Ranging is a very useful technique, because it can pinpoint the round-trip distance to the Cassini-Huygens spacecraft to an order of magnitude of a single meter. But that's only one dimension. Where is the spacecraft in three dimensions?

Precision Ranging

A method that is called "precision ranging" is used to pinpoint the three-dimensional whereabouts of the spacecraft. Once in a while, the Cassini-Huygens navigators schedule the DSN so that radio telescopes in both the Earth's northern hemisphere (California and/or Spain) and the southern hemisphere (Australia) spend time tracking the spacecraft.

This permits Cassini to be located by what is known as "triangulation": If you measure the range to Cassini from a DSN station in the northern hemisphere, and you do the same from a station in the southern hemisphere (and you have measured the precise distance between the two Earth stations, too) then trigonometry says you can locate the spacecraft in three dimensions.

If measurement A is equal to B in the illustration, the spacecraft is between the stations. If A is longer than B, Cassini must be further south, and so on.

There are other factors to consider, though. By its nature, ranging can only measure the total round-trip distance. Since the spacecraft is moving, the range from the Earth to the spacecraft will be a little different from the range back to Earth. And the spacecraft's speed is always changing, so the technique has to account for that as well. Moreover, precision ranging measurements do not need to be obtained at the same time from both hemispheres. Rather than immediate, "simple" triangulation, tracking data acquired over a period of weeks or months are fitted to a continuously refined model of the spacecraft's trajectory.