Date: Fri, 9 Jan 1998 17:41:50 +1030 From: Greg Lehey <grog@lemis.com> To: Tim Tsai <tim@futuresouth.com> Cc: David Kelly <dkelly@hiwaay.net>, FreeBSD Hackers <Hackers@FreeBSD.ORG> Subject: Re: GPS for xntpd Stratum 1 servers Message-ID: <19980109174150.03353@lemis.com> In-Reply-To: <19980109010737.63918@futuresouth.com>; from Tim Tsai on Fri, Jan 09, 1998 at 01:07:37AM -0600 References: <michaelh@cet.co.jp> <199801090340.VAA13302@nospam.hiwaay.net> <19980108232535.39313@futuresouth.com> <19980109172927.06125@lemis.com> <19980109010737.63918@futuresouth.com>
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On Fri, Jan 09, 1998 at 01:07:37AM -0600, Tim Tsai wrote: > On Fri, Jan 09, 1998 at 05:29:27PM +1030, Greg Lehey wrote: >>> You only need three satellites to get a 2D position. I believe 1 >>> satellite in view is all that is necessary to get the time. That is very >>> practical nowadays [in Mississippi we easily get 8-10 satellites in clear >>> view] even if you only have a clear view of the sky on one side of the >>> building. >> >> Recall that there are only 24 active satellites, 12 above the horizon >> and 12 below the horizon. If you block out half the sky, the most you >> can hope for is 6. But that's enough, and I've had plenty of success >> with a GPS receiver mounted just inside a window. > > Right, I didn't mean 8-10 satellites from one side of the building, > merely that with a full view of the sky we can easily get 8-10 here. Sorry, I mistunderstood. Yes, now we're in agreement. >>> It'd be easier to use a couple of RS232<->RS422/RS485 converters. At >>> the typical GPS baud rate (4800/9600 baud) you should be able to run the >>> wire hundreds of meters if not more (RS422 spec escapes me at the moment). >>> The converters run for about $30-$100 a piece. >> >> What sort of time accuracy are you hoping for here? To transmit a >> short datagram (say, 16 bytes) at 9.6 kb/s will take you 16 ms. > > Since I am no expert on NTP I will refrain from further comments on > that. Oops. Yes, you were talking about NTP, I'd forgotten that. Yes, I don't know the requirements off the top of my head either. > I kinda doubt the accuracy is dependent on the transmission latency > though (I'd think that a long but deterministic transmission time is > better than short but unpredictable transmission time), but what do > I know. I don't know how NTP can know the latency, and that's the limit of your accuracy. I suppose you could tell it. > Also, dependable transmission time over RS232 would be better than > unpredictable ethernet transmission time in this application, no? I think NTP does quite well with jitter, which is what we're talking about here. I don't know many machines which take 16 ms to deliver a local datagram (some Tandem systems are a notable exception). Greg
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