Doppler system with single frequency

例句：

1. 1.The Doppler system is used to measure the speed of sound in air.
   多普勒系统用于测量空气中声速的速度。
2. 2.In a Doppler system, the frequency of the transmitted signal changes with the movement of the source.
   在多普勒系统中，发射信号的频率随着源的运动而变化。
3. 3.The Doppler system was developed to improve communication over long distances.
   多普勒系统被开发出来以改善远距离通信。