  We report in this letter the outstanding frequency stability performances of
an autonomous cryogenique sapphire oscillator presenting a flicker frequency
noise floor below 2e-16 near 1,000 s of integration time and a long term Allan
Deviation (ADEV) limited by a random walk process of 1e-18/sqr(tau). The
frequency stability qualification at this level called for the implementation
of sophisticated instrumentation associated with ultra-stable frequency
references and ad hoq averaging and correlation methods.
