Doppler system with double frequency

例句：

1. 1. TheDoppler system is used to detectthe movement of objects in a moving vehicle.
   多普勒系统用于检测移动车辆中物体的运动。
2. 2. The Doppler systemcan be used to measure the speed ofsound in different mediums.
   多普勒系统可用于测量不同介质中声速的速度。
3. 3. The Doppler system has beendeveloped to improve communication between aircraft and groundstations.
   多普勒系统已被开发以改进飞机与地面站之间的通信。