signal encrypt system

例句：

1. 1. The policeman used a flashlight to signal the driver to stop.
   警察用手电筒示意司机停车。
2. 2.The traffic light is a common signal for pedestrians crossing the street.
   交通灯是行人过马路的常见信号。
3. 3.The GPS system uses signals from satellites to guide a vehicle's navigation.
   全球定位系统利用卫星发出的信号来引导车辆导航。