SELF-LOCATING REMOTE MONITORING SYSTEMS

Abstract: The system (750) includes a supervising base station (754) and remote monitoring unit(s) (752) including a navigational receiver (756) providing a remote unit location (759) and a transmitter (758) for communicating the location (759) to the base station (754) for display (772). The remote unit (752) includes sensor(s) (760) monitoring the remote location. A change in sensor status (761) results in the status and the location being transmitted to the base station (754), which includes alarms (776) and displays (772) responsive to the change in status. Various embodiments define a man-over-board system, an invisible fence system, and a weather surveillance system.

Description: SELF-LOCATING REMOTE MONITORING SYSTEMS Claim of Priority: This Apphcation claims priority from a copending International Patent Apphcation. No. PCT/US95/13823. filed October 26. 1995. having an International Publication No. WO 96/13819. and International Publication Date 9 May 1996. Technical Field: This invention relates to personal alarm systems and in particular to such systems transmitting at a higher power level during emergencies. Background Art: Personal alarm systems are well known in the an (see for example US Patents 4.777.478; 5.025.247: 5.1 15.223; 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 wom 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'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.. US Patent 4.~77.478. provide for a panic button to be activated by the child, or an alarm to be e . M ( Ms . eμ SYC Z.3S- -f 0$ given if someone attempts to remove the remote unit from the child's clothing. Banks. US Patent 5,025.247. teaches a base station which latches an alarm condition so that failure ofthe remote unit, once having given the alarm, will not cause the alarm to turn ofFbefore help is summoned. Moody. US Patent 5.1 15,223, teaches use of orbiting satellites and triangulation to limit the area of a search for a remote unit which has initiated an alarm. In OS Patent 4,952,928 to Carroll et al., and in US Patent 4,819.860 to Hargrove et al.. the apparatus provides for the remote monitoring ofthe vital signs of persons who are not confined to fixed locations. Ghahariiran. US Patent 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. US Patent 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'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 ofthe preventable injuries and deaths among our toddler age children. These hazards include the child's wandering away from a safe or supervised area, water immersion, 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'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's useful life is still very much a factor of its physical size, which, as stated above, is limited because ofthe 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. Additional US Patents of interest with respect to this continuation-in-part include: 3.646.583; 3.784.842: 3.828,306; 4.216.545; 4.598,272; 4,656.463; 4675.656; 5.043.736; 5,223.844; 5.31 1.197; 5,334,974; 5.378,865. Disclosure of Invention: It is an object ofthe 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 ofthe present invention to provide such a system which includes sensors for the hazardous conditions typically confronting young children. It is a further object ofthe 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 deteπnining 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 ofthe invention. the base station transmits a periodic polling signal and the remote unit monitors the field strength ofthe 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 mdication 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 ofthe 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 ofthe 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'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 ofthe 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 ofthe 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 ofthe 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 ofthe 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 fuπher understanding ofthe objects, features and advantages ofthe present invention. reference should be had to the following description ofthe 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 ofthe present invention and transmitting at selectable power levels. FIG. 2 is a block diagram of another embodiment ofthe personal alarm system illustrated in FIG. 1 including multiple remote units. FIG. 3 is a block diagram illustrating another embodiment ofthe 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 ofthe 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 ofthe personal alarm system of FIG. 1, in a typical child monitoring application. FIG. 8 is a pictorial diagTam 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. 1 1 is a block diagram illustrating a man-over-board alarm system in accordance with one aspect ofthe present invention. FIG. 12 is a block diagram illustrating another embodiment ofthe man-over-board alarm system. FIG. 13 is a block diagram illustrating an invisible fence monitoring system according to another aspect ofthe 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 ofthe invisible fence system. FIG. 18 is a block diagram showing a fixed-location environmental sensing system according to another aspect ofthe 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 plurahty 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 ofthe invention. Best Mode for Carrying Out the Invention: With reference to FIG. 1. there is shown a block diagram of a personal alarm system according to one embodiment ofthe 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 ofthe 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 accomphsh the shift to the higher power level, the remote unit receiver 18 generates a signal 26 which is proportional to the field strength ofthe received signal, transmitted by the base station 14. The remote unit 12 includes a comparitor 28 which compares the magnitude ofthe 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 altemative embodiment, transmission is selectable widiin a continuous range of transmission power levels. Within an operating range ofthe personal alarm system 10. the field strength ofthe base station 14 transmitted signal when received at the remote unit 12 is inversely proportional to the fourth power (approximately) ofthe 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 ofthe 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 ofthe 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. Figure 2 is a block diagram illustrating another embodiment ofthe personal alarm system ofthe 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 comparitor 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 LD- Status display 76. In one embodiment ofthe 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 apphcations, 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 ofthe type of hazard is displayed by the base station on the LD-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. Figure 3 is a block diagram illustrating another embodiment ofthe 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 uumber 92. a visual beacon 94. an audible beacon 96. a watchdog timer 98. a plurahty 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 1 10. an emergency switch ("panic button") 1 12. a battery 1 13. and a 'low battery power' sensor 1 14. The base station 84 includes a transmitter 1 16, a receiver 118 which produces a received field strength signal 120. a comparitor 122, a predetermined limit value 124, a comparitor output signal 126. an interval timer 128. control signals 130 and 132. a visual alarm 134. an audible alarm 136. an LD 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 plurahty ofthe 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 LD number 152. a plurahty 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 apphcations 172. Figure 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 plurahty of unassigned bits 190 reserved for a future apphcation. 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 plurahty 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 1 16 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 will 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 comparitor 122. The predetermined limit value 124 is also connected to the comparitor 122 which provides a comparitor output signal 126. If the received field strength 120 is less than the limit value 124, the comparitor 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 ofthe 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 ofthe watchdog timer 98. the remote unit transmitter 86 is commanded to transmit a non-polled message 150. In one embodiment ofthe 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 ofthe tamper switch 109 causes the remote unit to transmit a code or status bit to the base unit to identify the cause ofthe 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 ofthe remote unit from the child's person. In another embodiment, the remote unit 82 includes a circuit 1 14 which monitors battery power. The circuit 1 14 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 ofthe present invention. the base station receiver 1 18 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 ofthe status bits 166-170. The base station 84 also includes the status and LD display 138 used to display the status of all remote units in the personal alarm system 80. In another embodiment ofthe 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 "91 1 " number. FIG. 6 is a partial block diagram illustrating an embodiment ofthe 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 ofthe remote unit 202. and a watchdog timer 219. In a preferred embodiment ofthe 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 ofthe 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 world-wide coordinate system such as longitude and latitude. In one embodiment ofthe 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 ofthe remote unit 202 as a map for easy location ofthe transmitting remote unit 202. In another embodiment ofthe 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 ofthe 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 ofthe 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 LD number 21 1. The timer 209 is connected to cause the remote unit to transmit the LD 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 ofthe 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. Figures 7-10 are pictorial illustrations of alternative embodiments ofthe personal alarm system ofthe 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. 1 1 is a block diagram which illustrates a man-over-board system in accordance with one aspect ofthe 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-over-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 ofthe remote unit 302. a display 326 for displaying the status ofthe sensor 308, a circuit 328 for comparing the field strength ofthe 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 ofthe 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 sateUites 336. The navigational receiver 304 converts the navigational information into a location ofthe remote unit 302 and outputs the location 338 to the radio transmitter 3 14 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 ouφut 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 ofthe 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 comparitor circuit 328 compares the received field strength 334 with a predetermined limit 330 and produces an output signal 346 if the sign ofthe comparison is negative, indicating that the field strength ofthe 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's companions, who can then take appropriate action. In heavy seas or poor visibility, the base station 318 displays the current location ofthe 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 generaUy 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 ofthe 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 sateUites (not shown). The receiver provides the location 382 ofthe remote unit 352 for radio transmission to the base station 354. The remote unit radio transmitter 3 8 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 ofthe remote unit for display by the location display 370. and a signal 390 whose value is inversely proportional to the field strength ofthe signal received by the radio receiver 366. The received field strength signal 390 and the predetermined limit 374 are compared by the comparitor 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 iUustrates an invisible fence for monitoring a movable subject and designated generaUy 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 comparitor 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 π ή ld electrical shock. The base station 404 includes a radio receiver 424, a comparitor 426. storage circuits 428 for storing information defining a predetermined positional status, and an alarm 430. In the embodiment Ulustrated 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'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 apphcations are keeping a pet inside the yard, and applying a m d electrical shock to the pet if it wanders too close to a defined perimeter. Attaching the remote unit 402 to a chUd and alerting the caregiver in the event the chUd 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 ofthe 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 Ulustrating boundaries used to define geographical regions such as those used in a preferred embodiment ofthe 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 subregions. one subregion defining an area 490 whoUy within the boundary 462. and the other subregion defining an area 492 outside the boundary 462. FIG. 16 shows a geographical region 480 which includes subregions 482 and 484. Subregion 482 is entirely surrounded by subregion 484. whUe subregion 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 comparitor 412 (FIG. 13 ). The comparitor 412 also receives the location output 432 from the navigational receiver 406. The comparitor 412 compares the location ofthe 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 comparitor 412 also receives an input from the second storage circuits 414. These circuits store information defining a predetermined positional status. Some examples wiU be useful in explaining how the positional status is used. Referring to FIG. 14. remote unit locations 494 and 496 are Ulustrated 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 "within a defined geographical region." and that the location 496 is "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 comparitor 412 wiU define a positional status that "the location ofthe remote unit relative to the defined region is acceptable." This positional status wUl be transmitted to the base station 404 and wiU not result in activation ofthe alarm 430. For the next example, assume that that the navigational receiver 406 reports the location ofthe remote unit to be the location 496. and that the other assumptions remain the same. Then the comparitor 412 wUl define a positional status that "the location ofthe remote unit relative to the defined region is not acceptable." This positional status wiU be transmitted to the base station 404 and wUl result in activation ofthe 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 ofthe remote unit 402 from location 498 to location 500 to location 502. Assume that the area outside the boundary 488 defines an "acceptable" subregion. Assume further that the area between the boundaries 488 and 486 defines a "warning" subregion. Also assume that the area 482 inside the boundary 486 defines a "prohibited" subregion. FinaUy. 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 comparitor 412 wiU determine that the location 498 is acceptable and wUl take no further action. The comparitor 412 wiU determine that the location 500 is within the warning subregion 484 and wiU 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 comparitor 412 wUl determine that the remote unit has entered a prohibited zone and wUl activate the mild electric shock circuit 418 which makes contact with the skhi ofthe 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 necessar ." respectively. In another embodiment, no enforcement or warning are given by the remote unit 402. Instead, as when used to monitor the movements of chUdren 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 comparitor 426. The preferred embodiment for the storage and comparison circuits is the use of an embedded microprocessor. FIG. 17 is a block diagram Ulustrating a personal alarm system such as the invisible fence of FIG. 13. and designated generaUy 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 ceUular 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 generaUy 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 ofthe 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 ofthe remote unit 552, a comparitor 570. storage circuits 572 for storing information defining a predetermined sensor status, and an alarm 574. The environmental monitoring system 550 is useful for apphcations 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 apphcations would include use in forests for fire perimeter monitoring in which the sensor 558 was a heat sensor, or in monitoring for oU spUls when attached to a fixed buoy and the sensor 558 detecting oU. Other useful apphcations include any apphcation 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 LD numbers, such as those used in the systems Ulustrated in Figs. 2 and 3. The storage circuits 556 provide an output 576 defining the location ofthe 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 ofthe sensor status to the base station 554. The communications are received by the base station's radio receiver 566 which provides outputs representing both the location 580 ofthe remote unit 552 and the sensor status 582. The location 580 is connected to the display 568 so that the location ofthe remote unit 552 can be displayed. The comparitor 570 receives the sensor status 582 and the information defining the predetermined sensor status which is stored in the storage circuits 572. If the comparitor 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 iUustrates 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 ofthe remote unit. This altemative 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 ofthe remote unit 602. a display 620 for displaying the computed location, a second display (can be part ofthe first display) 622 for displaying a sensor status, a comparitor 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 sateUites (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 ofthe remote unit based upon the demodulated navigational information. In the case of GPS navigational information, geometric dUution of precision computations are done at the base station 604 to derive the actual location ofthe 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 aU 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 comparitor 624. Stored information defining a predetermined sensor status is provides by the storage circuits 626 as a second input to the comparitor 624. If the received sensor status and the stored sensor status do not agree, the comparitor 624 activates the alarm 628 to alert the base station operator. FIG. 20 is a block diagram which iUustrates an altemative embodiment ofthe invisible fence system in which the base station computes the location ofthe remote unit, and in which the fence definitions are stored at the base station rather than in the remote unit. The altemative system is designated generaUy 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 comparitor 682. a second comparitor 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 ofthe remote unit 652. The storage circuits 678 store information defining a geographical region. The first comparitor 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 predeteπnined positional status. The second comparitor 684 receives the positional status 706 and the predetermined positional status 708 and provides control output signals 710 based upon the results ofthe positional status comparison. When the location 702 is within a defined "warning" or "restricted" zone, the second comparitor 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 mUd electric shock ofthe embodiment shown in FIG. 13. The second comparitor 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 wUl command the remote unit 652 to provide an electric shock to enforce the restriction. FIG. 21 is a block diagram Ulustrating another embodiment of a man-over-board alarm system, designated generaUy 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 manuaUy 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 ofthe 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 normaUy in the low-power standby mode, thereby conserving operating power which is normaUy supphed 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 manuaUy 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 manuaUy 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 ofthe 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-ovei -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-controUer. 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 ofthe 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 ofthe 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- controUer. In another specific embodiment, the circuit 780 and the circuit 782 are implemented using an embedded, programmed micro- controUer. The functions performed by the circuits 780 and 782 are performed in different embodiments altematively by discrete integrated circuits, by a programmed micro-controUer. by an embedded, programmed micro- controUer. by a programmed micro-processor, and by an embedded, programmed micro-processor. In a specific embodiment ofthe man-over-board alarm system Ulustrated in FIG. 21. the sensor 760 includes a plurahty of environmental, physiological and hazard sensors providing output signals and defining a sensor status vector. In another specific embodiment. the sensor 760 provides a plurahty 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 ofthe man-over-board alarm system Ulustrated in FIG. 21. the manually operated switch 762 includes a plurahty of manuaUy 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 manuaUy 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 manuaUy 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 ofthe man-over-board alarm system Ulustrated in FIG. 21. and designated generaUy 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 ofthe 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 ofthe man-over-board alarm system shown in FIG. 21. The alarm svstem is designated generaUy 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 Ulustrating another embodiment of an invisible fence system, designated generaUy 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 aUows 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 altematively excluded from, a defined region between the hours of 6PM and 6AM. 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. hi a specific embodiment, the parolee is first warned that he has left a region of confinement at an unaUowed time. If the violation continues, the parolee is given a mUd electrical shock. If the violation continues, the intensity ofthe 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 Ulustrating boundaries used to define geographical regions such as those used in a preferred embodiment ofthe 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 subregions. one subregion defining an area 1002 wholly within the boundary, and the other subregion defining an area 1004 outside the boundary. In a specific embodiment of an invisible fence system, such as that Ulustrated in FIG. 24. a memory 860 stores information defining a geographical region, for example the region 1002. In an example ofthe operation ofthe 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 8PM and 7AM. and that at aU other times the parolee is permitted to be outside the region 1002. FIG. 26 is a table defining a relationship between the location ofthe remote unit 852 (FIG. 24) and the time-of-day for use in understanding a curfew feature of a specific embodiment ofthe invisible fence system 850. Each row ofthe table represents a different location, and each column ofthe table represents a subdivision ofthe 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 8PM and 7AM. If the parolee leaves home during the interval from 8PM to 7AM. 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 Ulustrated 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. hi another embodiment of he 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 estabhshed curfew. FIG. 27 is a block diagram Ulustrating another embodiment of an invisible fence system, designated generaUy 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 ofthe 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-controUer. A memory ofthe 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 ofthe 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 8PM and 7AM. The compare/control circuit 1038 compares the received information described above and determines whether the parolee is in violation ofthe defined curfew. The parolee is in violation ofthe curfew defined by the table in FIG. 26 when he is outside his home between the hours of 8PM and 7AM. In this example, the region 1002 (FIG. 25) corresponds to the parolee ' s home. Locations outside region 1002 are therefore outside his home. In this example, if the parolee is in violation ofthe 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 FIG ' s 13 and 16. In a specific embodiment ofthe invisible fence system shown in FIG. 27. the location ofthe 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 ofthe invisible fence system of FIG. 27. the time-of-day circuit 1037 is implemented as pan ofthe imbedded micro-processor. 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 generaUy 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 Ulustrated in FIG ' s 19. 20. 34 and 36. FIG. 28 is a partial block diagram Ulustrating 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 ofthe 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 whUe 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 sateUites. relay type radio hnks and other common carrier technologies such as ceUular telephone, wireless communications, and personal communications systems ("PCS"). FIG. 29 is a partial block diagram Ulustrating an altemative embodiment ofthe 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 iUustrates a radio transmitter 86, a circuit 90 for selecting a transmission power level for the transmitter 86. An oil/chemical sensor 1 13 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 1 1 1 to the transmitter 86 for transmission ofthe sensor status. The output of each sensor 100 is connected via line 1 17 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 1 17 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 generaUy 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 1 100 and a higher power level command circuit 1 102. FIG. 30 illustrates a system in which a sensor status 1095 is transmitted to the base station 1084 and generates an alarm 1 100. The command circuit 1 102 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 inteφreted by the control circuit 1090 to select a higher power transmission level 1092. FIG. 31 is a partial block diagram Ulustrating a circuit 1 130 including an analog-to- digital converter 1 132 and a read-only memory 1 134. The analog-to-digital converter 1 132 receives an analog input signal 1 131 and provides digital output signals 1133. The digital output signals 1 133 are connected to address input lines ofthe read-only-memory 1 134. The read-only- memory provides digital output signals of stored information from an addressed memory location on output lines 1 135. 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 Ulustrating a digital-to-analog converter 1 140. The digital-to- analog converter 1 140 receives digital input signals on lines 1 141 and provides an analog output signal on line 1 142. FIG. 33 is a block diagram Ulustrating an embodiment of a personal alarm system. designated generaUy 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 1 158. a circuit 1 160 for selecting transmission power level and a sensor 1162. The base station 1 154 includes a radio receiver 1164. a radio transmitter 1166. an alarm 1 168 and a command control circuit 1 170. The digital-to-analog converter illustrated in FIG. 32 is used in a specific embodiment ofthe circuit 1 160 of FIG. 33 for selecting one of a plurahty of transmission power levels, as commanded by the base station. The base station receiver 1 164 provides a signal 1 165 proportional to a received field strength. In a specific embodiment, the signal 1 165 is au analog signal and is converted to a digital form using the conversion circuit 1 130 of FIG. 31. The digital output signals 1 135 are used by the command control circuit 1 170 to generate a power-level command 1 171 for transmission to the remote unit 1 152. In one embodiment ofthe remote unit select power level circuit 1 160, the received digital power- level command is used directly to control the power level ofthe remote unit transmitter 1 156. In another embodiment, the received power-level command is converted to an analog signal which is used to control the power level ofthe remote unit transmitter 1 156. 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 1 165 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 Ulustrating a specific embodiment of a weather alarm system, designated generaUy by the numeral 1 180. The weather alarm system 1 180 includes a remote unit 1 182 and a base station 1184. The remote unit 1 182 includes a navigational receiver 1 186. a weather receiver 1 188. a radio transmitter 1 190. region defining circuits 1 192. weather threshold defining circuits 1 194. information combining circuits 1 196, and information comparison circuits 1 198. The base station 1 184 includes a radio receiver 1200, a display circuit 1202. and an alarm 1204 The weather alarm system 1180 operates generaUy as foUows. the remote unit 1 182 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 1 186 is used to determine the location ofthe aircraft at any point in time. A weather receiver 1 188 receives weather parameters broadcast by a Weather SurveiUance Radar System ofthe 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 ofthe monitored parameters exceeds the programmed limit, the remote unit transmitter 1 190 is activated and transmits the location 1 187 ofthe aircraft. Lu some embodiments, specific weather parameters are also transmitted. The base station 1 184 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 Ulustrating an example of a weather region useful in understanding the operation ofthe weather alarm system 1 180 and simUar embodiments. The weather region is designated generaUy by the numeral 1220 and 1220 includes a region 1222 in which weather parameters are received from a weather SurveiUance 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 ofthe lower continental United States the weather receiver 1 188 receives weather parameters relevant to the current location 1224 of the weather alarm system remote unit 1 182 (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 1 184 when any monitored parameter exceeds its programmed limit. FIG. 36 is a pictorial diagram Ulustrating an example of another weather region, designated generaUy by the numeral 1240. In this example, the weather region 1240 mcludes 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 1 192 are used to define the zone ( 1226 in FIG. 35. and 1248 in FIG. 26) which is moving relative to the aircraft. In a specific embodiment, the circuits 1 192 are a memory portion of a programmed micro- controUer. 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 1 194 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 generaUy by the numeral 1 195 and in a specific embodiment are stored in a memory portion of a programmed micro-controUer. As the aircraft proceeds on its flight, the navigational receiver 1 186 continues to provide a current location 1 187. whUe the weather receiver 1 188 continues to provide current weather information 1 189. The location 1 187 and the surrounding zone defining information 1 193 are combined bv circuits 1 1 6 and define a zone relative to the weather reporting region ( 1222 in the example of FIG. 35. and 1242 in the example of FIG. 36). This relative zone is compared by circuits 1 198 with the received weather parameters 1 189 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 1 199 is used to activate the remote unit transmitter 1 190 for transmitting the current location 1 187 and the result 1 199 of the comparison. FIG. 37 is a partial block diagram Ulustrating a specific embodiment of a remote unit for a weather alarm system. The portion ofthe remote unit is designated generaUy by the numeral 1250. and mcludes a navigational receiver 1252. a circuit 1254 for defining an activation threshold, and a comparison circuit 1256. In the embodiment Ulustrated 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 1 1252 which has been operating in a standby mode. Since current location is not avaUable untU 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 FIG. 35. and 1242 in the example of FIG. 36). In a specific embodiment, the circuits 1254 and 1256 are part of a programmed micro- controUer. 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 ofthe remote unit exceeds its limit. faUs below its defined threshold, and faUs inside/outside of a defined range. In the embodiment Ulustrated in FIG. 38 aU the inteUigence 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- controUer. FIG. 39 is a block diagram Ulustrating 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 ofthe first variable with the value ofthe condition, and a circuit 1312 responsive to the comparison for enabling the communications transmitter 1306 to transmit the value ofthe second variable and to transmit a function ofthe value ofthe first variable. Though the description of FIG. 39 is very abstract, the figure represents the essence of the major embodiments ofthe present invention. as the following examples wiU Ulustrate. In a simple man-over-board monitor as Ulustrated in FIG. 1 1. the value 3 10 ofthe first variable is provided by a sensor 308. the value 338 ofthe 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 3 10. In the same man-over-board monitor, when a panic button 3 12 is depressed, the transmitter 314 transmits the remote unit location 338 and the switch status 340. In an environmental monitor Ulustrated in FIG. 18, the value ofthe first variable is a sensor status 578 for a monitored environmental parameter, whUe the value ofthe second variable is a location 576 ofthe 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 ofthe remote unit and the sensor status 578. Altematively, the remote unit 552 defines a patient monitor, and the value ofthe second variable is stored information 556 which identifies the patient, such as name, room and bed number, patient identification code. The value ofthe 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 ofthe second variable and transmits and the sensor status 578 as the function ofthe 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 ofthe value ofthe second variable — dynamic location, patient LD. and static location, respectively — and a transmission of an appropriate function o the value ofthe first variable — sensor status. In a man-over-board monitor 752 Ulustrated in FIG. 21. the value ofthe second variable is provided by a dynamic location determining device, in this case the navigational receiver 756. Altemative embodiments use the World- wide LORAN navigation system, a satellite navigational system such as the GPS system, and other altemative global and regional navigational systems for providing a value ofthe second variable which is the location ofthe remote unit 752. Another example of a remote unit represented by the block diagram in FIG. 39 is a remote weather alarm 1182 Ulustrated in FIG. 34 in which the value ofthe second variable is a remote unit location 1 187. and in which the function ofthe first variable is defined by a circuit 1 198 to be the result 1199 of a comparison of a monitored weather parameter, within the defined zone relative to the weather alarm location 1 187, with a defined weather threshold 1195. Another example ofthe remote unit represented by FIG. 39 is an invisible fence monitor 852 as Ulustrated in FIG. 24. The value ofthe second variable is a location 859 provided by a navigational receiver 856. whUe the transmitted function ofthe first variable is a positional and time status 869. the result of a comparison by a circuit 864 ofthe 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 Ulustrating a remote alarm unit designated generaUy by the numeral 1320. The remote unit 1320 includes a circuit 1322 defining a first variable and providing a value 1 23 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 ofthe first variable with the value ofthe condition, and a circuit 1330 responsive to the comparison for enabling the communications transmitter 1324 to transmit a function ofthe value 1323 ofthe 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 altematively one of direct radio contact such as Ulustrated in a variety ofthe figures, wireless. ceUular. radio telephone, radio relay, to name a few representative communications channels as shown in FIG's 17 and 28. An example of a monitoring system such as illustrated in FIG. 40 is shown in FIG's 3. 30 and 33. In each instance, one or more sensors and switches provide the value for the first variable and the transmitted function ofthe value ofthe first variable is altematively the sensor value and the sensor/switch status. The circuits 1326. 1328 and 1330 find their equivalents in an activation ofthe transmitter upon a change ofthe sensor/switch status. The remote monitoring system Ulustrated in FIG. 3 includes both t> remote unit 82 ofthe class shown in FIG. 40 and a compatible base station 84. FIG. 41 is a partial block diagram which Ulustrates a plurahty 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 generaUy by the numeral 1350 and includes a plurahty of sensor/switch status bits 1352 defining a status vector 1354. A portion 1356 ofthe transmitted format 1350 is unused and marked reserved. FinaUy. FIG. 43 is a partial block diagram Ulustrating the temporary connection of an input device to a remote monitor ofthe 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 supphes 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 momtor 1362. and the remote monitor uses the value stored by the circuit 1364 as the value ofthe second variable. The remote momtor 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 FIG. 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. altematively, a patient's LD information. In one embodiment ofthe 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 ofthe personal alarm system in accordance with this invention. it is to be understood that the above description is Ulustrative only and not limiting ofthe disclosed invention. Thus, the invention is to be limited only by the claims as set forth below.

Claims: Claims: 1. A man-over-board alarm system, comprising: a remote unit including a navigational receiver for receiving navigational information defining a location ofthe remote unit, and a radio transmitter for transmitting the remote unit location; a base station including a radio receiver for receiving the remote unit location; the remote unit and the base station defining a separation distance between the remote unit and the base station: the base station including measuring means for deternώiing whether the separation distance exceeds a predetermined limit, and means responsive to the measuring means for giving an alarm and a display for displaying the remote unit location. whereby, a separation distance exceeding the predetermined limit causes a man-over¬ board alarm and the base station displays the location ofthe remote unit. 2. The man-over-board alarm system as set forth in Claim 1. wherein the remote unit further includes a sensor having an output signal, the sensor defining a sensor status, and the radio transmitter connected to the output signal for transmitting the sensor status, and the base station includes a display for displaying the sensor status, the navigational receiver further includes a low power standby mode and a normal operating mode, and the alarm system further includes means responsive to the sensor output signal for causing the navigational receiver to switch from the standby mode to the normal operating mode when a hazard is detected. 3. The man-over-board alarm system as set forth in Claim 1. wherein the remote unit further includes a sensor having an output signal, the sensor defining a sensor status, and the radio transmitter connected to the output signal for transmitting the sensor status, and the base station includes a display for displaying the sensor status, the remote unit further includes a beacon activated by the sensor output signal when a hazard is detected. 4. The man-over-board alarm system as set forth in Claim 1. wherein the remote unit further includes a sensor having an output signal, the sensor defining a sensor status, and the radio transmitter connected to the output signal for transmitting the sensor status, and the base station mcludes a display for displaying the sensor status, and means responsive to the sensor status for giving an alarm. 5. The man-over-board alarm system as set forth in Claim 1. wherein the remote unit further includes a sensor having an output signaL the sensor defining a sensor status, and the radio transmitter connected to the output signal for transmitting the sensor status, and the base station includes a display for displaying the sensor status, the sensor output signal is provided by a remote unit manuaUy operated switch, defining a panic button, and the system includes a beacon activated by the panic button. 6. The man-over-board alarm system as set forth in Claim 1 ' . including a one-way voice channel linking the remote unit with the base station. 7. The man-over-board alarm system as set forth in Claim 1. wherein the base station includes a radio transmitter and the remote unit includes a radio receiver defining two-way radio communication between the remote unit and the base station, including a two-way voice channel linking the remote unit and the base station. 8. An invisible fence system for monitoring a movable subject, comprising: a remote unit including, a navigational receiver providing a remote unit location, means for providing time-of-day, and a radio transmitter; a base station including. receiving means defining a one-way communication link with the remote unit. and an alarm; the remote unit further including, a first memory for storing information defining a geographic region, a second memory storing information defining a predetermined positional status and a predetermined time interval, and further defining a curfew, and a circuit for comparing the remote unit location, the defined geographic zone, the predetermined positional status, the time-of-day and the curfew, and defining a positional and time status, and the circuit connected to the transmitter for communicating the positional and time status: the base station being responsive to the communicated positional and time status and defining a curfew violation, and the alarm being responsive to the curfew violation. 9. The invisible fence system as set forth in Claim 8. wherein the remote unit transmits the remote unit location and the time-of-day, and the base station further includes means for displaying the remote unit location and the time-of-day. 10. The invisible fence system as set forth in Claim 8, wherein the communications link between the remote unit and the base station receiving means includes a modem for connection to a communications network, the network providing a portion ofthe completed communications link. 1 1. An invisible fence system, comprising: a remote unit including, a navigational receiver providing a remote unit location and a time-of-day. a radio transmitter connected for transmitting the remote unit location and the time-of-day. a radio receiver. alarm and enforcement means responsive to the radio receiver: a base station including, means for receiving the remote unit location and the time-of-day. a first memory storing information defining a geographical region, a second memory storing information defining a predetermined positional status and a time curfew. a circuit for comparing the remote unit location, the defined geographical region and the predetermined positional status, and the time-of-day and the time curfew and for providing a positional and curfew status, a control circuit responsive to the positional and curfew status and defining an enforcement command, and means for transmitting the enforcement command: and the remote unit alarm and enforcement means being responsive to the transmitted enforcement command. 12. The invisible fence system as set forth in Claim 1 1. wherein the base station further includes means for displaying the remote unit location and the time-of-day. and an alarm responsive to an enforcement command. 13. A stationary environmental monitor system, comprising: 43 a remote unit including, storage means for storing information defining the location ofthe remote unit, an environmental sensor providing a plurahty of output signals defining a sensor status vector, an output signal and defining a sensor status, and a radio transmitter connected for transmission ofthe location defining information and the sensor status; and a base station including, a radio receiver for receiving the location defining information and the sensor status, and means responsive to a predetermined change in the sensor status for displaying the location ofthe remote unit and for providing an alarm, whereby the location ofthe remote unit is stored in the storage means and a predetermined change in the sensor status causes the location to be displayed and an alarm to be given at the base station. 14. A personal alarm system, comprising: a remote unit including a navigational receiver for receiving navigational information, a demodulator for demodulating the received navigational information, timing circuits for providing precise time-of-day information, a manually operated switch, defining a panic button and havmg an output signal defining a switch status, operation ofthe panic button producing a change in the switch status, and a radio transmitter for transmitting the demodulated navigational information, the precise time-of-day information, and the switch status; a base station including a radio receiver for receiving the demodulated navigational information, the precise time-of-day information, and the switch status; the base station also including computational means connected for combining the received demodulated navigational information and the precise time-of-day information to determine a location ofthe remote unit, and a display for displaying the location ofthe remote unit; and the base station also including means for displaying the switch status and means responsive to a change in the switch status for giving an alarm, whereby, the remote unit location is displayed, and the alarm is responsive to the panic button. 15. A personal alarm system, comprising: a remote unit including a navigational receiver for receiving navigational information defining a location ofthe remote unit, a manuaUy operated switch defining a panic button and having an output signal defining a switch status, operation ofthe panic button producing a change in the switch status, and a radio transmitter for transmitting the remote unit location and the switch status; a base station including a radio receiver for receiving the remote unit location and the switch status; the base station also including a display for displaying the remote unit location and the switch status; and the base station also including means responsive to a change in the switch status for giving an alarm. whereby, the remote unit location is displayed and a change in the switch status produces an alarm. 16. A personal alarm system, comprising: a remote unit including a navigational receiver for receiving navigational information defining a location ofthe remote unit, the navigational receiver having a low power standby mode and a normal operating mode, the remote unit also including a sensor for detecting a personal hazard, the sensor having an output signal and defining a sensor status, means responsive to the sensor output signal for causing the navigational receiver to switch from the standby mode to the normal operating mode when a hazard is detected, and a radio transmitter for transmitting the remote unit location and the sensor status; a base station including a radio receiver for receiving the remote unit location and the sensor status; the base station also including a display for displaying the remote unit location and the sensor status: and the base station also including means responsive to a change in the sensor status for giving an alarm. whereby, the remote unit location is displayed and a change in the sensor status produces an alarm. 17. A personal alarm system, comprising: a remote unit including radio transmitting means, radio receiving means, at least one sensor means for detecting a personal hazard, the remote unit transmitting means responsive for communicating a detected hazard; the remote unit transmitting means being able to transmit at more than one power level and defining a higher power level, and the remote unit including means for enabling transmission at the higher power level when a personal hazard is detected; a base station including radio transmitting means and radio receiving means; the remote unit and the base station defining a two-way radio communication link, and also 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 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, and for indicating when a personal hazard is detected. 18. A personal alarm system, 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 plurahty of transmitting power levels; a base station including radio transmitting means and radio receiving means; the remote unit and the base station defining a two-way radio communication link, and the remote unit radio receiving means defining a received signal strength; the remote unit including control means responsive to the received signal strength for causing the remote unit to transmit at a power level selected by a predetermined power-level function ofthe received signal strength; the remote unit including at least one sensor means for detecting a personal hazard, and means for communicating the detected hazard to the base station; and the remote unit including means for communicating an alarm function ofthe received signal strength, and the base station including means responsive to the communication for giving an alarm. 19. The personal alarm system as set forth in Claim 18. wherein the received signal strength is further defined by a voltage level on a signal line and the control means includes an analog-to-digital converter connected to receive the signal line and to provide digital output signals connected to address input lines of a read-only memory, the memory containing information defining the power-level function, the memory having digital output lines connected for controlling the power level in response to the received signal strength. 20. The personal alarm system as set forth in Claim 18, wherein the received signal strength is further defined by a voltage level on a signal line and the control means includes an analog-to-digital converter connected to receive the signal line and to provide digital output signals connected to address input lines of a read-only memory, the memory containing information defining the power-level function, the memory having digital output lines connected to the inputs of a digital-to-analog converter, the digital-to-analog converter having an analog output line providing a control voltage for selecting the remote unit transmission power level. 21. A personal alarm system, comprising: a remote unit including a transmitter and a receiver, the remote unit transmitter being capable of transmitting at more than one power level and defining a plurahty of power levels. a base station including a transmitter and a receiver, and defining a two-way communications link with the remote unit. the base station receiver defining a received signal strength, the base station transmitting a command responsive to the received signal strength, the remote unit includmg a control circuit responsive to a received command for selecting the transmission power level, the remote unit including a sensor for detecting a hazard, the sensor defining a sensor status, and the remote unit transmitter connected for communicating the status, the base station including an alarm responsive to the communicated status for giving an alarm when a hazard is detected. 22 The personal alarm system as set forth in Claim 21. wherein the received signal strength is further defined by a voltage level on a signal line and the control circuit includes an analog-to-digital converter connected to receive the signal line and to provide digital output signals connected to address input lines of a read-only memory, the memory containing information defining a power-level function, the memory having digital output lines defining the command for selecting the transmission power level. 23. A weather alarm system, comprising: a remote unit including, a navigational receiver providing a remote unit location, a weather surveUlance radar receiver providing weather parameters within a predetermined weather region, and identifying the weather region, a first memory storing information defining a geographical zone relative to the remote unit location, a circuit combining the remote unit location and the geographical zone to define a local weather zone. a second memory storing information defining at least one weather parameter threshold. means for determining that the local weather zone is within the identified weather region, and that a received weather parameter exceeds the at least one weather arameter threshold, a transmitter connected to communicate the result ofthe determination: and a base station including means responsive to the communication for giving an alarm and for displaying the result ofthe determination. 24. The weather alarm system as set forth in Claim 23. wherein the navigational receiver also provides a time-of-day. and the transmitter also communicates the time-of-day for display by the base station. 25. The weather alarm system as set forth in Claim 23. wherein the transmitter also communicates weather parameters for display by the base station. 26. The weather alarm system as set forth in Claim 23. wherein the base station means responsive to the communication includes a radio receiver. 27. The weather alarm system as set forth in Claim 23. wherein the base station means responsive to the communication includes a modem 28 The weather alarm system as set forth in Claim 23. wherem the navigational receiver includes a low-power standby mode and a normal operating mode and is responsive to the determination for switching from the standby mode to the normal operating mode. 29. A personal alarm system remote unit, comprising: a radio transmitter and radio receiver for providing a two-way radio communication link; a navigational receiver for providing a location ofthe remote unit; a manuaUy operated switch defining a pair of electrical contacts for providing an output signal; the radio transmitter connected for transmitting the remote unit location and the switch output signal; and a microphone and speaker connected with the radio transmitter and receiver for providing a two-way voice channel via the two-way radio communication link. 30. The personal alarm system remote unit as set forth in Claim 29. wherein the radio transmitter and receiver comprise a wireless telephone for use with a wireless telephone network. 31. The personal alarm system remote unit as set forth hi Claim 30. further including means connected to the manuaUy operated switch for initiating a wireless telephone caU to the 911 dedicated pubhc safety help telephone number. 32. The personal alarm system remote unit as set forth in Claim 30, wherein the wireless telephone is a ceUular telephone for operation with a ceUular telephone network. 33. The personal alarm system remote unit as set forth in Claim 30. wherein the wireless telephone is a personal communications services telephone for operation with a personal communications services telephone network. 34. The personal alarm system remote unit as set forth in Claim 30, wherein the wireless telephone is a radio telephone for operation with a radio telephone network. 35. The personal alarm system remote unit as set forth in Claim 30, further including a plurahty of manuaUy operated switches connected for selectively initiating telephone caUs to any one of a plurahty of predetermined telephone numbers. 36. The personal alarm system remote unit as set forth in Claim 35, wherein one ofthe predetermined telephone numbers is the 91 1 dedicated pubhc safety help telephone number. 37. The personal alarm system remote unit as set forth in Claim 35. further including means for manuaUy programming at least some ofthe predetermined telephone numbers. 38. A remote unit, comprising: a communications transmitter; a circuit for providing a first variable having a value; -- — -» Λ\ a circuit for determining whether a predetermined change in the value ofthe first variable has occurred; a circuit for providing a second variable having a value; and the communications transmitter connected for transmitting the value ofthe second variable and the value of a function ofthe first variable when the predetermined change in the value ofthe first variable has occurred. 39. The remote unit as set forth in Claim 38. wherein the circuit for providing the first variable is a sensor having an output signal and the value ofthe first variable is an electrical parameter ofthe output signal and defines a sensor status, and the transmitted function ofthe first variable is the sensor status. 40. The remote unit as set forth in Claim 39. wherein the circuit for providing the first variable includes a plurahty of sensors, each having a sensor output signal having a value defined by an electrical parameter ofthe sensor output signal, and wherein the plurahty of sensor output signals defines a sensor status vector, and the communications transmitter is connected for transmitting the sensor status vector, and wherein the circuit for detemiining whether a predetermined change has occurred determines whether a predetermined change has occuπed within the defined status vector. 41. The remote unit as set forth in Claim 38. wherein the circuit for providing the first variable is a pair of electrical contacts defining a manuaUy operated switch, and wherein the value ofthe first variable is one of a closed circuit and an open circuit defining a switch status, and the transmitted function ofthe first variable is the switch status. 42. The remote unit as set forth in Claim 41, wherein the manuaUy operated switch defines a panic button. 43. The remote unit as set forth in Claim 41. wherein the circuit for providing the first variable is a plurahty of switches, and wherein the value ofthe first variable defines a vector of values, each value being one of a contact closure and an open circuit, defining a switch status vector, and the transmitted function ofthe first variable is the switch status vector. 44. The remote unit as set forth in Claim 43. wherein the plurality of switches defines a manuaUy operated numeric input device. 45. The remote unit as set forth in Claim 43. wherein the plurahty of switches defines a manuaUy operated alphanumeric input device. 46. The remote unit as set forth in Claim 38. wherein the circuit for providing the second variable is a means for storing a number, and the value ofthe second variable is the stored number. 47. The remote unit as set forth in Claim 46, further including means for providing a patient identification code for storage as the value ofthe second variable, and wherein the circuit for providing the first variable includes at least one sensor for monitoring a physiological/environmental parameter and defining a sensor status, the transmitted function ofthe first variable being the sensor status, and the remote unit defining a patient monitor. 48. The remote unit as set forth in Claim 46. further including means for connecting an input device for providing the location ofthe remote unit for storage as the value ofthe second variable, and wherein the circuit for providing the first variable includes a sensor for monitoring an environmental parameter and defining a sensor status, the transmitted function ofthe first variable being the sensor status, and the remote unit defining an environmental monitor. 49. The environmental monitor as set forth in Claim 48 in combination with a plurahty of manuaUy operated switches for providing the location ofthe remote unit. 50. The environmental monitor as set forth in Claim 48 in combination with a dynamic location deterrmning device for providing the location of the remote unit. 51. The environmental monitor as set forth hi Claim 50, wherein the dynamic location determining device is a navigational receiver. 52. The environmental monitor as set forth in Claim 51. wherein the navigational receiver operates with a satellite navigational system. 53. A method for remotely monitoring an environmental parameter, comprising the steps of: providing an environmental monitor as set forth in Claim 48; providing an input device for supplying a number representing a location; connecting the input device to the environmental monitor via the connecting means: determining the location ofthe environmental monitor; using the input device to provide a number corresponding to the location ofthe environmental monitor; storing the number in the number storing means; disconnecting the input device from the connecting means: monitoring an environmental parameter; activating the communications transmitter when a predetermined change in the value ofthe monitored parameter occurs; transmitting the sensor status and the stored location ofthe environmental momtor. 54. The method as set forth in Claim 53, wherein the input device is a plurahty of manuaUy operated switches and wherein the location ofthe environmental monitor is determined using a GPS receiver, and the number representing the location for storage in the number storing means is entered using the manuaUy operated switches. 55. The method as set forth in Claim 53, wherein the input device is a GPS receiver having means for connecting to the environmental monitor, the receiver being operated to determine the environmental monitor location and to provide a number representing the location for storage in the number storing means. 56. The remote unit as set forth in Claim 38, wherein the circuit for providing the second variable is a dynamic location deterrmning means, and the value ofthe second variable is the location ofthe remote unit. 57. The remote unit as set forth in Claim 56, wherein the dynamic location deteπnining means is a navigational receiver. 58. The remote unit as set forth in Claim 57. wherein the navigational receiver is a LORAN receiver. 59. The remote unit as set forth in Claim 57. wherein the navigational receiver is a satellite navigational system receiver. 60. The remote unit as set forth in Claim 59, wherein the satellite navigational receiver is a GPS receiver. 61. The remote unit as set forth in Claim 57, wherein the circuit providing the first variable is a water immersion sensor and wherein immersion ofthe remote unit in water activates the communications transmitter for transmitting the remote unit location, the remote unit defining a man-over-board monitor. 62. The man- over-board momtor as defined in Claim 61. further including a beacon activated when the monitor is immersed in water. 63. The man-over-board monitor as set forth in Claim 62. wherein the beacon is a visual beacon. 64. The man-over-board monitor as set forth in Claim 62. wherein the beacon is an audible beacon. 65. The man-over-board monitor as set forth in Claim 61. adapted for operation from a battery and enclosed in a waterproof floatation device. 66. The man-over-board monitor as set forth in Claim 65, wherein the waterproof floatation device is a life vest. 67. The remote unit as set forth in Claim 57. wherein the circuit for providing the first variable includes: a weather surveillance radar receiver providing weather parameters within a predetermined weather region, and identifying the weather region, a first memory storing information defining a geographical zone relative to the remote unit location. a circuit combining the remote unit location and the geographical zone to define a local weather zone. a second memory storing information defining at least one weather parameter threshold, means for determining that the local weather zone is within the identified weather region, and that a received weather parameter exceeds the at least one weather parameter threshold, and the communications transmitter connected to communicate the result ofthe determination and defining a remote weather alarm, whereby a geographical zone is specified and weather parameters within the zone are monitored and compared with parameter thresholds and the result ofthe comparison is transmitted, permitting remote monitoring of weather conditions within a predefined region. 68. The remote weather alarm as defined in Claim 67. further including the navigational receiver providing time-of-day and the communications transmitter connected to communicate the time-of-day. 69. The remote weather alarm as defined in Claim 67. further including the communications transmitter connected for communicating received weather parameters. 70. The remote weather alarm as defined in Claim 67. further including the first and second memories combined into a single memory. 71. The remote unit as set forth in Claim 57. wherein the circuit for providing the first variable includes: means for providing time-of-day, a first memory for storing information defining a geographic region. a second memory storing information defining a predetermined positional status and a predetermined time intervaL and further defining a curfew, and a circuit for comparing the remote unit location, the defined geographic zone, the predetermined positional status, the time-of-day and the curfew, and defining a positional and time status, the positional and time status defining the value ofthe first variable, the remote unit defining an invisible fence monitor, and the communications transmitter connected for communicating the positional and time status. 72. The invisible fence monitor as defined in Claim 71. wherein the positional and time status define a curfew violation and the monitor includes alarm and enforcement means responsive to the curfew violation. 73. The invisible fence momtor as defined in Claim 71, wherein the first and second memories are combined to form a single memory, so that the information defining a geographic region and the information defining a curfew are stored in the single memory. 74. The invisible fence monitor as defined in Claim 71. wherein the communications transmitter is connected to transmit the monitor location and the time-of-day. 75. The remote unit as set forth in Claim 38, further including a microphone connected to the communications transmitter for providing a one-way voice channel. 76. The remote unit as set forth in Claim 38. further including a communications receiver. 77. The remote unit as set forth in Claim 76, wherein the communications transmitter and the communications receiver are adapted for operation with a radio relay system. 78. The remote unit as set forth in Claim 76, wherein the communications transmitter and the communications receiver are adapted for operation with a radiotelephone system. 79. The remote unit as set forth in Claim 76, wherein the communications transmitter and the communications receiver are adapted for operation with a ceUular telephone system. 80. The remote unit as set forth in Claim 76. wherein the communications transmitter and the communications receiver are adapted for operation with a personal communicator system. 81. The remote unit as set forth in Claim 76. wherein the communications transmitter and the communications receiver are adapted for operation with a wireless communications system. 82. The remote unit as set forth in Claim 76, further including a microphone connected to the commumcations transmitter and a speaker connected to the communications receiver for providing a two-way voice link. 83. A remote unit, comprising: a communications transmitter; a circuit for providing a first variable having a value; a circuit for determining whether a predetermined change in the value ofthe first variable has occurred: the communications transmitter connected for transmitting the value of the first variable when the predetermined change in the value ofthe first variable has occuπed; and a communications receiver. 84. A remote monitoring system, comprising: a remote unit including, a communications transmitter, a circuit for providing a first variable having a value. a circuit for determining whether a predetermined change in the value ofthe first variable has occurred, the communications transmitter connected for transmitting the value ofthe first variable when the predetermined change in the value ofthe first variable has occurred, and a communications receiver; and a base station including, a communications transmitter, a communications receiver defining a two-way communications link with the remote unit, and the base station including alarm and display means responsive to a received value ofthe first variable.

Inventors: BARINGER WILLIAM B, SCHLAGER DAN, BARINGER, WILLIAM, B., SCHLAGER, DAN
Applicants: BARINGER WILLIAM B, SCHLAGER DAN, BARINGER, WILLIAM, B., SCHLAGER, DAN
Application year: 1996
Main classification: G08B21
IPC codes: B63C9, G08B13, G08B19, G08B21, G08B25, G08B26