The USO long-term frequency stability requirement of / 2·10E-10 is critical for a successful Doppler Wind Experiment. By "long-term" it is meant that the uncertainty in the frequency must be constrained within these limits for the maximum expected duration of the atmospheric descent (2.5 hours), i.e., the total frequency shift of the PRL signal is less than 0.4 Hz over the entire Huygens mission. Assuming a typical DWC angle of 65°, an unwanted frequency shift of this magnitude would be indistinguishable from a zonal wind with velocity of 0.15 m/s. This USO frequency stability is thus mandatory for achieving the primary DWE goal, a determination of Titan's zonal wind height profile with an accuracy well below the 1 m/s level. This must be maintained throughout the entire Huygens mission, in spite of many rather severe changes in the environmental conditions (temperature T, pressure P, acceleration A and magnetic field B). Generally, this requirement can be expressed by:

where "X" collectively labels other factors, e.g. the variation
of the USO supply voltage.
Typical values for the expected fractional change in the
USO output frequency are listed in Table 4.

Although not as critical as the long-term frequency stability, the short-term frequency stability is another criterion for the frequency quality of the USO. The short-term frequency stability is characterized by the mean fractional frequency deviation

This quantity, sometimes called the
"Allan deviation", depends on the integration time (s. also
eq. (26)).
The specified USO short-term frequency stabilities are shown in
Table 5.
These values are sufficient to detect PRL frequency modulations due
to atmospheric turbulence as well as pendulum and rotational
motion of the Huygens Probe.
Table 6 lists the specifications on the phase noise of the output signal
spectrum for various displacements from the center peak.