A cable, having two optical fibers, is buried in a two foot wide swath surrounding a facility containing buildings and grounds. The fibers are connected to directional couplers to form two fiber Sagnac interferometer loops. By making one loop longer than the other, with an additional fiber spool, not only can an intruder across the perimeter be detected but also the location of the intruder along the perimeter can be ascertained. The proposed system is described and analyzed. Simulation shows that such a system provides an excellent means of perimeter protection at low cost.