Exterior Intrusion Sensors

Description

An exterior intrusion sensor is a detection device that is used in an outdoor environment to detect intrusions into a protected area. These devices are designed to detect an intruder, and then communicate an alarm signal to an alarm system. The alarm system can respond to the intrusion in many different ways, such as by triggering an audible or visual alarm signal, or by sending an electronic signal to a central monitoring location that notifies security personnel of the intrusion. Alarm systems are discussed in detail in the Alarms Product Guide and will not be discussed in this document. Computer network intrusion detection is not discussed in this document. This document will focus only on the detection devices that detect unauthorized intruders attempting to enter into a protected area.

Intrusion sensors can be used to protect many kinds of assets. Intrusion sensors that protect physical space are classified according to whether they protect indoor, or “interior” space (i.e., an entire building or a room within a building), or outdoor, or “exterior” space (i.e., a fence line or perimeter). Interior intrusion sensors are designed to protect the interior space of a facility by detecting an intruder who is attempting to enter, or who has already entered a room or building. In contrast, exterior intrusion sensors are designed to detect an intrusion into a protected outdoor/exterior area. Exterior protected areas are typically arranged as zones or exclusion areas placed so that the intruder is detected early in the intrusion attempt before the intruder can gain access to more valuable assets (e.g., into a building located within the protected area). Early detection creates additional time for security forces to respond to the alarm. This document will focus only on exterior intrusion sensors; interior intrusion sensors will be discussed in a separate document.

Exterior Sensor Technologies

Exterior intrusion sensors are classified according to how the sensor detects the intrusion within the protected area. The three classes of exterior sensor technology include:

• Buried line sensors;
• Fence-associated sensors; and
• Freestanding sensors.

Each of these sensor types is discussed separately below.

Buried-Line Sensors

As the name suggests, buried line sensors are sensors that are buried underground, and are designed to detect disturbances within the ground – such as disturbances caused by an intruder digging, crawling, walking, or running on the monitored ground. Because they sense ground disturbances, these types of sensors are able to detect intruder activity both on the surface and below ground. Individual types of exterior buried line sensors function in different ways, including, by detecting motion, pressure, or vibrations within the protected ground, or by detecting changes in some type of field (e.g., magnetic field) that the sensors generate within the protected ground. Specific types of buried line sensors include pressure or seismic sensors, magnetic field sensors, ported coaxial cables, and fiber-optic cables. Details on each of these sensor types are provided below. More specific information on buried line exterior intrusion sensors can be found in the Exterior Intrusion Buried Sensors Product Guide.

• Buried Seismic Intrusion Detection Safeguards Technology

  Buried-line pressure or seismic sensors detect physical disturbances to the ground – such as vibrations or soil compression – caused by intruders walking, driving, digging, or otherwise physically contacting the protected ground. These sensors detect disturbances from all directions and, therefore, can protect an area radially outward from their location; however, because detection may weaken as a function of distance from the disturbance, choosing the correct burial depth for the design area will be crucial. In general, sensors buried at a shallow depth protect a relatively small area but have a high probability of detecting intrusion within that area, while sensors buried at a deeper depth protect a wider area but have a lower probability of detecting intrusion into that area. Information on the specific design, operation, detection, and pros and cons of individual types of buried line pressure and seismic sensors is provided in the table below.

• Buried line magnetic field sensors detect changes in a local magnetic field that are caused by the movement of metallic objects within that field. This type of sensor can detect ferric metal objects worn or carried by an intruder entering a protected area on foot as well as vehicles being driven into the protected area. Information on the specific design, operation, detection, and pros and cons of magnetic field sensors is provided in the table below.

• Perimitrax, Buried Cable Intrusion Detection Sensor, Magal-Senstar, Inc.

  Buried line ported coaxial cable sensors detect the motion of any object (i.e., human body, metal, etc.) possessing high conductivity and located within close proximity to the cables. An intruder entering into the protected space creates an active disturbance in the electric field, thereby triggering an alarm condition. Information on the specific design, operation, detection, and pros and cons of ported coaxial cable sensors is provided in the table below.

• Buried line fiber-optic cable sensors detect changes in the attenuation of light signals transmitted within the cable. When the soil around the cable is compressed, the cable is distorted, and the light signal transmitted through the cable changes, initiating an alarm. This type of sensor is easy to install because it can be buried at a shallow burial depth (only a few centimeters) and still be effective. Information on the specific design, operation, detection, and pros and cons of fiber optic cable sensors is provided in the table below.

Fence-Associated Sensors

Micronet, Perimeter Fence Detection System, Southwest Microwave, Inc.

• Fence-disturbance sensors detect the motion or vibration of a fence, such as that, that can be caused by an intruder attempting to climb or cut through the fence. In general, fence disturbance sensors are used on chain link fences or on other fence types where a moveable fence fabric is hung between fence posts. Information on the specific design, operation, detection, and pros and cons of fence disturbance sensors is provided in the table below.

• Infinity Taut-wire, Integrated Security Corporation

  Taut-wire sensor fences are similar to fence-disturbance sensors, except that instead of attaching the sensors to a loose fence fabric, the sensors are attached to a wire that is stretched tightly across the fence. These types of systems are designed to detect changes in the tension of the wire rather than vibrations in the fence fabric. Taut-wire sensor fences can be installed over existing fences, or as stand-alone fence systems. Information on the specific design, operation, detection, and pros and cons of taut-wire fence sensors is provided in the table below.

• Electric field or capacitance sensors detect changes in capacitive coupling between wires that are attached to, but electrically isolated from, the fence. As opposed to other fence-associated intrusion sensors, both electric field and capacitance sensors generate an electric field that radiates out from the fence line, resulting in an expanded zone of protection relative to other fence-associated sensors, and allowing the sensor to detect an intruders’ presence before they arrive at the fence line. Note: proper spacing is necessary during installation of the electric field sensor to deter a would-be intruder from slipping between largely spaced wires. Information on the specific design, operation, detection, and pros and cons of these sensors is provided in the table below.

Free-Standing Sensors

Free-standing sensors, which include active infrared, passive infrared, bistatic microwave, monostatic microwave, dual-technology, and video motion detection (VMD) sensors, consist of individual sensor units or components that can be set up in a variety of configurations to meet a user’s needs. They are installed above-ground, and depending on how they are oriented relative to each other, they can be used to establish a protected perimeter or a protected space. More details on each of these sensor types are provided below.

• Model 455, Outdoor Active Infrared Intrusion Sensor, Southwest Microwave, Inc.

  Active infrared sensors transmit infrared energy into the protected space, and monitor for changes in this energy caused by intruders entering that space. In a typical application, an infrared light beam is transmitted from a transmitter unit to a receiver unit. If an intruder crosses the beam, the beam is blocked, and the receiver unit detects a change in the amount of light received, triggering an alarm. Different sensors can use single- and multiple-beam arrays. Single-beam infrared sensors transmit a single infrared beam. In contrast, multiple-beam infrared sensors transmit two or more beams parallel to each other. This multiple-beam sensor arrangement creates an infrared “fence.” Information on the specific design, operation, detection, and pros and cons of these sensors is provided in the table below.
  
  

• Infinity Passive Infrared, Integrated Security Corporation

  Passive infrared (PIR) sensors monitor the ambient infrared energy in a protected area, and evaluate changes in that ambient energy that may be caused by intruders moving through the protected area. Detection ranges can exceed 100 yards on cold days with size and distance limitations dependent upon the background temperature. PIR sensors generate a non-uniform detection pattern (or “curtain”) that has areas (or “zones”) of more sensitivity and areas of less sensitivity. The specific shape of the protected area is determined by the detector’s lenses. The general shape common too many detection patterns is a series of long “fingers” emanating from the PIR and spreading in various directions. When intruders enter the detection area, the PIR sensor detects differences in temperature due to the intruder’s body heat, and triggers an alarm. While the PIR leaves unprotected areas between its fingers, an intruder would be detected if he passed from a non-protected area to a protected area. Information on the specific design, operation, detection, and pros and cons of these sensors is provided in the table below.

Model 380 Outdoor Microwave Transceiver, Southwest Microwave

• Microwave sensors detect changes in received energy generated by the motion of an intruder entering into a protected area. Monostatic microwave sensors incorporate a transmitter and a receiver in one unit, while bistatic sensors separate the transmitter and the receiver into different units. Monostatic sensors are limited to a coverage area of 400 feet, while bistatic sensors can cover an area up to 1,500 feet. For bistatic sensors, a zone of no detection exists in the first few feet in front of the antennas. This distance from the antennas to the point at which the intruder is first detected is known as the offset distance. Due to this offset distance, antennas must be configured so that they overlap one another (as opposed to being adjacent to each other), thereby creating long perimeters with a continuous line of detection. Information on the specific design, operation, detection, and pros and cons of these sensors is provided in the table below.

• MS15 / MS16, Dual Technology Motion Sensor, Southwest Microwave, Inc

  Dual-technology sensors consist of two different sensor technologies incorporated together into one sensor unit. For example, a dual technology sensor could consist of a passive infrared detector and a monostatic microwave sensor integrated into the same sensor unit. Information on the specific design, operation, detection, and pros and cons of these sensors is provided in the table below.

• Video motion detection (VMD) sensors monitor video images from a protected area for changes in the images. Video cameras are used to detect unauthorized intrusion into the protected area by comparing the most recent image against a previously established one. Cameras can be installed on towers or other tall structures so that they can monitor a large area.

VMD sensors can use either analog or digital technology. In an analog system, which is the older of the two technologies, the camera signal is monitored to detect changes in the brightness of the images over time. These changes in brightness occur when objects move within the monitored area. When a change is detected, an alarm is initiated. Newer digital VMDs convert the camera signal into a digital format, which allows digital processors to evaluate the video signal electronically. The video scene is divided into zones, and each zone is monitored separately for multiple parameters, including changes in brightness or contrast, movement between adjacent zones, changes in speed of motion across zone areas, changes in the size of objects within zones, or other global changes across most or all of the zones. Information on the design, operation, detection, advantages, and disadvantages of these sensors are provided in the table below.

Attributes and Features

Sensor Visibility

Sensors are designed either to be covert or to be visible. A covert sensor, such as a buried-line sensor, is not visible to a would-be intruder, and thus the intruder may not know the sensor is there and may not attempt to defeat it. Therefore, these types of sensors may be more likely to detect intruders. In contrast to covert sensors, visible sensors are located out in the open and are easily identified by any would-be intruders. Therefore, these types of sensors may also serve as a visual deterrent against unauthorized intrusion.

Sensor Configuration

Sensors either require line of sight or follow the terrain during performance. Some types of sensors require a clear line of sight (LOS) between the transmitter and receiver for optimal performance. For example, an active infrared sensor requires an uninterrupted LOS so that the transmitter can transmit the beam directly to the receiver. Any irregular topography or intervening objects can block the beam, reducing or eliminating the sensor’s effectiveness. This problem can be overcome by adding more units to the system to ensure that there is a clear LOS between the units, although this can become expensive with uneven terrain or over large distances. In contrast, a terrain-following sensor does not require a clear LOS between units, and functions well on both flat and jagged terrain. For example, buried-line sensors can be buried at angles to follow the terrain, while fence sensors are installed on a fence, so as long as the fence can be installed in the terrain, the fence sensor can be installed in that terrain.

Detection “Zone of Dimensions”

Different intrusion sensors monitor different “shapes” of space. A “volumetric” sensor, such as a PIR, monitors a wide three-dimensional zone, whereas a “line detection” sensor, such as an active infrared sensor, monitors only a very narrow protected zone (essentially two dimensional). Because line detection systems monitor only a narrow space, they may be most appropriate for high-traffic areas where people or vehicles pass close to the protected area or in more rural areas when wildlife may wander close to the protected area. On the other hand, volumetric sensors may be more appropriate when the user wants to monitor for intrusions from several directions at once and/or from further away from the actual sensor unit.

The table below summarizes the features and attributes discussed above for each of the sensors discussed in this document.

Classification of Exterior Sensors

Cost

Costs for exterior intrusion sensors can vary greatly depending on the size of the facility and its terrain. As discussed above, different sensors have different ranges, and users will need to purchase enough sensors to ensure that they cover all of the area they wish to monitor. In addition, some types of sensors require a clear LOS between individual units, and thus units may need to be spaced more closely to ensure a clear LOS, resulting in the need for more units than would normally be required to protect the same volume of flat terrain.

Buried-Line Sensors

Several example costs for buried-line sensors are provide below. More specific cost information is available from the Exterior Intrusion Buried Sensors Product Guide.

Ported coaxial cable sensors cost approximately $20 to $32 per foot. Typical installation times ranging from 300–1,300 feet per day. Fiber-optic cable sensors range from $10 to $15/ft. with a 1 day installation time per zone.

Fence-Associated Sensors

Several example costs for fence-associated sensors are provided below. More specific cost information is available from the Fence-Associated Exterior Intrusion Sensors Product Guide.

Fence-disturbance sensors range from $2 to $116 per foot, uninstalled. Installation time is estimated at about of 1.0 to 1.5 times the price of equipment. The price of a typical taut-wire sensor fence is between $150 and $250 per foot; installation generally takes 1 day per zone. Electric field and capacitance sensors cost around $23 to $58 per foot, uninstalled. Installation time is 1 day per zone (zones can range from 300 to 1000 feet).

Free-Standing Sensors

Costs for monostatic microwave intrusion sensors range from $7 to $13 per foot of protected space, uninstalled. Installation takes approximately 2 hours and costs 1.5 to 2.0 times equipment costs. Bistatic microwave intrusion sensors sell for $1 to $7 per foot of protected space, uninstalled. Typically installation takes 4 hours and costs 1.5 to 2.0 times the price of equipment. Passive infrared detectors cost approximately $5 to $6 per foot of protected space, uninstalled. Installation time is approximately 2 hours with an estimated cost of 1.5 to 2.0 times equipment costs. The cost of a video motion detection sensor ranges from $12,000 for a 4-channel unit and $30,000 for an 8-channel unit. The associated viewing station can cost between $7,000 and $8,000. The installation time varies depending on the number of cameras and features chosen for a given system.

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