Types of Perimeter Detection Sensor

8/4/2019 Types of Perimeter Detection Sensor

1/12

A Comparison and Contrast of Exterior Perimeter Security Systems

By John Trinckes

Introduction

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 paper is designed to answer the following questions: What are

the most common perimeter systems? What are the principles behind these systems? How do these systems

work in different applications? What are the considerations when deciding to use a certain system? And how

can these systems be defeated?

When classifying and comparing certain perimeter systems, there are five methods to consider. First toconsider 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-of-sight (LOS) or does the

sensor follow the terrain. A line-of-sight system usually refers to a clear line-of-sight 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).

Table 1. Types of Exterior Sensors and Characteristics. (Garcia, p. 67)

Modes Type of Sensors Passive or Active Covert or Visible LOS or Terrain Volumetric or Line

Detection

Buried Line Seismic Pressure 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

Sensor Fence Passive Visible Terrain Line DetectionElectric Field Active Visible Terrain Volumetric
http://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htmhttp://www.solarbeam.com/images/Compare/Hurricane6.htmhttp://www.solarbeam.com/images/Compare/Hurricane5.htmhttp://www.solarbeam.com/images/Compare/Hurricane4.htmhttp://www.solarbeam.com/images/Compare/Hurricane3.htmhttp://www.solarbeam.com/images/Compare/Hurricane2.htmhttp://www.solarbeam.com/images/Compare/Hurricane1.htm


2/12

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

(Garcia, 2001)

Buried Line Sensors

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

optic. 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 (see

Diagram 1). Some defeating activities are avoiding the detection zones, cushioning movement vibrations,

bridging over the zones, and any combination of these.

A seismic sensor consists of geophones that are made up of conducting coil and magnet (see Diagram 2). 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).

Diagram 1. Pressure Sensors. Diagram 2. Seismic Sensors.

( Perimeter Security Sensor Technologies Handbook , 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.


3/12

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 (see Diagram 3). 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).

Diagram 3. Ported Cable Sensor.


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 (see Diagram 4).

Diagram 4. Fiber Optic Cable.


4/12


Fence-Associated Sensors

T here are three types of sensors that attach to or mount on fences, these are: fence-disturbance sensors,sensor fences, and electric field or capacitance. 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. Fence-disturbance 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 (see Diagram 5.)

Diagram 5. Fence-Disturbance Sensors.


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 fence-disturbance sensors. The

wire is usually barbed wire and the transducers are mechanical switches, strain gauges, or piezoelectric

elements (see Diagram 6). 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).

Diagram 6. Taut-wire Sensor.


5/12


Taut wire fences can use coaxial cable (see Diagram 7a, 7b, and 7c) and fiber optics (see Diagram 8) as well as

being combined with other sensors to form a better detection system (see Diagram 9). All of these systems are

similar in the design principles with just slight variations of the mechanisms used for detection.

Diagram 7a. Sensor Fences or Taut-

Wire Sensors.

Diagram 7b. Magnetic. Diagram 7c. Applications.


6/12


Diagram 8. Fiber Optic Fence Sensor. Diagram 9. Combinations of Sensors.


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 (see Diagram 10).


7/12

Diagram 10. Electric Field Sensors.


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).

Microwave 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 thatrapidly turn on and off, first at the one frequency, then at the second frequency. The receiver temporarily shuts


8/12

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 (see Diagram 11a).

Diagram 11a. Monostatic Microwave Sensors.


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 (see Diagram 11b and 11c). 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 (see Diagram 11d) (Garcia, 2001).

Diagram 11b. Bistatic Microwave Layout. Diagram 11c. Bistatic Microwave Sensors.


Diagram 11d. Bistatic Microwave Patterns.


9/12


There 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 (see Diagram 12a). 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 thedetection pattern can assist an intruder ( ( Perimeter Security Sensor Technologies Handbook , 2001) .

Diagram 12a. Passive Infrared Sensor.


10/12


Active 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 (see Diagram

12b). 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).

Diagram 12b. Active Infrared Motion Sensors.


11/12


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 tomonitor 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 ever-changing world and there are always

work being done to develop better systems (Garcia, 2001).

Final Comment

Although there are many different systems to choose from, this author's recommendation is for the active

infrared perimeter detection system, specifically the one designed by Aressco Technologies, Inc. in Homestead

, FL. To get further information on this system, go tohttp://www.solarbeam.com . Out of all of the systems

talked about, the SolarBeam SPT2000 is one of the best, state-of-the-art perimeter security systems on the

market. "The SPT2000 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" (SolarBeam, 2001). This system consists of polycarbonate, weatherproof towers that have four RedNet I. R.
http://www.solarbeam.com/http://www.solarbeam.com/http://www.solarbeam.com/http://www.solarbeam.com/


12/12

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' (see Diagram 13). These towers can be placed up to 400 feet apart with line-of-sight and Aressco's

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 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

SolarBeams will give the best possible perimeter security available for the best price.

Diagram 13. SolarBeam Paths.

Documents

Types of Perimeter Detection Sensor