 Rather than using an underground wire, they emit a radio signal from a central unit, and activate when the pet travels beyond a certain radius from the unit.In another type, the collar uses GPS signals to determine proximity to a predetermined "virtual fence", without the need for any physical installation at all.This system allows some additional flexibility, such as simpler inclusion of "islands" within the containment area, and easier changes to the boundary, although location of the boundary is not as precise due to GPS tolerances.