Brooks Shera wrote: > In view of recent interest in the Allan Deviation of GPS-based 1 pps time, > it should be mentioned that the calculation of ADEV is based on a > statistical model which is not completely appropriate for noise sources > present in GPS signals and their decoding hardware/software. For example, > white phase noise, > which results from quantization effects in GPS receivers (sawtooth) and in > counter-based TICs. > > The result is that ADEV plots can be misleading if used to compare the > performance of clocks or signals with dissimilar noise properties. Allan > pointed this out in his 1981 paper in 'Proc. 35th Ann. Freq. Control > Symposium', and proposed a "modified" ADEV (MDEV) as a solution. He wrote: > > "A direct application for using the modified allan variance recently arose > in the analysis of atomic clock data as received from a GPS satellite. > ...Using Mod AV we can tell that the fundamental limiting noise process > involved in the system is white noise PM with the exciting result that > averaging for four minutes can allow one to ascertain time differences to > better that one nsec." > > The impact of time averaging to suppress white phase noise is illustrated > by a new plot TVB has created and placed on his website > http://www.leapsecond.com/pages/3gps/gps-adev.gif. > > These revealing plots show that just a few hundred-second time average has > largely removed the performance > difference between the aged VP and the newer M12+ and has almost entirely > removed the benefit of sawtooth correction (CNS-CNS II). These important > results are not evident in the usual ADEV plot. > > Such a time averaging effect is not entirely unexpected. In the long run, > the averaged time jitter in the transmission path between the satellite and > the ground must approach zero. What IS surprising is that the noise > averages to zero rather quickly and for purposes that require time > filtering, e.g. a GPSDO, it can safely be ignored. > > > Regards, and have fun, Brooks > > > _______________________________________________ > time-nuts mailing list > time-nuts@febo.com > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > Brooks
The plot you are referring to is surely: http://www.leapsecond.com/pages/3gps/gps-adev-mdev.gif The VP is still distinctly worse until tau > 1000sec. The plots are not conclusive evidence that correcting for the sawtooth error isn't advisable. What about "hanging bridges" and similar artifacts? Bruce _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts