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
