Outboard Detection Systems: Sensors and Coverage
What the candidate must know
Outboard detection covers every sensor mounted outside the protected hull: intrusion beams, radar and microwave motion detectors, pressure mats, magnetic reed switches on hatches and doors, vibration and shock sensors, and thermal or infrared trip devices. Candidates must be able to distinguish detection (sensing an event) from assessment (deciding whether it is a threat) and from annunciation (signalling). Know the vocabulary of coverage: detection zone, field of view, range, dead zone, masking, and line-of-sight.
Traps that appear in questions
- Confusing coverage overlap with redundancy; overlap improves probability of detection only if the sensors use different technologies.
- Assuming a PIR works through glass or around corners; passive infrared needs line of sight and is defeated by heat sources and rapid temperature change.
- Forgetting environmental masking: rain, fog, salt spray, insects, and foliage all reduce effective range.
- Mixing up normally open and normally closed alarm contacts and what a broken wire produces in each case.
- Ignoring tamper circuits; a sensor without tamper supervision is a coverage gap, not just a convenience issue.
Worked example / rule of thumb
A perimeter beam rated 100 m is installed across a 70 m gap. Derate for heavy rain by roughly 30 percent: effective range is about 70 m, so the link is marginal and should be reduced or split into two shorter spans. Rule of thumb: design to no more than 70 percent of the manufacturer's clear-weather range, and always verify detection with a walk test at the far edge of each zone, not at the centre.