Getting a sense for danger.
But the children weren't the only ones who saw an opportunity in the storm. Three inmates at a local maximum security prison had been waiting for just such an occasion to try out their escape plan. They knew that the heavy downfall and high winds would curtail the vision of security mobile patrols. The inmates also knew that the prison's CCTV cameras would be unable to show corrections officers anything more than a white wall of blowing snow.
At the height of the storm, the men broke out of their living unit and camouflaged themselves in white sheets. The inmates set out across the prison grounds, making it through the inner fence, and less than twenty feet from the outer fence, when they were detected by an electronic perimeter intrusion detection sensor - in this case, a buried ported coaxial cable. The blizzard breakout had ended in a bust.
DEVELOPMENT. Perimeter intrusion detection sensors were once unable to operate consistently in harsh outdoor environments, but technological refinements have enabled them to function reliably in all weather conditions and environments. These sensors now allow end-users to extend security boundaries away from the assets to issue distant early warning signals and provide time for the threat to be located, assessed, and responded to accordingly. Today's security manager can choose from a range of intrusion detection sensor technologies to address the spectrum of environmental conditions, site constraints, nuisance alarm considerations, and federal and state regulations.
Although human beings are indispensable as the response force, as well as for monitoring security systems and making decisions based on the information provided, they are not as effective at detection as these new-generation sensors. Additionally, the cost of manned guard positions continues to increase steadily. The estimated yearly cost of a twenty-four-hour manned perimeter security post is now between $100,000 and $150,000, and climbing. Meanwhile, the cost of sensor technology continues to decrease while a greater level of security is provided. It is not unusual for an end-user of perimeter intrusion detection sensors to see a payback on the investment within one to two years.
PERFORMANCE PARAMETERS. In selecting a sensor technology or a specific product, a security manager should look at performance with regard to several criteria including the probability of detection (PD), the false alarm rate (FAR), the nuisance alarm rate (NAR), and vulnerability to defeat (VD). These criteria equate to strengths and weaknesses, [TABULAR DATA OMITTED] and their understanding is vital to selecting the right technology for the application.
PD is the likelihood, expressed as a percentage, that an intruder will be recognized within the sensor's detection zone. The sensor's PD is conditional on target characteristics (such as size, speed, and orientation), sensitivity settings, weather conditions, site conditions, and proper maintenance of the equipment. It is considered impossible to have a 100 percent PD for any sensor, as this leads to an unacceptably high FAR or NAR (i.e. the sensor will be too sensitive). The PD for outdoor sensors is typically specified at 99 percent, with anything less than 90 percent considered unacceptable.
FAR is the rate of invalid alarms caused by unknown sources and, for today's sensors, the sensor's own self-generated noise. FAR is normally expressed as a number per day, month, or year. FAR is normally negligible and can range from one per month for low-end sensors to one per year for high-end. However, FAR can also be adversely affected by contributory problems that include temperature extremes above or below what the sensor's electronics were designed to withstand, faulty or improper electrical connections, and damage to equipment done during installation.
NAR is the rate of invalid alarms caused by nonthreat sources. It is also normally expressed as a number per day or month. NAR differs from FAR in that the source of the alarm can be assessed by the system operator as invalid for example, by remote CCTV. NAR is most commonly associated with small animals and nonthreat personnel in the sensor's detection zone. Environmental influences that are recognizable by the operator, such as blowing snow, can also be a factor. NAR will vary considerably depending on the technology and the application. A typical high-end sensor will have a NAR of one per month per zone, whereas low-end sensors can have several per day.
VD is the likelihood that a sensor can be defeated, for example, by bypass or spoofing. Each sensor technology has different vulnerabilities, although they also have some common vulnerabilities. For example, intruder foreknowledge of the system, both operational readiness and state of maintenance, could increase the overall vulnerability for any system.
Proper management of the system's FAR/NAR is critical to the effectiveness of a perimeter protection system. The total invalid alarm rate (IAR) for a sensor is the sum of the FAR and NAR. Without assessment, these nuisance alarms can result in an unacceptably high false dispatch rate and a security system that is either ignored or shut down by its human monitors.
Fortunately, techniques for managing invalid alarm rates have been developed. These include selecting the proper sensor technology for the application, supplementing intrusion detection sensors with CCTV for remote assessment of each event, paying proper attention to site specifics, and providing sufficient operation and maintenance support for the system.
CATEGORIES. Intrusion detection sensors, as defined by operation, are either active or passive. Active sensors transmit energy and detect a change in received energy, while passive sensors [TABULAR DATA OMITTED] detect energy caused by an intruder or a change in natural energy caused by an intruder.
Sensor types, as defined by installation method, are either visible or covert. By configuration, there are line-of-sight sensors requiring the device's components to have a clear, flat surface. Alternately, terrain-following sensors have no line-of-sight limitations, detecting equally well on even or uneven terrain.
[TABULAR DATA OMITTED]
Sensor types can also be defined by the type of detection zone. Volumetric sensors detect intruders in a volume of space that is generally invisible to the naked eye and, therefore, difficult for an intruder to pin down, while line sensors have a detection zone along well-defined parameters.
Finally, they can be categorized by application: freestanding, fence-mounted, or buried.
TECHNOLOGIES. Eight outdoor perimeter intrusion sensor technologies are currently available, each with its own benefits and drawbacks to consider and explore.
Microwave. The most common type of microwave sensor used in outdoor applications is bistatic radar. This technology uses separate transmitting and receiving antennas to define a line-of-sight, above-ground detection zone. These sensors operate at microwave frequencies of 10 GHz or 24 GHz and detect changes in signal that are caused by objects moving between the antennas. To overcome a blindspot located near each antenna, microwave sensors must be overlapped in what is often called a "basket weave" configuration. High detection zones can be created by stacking antennas vertically.
Some of the common FAR/NAR problems associated with microwave sensors are moving metal objects such as chain-link fences that flex in the wind, wandering animals and birds, blowing debris, surface water, vegetation, and blowing sand and snow. Overall, microwave sensors are considered to have a medium level of vulnerabilities.
Ported coax. This type of sensor, which operates in the Very High Frequency radio band, is also known as a "leaky cable" or "guided radar" sensor. It uses buried coaxial cables to establish a terrain-following covert detection zone. An electromagnetic detection field is created around the sensor cables and extends both above and below ground. The sensor looks specifically for the mass, speed, and conductivity characteristics of human intruders and vehicles.
A ported coax sensor can function in all weather conditions and can discern and discount the movement of small animals. The overall invalid alarm rate is very low, with moving surface water or metal objects in the detection zone the only consistent contributors to false and nuisance alarms. The sensor can provide a wide detection field, and ported coax cable can be installed in sand, clay, frozen ground, salt, asphalt, and concrete. The vulnerability to defeat is considered to be the lowest of the sensor technologies now available.
Active infrared. This sensor technology operates by the transmission of an invisible beam shot from an infrared light-emitting diode through a lens to a receiver at the other end of the detection zone. The sensor detects any break in received infrared energy because of an object passing through the beam. Single beam and multiple beam sensors are available.
The IAR of infrared sensors can be affected by weather conditions that cause reduced visibility, such as snow, rain, fog, and dust, as well as by animals, birds, and windborne debris breaking the beam. Multiple beam configurations are quite common, being particularly cost-effective for commercial applications. However, this technology has a high level of vulnerability.
Electric field. Electric field sensors operate by detecting a change in capacitance among a set of parallel, insulated sensor wires attached to a fence or in-smiled on their own posts. The detection zone is defined by the wires with the maximum detection capability in proximity to or between the wires. Fence-mounted, freestanding, and other configurations are possible, and high detection zones can be created by using multiple wires.
The sensors can be falsely set off because of ice-coating, melting snow, fence motion, climbing animals, and metal objects. Electrical grounding of the sensor is required for a low NAR. The technology is considered to have a medium level of vulnerabilities.
Fence disturbance. Fence disturbance sensors come in a variety of forms, including motion sensing, acoustic, or fiber optic. All types are mounted either on the fabric or inside the posts of a fence or pallisade and trigger an alarm at perceived attempts to breach the barrier. Wind, lightning, rain, snow, animals, blowing trash, and vegetation hitting the fence can trigger false or nuisance alarms. This technology is considered to have a medium VD level.
Taut wire. Taut wire sensors combine [TABULAR DATA OMITTED] barrier technology with sensors. They consist of rows of parallel high-tension wires connected to sensors capable of detecting displacements. These sensors can either be contact closure switches or strain measuring gauges. The sensor detects attempts by an intruder to cut, climb, or separate the wires. The wires, usually barbed, can be mounted on existing fences or placed on mounting structures. Animals and ice loading are common invalid alarm problems. Overall, the technology has a low invalid alarm rate; however, it has chronic maintenance issues, and its vulnerability rating is high.
Seismic/pressure. These underground sensors detect pressure waves on or vibration to the soil caused by an intruder. The components of a typical pressure sensor are tubes filled with pressurized liquid connected to a pressure switch. An intruder moving across the detection zone will cause a change in the pressure and trigger an alarm.
A typical seismic sensor consists of a set of coils and magnets called "geo-phones." During a seismic disturbance, caused perhaps by an intruder walking, running, or crawling over the sensor, an electrical current is generated by the coil and magnets, signaling an alarm.
The FAR/NAR of seismic and pressure sensors can be adversely affected by ground conditions and seismic noise, such as wind energy transmitted by fences, poles, trees, and nearby vehicles. The VD of these sensors is rated as medium.
Magnetic field. A magnetic field sensor consists of a series of buried wire loops or coils. Metal objects moving over the sensor induce a current and a subsequent alarm. Magnetic field sensors generally come in two types: those sensitive enough to detect humans and those designed primarily to detect vehicles.
Electromagnetic disturbances such as lightning, or any metallic object, can signal a false or nuisance alarm. The sensor's VD rating is high.
Video motion. Video motion detectors (VMDs) process standard video signals from CCTV cameras, looking for contrast changes in defined sensor zones within the camera's field of view. With the cost of solid-state CCTV cameras decreasing, VMD technology is gaining popularity, especially because of its ease of installation and re-configuration.
Two types of VMD sensors are available: one detects contrast changes and is best suited for indoors or for very short range applications outdoors; the other, an exterior model, uses digital signal processing to detect not only contrast changes but also factors such as target size, speed, and movement.
Sources of movement such as birds, animals, blowing debris, dust, snow, ram, fog, and cloud shadows, as well as camera shake, may cause invalid alarms, although individual VMDs vary in their susceptibility to these factors. The vulnerability rating for video motion sensors is moderate.
TESTING. Due to the wide range of environmental conditions encountered in outdoor security applications, it is difficult to obtain quantitative performance data. Accordingly, systems designers have to rely on references and long-term testing, typically conducted over several seasons, and published by independent testing agencies such as Sandia National Laboratories. Most sensor technologies can reliably detect an upright, walking intruder, but there is a wide variation in performance for other types of intrusions. In addition, each technology has its own particular performance in response to a range of common invalid alarm sources.
COST. This wide range of intrusion detection sensor technologies exists to allow security system designers to match the specific site with the most appropriate technologies. Understanding the strengths and weaknesses of each technology is critical to success. Summary tables, such as those included here, are used to select the appropriate technologies best matching the specific terrain, environmental conditions, and threat level of a site. For high-security applications, technologies are often used in combination - either to provide higher levels of PD or FAR/NAR performance or to reduce vulnerabilities.
In addition to providing higher levels of security - by increasing the standoff distance against explosive devices, for example - outdoor security systems can also provide a significant financial payback by reducing the dangers and costs of manned guarding. However, the security manager must be careful to look at the full life-cycle cost, often referred to as the "cost of ownership," when selecting a security system for a specific application. Major factors of life-cycle cost are initial equipment acquisition, installation, and ongoing maintenance.
Equipment acquisition costs can be misleading, and knowledgeable users must consider a number of factors. The site terrain is one: in rough terrain, line-of-sight sensors can be prohibitively expensive, because more of them are required to provide complete detection coverage. The sensor installation type is another major factor. Visible sensors, because they are relatively easy to defeat, are most cost-effective for low security applications where the intruder is not knowledgeable; otherwise, multiple sensors of different technologies must be used. For high-security applications, covert sensors are more cost-effective, as they have the lowest number of vulnerabilities and can be used alone. Yet another factor is susceptibility to perceive foliage, snow, debris, and other nonthreats as threats. For such sensors, the cost of keeping the zone clear must be included.
Perimeter intrusion detection is one of the most challenging fields in the security industry. But modem technological advances and years of real world experience have refined the art of designing and applying sensors in the outdoor environment, making it possible for security managers who select the right technology for the site to be fairly confident of their outermost defenses.
Ronald W. Clifton is president and CEO of Senstar Corporation, an international security products company with operations in the United States, Canada, the United Kingdom, and Germany. Clifton is a member of the ASIS Standing Committee on Physical Security, as well as the Executive Committee of the Carnahan Conference on Security Technology. Martin L. Vitch, CPP, is senior security consultant for C.H. Guernsey and Company of Stone Mountain, Georgia. He is a member of the ASIS Standing Committee on Physical Security.
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|Title Annotation:||electronic perimeter intrusion detection sensors|
|Author:||Clifton, Ronald W.; Vitch, Martin L.|
|Date:||Feb 1, 1997|
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