Perimeter protection vendors typically divide the sensing function into four areas – detection, delay, assessment and response – and they typically specify one or more of four different types of detection or sensing systems:
- Fence mounted sensors that detect the cutting, climbing or lifting of the perimeter fence;
- Volumetric field sensors such as microwave, that create an invisible field of detection with height and width, making it more difficult to defeat;
- Buried or covert cables that are located in the ground using an rf field; and
- Infrared sensors which, in their active mode, provide a light beam.
The decision on which of these should be utilized, should be based on the threat and risk assessment, and whether it is high, medium or low.
The most common types of fences used in perimeter protection, are chain link, welded wire, expanded metal, punched metal, ornamental and palisade.
While chain link is the most common and least expensive, it is the least aesthetically pleasing and offers the least amount of delay resistance to attempted breaches. Welded wire, expanded metal and punched metal constitute the second type of fence fabric, are more expensive and offer added protection against someone passing items through the fence of breaching it with ordinary tools. These products are typically vinyl coated to provide color and added longevity.
There are three prevailing standards used for vehicle anti-ram protection, said King. For embassies overseas the US Department of State utilizes anti-ram walls allowing very limited penetration, since the embassy is typically in an urban setting with limited setback from public roads. For military sites, the Department of Defense uses a broader standard that incorporates a number of threat size vehicles at varying speeds with varying amounts of permissible penetration, because site conditions typically include more standoff distance between the VBIED and the protected asset. For all other sites, the appropriate standard is the ASTM F2656 standard, which incorporates four different threat vehicles, four different vehicle speeds and four different zones of penetration.