We present design aspects of a cryogenic sapphire oscillator which is being developed for ultra-high short term stability and low phase noise in support of the Cassini Ka-band Radio Science experiment. With cooling provided by a commercial cryocooler instead of liquid helium, this standard is designed to operate continuously for periods of a year or more. Performance targets are a stability of 3x10(-15) (1 second less than or equal to tau less than or equal to 100 seconds) and a phase noise of -73dB/Hz @ 1Hz measured at 34 GHz. Test results are reported for several subsystems; including cryocooler, vibration isolation system, and ruby compensating element.