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Electric fences prevent livestock from wandering, but they are expensive.
Agritech Electronics of Kansas is patenting a system that makes the animals
carry their own fencing (WO 98/1023). Each wears a collar that contains a
battery and receiver for signals from the Global Positioning System satellites.
The GPS receiver determines the animal’s position. A chip in the collar is
preprogrammed with the area the animal is allowed to graze, and delivers an
electric shock if it strays outside these boundaries.