Date: Thu, 16 Dec 1999 13:50:12 -0700 From: Nate Williams <nate@mt.sri.com> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: nate@mt.sri.com (Nate Williams), 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: <199912162050.NAA21068@mt.sri.com> In-Reply-To: <17662.945377157@critter.freebsd.dk> References: <199912161810.LAA19919@mt.sri.com> <17662.945377157@critter.freebsd.dk>
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> >> Between the two of us Dave Mills and I have managed to get the > >> "nanokernel" to act sensibly in the domain inside +/- 1usec which > >> the old one didn't. (See http://gps.freebsd.dk for what kind of > >> performance this can result in, given appropriate hardware). > > > >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? > > I think there are several classes of GPS receivers: > > "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...). > "VP Marketing to VP engineering: Everybody else has a PPS signal > make sure our product has one too at no extra cost or schedule changes." > > You don't want to know. As bad as 1msec have been seen, > jitter as bad as 200nsec. > > "Straight PPS" > > Derived from the internal clock, typically in the "a few usec" > class. > > "Position hold PPS" > > State of the art 1 band GPS does a stddev of about 35nsec. The > Motorola Oncore UT+ is considered the leader of the pack I think, > other vendors have similar devices. > > "Postion hold PPS + OCXO" > > OEM products doing basically what the HP 58503A does. > We're into cesium like (or better!) quality here. > > I have *not* heard some rumours about carrier phase tracking low cost > receivers, and I was *not* told that they can practically uwiggle > the S/A when in position hold mode and I was *not* told to expect them > on the market in 1H2000 :-) As I mentioned to Warner, is there any way to know how good a particular model of a GPS receiver is? Nate To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-current" in the body of the message
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