主题：问个土问题----超市购物车锁定功能 -- 鹰击长空

Theft deterrent systems are becoming popular in many shopping centers. The system works by locking one of the wheels when the cart is rolled out of the "green zone" into the "red zone." Each shopping cart is fitted with an electronic locking wheel, or 'boot'. The boot locks when it receives a low power FM frequency from the transmission line. A transmitter with a thin wire is placed around the radius of the parking lot. The wire is placed under the cement or asphalt only a few inches. This wire has an effective broadcast range of within a few feet both vertically and horizontally. If the cart's wheel does lock, a remote is used that broadcasts a certain frequency to unlock the wheel. Usually a yellow line is placed several feet in front of the broadcast range to warn customers that their cart will abruptly stop if they roll their cart over the yellow line. Minor injuries have occurred to people's wrists and arms when the cart abruptly stops[citation needed]. If two nearby stores are both using this type of anti-theft system, it is possible to have conflicts.