Regardless of the frequency of a source of electromagnetic waves, they are subject to the Doppler effect. The Doppler effect causes the observed frequency of a source to differ from the radiated frequency of the source if there is motion that is increasing or decreasing the distance between the source and the observer. The same effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer, or vice-versa.
When the distance between the source and receiver of electromagnetic waves remains constant, the frequency of the source and received wave forms is the same. When the distance between the source and receiver of electromagnetic waves is increasing, the frequency of the received wave forms is lower than the frequency of the source wave form. When the distance is decreasing, the frequency of the received wave form will be higher than the source wave form.
The Doppler effect is routinely observed in the frequency of the signals received by ground receiving stations when tracking spacecraft. The increasing or decreasing of distances between the spacecraft and the ground station may be caused by the spacecraft's trajectory, its orbit around a planet, Earth's revolution about the sun, or Earth's daily rotation on its axis. A spacecraft approaching Earth will add a positive frequency bias to the received signal. However, if it flies by Earth, the received Doppler bias will become zero as it passes Earth, and then become negative as the spacecraft moves away from Earth. A spacecraft's revolutions around another planet such as Mars adds alternating positive and negative frequency biases to the received signal, as the spacecraft first moves toward and then away from Earth. The Earth's rotation adds a positive frequency bias to the received signal as the spacecraft rises in the east at a particular tracking station, and adds a negative frequency bias to the received signal as the spacecraft sets in the west.
If two widely-separated tracking stations on Earth observe a single spacecraft in orbit about another planet, they will each have a slightly different view, and there will be a slight difference in the amount of Doppler shift observed by each station. For example, if one station has a view exactly edge-on to the spacecraft's orbital plane, the other station would have a view slightly to one side of that plane. Information can be extracted from the differencing of the two received signals that describes the spacecraft's arc through space in three dimensions. This data type, differenced Doppler, is a useful form of navigation data which can yield a very high degree of spatial resolution. It is further discussed in Chapter 13, Spacecraft Navigation.
Recap
