Abstract:
A chemical, biological, radiological, and nuclear weapon detection system is disclosed that heightens its acuity and alertness when it senses that a chemical, biological, radiological, or nuclear weapon attack is more likely. For example, it is well understood that a chemical gas attack is likely to be less effective when it is raining than when it is clear because the rain will suppress and dilute the chemical agent. Therefore, the likelihood of a chemical gas attack is higher when it is clear. In light of this and similar knowledge, the illustrative embodiment checks for evidence of an attack more frequently and with great acuity than when the ambient environmental (e.g., meteorological, etc.) characteristics (e.g., whether is it precipitating or not, whether it is sunny or not, etc) suggest that an attack is more likely. This enables the embodiment to conserve consumables that are used in detecting attacks for when the attacks are more likely.

Description:
FIELD OF THE INVENTION 
   The present invention relates to civil defense in general, and, more particularly, to chemical, biological, radiological, and nuclear weapon detection systems. 
   BACKGROUND OF THE INVENTION 
   A chemical, biological, radiological, or nuclear attack on a civilian population is a dreadful event, and the best response requires the earliest possible detection of the attack so that individuals can flee and civil defense authorities can contain its effects. To this end, chemical, biological, radiological, and nuclear weapon detection systems are being deployed in many urban centers that will give civil defense authorities almost instant notification that an attack has occurred. 
   SUMMARY OF THE INVENTION 
   A terrorist seeks to impose his or her will on a government by convincing its citizenry that conciliation—and not confrontation—will make the threat disappear. If the government is able to protect its citizens from violence, the policy of confrontation will be deemed successful and the terrorist&#39;s agenda will be thwarted. In contrast, if the terrorist is able to strike wherever and whenever it desires, the policy of confrontation will be deemed unsuccessful and the terrorist&#39;s agenda will be promoted by those who favor conciliation. 
   In either case, the government and the terrorist are locked in a struggle to undermine the citizenry&#39;s respect and confidence in the other. It warrants repeating that the salient traits that the government and the terrorists vie for are respect and confidence, and, therefore, any factor—however apparently remote—that enhances or detracts either&#39;s respect and confidence is important. 
   One way that the government earns and maintains the respect and confidence of the citizenry is by quickly and accurately informing the public when an attack has occurred and by taking the appropriate action. This means that there are two ways in which the government can lose the respect and confidence of the citizenry. First, the government can fail to inform the public when an actual attack has occurred, and second, the government can inform the public that an attack has occurred when in fact there has been so such attack. Therefore, it&#39;s important for the government to inform the public of an attack when an attack has in fact occurred, but that it is also important for the government not to issue false alarms. To this end, the respect in the government is best enhanced by a chemical, biological, radiological, and nuclear weapon detection system that both: (1) quickly issues an alarm in the event of a real attack, and (2) accurately withholds false alarms. 
   The illustrative embodiment of the present invention incorporates a mechanism that heightens its acuity and alertness when it senses that a chemical, biological, radiological, or nuclear weapon attack is more likely. For example, it is well understood that a chemical gas attack is likely to be less effective when it is raining than when it is clear because the rain will suppress and dilute the chemical agent. Therefore, the likelihood of a chemical gas attack is higher when it is clear. 
   In light of this and similar knowledge, the illustrative embodiment checks for evidence of an attack more frequently and with great acuity than when the ambient environmental (e.g., meteorological, etc.) characteristics (e.g., whether is it precipitating or not, whether it is sunny or not, etc) suggest that an attack is more likely. This enables the embodiment to conserve consumables that are used in detecting attacks for when the attacks are more likely. 
   The illustrative embodiment comprises: a first environmental sensor for monitoring a first environmental factor; and a first hazardous material sensor for checking for the presence of a first hazardous material, wherein said first hazardous material sensor checks for the presence of said first hazardous material in accordance with a first schedule that is based on said first environmental factor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  depicts a city map and the location of the salient components of the illustrative embodiment of the present invention on that map. 
       FIG. 2  depicts a block diagram of the salient components of each of environmental sensor arrays  101 - 1  through  101 - 17 . 
       FIG. 3  depicts a block diagram of the salient components of each of video camera clusters  102 - 1  through  102 - 13 . 
       FIG. 4  depicts a block diagram of the salient components of each of hazardous material detection stations  103 - 1  through  103 - 11 . 
       FIG. 5  depicts a block diagram of the salient components of hazardous material sensor array  401 - k.    
       FIG. 6  depicts a block diagram of the salient components of hazardous material station processor  402 - k.    
       FIG. 7  depicts a block diagram of the salient components of system control center  110 . 
       FIG. 8  depicts a flowchart of the salient tasks associated with the deployment and operation of the illustrative embodiment. 
       FIG. 9  depicts a flowchart of the salient tests in task  805  of  FIG. 8 . 
       FIG. 10  depicts a flowchart of the salient tasks associated with the operation of hazardous material detection processor  402 - k.    
       FIG. 11  depicts the threshold for VX Gas in parts per million (ppm) as a function of both precipitation and whether or not it is sunny. 
   

   DETAILED DESCRIPTION 
     FIG. 1  depicts a city map and the location of the salient components of the illustrative embodiment of the present invention on that map. The illustrative embodiment comprises:
         i. seventeen (17) spatially-disparate environmental sensor arrays  101 - 1  through  101 - 17 ,   ii. thirteen (13) spatially-disparate video camera clusters  102 - 1  through  102 - 13 ,   iii. eleven (11) spatially-disparate hazardous material detection stations  103 - 1  through  103 - 11 , and   iv. system control center  110 .
 
Environmental sensor arrays  101 - 1  through  101 - 11  and video camera clusters  102 - 1  through  102 - 11  are not distinctly shown in  FIG. 1  because they are co-located with and contained within hazardous material detection stations  103 - 1  through  103 - 11 , respectively.
       
   Environmental sensor arrays  101 - 1  through  101 - 17 , video camera clusters  102 - 1  through  102 - 13 , and hazardous material detection stations  103 - 1  through  103 - 11  are deployed throughout city  100  to enable the comprehensive environmental, video, and hazardous material surveillance of city  100 . In accordance with the illustrative embodiment, all of environmental sensor arrays  101 - 1  through  101 - 17 , video camera clusters  102 - 1  through  102 - 13 , and hazardous material detection stations  103 - 1  through  103 - 11  are outdoors, but after reading this specification it will be clear to those skilled in the art how to make and use embodiments of the present invention in which some or all of the environmental sensor arrays, video camera clusters, and hazardous material detection stations are indoors. Furthermore, although the illustrative embodiment is depicted as deployed in an urban environment, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that are deployed or deployable in other environs (e.g., on ship board, in a rural area, in suburbia, etc.). 
   Each of environmental sensor arrays  101 - 1  through  101 - 17  monitors eight environmental characteristics (e.g., precipitation, humidity, sunlight, temperature, wind speed, wind direction, barometric pressure, ambient sound, etc.) at a different location and reports its findings to system control center  110 . Furthermore, each of environmental sensor arrays  101 - 1  through  101 - 11  reports its findings to hazardous material detection stations  103 - 1  through  103 - 11 , respectively. In accordance with the illustrative embodiment, the reporting is accomplished through wireline telemetry in well-known fashion. It will be clear to those skilled in the art, however, after reading this specification, how to make and use alternative embodiments of the present invention in which some or all of the reporting is accomplished through wireless telemetry. The details of environmental sensor arrays  101 - 1  through  101 - 17  are described below and with respect to  FIG. 2 . 
   Each of video camera clusters  102 - 1  through  102 - 13  monitors a location, in well-known fashion, and transmits its video signals to system control center  110  via wireline telemetry. It will be clear to those skilled in the art, however, how to make and use alternative embodiments of the present invention in which some or all of the video signals are transmitted via wireless telemetry. The details of video camera clusters  102 - 1  through  102 - 13  are described below and with respect to  FIG. 13 . 
   Each of hazardous material detection stations  103 - 1  through  103 - 11  measures the amount of six (6) hazardous materials (e.g., nuclear warfare agents, chemical warfare agents, biological warfare agents, etc.) and transmits an alarm status for each hazardous material to system control center  110  via wireline telemetry. It will be clear to those skilled in the art, however, how to make and use alternative embodiments of the present invention in which some or all of the alarms are transmitted via wireless telemetry. Although each of hazardous material detection stations  103 - 1  through  103 - 11  detects six (6) hazardous materials, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that detect any number of hazardous materials. The details of hazardous material detection stations  103 - 1  through  103 - 11  are described below and with respect to  FIGS. 4 through 6 . 
   Although the illustrative embodiment comprises 17 environmental sensor arrays, 13 video camera clusters, and 11 hazardous material detection stations, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that comprise any number of environmental sensor arrays, video camera clusters, and hazardous material detection stations. Furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which one or more of the hazardous material detection stations lacks a video camera cluster or an environmental sensor array or both. 
   System control center  110  receives the telemetry from environmental sensor arrays  101 - 1  through  101 - 17 , video camera clusters  102 - 1  through  102 - 13 , and hazardous material detection stations  103 - 1  through  103 - 11  and determines, in the manner described below, whether or not to issue a system-wide alarm. The operation of environmental sensor arrays  101 - 1  through  101 - 17 , video camera clusters  102 - 1  through  102 - 13 , hazardous material detection stations  103 - 1  through  103 - 11 , and system control center  110  are described in detail below and with respect to  FIGS. 8 through 11 . 
     FIG. 2  depicts a block diagram of the salient components of each of environmental sensor arrays  101 - 1  through  101 - 17 . Environmental sensor array  101 - i , for i=1 through 17, comprises:
         i. precipitation sensor  201 - i - 1 ,   ii. humidity sensor  201 - i - 2 ,   iii. sunlight sensor  201 - i - 3 ,   iv. temperature sensor  201 - i - 4 ,   V. wind speed sensor  201 - i - 5 ,   vi. wind direction sensor  201 - i - 6 ,   vii. barometric pressure sensor  201 - i - 7 , and   viii. ambient sound sensor  201 - i - 8 .
 
The illustrative embodiment measures these eight environmental factors because each of them can—for the reasons described below—be correlated to the efficacy, and, therefore, the likelihood of a chemical, biological, radiological, or nuclear weapons attack.
       
   In accordance with the illustrative embodiment, each of environmental sensor arrays  101 - 1  through  101 - 17  comprises the same eight sensors. It will be clear to those skilled in the art however, after reading this specification, how to make and use alternative embodiments of the present invention in which each sensor array has any subset of these sensors. Furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention that measure one or more additional environmental factors that can be correlated to the efficacy, and, therefore, the likelihood of a chemical, biological, radiological, or nuclear weapons attack. 
   The output of each sensor is multiplexed into environmental telemetry feed  202 - i  in well-known fashion and transmitted to system control center  110  and, for k=1 through 11 to hazardous material station alarms  402 - k , respectively. It will be clear to those skilled in the art how to make each of environmental sensor arrays  101 - 1  through  101 - 17 . 
     FIG. 3  depicts a block diagram of the salient components of each of video camera clusters  102 - 1  through  102 - 13 . Video camera cluster  102 - v , for v=1 through 13, comprises: video camera # 1 , video camera # 2 , and video camera # 3 . The output of each camera is multiplexed in well-known fashion and transmitted to system control center  110  via wireline telemetry feed  302 - v . It will be clear to those skilled in the art how to make each of video camera clusters  102 - 1  through  102 - 13 . 
   In accordance with the illustrative embodiment, each of video camera clusters  102 - 1  through  102 - 13  comprises three cameras. It will be clear to those skilled in the art however, after reading this specification, how to make and use alternative embodiments of the present invention in which each video camera cluster has any number of video cameras (including only one (1) camera). 
     FIG. 4  depicts a block diagram of the salient components of each of hazardous material detection stations  103 - 1  through  103 - 11 . Hazardous material detection station  103 - k , for k=1 through K, comprises:
         i. hazardous material sensor array  401 - k ,   ii. hazardous material station processor  402 - k ,   iii. environmental sensor array  101 - k , and   iv. video camera cluster  102 - k,  
 
interconnected as shown.
       
   Hazardous material sensor array  401 - k  comprises six hazardous material sensors for measuring the amount of alpha particles, beta particles, anthrax, small pox, sarin gas, and VX gas present at the array. In accordance with the illustrative embodiment of the present invention, hazardous material sensor array  401 - k  receives measurements on the current environmental factors from environmental sensor array  101 - k  and uses them to determine how frequently—and with what sensitivity—it should sample the ambient environment for the hazardous materials. This is because a chemical, biological, radiological, or nuclear attack is more likely to occur when some environmental factors are present than at other times, and, therefore, the illustrative embodiment is more diligent in looking for an attack when the environmental factors are more favorable for an attack. 
   Hazardous material sensor array  401 - k  does not determine whether the amount of a measured hazardous material should trip an alarm; this is performed by hazardous material station processor  402 - k . To this end, the measurements made by hazardous material sensor array  401 - k  are transmitted to hazardous material station processor  402 - k  via wireline feed  411 - k . The details of hazardous material sensor array  401 - k  are described below and with respect to  FIG. 5 . 
   Hazardous material station processor  402 - k  takes the measurements from hazardous material sensor array  401 - k  and the measurements from environmental sensor array  101 - k  and determines whether or not to transmit a “station” alarm to system control center  110  via wireline telemetry feed  412 - k . In accordance with the illustrative embodiment, an alarm is not issued when the measured amount of a hazardous material reaches a static threshold. Instead, an alarm is issued when the amount of a hazardous material reaches a dynamic threshold, wherein the threshold changes and is based on at least one environmental factor. The purpose of having the threshold change as a function of one or more environmental factors is to recognize that a chemical, biological, radiological, or nuclear attack is more likely to occur when some environmental factors are present than at other times, and, therefore, the threshold for issuing an alarm should lower when the environmental factors are more favorable for an attack than when the factors are unfavorable for an attack. The threshold for each hazardous material can be changed independently of the threshold for the other hazardous materials, and the threshold for each threshold can be determined using a different function of the environmental factors. The details of hazardous material station processor  402 - k  are described in detail below and with respect to  FIG. 6 . 
   Hazardous material station processor  402 - k  comprises a general-purpose digital processor that performs an adaptive algorithm that sets the dynamic threshold based on measurements from environmental sensor array  101 - k . In some alternative embodiments of the present invention, hazardous material station processor  402 - k  is a neural network. 
     FIG. 5  depicts a block diagram of the salient components of hazardous material sensor array  401 - k , which comprises:
         i. alpha particle sensor  501 - k - 1 ,   ii. beta particle sensor  501 - k - 2 ,   iii. anthrax sensor  501 - k - 3 ,   iv. small pox sensor  501 - k - 4 ,   V. sarin gas sensor  501 - k - 5 , and   vi. VX gas sensor  501 - k - 6 ,
 
interconnected as shown. Each of the six sensors is a point sensor and receives one or more measurements of the current ambient environment factors as observed by environmental sensor array  101 - k  and uses them to change the schedule or when—and with what care—it should sample the ambient environment for its specific hazardous material. In some alternative embodiments of the present invention, one or more of the sensors are stand-off sensors, in contrast to point sensors, and it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention which comprise point sensors, stand-off sensors, or a combination of point sensors and stand-off sensors.
       
   In general, a chemical, biological, radiological, or nuclear attack is more likely to occur:
         i. when it is not precipitating (e.g., raining, snowing, sleeting, etc.) because the precipitation frustrates the dissemination and enervates the efficacy of the hazardous materials;   ii. when it is lower humidity, for the same reasons;   iii. when it is night (i.e., there is no sunlight) because the sunlight tends to breakdown the biological and chemical agents, because attacks are more psychologically terrifying at night, and because inversion layers typically occur at night;   iv. when the temperature is not extreme;   V. when the wind is blowing because the wind helps to the disseminate the hazardous materials;   vi. when the wind is blowing in a constant direction because it also helps to disseminate the hazardous materials;   vii. when a rising barometric pressure suggests that fair weather is coming; and   viii. shortly after a sound that could be caused by a chemical explosion.
 
Therefore, the schedule for checking for each hazardous material should be faster or more frequent when the conditions are ripe for an attack with that type of material. The rate for checking for each hazardous material can be different than the rate for the other hazardous materials, and the rate for checking for each hazardous material can be a different function of environmental factors. After reading this specification, it will be clear to those skilled in the art how to make and use alpha particle sensor  501 - k - 1 , beta particle sensor  501 - k - 2 , anthrax sensor  501 - k - 3 , small pox sensor  501 - k - 4 , sarin gas sensor  501 - k - 5 , and VX gas sensor  501 - k - 6 .
       

     FIG. 6  depicts a block diagram of the salient components of hazardous material station processor  402 - k , which comprises:
         i. alpha particle station alarm  601 - k - 1 ,   ii. beta particle station alarm  601 - k - 2 ,   iii. anthrax station alarm  601 - k - 3 ,   iv. small pox station alarm  601 - k - 4 ,   V. sarin gas station alarm  601 - k - 5 , and   vi. VX gas station alarm  601 - k - 6 ,
 
interconnected as shown.
       
   Each of these six station alarms receives:
         i. one or more measurements of the current ambient environment factors as observed by environmental sensor array  101 - k , and   ii. a stream of measurements from its corresponding sensor in hazardous material sensor array  401 - k,  
 
and uses them to determine when an alarm for that hazardous material should be transmitted to system control center  110  via wireline  411 - k . Each of the six station alarms is issued when the amount of a hazardous material reaches a threshold, and an alarm is stopped when the amount of the hazardous material falls below the threshold. A station can issue one or more alarms concurrently.
       

   The thresholds are not static, however, but change and are at least partially based on one or more of the measurements of the current ambient environment factors as observed by environmental sensor array  101 - k . In particular, a chemical, biological, radiological, or nuclear attack is more likely to occur when some environmental conditions are present, and, therefore, the individual thresholds for each alarm are higher when those environmental conditions do not exist. For example, the threshold for sarin as is higher when it is precipitating than when it is not precipitating, lower when it is lower humidity than higher humidity, lower when it is night than when it is day, and lower when it is windy than when it is not windy. The operation of hazardous material station processor  402 - k  is described in detail below and with respect to  FIGS. 8 through 11 . 
     FIG. 7  depicts a block diagram of the salient components of system control center  110 , which comprises:
         i. hazardous material detection station map  701 ,   ii. system processor  702 ,   iii. video switch  703 , and   iv. video display  704 ,
 
interconnected as shown.
       
   One of the advantages of the illustrative embodiment is that it incorporates mechanisms that seek to thwart false system alarms. One of these mechanisms is based on the understanding that a chemical, biological, radiological, or nuclear weapon attack is more likely to issue when there are alarms from multiple stations that are near each other than when there are alarms from multiple stations that are not near each other (e.g., are randomly distributed around the area that is monitored, etc.). To facilitate this analysis, the illustrative embodiment comprises a map—hazardous material detection station map  701 —that associates each hazardous material detection station to its location (e.g., latitude and longitude, etc.). 
   Another of the mechanisms that the illustrative embodiments uses to prevent false system alarms is based on the understanding that alarms from multiple stations are more likely to occur temporally in the same direction as the wind—as the hazardous material is blown downwind and into contact with the various hazardous material detection stations. To facilitate this analysis, hazardous material detection station map  701  also associates each environmental sensor array to its location. 
   In accordance with the illustrative embodiment, hazardous material detection station map  701  is a data structure, such as that depicted in Table 1. 
                                               TABLE 1                   Hazardous Material Detection Station Map 701                Latitude   Longitude                        Hazardous Material   40° 35′ 56.03″ N.   140° 35′ 46.44″ E.       Detection Station 411-1       Hazardous Material   40° 34′ 26.83″ N.   140° 36′ 36.02″ E.       Detection Station 411-2       . . .   . . .   . . .       Hazardous Material   40° 36′ 36.14″ N.   140° 38′ 56.33″ E.       Detection Station 411-11       Environmental Sensor   40° 35′ 56.66″ N.   140° 33′ 14.03″ E.       Array 101-12       Environmental Sensor   40° 36′ 49.35″ N.   140° 35′ 06.55″ E.       Array 101-13       . . .   . . .   . . .       Environmental Sensor   40° 37′ 35.93″ N.   140° 35′ 52.83″ E.       Array 101-17                    
It will be clear to those skilled in the art how to make hazardous material detection station map  701 .
 
   System processor  702  receives the telemetry from hazardous material detection alarms  411 - 1  through  411 - 11 , the telemetry from environmental sensor arrays  101 - 1  through  101 - 17 , and the location data from hazardous material detection station map  701  and determines whether or not to issue a system alarm. In accordance with the illustrative embodiment, system processor  702  is a general-purpose processor that is programmed to perform the functionality described herein and with respect to  FIGS. 8 through 11 . 
   When system processor  702  determines that an attack has occurred or is occurring, it issues a system alarm to the personnel who monitor the illustrative embodiment (who are not shown in  FIG. 7 ) and it directs video switch  703  to automatically route the video feed(s) for the area(s) where the attack has occurred or is occurring to video display  704 . This enables the personnel who monitor the illustrative embodiment to further verify the attack. For example, if system processor  702  determines that a chemical gas attack is occurring in Times Square, then video of people collapsing and convulsing in Times Square will enable the personnel who monitor the illustrative embodiment to verify that indeed a gas attack has occurred. In contrast, if system processor  702  determines that a chemical gas attack is occurring in Times Square, then video showing people going about their business as usual will suggest to the personnel who monitor the illustrative embodiment that it is a false alarm or that it should be investigated more thoroughly. 
   Video switch  703  is controllable by system processor  702  as it is well known to those skilled in the art, and video display  704  is also well known to those skilled in the art. 
     FIG. 8  depicts a flowchart of the salient tasks associated with the deployment and operation of the illustrative embodiment. 
   At task  801 , hazardous material detection station map  701  is built and environmental sensor arrays  101 - 1  through  101 - 17 , video camera clusters  102 - 1  through  102 - 13 , and hazardous material detection stations  103 - 1  through  103 - 11  are deployed throughout city  100  in accordance with hazardous material detection station map  701 . It will be clear to those skilled in the art, after reading this specification, how to perform task  801 . 
   At task  802 , system processor  702  in system control center  110  continually receives the station alarm status from each of the six station alarms for each of the eleven hazardous material detection stations (i.e., system processor  702  periodically receives the station alarm status for all 11×6=66 station alarms). In the best of cases, system processor  702  does not receive any station alarms. 
   At task  803 , system processor  702  in system control center  110  continually receives the environmental telemetry transmitted from each of the eight environmental sensors for each of the sixteen environmental sensor arrays (i.e., system processor  702  periodically receives the environmental data for all 16×8=128 environmental sensors). 
   At task  804 , system processor  702  in system control center  110  continually receives the video signals from each of the thirteen video surveillance clusters. In accordance with the illustrative embodiment, tasks  802 ,  803 , and  804  are performed concurrently, but it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention in which tasks  802 ,  803 , and  804  are performed in any order. 
   At task  805 , system processor  702  in system control center  110  determines whether a system-wide alarm should be issued. In accordance with the illustrative embodiment, system processor  702  determines whether to issue a system-wide alarm based on:
         i. the number of station alarms that are received,   ii. the number of hazardous materials that are detected,   ii. the proximity of the station alarms, when there is more than one station alarm,   iv. the temporal sequence in which the station alarms are received, when there is more than one station alarm, and   v. the environmental conditions (including wind direction).
 
It will be clear to those skilled in the art, however, after reading this specification, how to make and use alternative embodiments of the present invention that omit one or more of these factors. When system processor  702  determines that an alarm should be issued, control passes to task  806 ; otherwise control returns to task  802 . The details of task  805  are described below and with respect to  FIG. 9 .
       

   At task  806 , system processor  702  issues a system-wide alarm and directs video switch  703  to direct the video telemetry from areas where the station alarms are coming to video display  704 . After task  806  has been performed, control returns to task  802 . 
     FIG. 9  depicts a flowchart of the salient tests in task  805  of  FIG. 8 . It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that omit one or more of the tests. 
   At test  901 , system processor  702  determines whether at least N of M neighboring hazardous material detection stations issued an alarm for a first hazardous material, wherein N and M are positive integers, wherein 2≦N≦M≦K, and wherein at least one of N and M change based on an environmental factor. Test  901  incorporates three different mechanisms for reducing the probability that a false system-wide alarm will be issued. 
   The first mechanism requires that at least N (wherein 2≦N) stations report an alarm for the same hazardous material within an interval of time. This prevents a false alarm from one hazardous material detection station from issuing a false system-wide alarm. If the probability of a station issuing a false alarm is p and the probability of each station issuing a false alarm is independent of another station issuing a false alarm, then the probability that the illustrative embodiment will issue a false system-wide alarm is no higher than p N . The implication is that the probability of issuing a false system-wide alarm is affected by the value of N. High values of N lower the likelihood of a false system-wide alarm, but also increase the likelihood that a real system-wide alarm will not issue. It will be clear to those skilled in the art, after reading this specification, how to select values for N based on the acceptable likelihood of a false system-wide alarm and on the likelihood that a real system-wide alarm will not issue. 
   The second mechanism requires that the N stations reporting an alarm for the same hazardous material within an interval of time be a subset of M neighboring stations (i.e., have some proximity to each other). For the purpose of this specification, M stations are “neighboring stations” if and only if a circle exists that contains all M stations and no other stations. System processor  702  uses Hazardous Material Detection Station Map  701  to determine if a circle exists that contains all M stations and no other stations. 
   The purpose of this mechanism is to issue a system-wide alarm only when the N stations reporting an alarm for the same hazardous material within an interval of time have some proximity to each other. This is based on the assumption that a real attack is more likely to be detected by stations that are near each other than by stations that have no proximity. Small values of M lower the likelihood of a false system-wide alarm, but also increase the likelihood that a real system-wide alarm will not issue. It will be clear to those skilled in the art, after reading this specification, how to select values for M based on the acceptable likelihood of a false system-wide alarm and on the likelihood that a real system-wide alarm will not issue. 
   The third mechanism changes the values of at least one of N and M based on at least one environmental factor (e.g., precipitation, wind speed, the amount of sunlight, etc.) to cause the threshold for a system-wide alarm to be higher when the environmental factor(s) suggest that an attack is less likely. For example, the ratio of N:M will be higher when it is precipitating, when it is not windy, and when it is sunny. It will be clear to those skilled in the art, after reading this specification, how to change the values of N and M based on environmental factors based on the acceptable likelihood of a false system-wide alarm and on the likelihood that a real system-wide alarm will not issue. 
   In some alternative embodiments of the present invention, test  901  determines whether A % of the hazardous material detection stations within B meters issued an alarm for a first hazardous material, wherein A and B are positive real numbers, wherein 0%≦A %≦100%, and wherein at least one of A and B change based on an environmental factor. 
   At test  902 , system processor  702  determines whether at least P of V neighboring hazardous material detection stations issued an alarm for the first hazardous material, wherein P and V are positive integers, 2≦P≦V≦K, N≦P and wherein at least one of P and V change based on an environmental factor. The purpose of test  902  is to ensure that a system-wide alarm is only issued when the extent of the stations reporting an alarm expands, as would be expected in a real attack. 
   Test  902  incorporates three different mechanisms for reducing the likelihood that a false system-wide alarm will be issued, and these three mechanisms are analogous to those in test  901 . Therefore, it will be clear to those skilled in the art, after reading this specification, how to select values for P and V and how to change them based on environmental factors based on the acceptable likelihood of a false system-wide alarm and on the likelihood that a real system-wide alarm will not issue. 
   In some alternative embodiments of the present invention, test  902  determines whether C % of the hazardous material detection stations within D meters issued an alarm for the first hazardous material, wherein C is a positive real number, wherein 0%≦C %≦100%, and wherein at least one of C and D change based on an environmental factor. 
   At test  903 , system processor  702  determines whether at least R of S neighboring hazardous material detection stations issued an alarm for a second hazardous material, wherein R and S are positive integers, wherein 2≦R≦S≦K, and wherein at least one of R and S change based on an environmental factor. The purpose of test  903  is to ensure that a system-wide alarm is only issued when a second hazardous material is detected in addition to the first hazardous material, as would be expected in some types of real attacks. For example, in a nuclear attack, the detection of alpha particles might be accompanied by the detection of beta particles. There are, of course, other kinds of attacks that involve only one type of hazardous material. 
   In some alternative embodiments of the present invention, test  903  determines whether E % of the hazardous material detection stations within F meters issued an alarm for a second hazardous material, wherein E is a positive real number, wherein 0%≦E %≦100%, and wherein at least one of E and F change based on an environmental factor. 
   Test  903  incorporates three different mechanisms for reducing the likelihood that a false system-wide alarm will be issued, and these three mechanisms are analogous to those in test  901 . Therefore, it will be clear to those skilled in the art, after reading this specification, how to select values for R and S and how to change them based on environmental factors based on the acceptable likelihood of a false system-wide alarm and on the likelihood that a real system-wide alarm will not issue. 
   At test  904 , system processor  702  determines whether the spread of station alarms is generally consistent with the prevailing wind direction, as would be expected in a real attack as the hazardous material is blown downwind. To do this processor  702  uses it knowledge of the position of the stations reporting alarms, hazardous material detection station map  701 , and its knowledge of the prevailing wind direction, which it gleans from the environmental sensor arrays in the vicinity of the stations reporting alarms. It will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that decide whether the spread of station alarms is generally consistent with the prevailing wind direction. 
     FIG. 10  depicts a flowchart of the salient tasks associated with the operation of hazardous material detection processor  402 - k.    
   At task  1001 , hazardous material detection processor  402 - k  receives the environmental data from environmental sensor array  101 - k . It will be clear to those skilled in the art how to make and use embodiments of the present invention that perform task  1001 . 
   At task  1002 , hazardous material detection processor  402 - k  receives the hazardous material measurements from hazardous material sensor array  401 - k . It will be clear to those skilled in the art how to make and use embodiments of the present invention that perform task  1002 . Furthermore, it will be clear to those skilled in the art, after reading this specification, how to make and use embodiments of the present invention that perform tasks  1001  and  1002 , concurrently or in any order. 
   At task  1003 , hazardous material detection processor  402 - k  determines, based on the measurements received in task  1002  and the environmental data received in task  1001 , whether the amount of a hazardous material has reached a threshold such that the station&#39;s alarm should be issued. When hazardous material detection processor  402 - k  determines that the alarm should be issued, control passes to task  1004 ; otherwise control returns to task  1001 . 
   Hazardous material detection processor  402 -k incorporates a mechanism to reduce the probability that a false station alarm will be issued. In particular, hazardous material detection processor  402 - k  changes the threshold for each hazardous material based, at least in part, on the environmental data received in task  1001 . For example,  FIG. 11  depicts the threshold for VX Gas in parts per million (ppm) as a function of both precipitation and whether or not it is sunny. From  FIG. 11 , it can be seen that the threshold is higher when it is precipitating and sunny than when it is not precipitation or not sunny or neither precipitating nor sunny. 
   At task  1004 , hazardous material detection processor  402 - k  transmits a station alarm to system control center  110 , via wireline  412 - k . After task  1004 , control returns to task  1001  to determine if an alarm for a second hazardous material should be issued and to determine if the amount of the first hazardous material has fallen (or the threshold raised) such that the alarm should be discontinued. 
   It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
   Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,”“in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.