70m antenna at the Canberra Deep Space Communications Complex, outside Canberra, Australia

The large dish-shaped antennas of the Deep Space Network (DSN) are radio telescopes measuring up to 70 meters in diameter. These huge structures must be pointed to within a small fraction of a degree of a spacecraft's location in the sky in order to be able to receive its downlink. The area of the sky to which the antenna points is no larger than the area seen through a soda straw, so Cassini's navigators must publish predictions of where the spacecraft will appear to be in the sky.

The Cassini-Huygens spacecraft spends much of its time at varying attitudes, pointing its body-fixed science instruments at points of interest while recording data. Then, based on previously developed commands stored on the spacraft, the spacecraft turns to point its 4-meter dish-shaped high-gain antenna toward Earth. The spacecraft's downlink can then be received by the Deep Space Network, and the stored data can be played back.

Once the downlink has been received, the DSN can begin transmitting the uplink at X-band frequency (about 8.5 Ghz) to send commands to provide a reference frequency for the spacecraft to set its downlink for coherent Doppler and carrying out ranging measurements.

In carrying out radio science experiments, some DSN stations can uplink at Ka-band frequency (about 32 GHz). The Ka-band uplink forms such a narrow beam that it must be pointed slightly ahead of where the spacecraft is in the sky. Then, by the time the signal has traveled the distance, the spacecraft will have moved into the Ka-band beam, and can receive it. This "leading" the Ka-band uplink pointing is accomplished automatically when needed.