Aside from the information on the downlink, the carrier itself is used for tracking and navigating the spacecraft, as well as for carrying out experiments such as radio science or gravity field mapping. For each of these uses, an extremely stable downlink frequency is required, so that Doppler shifts on the order of fractions of a Hertz may be detected out of many GHz over periods of many hours. But it would be impossible for any spacecraft to carry the massive equipment required to maintain such frequency stability. Spacecraft transmitters suffer wide temperature changes, which cause frequency drift.

The solution is to have the spacecraft generate a downlink that is coherent to the uplink it receives.

While Cassini's ultra-stable oscillator is effective, it does not provide consistent stable and accurate data. Down in the basement of each Deep Space Network tracking complex is a room carefully kept at a constant temperature all year long. In each of these rooms is a frequency reference called a hydrogen maser. The signal from the hydrogen maser is amplified in the tracking station's transmitter, and sent up to Cassini. When Cassini has been commanded NOT to use its ultra-stable oscillator, it automatically uses the uplink frequency that it receives to compute its downlink frequency. Then Cassini's transmitter sends that frequency as its downlink. That way, much of the uncertainty is removed, and the Doppler effect can be measured so precisely that Cassini's speed can be measured to within a few millimeters per second.

That scheme is called coherent mode, because Cassini's downlink frequency is coherent with (based on) the uplink frequency. When Cassini is using its ultra-stable oscillator or other means to generate its downlink frequency, it is known as non-coherent mode, because the downlink is not coherent with the uplink.