Date: Thu, 16 Dec 1999 21:54:47 +0100 From: Poul-Henning Kamp <phk@critter.freebsd.dk> To: nate@mt.sri.com (Nate Williams) Cc: Warner Losh <imp@village.org>, "FreeBSD Current Users' list" <freebsd-current@FreeBSD.ORG> Subject: Re: HEADSUP: ntp4 to replace xntpd Message-ID: <17729.945377687@critter.freebsd.dk> In-Reply-To: Your message of "Thu, 16 Dec 1999 13:35:23 MST." <199912162035.NAA20788@mt.sri.com>
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In message <199912162035.NAA20788@mt.sri.com>, Nate Williams writes: >> : You may not know the answer to this, but it's worth a shot. Wht kind of >> : accuracy can we expect using 'cheap' off-the-shelf GPS receivers? >> >> We're getting, with ntp4 on a 3.x kernel, about +- 4uSec with a cheap >> gps receiver + atomic clock on a i486 class machine. > >I've got the cheap gps receiver (Garmin 12XL), but what do you mean by >an 'atomic clock'? Should the GPS receiver's NMEA messages be adequate >enough to do the job? However, all I need is ms accuracy, so anything >below 500us is good enough for me. If you only have NMEA you will have a hard time. At 1200 baud you get 50 usec jitter just from the rxclock in the uart (16 * 1200), not to mention how exact the NMEA is transmitted in the first place. There is a pretty vanilla NMEA refclock, so the easist way to find out is to try it out next to (in a network sense) a stratum 1 NTP. -- 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
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