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Sensor System Comparisons

A Comparison and Contrast of Exterior Perimeter Security Systems By John Trinckes


It is hard to decide on the right security system since there are so many different types of exterior perimeter security systems available on the market. This article is designed to answer the following questions:

When classifying and comparing certain perimeter systems, there are five methods to consider. First to consider are passive or active sensors. Passive sensors detect energy submitted by the target, or detect a change in the environment associated with the presence of the target. While Active sensors transmit some form of energy and detect a change in the received energy created by a disturbance caused by a target.

A second type of comparison is whether or not the sensor is hidden (covert) or is it visible to the intruder. 

A third type of method of comparison is the requirement for the sensors to have a line-ofsight (LOS) or does the sensor follow the terrain. A line-of-sight system usually refers to a clear line-ofsight between the transmitter and receiver of an active sensor. A terrain-following sensor does exactly what it states by allowing the transducer elements and the radiated field to follow the terrain.

The fourth type of comparison is whether the sensors are volumetric (detect intrusions in a volume of space) or line detection (detect along a line).

And finally, the application method that classifies these systems into three modes: buried line, fence-associated, and freestanding (Garcia, 2001).

Types of Exterior Sensors and Characteristics. 

ModesType of SensorsPassive or ActiveCovert or VisibleLOS or TerrainVolumetric or Line Detection
Buried Line SeismicPressure Passive Covert Terrain Line Detection
Magnetic Field Passive Covert Terrain Volumetric
Ported Coaxial Cable Active Covert Terrain Volumetric
Fiber-Optic Cable Passive Covert Terrain Line Detection
Fence-Associated Fence-Disturbance Passive Visible Terrain Line Detection
Magnetic Field Passive Visible Terrain Line Detection
Ported Coaxial Cable Active Visible Terrain Volumetric
Freestanding Video Motion Detection Passive Covert LOS Volumetric
Bistatic Microwave Active Visible LOS Volumetric
Passive Infrared Passive Visible LOS Volumetric
Active Infrared Active Visible LOS Line Detection
Dual Technology Active/Passive Visible LOS Volumetric

Buried Line Sensors

There are four types of buried-line systems: pressure or seismic, magnetic field, ported coaxial cable, and fiber-optic.

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A pressure or seismic sensor responds to disturbances of the soil caused by an intruder walking, running, jumping, or crawling on the ground. Pressure sensors are sensitive to lower frequency pressure waves than seismic sensors. Pressure sensors consist of a hose filled with liquid and connected to a transducer. Some defeating activities are avoiding the detection zones, cushioning movement vibrations, bridging over the zones, and any combination of these.

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A seismic sensor consists of geophones that are made up of conducting coil and magnet .

Either the coil or magnet are fixed and let the other vibrate during a disturbance. These types of sensors are very dependent on the type of soil in which they are buried. The depth at which they are buried is also a factor and there is an inverse relation between the probability of detection and the width of detection. The shallower the depth, the higher the probability, but the detection width is narrower. The deeper the depth, the lower the probability, but the wider the detection width.

Frozen soil is not conducive to this type of sensor and seismic noise may cause nuisance alarms. Some seismic noise such as wind energy cause fences, poles, and trees to shake. Other sources include vehicular traffic and heavy industrial machinery. If the location of the sensor line is known, a bridge could be formed over the line to defeat this type of system (Garcia, 2001).

Magnetic field sensors respond to changes in the local magnetic field caused by movement of metallic material. They are ideal for detecting vehicles or intruders with weapons, thus if an intruder has no weapons, they would not be detected. They consist of series of wire loops or coils buried in the ground. They can be susceptible to lightning due to the electromagnetic disturbances.

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Ported coaxial cable is also known as leaky coax or radiating cable sensors. They respond to a high dielectric constant or high conductivity near the cable that can be produced by humans and metal vehicles. This type of sensor consists of an outer conductor that is not completely shielded from the inner conductor and lets some radiated signals leak through. Metal or water can cause problems with these sensors due to them being large targets and cause of nuisance alarms. Also, standing metal objects and water distort the radiated field. Bridging over or avoiding the zone is the principle method of defeat (Garcia, 2001).

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Fiber-optic cables are made up of long, hair-like strands of transparent glass or plastic. Along these strands, light travels from one end to the other reflecting off of the surface of the cladding material making it a sort of ‘light-pipe’(Garcia, 2001). The light diffraction and intensity is a function of the shape of the fiber over its length that is measured at the end of the cable by sensitive equipment. If there is a disturbance in the zone, this function is changed and causes an alarm. Bridging over the sensors is the defeat mechanism and experts recommend using a microwave in conjunction with fiber.

Fence-Associated Sensors

There are three types of sensors that attach to or mount on fences, these are: fence-disturbance sensors, sensor fences, and electric field or capacitance.

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Fence-disturbance sensors are installed on security fences and typically constructed with chain-link mesh. They use several kinds of transducers to detect motion or shock caused from an intruder attempting to climb or cut through the fence. Fencedisturbance sensors respond to all mechanical disturbances so noises such and wind and debris blown by wind, rain, and nearby traffic activity can cause nuisance alarms (Garcia, 2001).. Also, the fence must be tight with rigid fence posts. The most common defeat method is to avoid contact with the fence by bridging or digging underneath. Furthermore, overhanging trees and nearby structures can assist the intruder to gain access.

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Taut-wire sensor fences consist of high tensile strength parallel, horizontal wires that connect to transducers near the midpoint of the wire span. Since these wires are under tension, they are able to detect climbing or cutting of the wires. They are much less susceptible to nuisance alarms than fencedisturbance sensors. The wire is usually barbed wire and the transducers are mechanical switches, strain gauges, or piezoelectric elements. The probability of detection of taut-wire is dependent upon the tension of the wires, wire friction, and wire spacing. If the space between the two wires is large enough, it could allow an intruder to pass through undetected. Like other type of fence-associated sensors, bridging over or tunneling underneath can defeat this system (Garcia, 2001).

Taut wire fences can use coaxial cable and fiber optics as well as being combined with other sensors to form a better detection system. All of these systems are similar in the design principles with just slight variations of the mechanisms used for detection.

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sensor system comparisons9 2 Electric field or capacitance sensors can be set to detect up to 1 meter beyond the wire of the fence. These sensors are susceptible to lightning, rain, fence motion, and small animals. Also, ice storms may cause damage to the wires. Since the detection volume extends beyond the plane of the fence, they are more difficult to defeat than other sensors by digging or bridging over.

Freestanding Sensors

The most common freestanding sensors used for exterior perimeter control are video motion detection, microwave, and infrared. Video motion detection uses existing closed-circuit television (CCTV) cameras to sense a change in the video signal level viewed on the scene. A computer that is set to recognize movement in a particular area, point, or grid on an image usually accomplishes motion detection. When the computer detects a change in the set image under the set parameters, then an alarm is generated that indicates there was movement in that set zone. Video motion detection only works with fixed cameras, thus pan/tilt/zoom cameras could not be used since any motion of the camera and the change of the video image will produce an alarm. Also, reliability of the detection could be lowered during conditions of reduced visibility caused by fog, snow, heavy rain, or loss of lighting at night (Garcia, 2001).

sensor system comparisons10Microwave sensors use transmitters and receivers working in the 10 Gigahertz (GHz) or 24 GHz microwave range. There are two types of microwave sensors, monostatic and bistatic. Monostatic units combine the transmitter and receiver in a single dual function unit. A monostatic unit uses two different frequencies that rapidly turn on and off, first at the one frequency, then at the second frequency. The receiver temporarily shuts off after transmission. Since microwave has a constant speed, the receiver is programmed to detect the reflected energy within a specific time period. Of course if it does not receive the reflection back in a certain time period due to an intruder’s movement, then an alarm is activated.

A bistatic microwave sensor uses two separate units, one for the transmitter, and one for the receiver to create a detection zone between the two units. The bistatic microwave sensors respond to changes in the vector sum caused by objects moving in the zone. The vector sum is made up of the microwave signal reflected from the ground surface and other objects in the zone. Three important considerations for microwave are that the ground must be flat so that objects do not shadow the beams; antennas should not be placed greater than 120 yards between each other since a crawler may not be detected due to dead zones created in the first few meters in front of the antennas; and there should not be any standing water in the area since this will create a moving reflective zone (Garcia, 2001).

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sensor system comparisons12There are two types of exterior infrared systems. The first is the passive infrared (PIR) sensor that detects electromagnetic radiated energy generated by sources that produce temperatures below that of visible light. The sensors focus on a narrow bandwidth measured in microns with the human body producing energy in the region of 7-14 microns. These sensors use the Rate of Change measurement to process and evaluate an unshielded/unprotected intruder walking through a designated zone. When the radiation change captured by the lens exceeds parameters, an alarm is signaled. Some defeat mechanisms deployed are to shadow, cloak or mask the intruders heat signature from the field of view. Also knowing the dead spots of the detection pattern can assist an intruder ((Perimeter Security Sensor Technologies Handbook, 2001).

sensor system comparisons13Active infrared sensors use two units, a transmitter and a receiver much like the bistatic microwave units. Unlike the microwave units, the active infrared sensors use a transmitter that generates a multiple frequency straight-line beam to the receiving unit that creates a sort of ‘fence’ between these two units. An intruder passing through this field of detection will interrupt the signal and cause an alarm. Some consideration for the active infrared sensors is the ability to align the two towers together and obtain line of sight. The sensitivity levels on most of the newer sensors can be adjusted for animals and vegetation to pass through without being detected, but when a human intruder or vehicle pass through, the alarm will activate. Attempts to defeat these sensors may be accomplished by bridging or tunneling over the ‘fence’ or by using the towers that house the beams as columns to support an intruder from vaulting over the beams (Garcia, 2001).

Dual-technology systems are a combination of any of the two above sensors. This gives the entire system an extra layer of protection, however, the system will only be as good as the weakest part. Special attention should be made to use such sensors that will enhance each other and attempt to strengthen each other’s weakest part. For instance, passive infrared could be used with microwave sensors to lower the false alarm rates. Both sensing elements could be used in a combined unit and through a logical ‘AND’ function, an alarm could be set off if certain parameters are met. For example, the infrared sensor would have to activate ‘AND’ the microwave sensor would have to activate to create an alarm that an intruder was detected (Garcia, 2001).

Emerging Technology

Some other emerging technologies for exterior perimeter control systems are using 3-D video motion detection and airborne intrusion detection. In the video realm, thermal-imagers are being deployed in an attempt to monitor very wide areas. For airborne detection, radar, vibration, imaging, and other technologies are being applied to detect an intruder through the air. Most of these technologies are under development and have not been fully tested. They are cited here to show that security is an everchanging world and there are always work being done to develop better systems (Garcia, 2001).

Final Comment

sensor system comparisons14Although there are many different systems to choose from, this author’s recommendation is for the active infrared perimeter detection system.

Out of all of the systems talked about, the S3 1000 Tower is one of the best, stateof- the-art perimeter security systems on the market.

The S3 1000 is a totally wireless, solar powered outdoor perimeter security system designed to provide perimeter protection where power and wiring are difficult or virtually impossible to install

 S3 Services Group, 2001

This system consists of polycarbonate, weatherproof towers that have four RedNet I. R. Beams installed inside. 12-volt batteries power the beams and the batteries are recharged by the included solar panel. The beams comprise of four transmitters and four receivers that see each other making a 16-beam path ‘fence’. These towers can be placed up to 400 feet apart with line-of-sight and patented RF transmitter/receiver can transmit up to 5 miles (and longer with repeaters), making this system totally portable and reusable. An entire system can be installed in a few hours (depending on the number of towers required). 

Regarding the price, this system is one of the most cost-effective and efficient systems on the market. The author knows from first hand experience that this system operates above a 95% probability of detection rate with over 95% confidence level. Since there is a black acrylic cover, no one can see how the beams are mounted or where the beams are pointing. Placing an anti-tamper proof switch in the tower stops all attempts of using the towers themselves as avenues of intrusions (Intrusion Detection Systems, 2001). This system is not susceptible to lightning strikes and/or electromagnetic interference. Due to its 15 protective coverings on the outside and inside, this system is totally weatherproof and can handle all types of environments. The towers can be utilized in many different applications ranging from nuclear power plant protection to airport perimeter security.

The goal of any perimeter system is to have the best possible detection rate in the most cost effective manner. This can be a very difficult task to accomplish, however, the S3 1000 will give the best possible perimeter security available for the best price.


  1. Garcia, Mary L. (2001). The Design and Evaluation of Physical Protection Systems. Woburn, MA: Butterworth-Heinemann.
  2. Infrared Perimeter Intrusion Detection System (IPID) (2001, December 9). Intrusion Detection Systems. Retrieved December 9, 2001
  3. Section Two: Technology Reviews (2001, December 9). Perimeter Security Sensor Technologies Handbook. Retrieved December 9, 2001