Michael Rack continues Security Management Today’s ‘design primer’ series for end users looking to procure a suitable perimeter intrusion detection system by evaluating the remaining sensor systems available.

Outdoor security sensors detect intruders as soon as they enter a protected area, and before they can gain access to people or valuable assets. In a correctional environment, security sensors may be used to immediately detect prison escape attempts. A quality outdoor security system demonstrates a significant return on investment by reducing the risk of theft, damage or personal injury.

Outdoor sensors may be deployed either to complement indoor security sensors or as primary security in situations where indoor security isn’t feasible. Able to protect assets at both manned and unmanned sites, outdoor security sensors are something of a practical solution for remote locations where security officers aren’t really a viable alternative.

The most common type of microwave sensor used in outdoor perimeter applications is bistatic radar. This technology uses separate transmitting and receiving antennas to define an active, visible, line-of-sight, above ground volumetric detection zone. Such zones can be quite wide midway between the antennas, and may require a significant clear zone. They operate at maximum frequencies of 10 GHz or 24 GHz, and actively detect changes in signals that are caused by objects moving inbetween the antennas.

To overcome a blind spot located near each antenna, microwave zones must be overlapped in what’s often referred to as a ‘basket weave’ configuration. Stacking antennas vertically can create high detection zones. Monostatic microwave sensors incorporate a transmitter and receiver in the same unit, the latter detecting microwave energy reflected from objects moving in the detection field. These units are most commonly used for short zones or as a gap filler for other sensors.

Recent developments in microwave sensors include a digital processor that enhances the networking of processors and enables remote maintenance and diagnostics.

End users should remember that the distance between the transmitter and receiver (bistatic units) can sometimes affect the probability of detection. Some of the more common nuisance alarm rate 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, moving vegetation and blowing sand and snow. Overall, microwave sensors are considered to have a medium level of vulnerability.

Passive (PIR) and active infrared

Passive infrared (PIR) sensors have been used for indoor applications for a long time now. Only recently have inexpensive outdoor versions been offered. PIR senses the presence of infrared radiation (heat) above background and detects primarily transverse motion. The technology is classified as passive, visible, line-of-sight, multi-line and free-standing.

Nuisance alarm rates can be significant if the units aren’t installed properly and the site left uncontrolled. Some manufacturers suggest linking two units in order to minimise the rate. Common sources of nuisance alarms here will be animals, flocks of birds, heavy snowfall, blowing dust, hail, sudden outbreaks of rain, direct sunlight and moving vegetation.

PIR sensors are considered to have a high vulnerability and high nuisance alarm rate, and are most often used in low-scale commercial applications or residential scenarios.

Active infrared 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 nuisance alarm rate from smaller animals and birds. Several units have to be stacked in heated enclosures in order to achieve significant vertical coverage. The end result is active, visible, line-of-sight, free-standing coverage in a single plane.

The probability of detection for infrared beam sensors can be affected by weather conditions that cause reduced visibility, such as heavy snow, sheeting rain, fog, blowing sand/earth and dust. Animals, flocks of birds, blowing debris and leaves – in addition to reflected sunlight from shiny surfaces – can all affect the nuisance alarm rate. Vulnerability for this type of sensor is high because of the ease of tunnelling under or bridging over the beams.

Electric field and taut wire sensors

Electric field sensors operate by detecting a change in capacitance among a set of parallel, insulated sensor wires attached to stand-offs on a fence or installed on their own posts. The detection zone is defined by the wires, with detection occurring in proximity to (or inbetween) the wires. Configurations can also be used that will protect rooftops and the sides of buildings. Using multiple wires can create high detection zones.

This technology is classified as active, visible, terrain-following, volumetric and either free-standing or fence-mounted (depending on the application). Its imposing configuration has a significant deterrent value. Recent developments here include using digital signal processing to enhance detection and minimise nuisance alarm rates, as well as improvements to the insulators and insulator types that support the wires.

Nuisance alarm rates 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 needed for a low nuisance alarm rate. Much like active infrared, this type of technology is considered to have a medium vulnerability to defeat level because of the possibility of bridging over or tunnelling under the detection field.

What about taut wire? Taut wire sensors effectively combine barrier technology with sensors. They consist of rows of parallel tensioned wires connected to sensors capable of detecting any displacements. These sensors may be either contact closure switches, piezoelectric sensors or strain measuring gauges. The sensor detects any attempts by intruders to cut, climb or separate the wires.

Sometimes barbed, the wires are supported on mounting structures that may be installed on existing fences or free-standing poles. The technology is classified as passive, visible, terrain-following, line and either free-standing or fence-mounted according to the specific end user application.

A quality outdoor security system demonstrates a significant return on investment by reducing the risk of theft, damage or personal injury. Able to protect assets at both manned and unmanned sites, outdoor sensors are a practical solution for those remote locations where guards aren't a viable alternative

Interestingly, taut wire has the lowest nuisance alarm rate of any sensor technology, although ice ‘loading’ can contribute to their occurrence. Typically, installations will involve the addition of a buried concrete wall to prevent tunnelling. Note that wire tensions have to be checked at least twice every year, and that the technology is considered to have a medium vulnerability to defeat rating because of the possibility of bridging.

Electrified barriers: the facts

In essence, an electrified barrier consists of a set of parallel, insulated wires that apply a non-lethal shock to would-be intruders when they come into contact with the wires. At the ‘point of contact’ an alarm will also be declared.

The wires might be fence-mounted or freestanding. In a few jurisdictions (such as prisons) lethal shocks may be permitted. Such systems boast a significant deterrent value, but can be defeated by a sophisticated intruder. They may not be legal in all jurisdictions.

This particular 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 nuisance alarms will often include casual contact by animals and birds, rain, snow and lightning. The vulnerability to defeat level is said to be medium because of the possibility of bridging over (or tunnelling under) the wires.

Surface wave sensors operate by setting up an electromagnetic field around a pair of parallel wires that are supported by fibre glass poles – a field that’s 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’s classified as active, visible, terrain-following, volumetric and freestanding.

For their part, seismic sensors are buried to detect any vibration to the soil caused by an intruder. A typical sensor consists of a set of coils and magnets known as ‘geophones’. During a seismic disturbance – perhaps caused by an intruder walking, running or crawling over the sensor – an electrical current is generated by the coil and magnets signalling an alarm. Here, the technology is classified as passive, covert, terrain-following, volumetric and buried.

The probability of detection might well be affected by soil conditions, in particular frozen ground. Meantime, the movement of a number of large animals, tree roots, fences and poles because of wind and the disturbances caused by nearby vehicles all contribute to nuisance alarm rates. This technology has a medium vulnerability to defeat because, although difficult to detect, if the location is known it may be easily bridged.

Pressure and magnetic fields

Pressure sensors are buried to detect pressure waves in the soil caused by an intruder. Typically, the sensor will either be a tube filled with a pressurised liquid connected to a pressure sensor, or a fibre optic (or other) cable that’s zig-zagged below the surface of the detection area. An intruder moving across that 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.

This technology is classified as passive, covert, terrain-following, volumetric and buried. As is the case with seismic sensors, the probability of detection may be affected by soil conditions, and frozen soil in particular. The movement of tree roots, fences and poles engendered by high winds and the disturbances caused by nearby vehicles all contribute towards nuisance alarm rates.

We’re talking about medium vulnerability to defeat with pressure sensors.

A magnetic field sensor comprises a series of buried wire loops or coils. Ferrous metal objects moving over the sensor induce a current and a subsequent alarm. These sensors are generally supplied in two types: those sensitive enough to detect humans and those designed primarily to detect vehicles. Passive, covert, terrain-following, volumetric and buried is how you would classify them.

Magnetic field sensors have a high vulnerability to defeat, while electromagnetic disturbances such as lightning or any metallic object can often be a source of nuisance alarm rates increasing.

For their part, video motion detectors 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 video motion detectors use digital signal processing to detect not only contrast changes but also factors such as target size, speed and movement. It should be borne in mind that attempts to use indoor video motion detectors in outdoor applications have produced a very poor performance.

Since CCTV projects a two-dimensional image of a three-dimensional reality, sources of movement can replicate the movement of humans and result in significant numbers of nuisance alarms. Camera shake in strong winds will also contribute towards this problem.

The probability of detection may be significantly reduced by heavy fog or snow. Vulnerability to defeat is rated as medium-to-high depending on the system used.