  The cryogenic sapphire oscillator (CSO) is a highly specialized machine,
which delivers a reference signal exhibiting the lowest frequency fluctuations.
For the best units, the Allan deviation (ADEV) is <1e-15 for integration time
between 1 and 10,000 s, with a drift <1e-14 in one day.The oscillator is based
on a sapphire monocrystal resonating at 10 GHz in a whispering-gallery mode,
cooled at 6 K for highest Q-factor and zero thermal coefficient. We report on
the progress accomplished implementing eleven CSOs in about 10 years since the
first sample delivered to the ESA station in Argentina.
  Short-term stability is improved by a factor of 3-10, depending on the
integration time, and the refrigerator's electric power is reduced to 3 kW.
Frequency stability and overall performances are reproducible, with unattended
operation between scheduled maintenance every two years. The CSO is suitable to
scientific applications requiring extreme frequency stability with reliable
long-term operation. For example, the flywheel for primary frequency standards,
the ground segment of GNSS, astrometry, VLBI, and radio astronomy stations.
