火车汽笛的音调,在驶近的时候会升高,而远离的时候会降低。像这种声源在相对运动中频率变化的现象称为多普勒效应。无线电波同样也有多普勒效应。美国霍普金斯大学利用多普勒效应对苏联第一颗人造卫星进行了跟踪试验,结果发现,当卫星在近地点时信号频率就增加,远地点时信号频率就降低。因为卫星轨道是已知的,所以接收卫星信号的接收机不论处于何方,它的位置都能被测定。这样,卫星就起着全天候北极星 The tone of a train whistle rises as it approaches, and decreases as it moves away. Such as the sound source in the relative frequency of movement in the phenomenon known as the Doppler effect. Radio waves also have the Doppler effect. The Hopkins University tracked the first Soviet satellite using the Doppler effect and found that the signal frequency increased when the satellite was near the ground and the signal frequency decreased at apogee. Because the orbit of the satellite is known, the receiver receiving the satellite signal can be located no matter where it is located. In this way, the satellite plays an all-weather Polaris