at the moment the condition is detected. In another embodiment, the hazardous condition is reported when the response to the periodic electronic handshake occurs. 
     In one embodiment, the base station  14  includes an audible alarm  38  which is activated by the receiver  22 . If the remote unit fails to complete the electronic handshake or reports a detected hazard or indicates it is out of range by sending an appropriate code, the base station alarm  38  is activated to alert the operator. 
       FIG. 2  is a block diagram illustrating another embodiment of the personal alarm system of the present invention. The alarm system is indicated generally by the numeral  40  and includes a first remote unit  42 , a second remote unit  44  and a base station  46 . The first remote unit  42  includes a transmitter  48 , a receiver  50 , an identification number  52 , a received field strength signal  54 , a comparator  56 , a predetermined limit value  58 , a control signal  60 , a power level select circuit  62  and a hazard sensor  64 . 
     The second remote unit  44  includes a separate identification number  66 , but is otherwise identical to the first remote unit  42 . 
     The base station  46  includes a transmitter  68 , an interval timer  70 , a receiver  72 , an alarm  74  and an ID-Status display  76 . 
     In one embodiment of the invention illustrated in  FIG. 2 , the radio transmission between the first remote unit  42  and the base station  46  includes the identification number  52 . The transmission between the second remote unit  44  and the base station  46  includes the identification number  66 . It will be understood by those skilled in the art that the system may include one or more remote units, each having a different identification number  52 . 
     It will also be understood that each remote unit  42  may have a different predetermined limit value  58 , The limit value  58  defines a distance between the remote unit  42  and the base station  46  beyond which the remote unit will transmit at its higher power level. If a number of remote units are being used to monitor a group of children, in a school playground for example, the limit values of each remote unit may be set to a value which will cause high power transmission if the child wanders outside the playground area. In other applications, the limit value  58  of each remote unit  42  may be set to a different value corresponding to different distances at which the individual remote units will switch to high power transmission. 
     In one embodiment, the base station  46  will provide an alarm  74  whenever a remote unit transmits at high power or reports the detection of a hazard. The identification number of the reporting remote unit and an indication of the type of hazard is displayed by the base station on the ID-Status display  76 . This information can be used by the operator, for example a day-care provider, to decide what response is appropriate and whether immediate caretaker notification is required. If a child has merely wandered out of range, the provider may simply send an associate out to get the child and return her to the play area. On the other hand, a water immersion hazard indication should prompt immediate notification of caretakers and emergency personnel and immediate action by the day-care employees. 
     In another embodiment, the remote unit receiver  50  determines that the separation distance between the remote unit  42  and the base station  46  exceeds the predetermined threshold. The remote unit transmitter  48  transmits a code or status bit to indicate that fact. 
     In an embodiment illustrated in  FIG. 1 , the polling message transmitted periodically by the base station  14  is an RF carrier. The carrier frequency is transmitted until a response from the remote unit  12  is received or until a watchdog timer (not illustrated) times out, resulting in an alarm The information contained in the remote unit response must include whether transmission is at low power or at high power, and whether a hazard has been detected, since the base station provides an alarm in either of these instances. 
     In an embodiment illustrated in  FIG. 2 , however, additional information must be reported and the advantages of a digitally formatted remote unit response will be apparent to those possessing an ordinary level of skill in the art. 
       FIG. 3  is a block diagram illustrating another embodiment of the personal alarm system in accordance with the present invention and generally indicated by the numeral  80 . Personal alarm system  80  includes a remote unit  82  and a base station  84 . 
     The remote unit  82  includes a transmitter  86 , a receiver  88 , a power level select circuit  90 , an ID number  92 , a visual beacon  94 , an audible beacon  96 , a watchdog timer  98 , a plurality of hazard sensors  100  including a water immersion sensor  102 , a smoke sensor  104 , a heat sensor  106 , a carbon monoxide sensor  108 , a tamper switch  109 , and an electrical shock sensor  110 , an emergency switch (“panic button”)  112 , a battery  113 , and a ‘low battery power’ sensor  114 . 
     The base station  84  includes a transmitter  116 , a receiver  118  which produces a received field strength signal  120 , a comparator  122 , a predetermined limit value  124 , a comparator output signal  126 , an interval timer  128 , control signals  130  and  132 , a visual alarm  134 , an audible alarm  136 , an ID and Status display  138 , a circuit  140  for initiating a phone call and a connection  142  to the public telephone system. 
     The base station  84  and a plurality of the remote units  82  illustrated in the embodiment of  FIG. 3  communicate using a digitally formatted message. One message format is used by the base station  84  to command a specific remote unit  82 , and a second message format is used by a commanded remote unit  82  to respond to the base station  84 . These message formats are illustrated in  FIGS. 5 and 4 , respectively. 
     With reference to  FIG. 4  there is shown a pictorial diagram of a preferred digital format for a response from a remote unit in a personal alarm system in accordance with the present invention, indicated generally by the numeral  150 . The digital response format  150  includes a remote unit ID number  152 , a plurality of hazard sensor status bits  154  including a water immersion status bit  156 , a smoke sensor status bit  158 , a heat sensor status bit  160 , an excessive carbon monoxide concentration status bit  162 , and an electrical shock status bit  164 . The response  150  also includes a high power status bit,  166 , a panic button status bit  168 , a low battery power detector status bit  170 , a tamper switch status bit  171 , and bits reserved for future applications  172 . 
       FIG. 5  is a pictorial diagram of a preferred digital format for a base station to remote unit transmission, generally indicated by the numeral  180 . The digital message format  180  includes a command field  182  and a plurality of unassigned bits  190  reserved for a future application. The command field  182  includes a coded field of bits  184  used to command a specific remote unit to transmit its response message (using the format  150 ). The command field  182  also includes a single bit  186  used to command a remote unit, such as the embodiment illustrated in  FIG. 3 , to transmit at high power. The command field  182  includes command bit  188  used to command a remote unit to activate a beacon, such as the visual beacon  94  and the audible beacon  96  illustrated in  FIG. 3 . The command field  182  also includes command bit  189 , used to command a remote unit to activate a GPS receiver, such as illustrated in  FIG. 6 . 
     In an alternative embodiment, the remote unit transmitter is adapted to transmit at one of a plurality of transmission power levels and the single command bit  186  is replaced with a multi-bit command sub-field for selection of a power level. In another embodiment, the remote unit transmitter is adapted to transmit at a power level selected from a continuum of power levels and a multi-bit command sub-field is provided for the power level selection. 
     Again with respect to  FIG. 3 , the Base station  84  periodically polls each remote unit  82  by transmitting a command  180  requiring the remote unit  82  to respond with message format  150 . The polling is initiated by the interval timer  128  which causes the base station transmitter  116  to transmit the outgoing message  180 . The numerals  150  and  180  are used to designate both the format of a message and the transmitted message. A specific reference to the format or the transmitted message will be used when necessary for clarity. As is common in the communications industry, the message win sometimes be referred to as a ‘signal,’ at other times as a ‘transmission,’ and as a ‘message;’ a distinction between these will be made when necessary for clarity. 
     The message  180  is received by all remote units and the remote unit to which the message is directed (by the coded field  184 ) responds by transmitting its identification number  152  and current status, bits  154 - 170 . The remote unit identification number  92  is connected to the transmitter  86  for this purpose. 
     In the embodiment illustrated in  FIG. 3 , the function of measuring received field strength to determine whether a predetermined separation distance is exceeded is performed in the base station  84 . The base station receiver  118  provides a received field strength signal  120  which is connected to the comparator  122 . The predetermined limit value  124  is also connected to the comparator  122  which provides a comparator output signal  126 . If the received field strength  120  is less than the limit value  124 , the comparator output signal  126  is connected to assert the “go-to-high-power” command bit  186  in the base unit  84  outgoing message  180 . The limit value  124  is selected to establish the predetermined separation distance beyond which transmission at high power is commanded. 
     In one embodiment, the selection of the limit value  124  is accomplished by the manufacturer by entering the value into a read-only memory device. In another embodiment, the manufacturer uses manually operated switches to select the predetermined limit value  124 . In another embodiment, the manufacturer installs jumper wires to select the predetermined limit value  124 . In yet another embodiment, the user selects a predetermined limit value  124  using manually operated switches. 
     The remote unit transmitter  86  is capable of transmitting at a power-conserving lower power level and also at an emergency higher power level. Upon receiving a message  180  including the remote unit identification number  184 , the remote unit receiver passes the “go-to-high-power” command bit  186  to the power level select circuit  90  which is connected to command the remote unit transmitter  86  to transmit a response  150  at the higher power level. The response  150  includes status bit  166  used by the remote unit  82  to indicate that it is transmitting at high power. 
     In one embodiment, the remote unit includes the watchdog timer  98  (designated a ‘No Signal Timeout’) which is reset by the receiver  88  each time the remote unit  82  is polled. If no polling message  180  is received within the timeout period of the watchdog timer  98 , the remote unit transmitter  86  is commanded to transmit a non-polled message  150 . 
     In one embodiment of the invention, the remote unit  82  includes a manually operated switch (“panic button”)  112  which is connected to the transmitter  86  to command the transmission of a non-potted message  150 . The panic button status bit  168  is set in the outgoing message  150  to indicate to the base station  84  that the panic button has been depressed. Such a button can be used by a child or invalid or other concerned person to bring help. 
     In another embodiment, the remote unit includes a tamper switch  109  which is activated if the remote unit is removed from the child, or is otherwise tampered with. The activation of the tamper switch  109  causes the remote unit to transmit a code or status bit to the base unit to identify the cause of the change of status (‘Tamper’ status bit  171  illustrated in  FIG. 4 ). In one related alternative, the remote unit transmits at the higher power level when the switch is activated by removal of the remote unit from the child&#39;s person. 
     In another embodiment, the remote unit  82  includes a circuit  114  which monitors battery power. The circuit  114  is connected to initiate a non-polled message  150  if the circuit determines that battery power has fallen below a predetermined power threshold. The message  150  will include the “low-battery-power” status bit  170 . In an alternative embodiment, a low battery power level will initiate a remote unit transmission at the higher power level (see  FIG. 3 ). 
     In the embodiment illustrated in  FIG. 3 , the remote unit  82  includes several hazard sensors  100 . These sensors are connected to report the detection of common hazards and correspond to the sensor status bits  154  in the remote unit response message  150 . 
     In another embodiment of the present invention, the base station receiver  118  is connected to a visual alarm  134  and an audible alarm  136  and will give an alarm when a message  150  is received which includes any hazard sensor report  154  or any of the status bits  166 - 170 . 
     The base station  84  also includes the status and ID display  138  used to display the status of all remote units in the personal alarm system  80 . 
     In another embodiment of the personal alarm system  80 , the base station  84  includes a circuit  140  for initiating a telephone call when an emergency occurs. The circuit  140  includes the telephone numbers of persons to be notified in the event of an emergency. A connection  142  is provided to a public landline or cellular telephone system. The circuit  140  can place calls to personal paging devices, or alternatively place prerecorded telephone messages to emergency personnel, such as the standard “911” number. 
       FIG. 6  is a partial block diagram illustrating an embodiment of the invention having a base station  200  and at least one remote unit  202 . The partially illustrated remote unit  202  includes a transmitter  204 , hazard sensors  201 , 203 , 205 , a circuit  208  for causing the transmitter to transmit at a higher power level, a transmit interval timer  209 , and a Global Positioning System (‘GPS’) receiver  210 . The partially illustrated base station  200  includes a receiver  212 , an alarm  213 , a display  214  for displaying global positioning coordinates of longitude and latitude, a circuit  216  for converting the global positioning coordinates into predefined local coordinates, a map display  218  for displaying a map in the local coordinates and indicating the location of the remote unit  202 , and a watchdog timer  219 . 
     In a preferred embodiment of the alarm system, the remote unit transmitter  204  is connected to receive the global positioning coordinates from the GPS receiver  210  for transmission to the base station  200 . 
     The GPS receiver  210  determines its position and provides that position in global positioning coordinates to the transmitter  204 . The global position coordinates of the remote unit  202  are transmitted to the base station  200 . The base station receiver  212  provides the received global positioning coordinates on line  222  to display  214  and to coordinate converter  216 . The display  214  displays the global coordinates in a worldwide coordinate system such as longitude and latitude. 
     In one embodiment of the alarm system, the coordinate converter  216  receives the global positioning coordinates from line  222  and converts these into a preferred local coordinate system A display  218  receives the converted coordinates and displays the location of the remote unit  202  as a map for easy location of the transmitting remote unit  202 . 
     In another embodiment of the alarm system the GPS receiver  210  includes a low power standby mode and a normal operating mode. The GPS receiver  210  remains in the standby mode until a hazard is detected and then switches to the normal operating mode. 
     In another embodiment of the alarm system, the GPS receiver  210  remains in the standby mode until commanded by the base station  200  to enter the normal operating mode (see command bit  189  illustrated in  FIG. 5 ). 
     In another embodiment of the alarm system, the remote unit transmitter  204  is connected to the hazard sensors  201 - 205  for transmission of detected hazards. The base station receiver  212  is connected to activate the alarm  213  upon detection of a hazard. 
     In one embodiment, a conventional electrical shock sensor  205  includes a pair of electrical contacts  207  which are attached to the skin of a user for detection of electrical shock. 
     In another embodiment, the remote unit  202  includes a transmit interval timer  209  and an ID number  211 . The timer  209  is connected to cause the remote unit to transmit the ID number at predetermined intervals. The base station  200  includes a watchdog timer  219  adapted to activate the alarm  213  if the remote unit fails to transmit within the prescribed interval. 
     In another embodiment of the alarm system, the remote unit  202  includes a carbon monoxide concentration sensor (see  108  of  FIG. 3 ) having an output signal connected to activate a sensor status bit (see  162  of  FIG. 4 ) for transmission to the base station  200 . 
       FIGS. 7-10  are pictorial illustrations of alternative embodiments of the personal alarm system of the present invention.  FIG. 7  illustrates a base station  250  in two-way radio communication with a remote unit  252  worn by a child. The child is running away from the base station  250  such that the separation distance  256  has exceeded the preset threshold. The base station has determined that an alarm should be given, and an audible alarm  254  is being sounded to alert a responsible caretaker.  FIG. 8  illustrates a remote unit worn at the waist of a workman whose location and safety are being monitored.  FIG. 9  illustrates a mobile base station  270  equipped with a cigarette lighter adapter  272  for operation in a vehicle.  FIG. 10  illustrates a base station  280  adapted for operation from ordinary household current  282 . 
       FIG. 11  is a block diagram which illustrates a man-over-board system in accordance with one aspect of the present invention, and designated generally by the numeral  300 . 
     The man-over-board system  300  includes a remote unit  302 , having a navigational receiver  304  and antenna  306  for receiving navigational information, a sensor  308 , having an output signal  310 , a manually operated switch  312 , a radio transmitter  314  having an antenna  316 . The man-aver-board system  300  also includes a base station  318  having a radio receiver  320  connected to an antenna  322  for receiving radio transmissions from the remote unit  302 . The base station  318  also includes a display  324  for displaying the navigational location of the remote unit  302 , a display  326  for displaying the status of the sensor  308 , a circuit  328  for comparing the field strength of the received radio transmission with a predetermined limit  330 , and an alarm  332  which is activated when the received field strength  334  falls below the value of the limit  330 . 
     In use, the remote unit  302  is worn by a user and an alarm will be given if the user falls over board and drifts too far from the boat. The navigational receiver  304  receives navigational information, as for example from global positioning satellites  336 . The navigational receiver  304  converts the navigational information into a location of the remote unit  302  and outputs the location  338  to the radio transmitter  314  for transmission to the base station  318 . 
     The sensor  308  provides an output signal  310  and defines a sensor status. The output signal  310  is connected to the radio transmitter  314  for transmitting the sensor status to the base station  318 . 
     The manually operated switch  312  includes an output  340  which is connected to the radio transmitter  314  and permits the user to signal the base station  318  by operating the switch  312 . In a preferred embodiment, the manually operated switch  312  defines a panic button. 
     The radio receiver  320  provides three outputs, the received location  342  of the remote unit  302 , the received sensor status  344 , and an output signal  334  proportional to the field strength of the received radio transmission. As described above with respect to  FIGS. 1-3 , the remote unit  302  and the base station  318  define a separation distance which is inversely proportional to the received field strength. The comparator circuit  328  compares the received field strength  334  with a predetermined limit  330  and produces an output signal  346  if the sign of the comparison is negative, indicating that the field strength of the received signal is less than the limit  330 . If the user drifts beyond a separation distance from the boat defined by the limit  330 , the alarm  332  is activated to alert the user&#39;s companions, who can then take appropriate action. 
     In heavy seas or poor visibility, the base station  318  displays the current location of the remote unit  302  on a suitable display  324 . This is done in some appropriate coordinate system, such as standard longitude and latitude. This feature permits the base station to maintain contact with the man-over-board despite failure to maintain direct eye contact. 
       FIG. 12  is a block diagram which illustrates a man-over-board system including a two-way radio communication link and designated generally by the numeral  350 . The man-over-board system  350  includes a remote unit  352  and a base station  354 . 
     The remote unit  352  includes a navigational receiver  356 , a radio transmitter  358 , a circuit  360  for causing the radio transmitter  358  to transmit at a high power level, a radio receiver  362 , and circuits  364  for activating a beacon. 
     The base station  354  includes a radio receiver  366 , a radio transmitter  368 , a display  370  for displaying the location of the remote unit  352 , a compactor circuit  372 , a predetermined limit  374 , an alarm  376 , and control circuits  378  for activating the radio transmitter  368 . 
     The navigational receiver  356  is connected to an antenna  380  for receiving navigational information, such as from global positioning system satellites (not shown). The receiver provides the location  382  of the remote unit  352  for radio transmission to the base station  354 . 
     The remote unit radio transmitter  358  and radio receiver  362  are connected to an antenna  384  for communication with the base station  354 . The base station radio receiver  366  and radio transmitter  378  are connected to an antenna  386  for communication with the remote unit  352 . 
     The base station radio receiver  366  provides two outputs, the location  388  of the remote unit for display by the location display  370 , and a signal  390  whose value is inversely proportional to the field strength of the signal received by the radio receiver  366 . 
     The received field strength signal  390  and the predetermined limit  374  are compared by the comparator circuit  372  to determine whether the remote unit  352  is separated from the base station  354  by a distance greater than the predetermined limit  374 . An alarm  376  is given when the separation distance exceeds the limit. 
     The control circuits  378  are used to cause the radio transmitter  368  to send a control signal to the remote unit  352  for selecting high-power remote unit radio transmission, or activating a visual or audible beacon for use in locating the user in heavy seas or bad visibility. 
       FIG. 13  is a block diagram which illustrates an invisible fence for monitoring a movable subject and designated generally by the numeral  400 . The invisible fence  400  includes a remote unit  402  and a base station  404  in one-way radio communication. 
     The remote unit  402  includes a navigational receiver  406 , a radio transmitter  408 , storage circuits  410  for storing information defining a geographical region, a comparator  412 , second storage circuits  414  for storing information defining a predetermined positional status, an alarm  416 , and a circuit  418  and having a pair of electrical contacts  420 ,  422  for providing a mild electrical shock. 
     The base station  404  includes a radio receiver  424 , a comparator  426 , storage circuits  428  for storing information defining a predetermined positional status, and an alarm  430 . 
     In the embodiment illustrated in  FIG. 13 , the invisible fence  400  defines a geographical region, for example the outer perimeter of a nursing home in which elderly persons are cared for. If a particular patient tends to wander away from the facility, creating an unusual burden upon the staff the remote unit  402  is attached to the patient&#39;s clothing. If the patient wanders outside the defined perimeter, the base station  404  alerts the staff before the patient has time to wander too far from the nursing home. 
     Other applications are keeping a pet inside the yard, and applying a mild electrical shock to the pet if it wanders too close to a defined perimeter. Attaching the remote unit  402  to a child and alerting the caregiver in the event the child strays from a permitted area. Placing the remote unit around the ankle of a person on parole or probation and giving an alarm if the parolee strays from a permitted area. The invisible fence can also be used to monitor movement of inanimate objects whose locations may change as the result of theft. 
     The remote unit navigational receiver  406  provides the location  432  of the remote unit. In a preferred embodiment, the storage circuits  410  are implemented using ROM or RAM, as for example within an embedded microprocessor. Consideration of  FIGS. 14-16  is useful to an understanding of how the invisible fence operates. 
       FIGS. 14 ,  15  and  16  are pictorial diagrams illustrating boundaries used to define geographical regions such as those used in a preferred embodiment of the invisible fence  400 . 
       FIG. 14  shows a portion  440  of a city, including cross streets  442 - 454  and a defining boundary  456 . The boundary  456  divides the map  440  into two portions, one portion above boundary  456 , the other portion below. 
       FIG. 15  shows a portion  460  of a city, including cross streets (not numbered) and a closed boundary  462  made up of intersecting line segments  464 ,  466 , 468 , 470 , 472  and  474 . The boundary  462  divides the city map  460  into two sub regions, one sub region defining an area  490  wholly within the boundary  462 , and the other sub region defining an area  492  outside the boundary  462 . 
       FIG. 16  shows a geographical region  480  which includes sub regions  482  and  484 . Sub region  482  is entirely surrounded by sub region  484 , while sub region  484  is enclosed within a pair of concentric closed boundaries  486  and  488 . 
     The information which defines these geographical regions and boundaries is stored in the storage circuits  410 , and serve as one input to the comparator  412  ( FIG. 13 ). The comparator  412  also receives the location output  432  from the navigational receiver  406 . The comparator  412  compares the location of the remote unit  402  with the defined geographical region and defines a relationship between the location and the defined region which is expressed as a positional status. The comparator  412  also receives an input from the second storage circuits  414 . These circuits store information defining a predetermined positional status. 
     Some examples will be useful in explaining how the positional status is used. Referring to  FIG. 14 , remote unit locations  494  and  496  are illustrated as dots, one location  494  being above the boundary  456 , the other location  496  being below the boundary. 
     For the first example, assume that the location  494  is 1I within a defined geographical region,” and that the location  496  is 1I outside the defined geographical region.” Assume also that the predetermined positional status is that “locations within the defined region are acceptable.” Next assume that the navigational receiver  406  reports the location  494  for the remote unit. Then the comparator  412  will define a positional status that “the location of the remote unit relative to the defined region is acceptable.” This positional status will be transmitted to the base station  404  and will not result in activation of the alarm  430 . 
     For the next example, assume that the navigational receiver  406  reports the location of the remote unit to be the location  496 , and that the other assumptions remain the same. Then the comparator  412  will define a positional status that “the location of the remote unit relative to the defined region is not acceptable.” This positional status will be transmitted to the base station  404  and will result in activation of the alarm  430 . 
     For the next example refer to  FIG. 16  which includes three successive locations  498 ,  500  and  502 , shown linked by a broken line, as for example by movement of the remote unit  402  from location  498  to location  500  to location  502 . Assume that the area outside the boundary  488  defines an “acceptable” sub region. Assume further that the area between the boundaries  488  and  486  defines a t1 waming” sub region. Also assume that the area  482  inside the boundary  486  defines a “prohibited” sub region. Finally, assume that the navigational receiver  406  provides three successive locations  498 ,  500  and  502 . 
     In a preferred embodiment, and given these assumptions in the preceding paragraph, the comparator  412  will determine that the location  498  is acceptable and will take no further action. The comparator  412  will determine that the location  500  is within the warning sub region  484  and will activate the remote unit alarm  416  to warn the person whose movements are being monitored that he has entered a warning zone. When the remote unit  402  arrives at the location  502 , the comparator  412  will determine that the remote unit has entered a prohibited zone and will activate the mild electric shock circuit  418  which makes contact with the skin of the monitored person through the electrical contacts  420 ,  422 . The positional status reported by the remote unit  402  for the successive locations  498 , 500  and  502  is “acceptable,” “warning given,” and “enforcement necessary,” respectively. 
     In another embodiment, no enforcement or warning are given by the remote unit  402 . Instead, as when used to monitor the movements of children or elderly patients, the positional status is transmitted to the base station  404 . There it is compared with a stored predetermined positional status and used to set an alarm  430  if the positional status is not acceptable. The predetermined positional status is stored in storage circuits  428  and the comparison is made by the comparator  426 . 
     The preferred embodiment for the storage and comparison circuits is the use of an embedded microprocessor. 
       FIG. 17  is a block diagram illustrating a personal alarm system such as the invisible fence of  FIG. 13 , and designated generally by the numeral  520 . Personal alarm system  520  includes a remote unit  522  and a base station  524 . 
     The remote unit  522  includes a radio transmitter  526  and a radio receiver  528  connected to a shared antenna  530 . The base station  524  includes a radio receiver  532  and a radio transmitter  534  connected to a shared antenna  536  and defining a two-way communication link with the remote unit  522 . 
     In one preferred embodiment, the communication link is direct between the respective transmitters  526 ,  534  and the corresponding receivers  528 ,  532 . Other embodiments include access to existing commercial and private communications networks for completing the communication link between the remote unit  522  and the base station  524 . Typical networks include a cellular telephone network  538 , a wireless communications network  540 , and a radio relay network  542 . 
       FIG. 18  is a block diagram showing an environmental monitoring system for use in fixed locations, designated generally by the numeral  550 . The environmental monitoring system  550  includes a remote unit  552  and a base station  554 . 
     The remote unit  552  includes storage circuits  556  for storing information defining the location of the remote unit  552 , at least one sensor  558 , a radio transmitter  560 , and an antenna  562 . 
     The base station  554  includes an antenna  564 , a radio receiver  566 , a display  568  for displaying the location of the remote unit  552 , a comparator  570 , storage circuits  572  for storing information defining a predetermined sensor status, and an alarm  574 . 
     The environmental monitoring system  550  is useful for applications in which the remote unit  552  remains in a fixed location which can be loaded into the storage circuits  556  when the remote unit  552  is activated. Such applications would include use in forests for fire perimeter monitoring in which the sensor  558  was a heat sensor, or in monitoring for oil spills when attached to a fixed buoy and the sensor  558  detecting oil. Other useful applications include any application in which the location is known at the time of activation and in which some physical parameter is to be measured or detected, such as smoke, motion, and mechanical stress. The environmental monitoring system  550  offers an alternative to pre-assigned remote unit ID numbers, such as those used in the systems illustrated in  FIGS. 2 and 3 . 
     The storage circuits  556  provide an output  576  defining the location of the remote unit  552 . This output is connected to the radio transmitter  560  for communication with the base station  554 . The sensor  558  provides an output signal  578  defining a sensor status. The output signal is connected to the radio transmitter  560  for communication of the sensor status to the base station  554 . 
     The communications are received by the base station&#39;s radio receiver  566  which provides outputs representing both the location  580  of the remote unit  552  and the sensor status  582 . The location  580  is connected to the display  568  so that the location of the remote unit  552  can be displayed. The comparator  570  receives the sensor status  582  and the information defining the predetermined sensor status which is stored in the storage circuits  572 . If the comparator  570  determines that the sensor status indicates an alarm situation, it activates the alarm  574  to alert a base station operator. 
       FIG. 19  is a block diagram which illustrates an alternative embodiment of a personal alarm system in which the remote unit transmits demodulated navigational and precise time-of-day information to the base station, and the base station uses that information to compute the location of the remote unit. This alternative embodiment is designated generally by the numeral  600  and includes a remote unit  602  and a base station  604 . 
     The remote unit  602  includes a navigational receiver  606 , a demodulator circuit  608 , a precise time-of-day circuit  610 , a sensor  612 , and a radio transmitter  614 . 
     The base station  604  includes a radio receiver  616 , computational circuits  618  for computing the location of the remote unit  602 , a display  620  for displaying the computed location, a second display (can be part of the first display)  622  for displaying a sensor status, a comparator  624 , storage circuits  626  for storing information defining a predetermined sensor status, and an alarm  628 . 
     In a preferred embodiment, the navigational receiver  606  receives navigational information from global positioning system satellites (not shown). In this embodiment, the raw navigational information is demodulated by the demodulator circuit  608  and the output of the demodulator  608  is connected to the radio transmitter  614  for communication to the base station  604 . 
     The precise time-of-day circuits  610  provide the time-of-day information needed to compute the actual location of the remote unit based upon the demodulated navigational information. In the case of GPS navigational information, geometric dilution of precision computations are done at the base station  604  to derive the actual location of the remote unit  602 . 
     The sensor  612  provides an output signal defining a sensor status. The demodulated navigational information, the precise time-of-day information and the sensor status are all connected to the radio transmitter  614  for communication to the base station  604 . 
     At the base station  604 , the radio receiver  616  provides the navigational and precise time-of-day information to the computation circuits  618  for determining the actual location. In a preferred embodiment, the computation is made using an embedded microprocessor. The computed location is displayed using the display  620 . 
     The radio receiver  616  also provides the received sensor status which forms one input to the comparator  624 . Stored information defining a predetermined sensor status is provides by the storage circuits  626  as a second input to the comparator  624 . If the received sensor status and the stored sensor status do not agree, the comparator  624  activates the alarm  628  to alert the base station operator. 
       FIG. 20  is a block diagram which illustrates an alternative embodiment of the invisible fence system in which the base station computes the location of the remote unit, and in which the fence definitions are stored at the base station rather than in the remote unit. The alternative system is designated generally by the numeral  650  and includes a remote unit  652  and a base station  654 . 
     The remote unit  652  includes a navigational receiver  656 , a demodulator circuit  658 , a precise time-of-day circuit  660 , a radio transmitter  662 , a radio receiver  664 , a shared antenna  666 , and control status circuits  668 . 
     The base station  654  includes a radio receiver  670 , a radio transmitter  672 , a shared antenna  674 , computation circuits  676 , storage circuits  678 , second storage circuits  680 , a first comparator  682 , a second comparator  684 , a display  686 , an alarm  688 , and control circuits  690 . 
     The navigational receiver  656  provides raw navigational information  692  to the demodulator circuit  658 . The demodulator circuit  658  demodulates the raw navigational information and provides demodulated navigational information  694  to the radio transmitter  662  for communication to the base station  654 , The precise time-of-day circuit  660  provides time-of-day information  696  to the radio transmitter  662  for communication to the base station  654 . 
     The base station radio receiver  670  provides received navigational information  698  and received time-of-day information  700  to the computation circuits  676  for conversion to an actual location  702  of the remote unit  652 . The storage circuits  678  store information defining a geographical region. 
     The first comparator  682  receives the location  702  and the region defining information  704  and provides a positional status  706 , as described above with respect to  FIGS. 13-16 . 
     The second storage circuits  680  store information  708  defining a predetermined positional status. The second comparator  684  receives the positional status  706  and the predetermined positional status  708  and provides control output signals  710  based upon the results of the positional status comparison. When the location  702  is within a defined “warning” or “restricted” zone, the second comparator  684  activates the alarm  688  and causes the location  702  to be displayed by the display  686 . 
     In one preferred embodiment, the remote unit includes circuits  668  which provide a means by which the base station  654  can warn the remote unit user or enforce a restriction, as for example, by applying the mild electric shock of the embodiment shown in FIG. B. The second comparator  684  uses a control signal  710  to activate the control circuits  690  to send a command via the radio transmitter  672  to the remote unit  652  for modifying the remote unit control status. For example, if the remote unit location is within a restricted zone, the base station  654  will command the remote unit  652  to provide an electric shock to enforce the restriction. 
       FIG. 21  is a block diagram illustrating another embodiment of a man-over-board alarm system, designated generally by the numeral  750 . The man-over-board alarm system  750  includes a remote unit  752  and a base station  754 . 
     The remote unit  752  includes a navigational receiver  756 , a radio transmitter  758 , an environmental sensor  760 , at least one manually operated switch  762 , a beacon  764 , a circuit  766  for activating the navigational receiver  756 , and a control circuit  768 . 
     The base station  754  includes a radio receiver  770 , a remote-unit location display  772 , a sensor status display  774 , an alarm  776 , a switch status display  778 , a control circuit  780 , and storage  782  for a predetermined limit value. 
     The navigational receiver  756  receives navigational information via an antenna  757  and provides a location  759  of the remote unit to the radio transmitter  758  for transmitting the remote unit location  759 . The navigational receiver  756  has a normal operational mode and a low-power standby mode. In a preferred embodiment, the navigational receiver  756  is normally in the low-power standby mode, thereby conserving operating power which is normally supplied by batteries. 
     The circuit  766  is responsive to the control circuit  768  for selecting the operational mode and thereby “activating” the navigational receiver. In a specific embodiment, the control circuit  768  is responsive to a hazard sensor  760 , such as a water-immersion sensor, for controlling the circuit  766  to activate the navigational receiver  756 . In another embodiment, the control circuit  768  is responsive to a manually operated switch  762 , such as a manually operated panic button, for activating the navigational receiver  756 . 
     In a specific embodiment, the sensor  760  provides an output signal  761 ˜ and defines a sensor status. The manually operated switch  762  provides an output signal  763 , and defines a switch status. The control circuit  768  receives the sensor output signal  761  and the switch output signal  763 , and connects each to the radio transmitter  758  for communication of the sensor status and the switch status to the base station  754 . 
     In another specific embodiment, the control circuit  768  is connected for activating the remote unit beacon  764  in response to a change in the sensor status  761 . In another embodiment, the control circuit  768  activates the beacon  764  in response to a change in the switch status  763 . In one embodiment, the beacon  764  is a visual beacon, such as a flashing light. In another embodiment, the beacon  764  is an audible beacon which emits a periodic sound. The beacon  764  aids searchers in locating a man-over-board. 
     In a specific embodiment, the control circuit  768  is implemented using a programmed micro-processor. In another specific embodiment, the control circuit  768  is implemented using an imbedded, programmed micro-processor. In another embodiment, the control circuit  768  is implemented using a programmed micro-controller. 
     The base-station radio receiver  770  receives the remote unit location  759 , the sensor status, and the switch status. The radio receiver  770  is connected to the display  772  for displaying the received remote unit location, is connected to the display  774  for displaying the received sensor status, and is connected to the display  778  for displaying the switch status. In a specific embodiment, the radio receiver  770  is connected to the alarm  776  which is activated by a change in the sensor status, such as the detection of immersion in water. In another specific embodiment, the alarm is activated by a change in the switch status, such as a manual operation of the panic button. 
     The radio receiver  770  provides a signal  771  corresponding to a field strength of a received radio communication. The control circuit  780  compares the received field strength  771  with a predetermined limit value  783  provided by circuit  782 . The control circuit  780  is connected to activate the alarm  776  when the received field strength is less than the predetermined limit value  783 . The received field strength  771 , the control circuit  780 , and the predetermined limit value  783  define a separation distance between the remote unit  752  and the base station  754 , as discussed above with respect to other embodiments of the invention. 
     In a specific embodiment, the control circuit  780  and the circuit  782  for providing the predetermined limit value  783  are implemented using a programmed micro-controller. In another specific embodiment, the circuit  780  and the circuit  782  are implemented using an embedded, programmed micro-controller. The functions performed by the circuits  780  and  782  are performed in different embodiments alternatively by discrete integrated circuits, by a programmed micro-controller, by an embedded, programmed microcontroller, by a programmed micro-processor, and by an embedded, programmed microprocessor. 
     In a specific embodiment of the man-over-board alarm system illustrated in  FIG. 21 , the sensor  760  includes a plurality of environmental, physiological and hazard sensors providing output signals and defining a sensor status vector. In another specific embodiment, the sensor  760  provides a plurality of output signals  761  defining another status vector. In another specific embodiment, the sensor  760  provides an analog output signal  761 , and the control circuit  768  converts the analog signal  761  for radio transmission as a sensor status vector. The base station  754  displays the sensor status vector using the display  774 . 
     In another specific embodiment of the man-over-board alarm system illustrated in  FIG. 21 , the manually operated switch  762  includes a plurality of manually operated switches providing multiple output signals  763 . The multiple output signals  763  define a switch status vector which is connected to the control circuit  768  for radio transmission to the base station  754 . The base station  754  displays the switch status vector using the display  778 . In a specific embodiment, the remote unit manually operated switches  762  define a numeric keypad, and the base station  754  displays a manual entry made using the numeric keypad. In another specific embodiment, the manually operated switches  762  define an alpha numeric keypad, and the base station  754  displays manually entered alpha numeric information. 
       FIG. 22  is a partial block diagram of the man-over-board alarm system illustrated in  FIG. 21 , and designated generally by the numeral  800 . The alarm system  800  includes a remote unit  802  and a base station  804 . The remote unit  802  includes a radio transmitter  806  and a microphone  808 . The base station  804  includes a radio receiver  810  and a speaker  812 . In this embodiment of the alarm system  800 , the microphone  808  is connected to the transmitter  806  for defining a one-way voice radio communication channel with the base station receiver  810  and speaker  812 . In a specific embodiment, the radio transmitter  806  is also used to transmit the remote unit location, the sensor status vector, and the switch status vector as discussed above with respect to  FIG. 21 . In another specific embodiment, the radio receiver  810  is also used to receive the remote unit location, the sensor status vector, the switch status vector, and to provide the received signal strength signal. 
       FIG. 23  is also a partial block diagram of the man-over-board alarm system shown in  FIG. 21 . The alarm system is designated generally by the numeral  814 . The alarm system  814  includes a remote unit  816  and a base station  818 . The remote unit  816  includes a radio transmitter  820 , a microphone  822 , a radio receiver  824  and a speaker  826 . The base station  818  includes a radio receiver  828 , a speaker  830 , a radio transmitter  832  and a microphone  834 . These elements are configured to provide a two-way voice communication channel between the remote unit  816  and the base station  818 . In a specific embodiment, the radio transmitter  820  and radio receiver  828  are also used to communicate the remote unit location, the sensor status vector, and the switch status vector. In another specific embodiment, the radio receiver  828  also provides a received signal strength signal. 
       FIG. 24  is a block diagram illustrating another embodiment of an invisible fence system, designated generally by the numeral  850 . The invisible fence system  850  includes a remote unit  852  and a base station  854 . 
     The remote unit  852  includes a navigational receiver  856 , a radio transmitter  858 , a memory  860  for storing information defining a geographic region, a memory  862  for storing information defining a predetermined positional and time status, a circuit  863  for providing time-of-day information, a comparison circuit  864 , and an enforcement and alarm circuit  865 . 
     The base station  854  includes a radio receiver  866 , a memory  868  for storing a predetermined positional and time status, a comparison circuit  870  and an alarm  872 . 
     The invisible fence system illustrated in  FIG. 24  differs from the embodiment of  FIG. 13  by providing an alarm and enforcement based upon both time and location. The embodiment of  FIG. 24  allows the defining of zones of inclusion, and alternatively zones of exclusion, which are defined in terms of location and time-of-day. For example, a parolee equipped with the remote unit  852  may be confined to, and alternatively excluded from, a defined region between the hours of 6 PM and 6 AM. If the parolee leaves the region of confinement, or enters the region of exclusion, between those two time limits, a radio transmission activates the alarm  872  at the base station  854 , and simultaneously activates an alarm and enforcement process  865  at the remote unit  852 . In a specific embodiment, the parolee is first warned that he has left a region of confinement at an un-allowed time. If the violation continues, the parolee is given a mild electrical shock. If the violation continues, the intensity of the electrical shock is increased. The authorities are put on notice by the base station alarm  872  that the parolee has violated his defined restrictions. 
       FIG. 25  is a pictorial diagram illustrating boundaries used to define geographical regions such as those used in a preferred embodiment of the invisible fence system  850 .  FIG. 25  shows a portion  1000  of a city, including cross streets (not numbered) and a closed boundary made up of intersecting line segments  1006 ,  1008 ,  1010  and  1012 . The boundary divides the city map  1000  into two sub regions, one sub region defining an area  1002  wholly within the boundary, and the other sub region defining an area  1004  outside the boundary. 
     In a specific embodiment of an invisible fence system, such as that illustrated in  FIG. 24 , a memory  860  stores information defining a geographical region, for example the region  1002 . In an example of the operation of the specific embodiment, assume the region  1002  represents a specific city block, surrounded by the city streets  1006 ,  1008 ,  1010  and  1012 . Further assume that a parolee is wearing the remote unit  852 , and that the parolee is required by the terms of his parole to remain within the city block  1002  between the hours of 8 PM and 7 AM, and that at all other times the parolee is permitted to be outside the region  1002 . 
       FIG. 26  is a table defining a relationship between the location of the remote unit  852  ( FIG. 24 ) and the time-of-day for use in understanding a curfew feature of a specific embodiment of the invisible fence system  850 . Each row of the table represents a different location, and each column of the table represents a subdivision of the time-of-day. The relationship defined by the table represents an example of a curfew requiring the parolee (in the preceding example) to remain at home, i.e., within the city block  1002 , between 8 PM and 7 AM. If the parolee leaves home during the interval from 8 PM to 7 AM, an alarm  872  is activated at the base station  854 . The information represented by the table is stored in a memory  862  in the remote unit  852 , and is referred to as a predetermined positional and time status. 
     With respect to the specific embodiment illustrated in  FIG. 24 , the memory  860  stores information defining the geographical region  1002  ( FIG. 25 ). The comparison circuit  864  receives the remote unit location  859 , the time-of-day  861 . the information defining the geographical region  1002 , and the curfew defining information  867 . The comparison circuit  864  compares the named items of information and provides a positional and time status  869  to the radio transmitter  858  for communication to the base station  854 . In another embodiment of the invisible fence system  850 , the transmitter  858  periodically transmits the remote unit location  859  and time-of-day  861 . This information is received at the base station  854  where the predetermined positional and time status is stored in a memory  868 . The base station  854  makes an independent determination of whether or not the curfew is violated. The positional and time status is compared by circuit  870  with the received location and time-of-day information. An alarm  872  is given if the remote unit violates the established curfew. 
       FIG. 27  is a block diagram illustrating another embodiment of an invisible fence system, designated generally by the numeral  1020 . The invisible fence system  1020  includes a remote unit  1022  and a base station  1024 . The remote unit  1022  includes a navigational receiver  1026 , a radio transmitter  1028 , a radio receiver  1030  and an enforcement and alarm circuit  1032 . The base station  1024  includes a radio receiver  1034 , a radio transmitter  1036 , a memory  1040  for storing information defining a geographical region, a memory  1042  for storing information defining a predetermined positional and time status, a display  1044  and an alarm  1046 . 
     The navigational receiver  1026  provides information  1027  defining a location of the remote unit  1022 , and is connected to the remote unit radio transmitter  1028  for communicating the remote unit location to the base station  1024 . The transmitted remote unit location is received by the base station radio receiver  1034  and provided on line  1035  to the control/compare circuit  1038 . The base station includes a circuit  1037  for providing time-of-day information  1039  to the control/compare circuit  1038 . 
     In a specific embodiment, the control/compare circuit  1038  is implemented as part of a programmed, imbedded micro-processor/micro-controller. A memory of the imbedded micro-processor provides the memory  1040  for storage of information  1041  defining a geographical region, and the memory  1042  for storage of information  1043  defining a predetermined positional and time status. The imbedded micro-processor implementation of the control/compare circuit  1038  receives the remote unit location  1035 , the time-of-day  1039 , the information  1041  defining a geographical region, and the information  1043  defining a predetermined positional and time status. 
     In the previous example, the defined geographical region corresponded to the region  1002  ( FIG. 25 ), and the predetermined positional and time status corresponded to the relationship defined by the table in  FIG. 26 . The parolee was required to be within the region  1002  between the hours of 8 PM and 7 AM. The compare/control circuit  1038  compares the received information described above and determines whether the parolee is in violation of the defined curfew. The parolee is in violation of the curfew defined by the table in  FIG. 26  when he is outside his home between the hours of 8 PM and 7 AM. In this example, the region  1002  ( FIG. 25 ) corresponds to the parolee&#39;s home. Locations outside region  1002  are therefore outside his home. In this example, if the parolee is in violation of the curfew, the control/compare circuit  1038  generates a signal  1045 , connected to the base station radio transmitter  1036  for activating an alarm/enforcement device  1032  at the remote unit  1022 . Such a device and an alarm/enforcement protocol have been described above with respect to  FIGS. 13 and 16 . 
     In a specific embodiment of the invisible fence system shown in  FIG. 27 , the location of the remote unit is displayed  1044  at the base station  1024 . In one embodiment, the control/compare circuit  1038  continuously displays the remote unit location. In another embodiment, the control/compare circuit  1038  provides and alarm  1046  and displays the remote unit location when the parolee has violated the curfew. 
     In a specific embodiment of the invisible fence system of  FIG. 27 , the time-of-day circuit  1037  is implemented as part of the imbedded microprocessor. When several remote units are transmitting their locations from different time zones, the base station time-of-day is adjusted at the base station to use the correct time-of-day for each transmitting remote unit. For a curfew type process, it is not necessary generally to use a precise time-of-day. However, when a precise time-of-day is required, the remote unit transmitter is connected to receive both a location and a precise time-of-day from the navigational receiver, or other precise time-of-day circuit, for transmission to the base station. Such arrangements are illustrated in  FIGS. 19 ,  20 ,  34  and  36 . 
       FIG. 28  is a partial block diagram illustrating an alarm system, designated generally by the numeral  1050 . The alarm system  1050  includes a remote unit  1052  and a base station  1054  and is intended to be representative of many of the alarm systems in accordance with aspects of this invention. The remote unit  1052  includes a radio transmitter  1056  and a radio receiver  1058 . The base station  1054  includes a modem  1060 . Through its modem  1060 , the base station  1054  is connected to a standard communications channel, designated  1064  and a two-way radio link  1062 , permitting a two-way communication between the base station  1054  and the remote unit  1052 . 
     Such an arrangement provides a radio link for communicating with the remote unit  1052  while not requiring the base station  1054  to include the necessary radio receiver and radio transmitter. In such a case, the base station includes a communications receiver and a communications transmitter which in one embodiment includes a radio communications facility and in another embodiment provides the modem capability. The modem  1060  permits the base station to be connected via standard land line communications, such as a commercial telephone network. Thus the standard communication channel  1064  includes a standard telephone network, communications satellites, relay type radio links and other common carrier technologies such as cellular telephone, wireless communications, and personal communications systems (“PCS”). 
       FIG. 29  is a partial block diagram illustrating an alternative embodiment of the personal alarm system  80  as depicted in  FIG. 3 . Parts shown in  FIG. 29  which correspond to parts shown in  FIG. 3  have the same identification numerals. 
       FIG. 29  illustrates a radio transmitter  86 , a circuit  90  for selecting a transmission power level for the transmitter  86 . An oil/chemical sensor  113  is added to the hazard sensors  100 . Each sensor provides an output signal defining a sensor status. The sensor status of all sensors is connected via a line  117  to the transmitter  86  for transmission of the sensor status. The output of each sensor  100  is connected via line  117  to the selection circuit  90  for selecting a transmission power level. The transmitter  86  normally operates at a reduced power level to conserve battery power. When a hazard sensor  100  detects a hazardous condition, the line  117  communicates that fact to the circuit  90  which causes the transmitter  86  to transmit at a higher power level. 
       FIG. 30  is a block diagram illustrating a specific embodiment of a personal alarm system, designated generally by the numeral  1080 , and including a remote unit  1082  and a base station  1084 . The remote unit  1082  includes a radio transmitter  1086 , a radio receiver  1088 , a control circuit  1090 , a transmission power level selection circuit  1092  and a sensor  1094 . The base station  1084  includes a radio receiver  1096 , a radio transmitter  1098 , an alarm  1100  and a higher power level command circuit  1102 . 
       FIG. 30  illustrates a system in which a sensor status  1095  is transmitted to the base station  1084  and generates an alarm  1100 . The command circuit  1102  is responsive to the received sensor status and causes the base station transmitter  1098  to transmit a command to the remote unit  1082  causing the remote unit to transmit at a higher power level. The command is received by the remote unit receiver  1088  and is interpreted by the control circuit  1090  to select a higher power transmission level  1092 . 
       FIG. 31  is a partial block diagram illustrating a circuit  1130  including an analog-to-digital converter  1132  and a read-only memory  1134 . The analog-to-digital converter  1132  receives an analog input signal  1131  and provides digital output signals  1133 . The digital output signals  1133  are connected to address input lines of the read-only-memory  1134 . The read-only-memory provides digital output signals of stored information from an addressed memory location on output lines  1135 . 
     The circuit shown in  FIG. 31  is used to convert a received field strength signal such as signal  771  in the base station  754  of  FIG. 21 , to a predetermined digital output vector on lines  1135 . 
       FIG. 32  is a partial block diagram illustrating a digital-to-analog converter  1140 . The digital-to-analog converter  1140  receives digital input signals on lines  1141  and provides an analog output signal on line  1142 . 
       FIG. 33  is a block diagram illustrating an embodiment of a personal alarm system designated generally by the numeral  1150 , and including a remote unit  1152  and a base station  1154 . The remote unit  1152  includes a radio transmitter  1156 , a radio receiver  1158 , a circuit  1160  for selecting transmission power level and a sensor  1162 . The base station  1154  includes a radio receiver  1164 , a radio transmitter  1166 , an alarm  1168  and a command control circuit  1170 . The digital-to-analog converter illustrated in  FIG. 32  is used in a specific embodiment of the circuit  1160  of  FIG. 33  for selecting one of a plurality of transmission power levels, as commanded by the base station. The base station receiver  1164  provides a signal  1165  proportional to a received field strength. In a specific embodiment, the signal  1165  is an analog signal and is converted to a digital form using the conversion circuit  1130  of  FIG. 31 . The digital output signals  1135  are used by the command control circuit  1170  to generate a power-level command  1171  for transmission to the remote unit  1152 . In one embodiment of the remote unit select power level circuit  1160 , the received digital power-level command is used directly to control the power level of the remote unit transmitter  1156 . In another embodiment, the received power-level command is converted to an analog signal which is used to control the power level of the remote unit transmitter  1156 . In this manner, the alarm system is able to compensate for an increase in separation distance, low remote unit battery power or other conditions which cause the received signal strength  1165  to be reduced. The circuits are also able to command a reduction of the remote unit transmitting power level to conserve remote unit battery power. 
       FIG. 34  is a block diagram illustrating a specific embodiment of a weather alarm system, designated generally by the numeral  1180 . The weather alarm system  1180  includes a remote unit  1182  and a base station  1184 . 
     The remote unit  1182  includes a navigational receiver  1186 , a weather receiver  1188 , a radio transmitter  1190 , region defining circuits  1192 , weather threshold defining circuits  1194 , information combining circuits  1196 , and information comparison circuits  1198 . 
     The base station  1184  includes a radio receiver  1200 , a display circuit  1202 , and an alarm  1204 . 
     The weather alarm system  1180  operates generally as follows, the remote unit  1182  is deployed in the field, such as in a small, private aircraft and is used to monitor the weather within a zone surrounding the aircraft. As the aircraft moves, the zone surrounding the aircraft moves also. A navigational receiver  1186  is used to determine the location of the aircraft at any point in time. A weather receiver  1188  receives weather parameters broadcast by a Weather Surveillance Radar System of the US Weather Service, providing up-to-date weather information for the United States. The remote unit is programmed to monitor specific weather parameters within the zone surrounding the aircraft and to compare those parameters with programmed limits. In the event that one or more of the monitored parameters exceeds the programmed limit, the remote unit transmitter  1190  is activated and transmits the location  1187  of the aircraft. In some embodiments, specific weather parameters are also transmitted. The base station  1184  receives the transmission, displays  1202  the location and any transmitted weather parameters, and, if appropriate, gives an alarm  1204 . 
       FIG. 35  is a pictorial diagram illustrating an example of a weather region useful in understanding the operation of the weather alarm system  1180  and similar embodiments. The weather region is designated generally by the numeral  1220  and  1220  includes a region  1222  in which weather parameters are received from a weather surveillance radar system. Within the region  1222  is a weather alarm system remote unit at a moving location  1224  and surrounded by a moving zone  1226  having a constant radius  1228 . It is perhaps more relevant to state that at any point in the contiguous 48 states of the lower continental United States the weather receiver  1188  receives weather parameters relevant to the current location  1224  of the weather alarm system remote unit  1182  (the aircraft, in our example above). The aircraft is surrounded by a moving zone  1226  and the remote unit is monitoring specified weather parameters within the moving zone, notifying the base station  1184  when any monitored parameter exceeds its programmed limit. 
       FIG. 36  is a pictorial diagram illustrating an example of another weather region, designated generally by the numeral  1240 . In this example, the weather region  1240  includes an area of weather reporting  1242 . The aircraft is located at point  1244  and is moving in a direction and at a velocity shown by a vector  1246 . In this example, the defined zone of weather parameter monitoring is 1248. 
     With respect once again to  FIG. 34 , the remote unit circuits  1192  are used to define the zone ( 1226  in  FIGS. 35 , and  1248  in  FIG. 26 ) which is moving relative to the aircraft. In a specific embodiment, the circuits  1192  are a memory portion of a programmed microcontroller, and the zone is defined by information stored in the memory portion. The defined zone is designated by the numeral  1193 . 
     The remote unit circuits  1194  define specific weather parameters to be monitored and also define specific threshold values, limits and ranges for use in monitoring the weather parameters. The defined values are designated generally by the numeral  1195  and in a specific embodiment are stored in a memory portion of a programmed micro-controller. 
     As the aircraft proceeds on its flight, the navigational receiver  1186  continues to provide a current location  1187 , while the weather receiver  1188  continues to provide current weather information  1189 . The location  1187  and the surrounding zone defining information  1193  are combined by circuits  1196  and define a zone relative to the weather reporting region ( 1222  in the example of  FIGS. 35 , and  1242  in the example of  FIG. 36 ). This relative zone is compared by circuits  1198  with the received weather parameters  1189  and the selected weather parameters and limit values  1195  to determine whether or not any monitored parameter within the moving zone exceeds it limit. The line  1199  is used to activate the remote unit transmitter  1190  for transmitting the current location  1187  and the result  1199  of the comparison. 
       FIG. 37  is a partial block diagram illustrating a specific embodiment of a remote unit for a weather alarm system. The portion of the remote unit is designated generally by the numeral  1250 , and includes a navigational receiver  1252 , a circuit  1254  for defining an activation threshold, and a comparison circuit  1256 . In the embodiment illustrated here, received weather parameters  1258  are compared with limit values, threshold values and ranges stored in the circuit  1254 . If any specified weather parameter exceeds its individual limit value, the comparison circuit  1256  activates the navigational receiver  11252  which has been operating in a standby mode. Since current location is not available until the navigational receiver is activated, the received weather parameters  1258  are not limited to a moving zone around the aircraft, but apply to the entire weather reporting region ( 1222  in the example of  FIGS. 35 , and  1242  in the example of  FIG. 36 ). In a specific embodiment, the circuits  1254  and  1256  are part of a programmed microcontroller. 
       FIG. 38  is a block diagram of another specific embodiment of a weather alarm system, designated generally by the numeral  1270 . The weather alarm system  1270  includes a remote unit  1272  and a base station  1274 . 
     The remote unit  1272  includes only a navigational receiver  1276 , providing a current location to a radio transmitter  1278  for transmission to a base station. 
     The base station  1274  includes a radio receiver  1280  for receiving the current location  1281 , a weather receiver  1282  for receiving weather parameters, a region defining circuit  1284  for defining a zone relative to the current remote unit location, a weather threshold defining circuit  1286  for selecting specific weather parameters and for defining limits, thresholds, and ranges for the each selected weather parameter, an information combining circuit  1288  for combining the current location and the zone defining information, a comparison circuit  1290  for selecting the specified parameters within the zone relative to the current location, comparing the selected parameters within the zone with their individual limits, and activating an alarm  1294  and displaying  1292  the current location and comparison results when a monitored weather parameter within the defined distance of the remote unit exceeds its limit, falls below its defined threshold, and falls inside/outside of a defined range. 
     In the embodiment illustrated in  FIG. 38  all the intelligence is placed into the base station  1274 , including the weather receiver  1282 . In a specific embodiment, the circuits  1284 ,  1286 ,  1288  and  1290  are part of a programmed micro-controller. 
       FIG. 39  is a block diagram illustrating a self-locating remote alarm unit designated generally by the numeral  1300 . The remote unit  1300  includes a circuit  1302  defining a first variable and providing a value  1303  for the first variable, a circuit  1304  defining a second variable and providing a value  1305  for the second variable, a communications transmitter  1306 , a circuit  1308  defining a condition and providing a value for the condition, a circuit  1310  for comparing the value of the first variable with the value of the condition, and a circuit  1312  responsive to the comparison for enabling the communications transmitter  1306  to transmit the value of the second variable and to transmit a function of the value of the first variable. 
     Though the description of  FIG. 39  is very abstract, the figure represents the essence of the major embodiments of the present invention, as the following examples will illustrate. 
     In a simple man-over-board monitor as illustrated in  FIG. 11 , the value  310  of the first variable is provided by a sensor  308 , the value  338  of the second variable is provided by a navigation receiver  304 . When the sensor status  310  changes, a transmitter  314  transmits the remote unit location  338  and the sensor status  310 . 
     In the same man-over-board monitor, when a panic button  312  is depressed, the transmitter  314  transmits the remote unit location  338  and the switch status  340 . 
     In an environmental monitor illustrated in  FIG. 18 , the value of the first variable is a sensor status  578  for a monitored environmental parameter, while the value of the second variable is a location  576  of the remote unit stored in a memory. When the sensor  558  detects a predetermined change in the monitored environmental parameter, the transmitter  560  transmits the stored location of the remote unit and the sensor status  578 . Alternatively, the remote unit  552  defines a patient monitor, and the value of the second variable is stored information  556  which identifies the patient, such as name, room and bed number, patient identification code. The value of the first variable is the output of a sensor  558  which monitors a physiological parameter, and defines a sensor status  578 . When a predetermined change in the monitored physiological parameter occurs, the transmitter  560  is activated and transmits the patient identification information  576  as the value of the second variable and transmits and the sensor status  578  as the function of the first variable. 
     The circuits  1308 ,  1310  and  1312  of  FIG. 39  find their equivalents in the man-over-board monitor, the patient monitor and in the environmental monitor in that a change in a sensor or switch status activates a transmission of the value of the second variable-dynamic location, patient ID, and static location, respectively—and a transmission of an appropriate function of the value of the first variable—sensor status. 
     In a man-over-board monitor  752  illustrated in  FIG. 21 , the value of the second variable is provided by a dynamic location determining device, in this case the navigational receiver  756 . Alternative embodiments use the World-wide LORAN navigation system, a satellite navigational system such as the GPS system, and other alternative global and regional navigational systems for providing a value of the second variable which is the location of the remote unit  752 . 
     Another example of a remote unit represented by the block diagram in  FIG. 39  is a remote weather alarm  1182  illustrated in  FIG. 34  in which the value of the second variable is a remote unit location  1187 , and in which the function of the first variable is defined by a circuit  1198  to be the result  1199  of a comparison of a monitored weather parameter, within the defined zone relative to the weather alarm location  1187 , with a defined weather threshold  1195 . 
     Another example of the remote unit represented by  FIG. 39  is an invisible fence monitor  852  as illustrated in  FIG. 24 . The value of the second variable is a location  859  provided by a navigational receiver  856 , while the transmitted function of the first variable is a positional and time status  869 , the result of a comparison by a circuit  864  of the location  859 , a time-of-day  861  and a defined curfew  860 ,  862 . 
     When a microphone  808  is connected to the remote unit transmitter  806 , as shown in  FIG. 22 , the remote unit of  FIG. 39  includes a one-way voice channel. 
       FIG. 40  is a block diagram illustrating a remote alarm unit designated generally by the numeral  1320 . The remote unit  1320  includes a circuit  1322  defining a first variable and providing a value  1323  for the first variable, a communications transmitter  1324 , a circuit  1326  defining a condition and providing a value for the condition, a circuit  1328  for comparing the value of the first variable with the value of the condition, and a circuit  1330  responsive to the comparison for enabling the communications transmitter  1324  to transmit a function of the value  1323  of the first variable. The remote unit  1320  also includes a communications receiver  1332  for defining a two-way communications link. 
     When the remote unit shown in  FIG. 39  includes a communications receiver, such as the receiver  1332  of  FIG. 40 , the communications channel is alternatively one of direct radio contact such as illustrated in a variety of the figures, wireless, cellular, radio telephone, radio relay, to name a few representative communications channels as shown in  FIGS. 17 and 28 . 
     An example of a monitoring system such as illustrated in  FIG. 40  is shown in  FIGS. 3 ,  30  and  33 . In each instance, one or more sensors and switches provide the value for the first variable and the transmitted function of the value of the first variable is alternatively the sensor value and the sensor/switch status. The circuits  1326 ,  1328  and  1330  find their equivalents in an activation of the transmitter upon a change of the sensor/switch status. The remote monitoring system illustrated in  FIG. 3  includes both a remote unit  82  of the class shown in  FIG. 40  and a compatible base station  84 . 
       FIG. 41  is a partial block diagram which illustrates a plurality of sensor/switches designated by the numeral  1340 . Each sensor/switch  1342  provides an output signal  1343  defining a sensor/switch status. A typical transmission format for a sensor/switch status and defining a sensor/switch vector is shown in the partial pictorial diagram of  FIG. 42 . The transmitted format is designated generally by the numeral  1350  and includes a plurality of sensor/switch status bits  1352  defining a status vector  1354 . A portion  1356  of the transmitted format  1350  is unused and marked reserved. 
     Finally,  FIG. 43  is a partial block diagram illustrating the temporary connection of an input device to a remote monitor of the type providing a stored value for the second variable. The figure includes the removable input device  1350  temporarily connected to the remote monitor  1362 . The remote monitor  1362  includes a circuit  1364  for storing a value for the second variable. The input device  1350  is connected to the remote monitor  1362  and supplies a value  1361  for storage in the circuit  1364 . Once the value  1361  has been stored, the input device  1360  is disconnected from the remote monitor  1362 , and the remote monitor uses the value stored by the circuit  1364  as the value of the second variable. The remote monitor  1362  corresponds to the self-locating remote alarm unit  1300  of  FIG. 39 , and the storage circuit  1364  of  FIG. 43  corresponds to the circuit  1304  of  FIG. 39 . 
     The two examples that are provided above for a self-locating remote alarm unit which provides a stored value for the second variable are the environmental monitor of  FIG. 18  and its other embodiment, the patient monitor. Both embodiments require that a value be provided for the second variable. A method for doing so is to connect an input device  1360  to the remote monitor  1362 , to use the input device to load a value for the second variable into the storage circuit  1364  ( 1304  of  FIGS. 39 , and  556  of  FIG. 18 ), then to disconnect the input device and to monitor the specified environmental/physiological parameters. In one embodiment, the input device is a keypad of manually operated switches. The keypad is used to input an environmental monitor location, or, alternatively, a patient&#39;s ID information. In one embodiment of the procedure, a navigational receiver is used to provide a user with the environmental monitor location, which the user then enters by hand using the keypad input device  1360  attached to the environmental monitor  1362  ( 552  of  FIG. 18 ). In another embodiment, the temporarily connected input device  1360  is a navigational receiver and the location  1361  is stored in the storage circuit  1364  ( 556  of  FIG. 18 ,  1304  of  FIG. 39 ). After the location has been stored in the storage circuit, the navigational receiver  1360  is disconnected and the environmental monitor left to do its job. 
     While the foregoing detailed description has described several embodiments of the personal alarm system in accordance with this invention, it is to be understood that the above description is illustrative only and not limiting of the disclosed invention. Thus, the invention is to be limited only by the claims as set forth below. 
       FIG. 44  is a block diagram illustrating a specific embodiment of a personal alarm system remote unit. The remote unit is designated generally by the reference numeral  1410 , and includes a satellite global positioning receiver (navigational receiver)  1412 , a radio transmitter  1414 , a sensor and threshold detector  1416 , a microphone  1418 , and a voice activated detector  1420 . 
     The navigational receiver  1412  receives positioning information from geo-synchronous satellites via antenna  1422 , and provides a global location  1424  of the remote unit for transmission by the radio transmitter  1414 . The location  1424  is represented in appropriate coordinates. 
     The sensor and threshold detector  1416  provides an output signal  1426  that is activated when the sensor detects a condition that exceeds a predetermined threshold level. A variety of specific sensors is contemplated, including but not limited to the following; a glucose sensor for monitoring the blood-glucose level of a patient; an oxygen sensor for monitoring the oxygen level of the ambient air; a motion sensor for detecting movement in excess of a predetermined threshold; a light sensor for detecting ambient light in excess of a predetermined threshold; a liquid immersion sensor, a heat sensor for detecting temperature in excess of a predetermined threshold; a carbon-monoxide sensor; and a smoke detector. 
     The microphone  1418  and the voice-activated detector  1420  provide an output signal  1430  that becomes active when the voice-activated detector  1420  detects a predetermined spoken distress phrase such as “HELP!” 
     In a specific embodiment of the personal alarm system remote unit  1410 , no sensor and threshold detector are included. In this embodiment, the radio transmitter  1414  is connected to transmit the remote unit location  1424  when the voice-activated detector output signal  1430  is active. This specific embodiment of the invention permits the remote unit to be worn or carried by a person and the person&#39;s global location will be transmitted via antenna  1428  when a predetermined distress phrase is detected. 
     In another specific embodiment of the personal alarm system remote unit  1410 , the sensor and threshold detector  1416  are included and the threshold detector portion is disabled. The radio transmitter is connected to transmit the sensor output signal (sensor status)  1426  when the remote unit location is transmitted. In yet another embodiment of the personal alarm system remote unit  1410 , the threshold detector is enabled and the radio transmitter is connected for transmitting a sensor status  1426  and a remote unit location  1424  when either of the sensor and threshold detector output signal  1426  and the voice-activated detector output signal  1430  is active. 
     In various specific embodiments, the navigational receiver is compatible with one of a geo-synchronous satellite global navigation system, the infrastructure-based TDOA and RSSI systems, the SATNAV system, and the LORAN system. The preferred embodiment is that the navigational receiver  1412  is compatible with the U.S. GPS system. 
       FIG. 45  is a block diagram illustrating a specific embodiment of a base station for use with a remote unit such as shown in  FIG. 44 . The base station is designated generally by the reference numeral  1432  and includes an antenna  1434 , a radio receiver  1436 , a display  1440  for displaying the remote unit location, and an alarm  1442 . In normal use, the radio receiver  1436  receives a radio transmission from a remote unit via the antenna  1434 . The radio receiver provides two output signals. A first output  1438  provides the global coordinates of the remote unit location for display while a second output  1439  becomes active when a transmission is received from a remote unit. The output  1439  is used to activate the alarm  1442 , In another specific embodiment of the base station  1432 , the output signal(s)  1438  includes both the remote unit location information and sensor status information. 
       FIG. 46  is a block diagram of a personal alarm system according to another aspect of the present invention. The personal alarm system is designated generally by the reference numeral  1500  and includes a remote unit  1502  and a base station  1504 . 
     The remote unit  1502  includes a navigational receiver  1506 , a demodulator circuit  1508 , a precise time-of-day circuit  1510 , a voice-activated detector circuit  1512 , a microphone  1514 , a radio transmitter ISIS, a navigational receiver antenna  1516 , and a radio transmitter antenna  1518 . 
     The navigational receiver provides modulated navigational information  1530  to the demodulator circuit  1508 . The demodulator circuit  1508  “demodulates” the modulated navigational information  1530  and provides demodulated navigational information  1532  to the radio transmitter  1515 . The precise time-of-day circuit  1510  provides a precise time-of-day signal  1534  to the radio transmitter. 
     The microphone  1514  is connected to the voice-activated detector circuit  1512  permitting the detector circuit  1512  to activate an output signal  1536  when a predetermined distress phrase is detected, for example “HELP!” 
     The radio transmitter  1515  is connected to transmit the demodulated navigational information  1532  and the precise time-of-day information  1534  when the voice-activated output signal  1536  becomes active. 
     The base station  1504  includes an antenna  1520 , a radio receiver  1522  circuits  1524  for computing the remote unit location, a display  1526  and an alarm  1528 . 
     Radio transmissions from the remote unit  1502  are received via the antenna  1520  and converted by the radio receiver into demodulated navigational information  1538 , and precise time-of-day information  1540 . The circuits  1524  receive the demodulated navigational information and the precise time-of-day information and compute a global location  1544  for the transmitting remote unit  1502 . The computed global location (in appropriate coordinates) is displayed on the display  1526 . The alarm  1528  is activated by a receiver output signal  1542  when a radio transmission from the remote unit is received. 
       FIG. 47  is a block diagram that illustrates another embodiment of a personal alarm system remote unit. The remote unit is designated generally by the reference numeral  1600  and includes a navigational antenna  1616 , a navigational receiver  1602 , a microphone  1610 , a voice-activated detector  1604 , a radio transmitter  1606  a radio antenna  1618 , a radio receiver  1608 , and a speaker  1612 . 
     The navigational receiver  1602  receives navigational information via the navigational antenna  1616  and provides a location  1620  of the remote unit in appropriate coordinates. 
     The microphone  1610  and the voice-activated detector  1604  are connected to provide a Transmit Location signal  1628  that becomes active when the detector  1604  recognizes an audible predetermined distress phrase such as “HELP!” The radio transmitter  1606  is connected with the Transmit Location signal  1628 , and with the remote unit location information  1620  so that the location information is transmitted when the signal  1628  becomes active. Thus, in normal use, the remote unit  1600  transmits its own location (in appropriate coordinates) when an audible, predetermined distress phrase is detected. The predetermined distress phrase is preset to a specific language. In another embodiment, the predetermined distress phrase is programmed into a programmable storage unit (not illustrated) that is connected with the voice-activated detector  1604 . 
     The remote unit  1600  includes a switch  1614  that connects the microphone  1610  with the radio transmitter  1606  for transmitting one-half of a two-way radio communication. The switch  1614  also is connected to generate a Transmit Voice signal  1626  that becomes active when the switch  1614  is operated. The radio transmitter  1606  is connected with the Transmit Voice signal  1628  so that when the switch is operated, the microphone is connected for voice transmission in a push-to-talk arrangement (half-duplex mode), and the radio transmitter transmits the voice via radio antenna  1618 . The other half of the two-way radio communication is received by the radio antenna  1618 , then converted to audible sound by the radio receiver  1608  and the speaker  1612 . 
       FIG. 48  is a partial block diagram that illustrates the use of a wireless phone within a personal alarm system remote unit according to a specific embodiment of the present invention. The personal alarm system remote unit is designated generally by the reference numeral  1700 , and includes a wireless phone  1702 , a wireless phone antenna  1704 , remote unit location information  1706 , and a Transmit Location signal  1710 . 
     The wireless phone  1702  typically includes elements necessary for two-way radio communication (full-duplex mode), such as a microphone ( 1610  of  FIG. 47 ) and a speaker ( 1612  of  FIG. 47 ). 
     When the Transmit Location signal  1710  becomes active, the wireless phone  1702  transmits the remote unit location information  1706 . 
       FIG. 49  is a partial block diagram illustrating the wireless phone of  FIG. 48  and including a circuit that automatically dials “911” for transmitting the remote unit location. The wireless phone is designated by the reference numeral  1720 , while the circuit that automatically dials “911” is designated by the reference numeral  1722 . When the Transmit Location signal  1724  becomes active, the circuit  1722  automatically dials the dedicated public safety help telephone number “911” via connection  1726  with the wireless phone  1720 . Once the telephone connection with the 911 service is established, the wireless phone  1720  transmits the remote unit location information ( 1706  of  FIG. 48 ). Recently, additional public safety help telephone numbers have been contemplated and, in some cases, assigned. A person having an ordinary level of skill in the relevant arts will appreciate that (1) the use of these additional telephone numbers is also contemplated by the present invention, and (2) a typical wireless phone includes a keypad permitting a user to place a call in the normal manner, including a call placed to a dedicated public safety help telephone number. 
       FIG. 50  is a partial block diagram that illustrates the use of a cellular telephone  1730  for transmitting the remote unit location and for two-way radio communication. In the illustrated embodiment, the wireless phone of  FIGS. 48 ,  49  is the cellular telephone  1730 .  FIG. 51  is a partial block diagram that illustrates the use of a PCS telephone for transmitting the remote unit location and for two-way radio communication. In the illustrated embodiment, the wireless phone of  FIGS. 48 ,  49  is the PCS telephone  1740 .