Another use of uplink and downlink, working together for tracking, takes advantage of what is called the "Doppler effect" to determine the speed at which the spacecraft is moving toward or away from Earth. If an object is sending out waves, whether they are sound, radio, or light, you can sense the motion of the object by observing the frequency of the waves received, provided you know the frequency at which they started out.

To make sense of how the Doppler effect works, imagine a clock ticking loudly across a room. Assume the clock ticks once per second, so normally, you can hear the ticks arrive once per second from where you stand. Each tick travels at the speed of sound, across the same distance.

Now, suppose the clock begins moving toward you. As the clock approaches, each tick arrives a little sooner, because each one has less distance to travel to you than the previous tick did. Since the intervals between received ticks have diminished, we can say their perceived frequency increases: they arrive more frequently.

On the other hand, if the clock moves away from you, each tick has a greater distance to travel to reach where you are standing. The intervals between the ticks have increased, and the ticks arrive less frequently-- that is, at a lower frequency.

If you measured the precise interval of ticks received (and if you know the clock creates ticks exactly once every second - at one Hertz) you could calculate the speed the clock is moving toward or away from you. If the clock was ticking a thousand times per second (one kHz, an audio frequency), you could easily hear the frequency change as the clock moves toward or away from you. Precise measurement could then tell you the speed the clock is traveling.

The same is true with the radio signal that the Cassini-Huygens spacecraft sends to Earth. Instead of sending one tick per second, the spacecraft's transmitter sends a signal that oscillates, or "ticks," approximately eight and a half billion times per second (Cassini's ticks travel across the distance at the speed of light). If the spacecraft is moving away, then the oscillations appear to arrive less frequently than that, similar to the clock example.

The Doppler effect is an important method of tracking the Cassini-Huygens spacecraft throughout its mission. An important point to realize though, is that in order to calculate the speed of the spacecraft, it is necessary to know the original frequency the spacecraft is sending. This is a challenge for any spacecraft, and engineers have developed ways to determine what frequency the spacecraft's transmitter is sending, independently of what is received. Cassini's Ultra-Stable Oscillator allows for fairly accurate measurements when there is only a downlink, and Coherent Mode provides the most accurate Doppler measurement when both the uplink and downlink are available.