Date: Thu, 16 Dec 1999 22:01:43 +0100 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: nate@mt.sri.com (Nate Williams) Cc: mjacob@feral.com, Ollivier Robert <roberto@eurocontrol.fr>, "FreeBSD Current Users' list" <freebsd-current@FreeBSD.ORG> Subject: Re: HEADSUP: ntp4 to replace xntpd Message-ID: <17769.945378103@critter.freebsd.dk> In-Reply-To: Your message of "Thu, 16 Dec 1999 13:50:12 MST." <199912162050.NAA21068@mt.sri.com>
next in thread | previous in thread | raw e-mail | index | archive | help
In message <199912162050.NAA21068@mt.sri.com>, Nate Williams writes: >> >> "What is a PPS signal ?" >> >> Typically handheld/boat naviation stuff. The NMEA or other >> serial timecodes are at best in the 1msec class. > >Again, for me this is acceptable. It would be nice to have it better >than this, but the kernel's of all the OS's I'm using have at best 1ms >precision for all of the applications being used (FS timestamps, >application program timestamps, etc...). Well, when I say "at best" I mean it. One NMEA boat-navigation unit I had access to over last winter had +/- 400msec performance. >As I mentioned to Warner, is there any way to know how good a particular >model of a GPS receiver is? measure it. It's not that hard actually, because you can trust the FreeBSD clock to be allright over short time intervals, so you timestamp the events (NEMA / PPS / Whatever) and analyse the pairwise difference between them: for instance: xxxxx2.100000 xxxxx3.140000 xxxxx4.120000 gives you *two* datapoints: +.040000 second and -.020000 second. Find the stddev of a couple of thousand samples and you have a good number which is correct to within a factor sqrt(two) or so of the real jitter. -- Poul-Henning Kamp FreeBSD coreteam member phk@FreeBSD.ORG "Real hackers run -current on their laptop." FreeBSD -- It will take a long time before progress goes too far! To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?17769.945378103>