Part 2 of a primer on outdoor perimeter protection by Michael E Rack, Vice president, Marketing and Business Development, Senstar-Stella Corporation
In last month's edition we looked at the history and basics of outdoor perimeter intrusion detection.

We stressed that security professionals who have to select a technology should look at strengths and weaknesses with regard to specific criteria including:

  • the probability of detection (PD)
  • the vulnerability to defeat (VD)
  • the false alarm rate (FAR)
  • the nuisance alarm rate (NAR)

We examined the first four of thirteen categories of sensor technology: Ported coax; Fence Disturbance; Microwave and Passive Infrared (PIR).

Here we continue with a look at the remaining eleven categories of sensor technology ...

  • Active Infrared: This sensor technology operates by the transmission of an invisible beam projected 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 the received infrared energy because of an object passing through the beam. Multiple beam units (dual or quad) are most common outdoors to minimise the NAR from small animals and birds. Several units have to be stacked in heated enclosures in order to achieve significant vertical coverage. The result is active, visible, line-of-sight freestanding coverage in a single plane.

    The PD of infrared beam sensors can be affected by weather conditions that cause reduced visibility, such as heavy snow, sheeting rain, fog, blowing sand and dust. Animals, flocks of birds, blowing debris and leaves, and reflected sunlight from shiny surfaces can affect NAR. The vulnerability is rated as high because of the ease of tunnelling under or bridging over the beams.

  • Electric Field: Electric field sensors operate by detecting a change in capacitance among a set of parallel, insulated sensor wires attached to standoffs on a fence or installed on their own posts. The detection zone is defined by the wires, with detection occurring in proximity to or in between the wires. Configurations can also be used to protect rooftops and the sides of buildings. Using multiple wires can create high detection zones. The technology is classified as active, visible, terrain following, volumetric, and either freestanding or fence-mounted depending on the application. Its imposing configuration has a significant deterrent value.

    Recent developments include using DSP to enhance detection and minimise NAR, and improvements in the insulators and insulator types that support the wires. NAR can be affected by rain, snow, ice coating of the wires, fence motion and animals. Electrical grounding of the sensor and the mounting poles is required for a low NAR. This technology is considered to have a medium VD level because of the possibility of bridging over or tunnelling under the detection field.

  • Taut wire sensors combine barrier technology with sensors. They consist of rows of parallel tensioned wires connected to sensors capable of detecting displacements. These sensors can be either contact closure switches, piezoelectric sensors or strain measuring gauges. The sensor detects attempts by an intruder to cut, climb or separate the wires. The wires, sometimes barbed, are supported on mounting structures that can be installed on existing fences or on freestanding poles. The technology is classified as passive, visible, terrain following, line, and either freestanding or fence-mounted depending on the application.

    Taut wire has the lowest NAR of any sensor technology. Animals and ice loading can contribute to NAR. Installation often includes a buried concrete wall to prevent tunnelling. The wire tension has to be checked twice a year. This technology is considered to have a medium VD level because of the possibility of bridging.

  • Electrified Barrier: An electrified barrier consists of a set of parallel, insulated wires that apply a non-lethal shock to intruders when they come in contact with the wires. An alarm is also declared. The wires may be fence-mounted or freestanding. In a few jurisdictions (e.g. prisons) lethal shocks may be permitted. This system has a significant deterrent value, but can be defeated by a sophisticated intruder. It may not be legal in all jurisdictions. The technology is classified as active, visible, terrain following, line, and either freestanding or fence-mounted depending on the application. Recent developments include more sophisticated means of varying the applied voltage and exciting the wires to prevent spoofing. Sources of NAR include casual contact by animals and birds, rain, snow, and lightning. This technology is considered to have a medium VD level because of the possibility of bridging over or tunnelling under the wires.

  • Surface Wave: Surface wave sensors operate by setting up an electromagnetic field around a pair of parallel wires that are supported by fibreglass poles. The field is particularly sensitive to human-sized intruders. Surface wave technology is used primarily in rapid deployment applications, but has also been used for rooftop protection. It is classified as active, visible, terrain following, volumetric, and freestanding.

    Sources of NAR include wind, heavy rain, wet snow, animals and flocks of birds. PD can be affected by installation quality. This technology is considered to have a medium VD level.

    Seismic sensors are buried to detect vibration to the soil caused by an intruder. A typical sensor consists of a set of coils and magnets called "geophones". 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, signalling an alarm. The technology is classified as passive, covert, terrain following, volumetric and buried.

    The security professional must be careful to look at the full life-cycle cost - the cost of ownership - when selecting a system

    The PD can be affected by soil conditions, especially frozen ground. The movement of a number of large animals, the movement of tree roots, fences and poles because of wind, and the disturbances caused by nearby vehicles all contribute to NAR. This technology has a medium VD because, although difficult to detect, if its location is known, it can be easily bridged.

  • Pressure: Pressure sensors are buried to detect pressure waves in the soil caused by an intruder. The sensor is either a tube filled with a pressurised liquid connected to a pressure sensor, or a fibre optic (or other) cable that is zigzagged below the surface of the detection area. An intruder moving across the detection area will compact the soil and cause either a pressure change in the tube or a minute deformation of the fibre optic cable that changes the received signal. The technology is classified as passive, covert, terrain following, volumetric and buried. As with seismic sensors, the PD can be affected by soil conditions, especially frozen ground. The movement of a number of large animals, the movement of tree roots, fences and poles because of wind, and the disturbances caused by nearby vehicles all contribute to NAR. This technology has a medium VD because, although difficult to detect, if its location is known, it can be easily bridged.

  • Magnetic: A magnetic field sensor consists of a series of buried wire loops or coils. Ferrous 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. The technology is classified as passive, covert, terrain following, volumetric and buried. Electromagnetic disturbances such as lightning or any metallic object can be a source of NAR. This technology has a high VD.

  • Video Motion: Video motion detectors (VMDs) process standard video signals from CCTV cameras, looking to qualify contrast changes in defined sensor zones within the camera's field of view as valid intrusions. Sophisticated outdoor VMDs use DSP to detect not only contrast changes but also factors such as target size, speed and movement. Attempts to use indoor VMDs in an outdoor application have given very poor performance. Because CCTV projects a two-dimensional image of a three-dimensional reality, sources of movement such as birds, animals, insects on the lens, blowing debris, dust, snow, rain, fog, and cloud shadows can mimic human movement and cause significant nuisance alarms. Camera shake in the wind also contributes to NAR. VMDs vary greatly in their susceptibility to these factors. PD can be significantly reduced (even to zero) by heavy fog or snow. The VD is rated as medium to high depending on product.

    Combining Sensor Technologies
    The wide range of intrusion detection sensor technologies exists to allow security professionals to match the most appropriate technologies to specific site requirements. For high security applications, technologies are often used in combination – either to provide higher levels of PD performance or to reduce FAR/NAR or VD.

    The alarm information from two sensor technologies is usually combined in one of two ways. The first way is to declare an alarm only if BOTH sensors go into alarm either simultaneously or within a pre-defined time window. This is referred to as an "AND" configuration. The second way is to declare an alarm if EITHER sensor goes into alarm. This is referred to as an "OR" configuration.

    Sensors are connected in an AND configura-tion primarily to lower the NAR, because environ-mental effects have to trigger both sensors before an invalid alarm is declared. However, the PD is lowered and the VD is made worse because a missed detection or the defeat of either sensor results in no alarm. Each sensor in this configuration must have both a very high PD and a very low VD.

    Sensors are connected in an OR configuration primarily to use their complimentary strengths, i.e. the strength of one technology covers or makes up for the weakness of the other and vice versa. In this case, the PD is higher because, in effect, there are two chances to catch the intruder. VD is lower because both sensors would have to be defeated to beat the system. However, NAR is higher because either sensor can report an environmental alarm. The key is to select sensors with complimentary strengths and good NAR performance.The most common combination available in a single package and used outdoors employs PIR and monostatic microwave using an AND configuration. Other possible combinations in separate packages include: active infrared / bistatic microwave; fence disturbance /ported coax; and fence disturbance /bistatic microwave.

    Testing
    Due to the wide range of environmental conditions encountered in outdoor security applications, it is difficult to obtain quantitative performance data. Summary tables, such as those included here, can be used to select the appropriate technologies best matching the specific terrain, environmental conditions and threat level for the site. However, in their decision-making, security professionals also have to rely on site references, the reputation of the manufac-turer, and the results of long-term testing, typically conducted over several seasons, and published by independent testing agencies such as Sandia National Labs in the US or PSDB in the UK.

    Most, if not all, 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. So understanding the strengths and weaknesses of each technology is critical to success.

    Assessment and Response
    Proper assessment and timely response are often the keys to a successful outdoor perimeter security system. There is little efficiency in having a highly sophisticated detection system if the alarms that are received are not properly assessed and/or if the response is too little or too late to be effective.

    Most, if not all, sensor technologies can reliably detect an upright walking intruder, but there is a wide variation in performance for other types of intrusions

    CCTV is the most common tool for assess-ment. It is also very safe and reliable, provided there is full perimeter coverage day and night. Some sites prefer to employ local personnel to assess all alarms, but a high dispatch rate can lower the efficiency of the assessment.

    Response should be initiated only after assessment has identified an unauthorised intruder.

    The response should be in time to apprehend the intruder if at all possible, or at least to prevent the intruder gaining access to the assets being protected.

    Turning on lights and a siren may work to scare off an unsophisticated intruder at first, but without a human response as well, they quickly learn to ignore these indicators.

    It is worth noting that many knowledgeable intruders prefer not to try to defeat the sensors but to attack poorly designed assessment and response capabilities.

    Costing
    In addition to providing higher levels of security (e.g. by increasing the standoff distance against explosive devices), outdoor security systems can also provide a significant financial payback by reducing the dangers and costs of manned guarding. However, the security professional 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. The major components of life-cycle cost are initial equipment acquisition, installation and ongoing maintenance.

    The acquisition cost of a single sensor can be misleading without considering a number of factors. 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 total cost may be less using fewer, more expensive terrain following sensors. The sensor installation type is another major factor. Visible sensors, because they are relatively easy to defeat, are more cost-effective for low security applications where the intruder is unsophisticated. 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. The cost of the assessment system (e.g. CCTV) and the alarm integration system must also be included. Installation costs can exceed the acquisition cost of the equipment. Such factors as site preparation (e.g. levelling the ground for line-of-sight sensors, or digging a trench for a buried sensor), providing power to the sensors, and providing a means to communicate alarm information back to a "head end" must be considered.

    The installation costs of a CCTV system with adequate lighting can also be significant.

    Maintenance costs
    Ongoing maintenance costs are often forgotten in annual budgets after the installation of the perimeter security system. Ongoing maintenance refers not only to the sensor system but also to the site. Sensor maintenance should include cleaning, testing and recalibration on a regular basis. Site maintenance can be critical to keeping environmental alarms as low as possible. Site maintenance can include cleaning up debris, leaves and snow, cutting grass, removing vegetation from fences, keeping fences in a good state of repair, and removing objects that would allow an intruder to hide from detection or assessment.

    Conclusion
    Outdoor perimeter intrusion detection is one of the most challenging fields in the security industry. Operating a successful system is as much selecting the right sensor technology for the application as managing and controlling the FAR/NAR to a manageable level so that the system does not get turned off or, worse, ignored.

    Modern 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 professionals to be confident in their outermost defenses.

    • Source

    Security Installer