Spacecraft navigators do the same thing that ocean-going ship navigators do -- except that
the ocean of space is much, much bigger and more dangerous. How do the navigators know
where the spacecraft is once it leaves Earth?
The path of the spacecraft -- the trajectory -- is planned well before launch. But once the
spacecraft leaves Earth, knowing where it is, and predicting where it will be at a certain time, is a
complicated process.
We can't see the spacecraft, even with a telescope, so we use an indirect method to find it.
If we know the velocity -- the speed and direction -- of the spacecraft, we can figure out where it
is. As the spacecraft travels away from Earth, the radio signals it sends to us appear to change
frequency. By monitoring this change, we can calculate speed and direction -- and thus, the exact
location of the spacecraft. This shift in frequency, called the Doppler shift after the physicist who
first described it, is caused by the motion of the spacecraft in relation to Earth. You hear the
Doppler affect in a police siren when it changes to a higher pitch as it approaches you, then
lowers after it passes by.
To track the spacecraft, NASA located the antennas of the
Deep Space Network
equally around Earth. The huge antennas are located in California, Spain, and Australia. As Earth
turns, an antenna at one location hands over the spacecraft signal to the next antenna.
Locations of the Deep Space Network antennas
Next: Commanding the Spacecraft 
