Some radio transmissions to and from Cassini –known as tracking data -- provide a kind of interplanetary Global Positioning System to determine Cassini's whereabouts. "Uplinked" transmissions originate at the powerful radio transmitters of NASA's Deep Space Network. The Network's sensitive receivers capture the "downlinked" transmissions from Cassini.
The change in pitch of the signal, called Doppler shift, provides the information on the spacecraft's speed toward or away from Earth.
To determine Cassini's range, or distance from Earth, specially coded signals (called ranging tones) are added to the radio transmission that is uplinked to the spacecraft, and the exact time of transmission is recorded. When the spacecraft receives these ranging tones, it immediately sends them back to Earth. Their exact time of arrival is recorded by the Deep Space Network. The round-trip distance can then be computed because the radio signals travel at the speed of light (about 299,792,458 meters per second, or 186,282 miles per second).
There are other factors to consider, too. How long did it take for the ranging tones to "turn around" inside the spacecraft's electronics? That miniscule delay is known from pre-launch testing. How long did it take the ranging tones to travel through the cable from the computer in the Deep Space Network signal-processing center out to the radio antenna before leaving Earth? The Deep Space Network measures that time while calibrating the system prior to each tracking period. And how far did the Earth move while the ranging pulses were traveling to the spacecraft? The navigators draw upon data gathered from years of observations by the astronomical community.
Special computer programs integrate all these data to determine Cassini's precise flight path in orbit around Saturn, just as they did during the nearly seven-year cruise from Earth.
Just as sailors use the stars to know their location, Cassini navigators use the stars to better know the spacecraft's location relative to targets in the Saturn system. "Optical navigation" images are taken by Cassini's cameras to show Saturn's moons against a field of background stars. The positions of those stars are well-known from astronomical measurements and add precision to knowledge of Cassini's location. The optical navigation data are then used to fine-tune the timing and pointing of instrument observations.
Flight Path Control
Once Cassini's position in space is known, mission controllers may adjust the spacecraft's flight path if it is found to be off-target. On a rapid schedule, maneuver commands are designed and transmitted. These operate Cassini's main rocket engine or its smaller rocket thrusters. Called Orbit Trim Maneuvers, they make small adjustments to the spacecraft's path in Saturn orbit. The flight team computes the direction and magnitude of the change that is required to adjust the spacecraft's flight path, and the burns usually last just minutes or seconds, to achieve the desired change. The resulting change in velocity, called delta-V, is generally only a few meters per second at most. These maneuvers serve to fine-tune the spacecraft's course and improve an instrument's view of its target, or meet a flyby distance or closest approach timing more precisely.