In this paper, we describe in detail an airport Perimeter Intrusion Detection System (PIDS) based on a wireless sensor network (WSN). The main purpose is to detect open field perimeter intrusions via ultra-small, power self-sustaining, accelerometer-based sensor nodes. The proposed system is inexpensive, easy to deploy and maintain, self-configurable and self-recoverable. By having no single point of failure component, this system is also exceptionally reliable. The proposed system is specifically designed for perimeter intrusion detection, preferably to supplement already-existing physical protections such as chain-link fences. While our system addresses airport security concerns, it can be extended to other critical infrastructures such as railway stations and ship ports.