Date: Fri, 9 Jan 1998 01:07:37 -0600 From: Tim Tsai <tim@futuresouth.com> To: Greg Lehey <grog@lemis.com> Cc: David Kelly <dkelly@hiwaay.net>, FreeBSD Hackers <Hackers@FreeBSD.ORG> Subject: Re: GPS for xntpd Stratum 1 servers Message-ID: <19980109010737.63918@futuresouth.com> In-Reply-To: <19980109172927.06125@lemis.com>; from Greg Lehey on Fri, Jan 09, 1998 at 05:29:27PM %2B1030 References: <michaelh@cet.co.jp> <199801090340.VAA13302@nospam.hiwaay.net> <19980108232535.39313@futuresouth.com> <19980109172927.06125@lemis.com>
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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. > > 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. 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. Also, dependable transmission time over RS232 would be better than unpredictable ethernet transmission time in this application, no? Tim
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