Life on the edge

Today’s wide range of intrusion detection sensor technologies exists to aid security professionals in matching the most appropriate of those technologies with specific site requirements. For high security applications, detection technologies are often used in combination – either to provide higher levels of probability of detection performance, or to reduce the false alarm/nuisance alarm rate (or perhaps the vulnerability to defeat).

The alarm information from two sensor technologies is usually combined in one of two ways. The first method is to declare an alarm only if both sensors pass 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 condition if either sensor happens to pass into alarm. That would then be termed an OR configuration.

Sensors are primarily connected in an AND configuration in a bid to lower the nuisance alarm rate because environmental effects have to trigger both sensors before an invalid alarm is declared. However, the probability of detection is lowered – and the vulnerability to defeat worsened – because a missed detection or the defeat of either sensor results in no alarm. In such a configuration, each sensor must possess both a very high probability of detection and a low vulnerability to defeat.

Intrusion detection sensors are connected in an OR configuration mainly so that their dovetailing strengths might be exploited (ie the strength of one technology covers or makes up for the weakness of the other and vice versa). In this case, the probability of detection is higher because, in effect, there are now two chances of catching the intruder. Vulnerability to defeat is lowered as both sensors would have to be ‘beaten’ in order to defeat the system.

However, nuisance alarm rates will be higher here because either sensor is capable of reporting an environmental alarm. The key is to select sensors with complementary strengths and good nuisance alarm rate performance.

End users should note that the most common combination available in a single package and used outdoors employs passive infrared and monostatic microwave using an AND configuration. Other combinations in separate packages include active infrared and bistatic microwave, fence disturbance and ported coax and fence disturbance in tandem with bistatic microwave sensors.

The importance of testing

Due to the wide range of environmental conditions encountered in outdoor security system applications, it’s usually pretty difficult to obtain quantitative performance data.

Summary tables can be used to select appropriate technologies that best match the specific terrain, environmental conditions and threat levels on site. However, in their decision-making process, security managers must also rely heavily upon site references, the reputation of the manufacturer and the results of long-term testing (typically conducted over several seasons, and then published by independent testing agencies such as the Police Scientific Development Branch).

Most – if not all – sensor technologies can reliably detect an upright walking intruder, but there’s 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. It stands to reason, then, that the security manager’s ability to understand the strengths and weaknesses of each technology is critical to success on site.

Proper assessment, timely response

Proper assessment and timely response are often the major factors in a successful outdoor perimeter security system. There’s little efficiency in having a highly sophisticated detection system in place if the alarms received aren’t properly assessed and/or if the response to them is too little or too late to be effective.

CCTV is the most common tool for assessment. Generally speaking, surveillance technology is also very safe and reliable, provided that the site enjoys full perimeter coverage during the daytime and throughout the night as well. Some clients prefer to employ local personnel to assess all alarms, but a high dispatch rate can actively lower the efficiency of that assessment.

For high security applications, detection technologies are often used in combination – either to provide higher levels of probability of detection performance or to reduce the false alarm/nuisance alarm rate (or perhaps the vulnerability to defeat)

Response should be initiated only after the assessment has identified an unauthorised intruder. The response should be made in time to apprehend the intruder if at all possible, or at the very least to prevent him or her from gaining access to the assets being protected.

Turning on lights and a siren may work in scaring off an unsophisticated intruder in the first instance, but without a human response on top of that they’ll quickly learn to ignore such indicators.

It’s worth noting that many knowledgeable intruders prefer not to try and defeat the sensors, but to attack poorly-designed assessment and response capabilities instead.

Cost of system ownership

In addition to providing higher levels of security (eg by increasing the stand-off distance against explosive devices), outdoor perimeter security systems can also provide a significant financial payback for end users by reducing the costs associated with manned security.

However, the security manager must be careful here. They must look at the full life-cycle cost – often referred to as the ‘cost of ownership’ – when selecting a security system for a specific application. Components of life-cycle costing are initial equipment acquisition, installation and outgoing maintenance.

The acquisition cost of a single sensor can be misleading if a number of factors aren’t considered. Site terrain is one of them. In rough terrain, line-of-sight sensors may be prohibitively expensive as more of them will be needed to provide complete detection coverage. The total cost may be lessened by the end user deploying fewer, more expensive terrain-following sensors.

Sensor installation type is another central factor that mustn’t be overlooked. Since they’re relatively easy to defeat, visible sensors are far more cost-effective for lower scale security applications where the intruder is unsophisticated. Otherwise, multiple sensors of different technologies must be used.

For high-end security applications, covert sensors will prove to be more cost-effective as they have the lowest number of vulnerabilities (and can be used in stand-alone mode). The cost of the assessment system (eg CCTV) and the alarm integration system should also be factored-in at some stage.

Installation costs can exceed the acquisition cost of the equipment. Such factors as site preparation (eg levelling the ground for line-of-sight sensors, or digging a trench for a buried sensor), providing power to the sensors and offering a means to communicate alarm information back to a ‘head end’ are vital. The installation costs of a CCTV system with adequate lighting could also be significant.

Ongoing maintenance costs are all-too-often forgotten in annual budgets subsequent to 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.
Michael Rack is managing director of Senstar-Stellar’s UK operation (www.senstarstellar.com)