phk has a Stanford Research Systems PRS10 Rubidium Frequency Standard (0 MHz rubidium oscillator)<http://www.thinksrs.com/products/PRS10.htm> http://www.thinksr

phk has a Stanford Research Systems PRS10 Rubidium Frequency Standard (0 MHz rubidium oscillator)<http://www.thinksrs.com/products/PRS10.htm> http://www.thinksrs.com/products/PRS10.htm> at his home. Serial # 005597. The short-term stability of this unit is about 5 × 10-12 This particular PRS10 is GPS referenced _and_ drives one of his GPS referenced NTP servers. He also has (or had) an array of 10 Motorola Oncore UT+ or M12+T GPS receivers sitting on a row 20cm apart, mounted on a shed roof. UTC(USNO) is provided by the USNO Reference System #2 (Master Clock #2), which is a Datum Inc. hydrogen maser http://tycho.usno.navy.mil/maser.html http://tycho.usno.navy.mil/maser.html The frequency stability "sigma" of MC #2 has been < 2 x 10-15 during the last year. NIST-F1, the nation's primary time and frequency standard, is a cesium fountain atomic clock developed at the NIST laboratories. The uncertainty of NIST-F1 is continually improving. In 2000 the uncertainty was about 1 x 10-15, but as of January 2013, the uncertainty has been reduced to about 3 x 10-16 So, no, not "more precise" than the clocks you list, but how many of us have a Rubidium standard in our home? (And when compared to the large number of clocks in the world, yes, I think that the PRS10 is "one of the most precise".)