Perimeter detection technologies fall into a few families: sensors on the fence, sensors in the ground, beams across open ground, video analytics and radar. Each detects some kinds of intrusion well and is fooled by some kinds of weather, wildlife or terrain. Good perimeter design picks technologies zone by zone and often combines two that fail in different ways.
What a perimeter system has to do
A perimeter system is judged on two measures that pull against each other. The probability of detection is how likely it is to catch a real intrusion. The nuisance alarm rate is how often it alarms on something harmless, such as wind, an animal or a passing truck. Turn the sensitivity up and both rise; turn it down and both fall. The art is choosing technology and settings so that real intrusions are caught while nuisance alarms stay low enough that operators keep trusting the system.
Detection also has to happen early enough to matter. The time between the alarm and the intruder reaching the target must be longer than the time it takes to assess the alarm and get a response there. Fences, gates and other barriers add delay; detection at or before the barrier makes that delay useful.
The main technologies
The technologies differ mainly in where they detect: in the approach zone outside the fence, on the fence itself, in a clear zone inside it, or across a wide area from a single point. The section below shows how they sit on a typical site.
Fence-mounted sensors
Cables or sensors attached to the fence fabric detect the vibration and movement of climbing, cutting or lifting. Types include electromechanical and electronic vibration sensors, fibre optic cable, and taut-wire systems. They need a fence in good condition: loose fabric, rattling gates and slack ties produce nuisance alarms, while a fence that can be bridged with a ladder without touching it defeats them. More on this in fence-mounted and buried perimeter sensors.
Buried sensors
Cables buried a short distance below the surface detect a person crossing above them, either through a disturbance of a radio field generated between two ported cables or through pressure and vibration in the soil. They are invisible and follow terrain, but they need stable, well-drained soil, and are affected by tree roots, standing water, frost and heavy snow cover in ways the design must account for.
Microwave and infrared beams
Bistatic microwave links create an invisible detection zone between a transmitter and receiver along a straight, level, clear path. Active infrared beams do the same with several stacked light beams and suit gates, short runs and roof edges. Both need flat ground or careful stacking over dips, and beams need overlaps at their ends so nobody can pass under or around them. Tall grass, drifting snow and fog can cause problems.
Video analytics
Software on visible or thermal cameras detects people and vehicles entering a defined zone or crossing a line, and the same camera then provides the image to assess the alarm. Analytics depend on lighting, view and image quality, and on how well the software can tell a person from a moving shadow, a deer or swaying branches. See perimeter detection with video analytics.
Ground radar
Radar scans a wide area from one point and tracks targets moving across it, reporting their position and heading. It suits large open sites, such as yards and airfield-style areas, and can cue cameras to follow a target. It needs a clear view across the ground and careful setup of zones to ignore traffic, water and vegetation.
Side by side
| Technology | Good at | Watch for |
|---|---|---|
| Fence-mounted | Detecting climb and cut on an existing fence | Fence condition, wind, loose gates, bridging over the top |
| Buried | Covert detection that follows terrain | Roots, drainage, frost, snow depth, digging nearby |
| Microwave beam | Long, flat, clear zones | Uneven ground, vegetation, drifting snow, gaps at beam ends |
| Infrared beam | Gates, short runs, roof edges | Fog, heavy snow, alignment, sun |
| Video analytics | Detection with built-in assessment | Lighting, weather, vegetation, animals, camera shake |
| Radar | Wide open areas, tracking | Obstructions, water surfaces, zone setup |
Combining technologies
Because each technology has its own weaknesses, many sites pair two whose nuisance alarm causes differ. A fence sensor paired with video analytics is a common example: the fence sensor is not bothered by shadows and the analytics are not bothered by a rattling fence. Some systems require both to alarm before raising a high-priority alarm, which cuts nuisance alarms but also lowers the probability of detection, so that choice should be deliberate. Others raise an alarm on either and use the second to help the operator assess it.
Detection technology is one layer. Lighting, barriers, access control at gates and the response plan are others, as described in layered security design. Lighting in particular decides whether visible cameras can assess alarms at night; see exterior and security lighting.
Questions that decide the choice
- Is there a fence, and is it in good enough condition to carry sensors?
- What is the ground like: flat or rolling, paved or soil, wet or well drained, treed or open?
- How close are roads, railways or neighbouring operations that cause vibration or movement?
- What wildlife is present, and how much snow and wind does the site get?
- Must detection be covert, or is a visible deterrent useful?
- How will alarms be assessed, and how long does a response take?
- Who will maintain the system, and how often can vegetation and fence repairs be done?
A typical case
A storage yard on the edge of town has a chain-link fence along three sides and a treed ravine along the fourth. The fence sides get fence-mounted sensors, after loose fabric and gates are repaired, with thermal cameras and analytics assessing each zone. The ravine side has no fence and too much vegetation for analytics to work reliably, so a buried sensor follows the top of the slope, with a camera positioned to view the crossing. The main gate gets an infrared beam set that is masked during business hours and armed after closing. After commissioning, alarm logs show nuisance alarms concentrated in one fence zone near a rail line, which is retuned rather than turned off.
How ehZee can help
We survey sites, choose technologies zone by zone, and design and oversee installation, tuning and walk testing of perimeter systems. Design drawings are sealed by a licensed professional engineer where required, and installation is carried out by licensed electrical contractors and qualified security installers. See our perimeter intrusion detection service.
Common questions
Which technology is best?
None is best everywhere. The right choice depends on the fence, ground, weather, wildlife and how alarms will be assessed, and it often differs from one zone to the next on the same site.
Can perimeter sensors be added to an old fence?
Often, after repairs. Loose fabric, broken ties, leaning posts and rattling gates must be fixed first, or the sensors will alarm constantly. A badly damaged fence may be better served by a buried or video-based approach.
How do we know the system still works?
By testing each zone on a schedule with realistic intrusion attempts and recording the results, and by reviewing alarm logs. A zone that has not alarmed for months may be healthy or may be broken; only a test tells you which.