From owner-freebsd-hackers Thu Jan 8 23:08:06 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.7/8.8.7) id XAA23079 for hackers-outgoing; Thu, 8 Jan 1998 23:08:06 -0800 (PST) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from shell.futuresouth.com (shell.futuresouth.com [207.141.254.20]) by hub.freebsd.org (8.8.7/8.8.7) with ESMTP id XAA23040 for ; Thu, 8 Jan 1998 23:07:58 -0800 (PST) (envelope-from tim@shell.futuresouth.com) Received: (from tim@localhost) by shell.futuresouth.com (8.8.8/8.8.8) id BAA26882; Fri, 9 Jan 1998 01:07:37 -0600 (CST) Message-ID: <19980109010737.63918@futuresouth.com> Date: Fri, 9 Jan 1998 01:07:37 -0600 From: Tim Tsai To: Greg Lehey Cc: David Kelly , FreeBSD Hackers Subject: Re: GPS for xntpd Stratum 1 servers References: <199801090340.VAA13302@nospam.hiwaay.net> <19980108232535.39313@futuresouth.com> <19980109172927.06125@lemis.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii X-Mailer: Mutt 0.88 In-Reply-To: <19980109172927.06125@lemis.com>; from Greg Lehey on Fri, Jan 09, 1998 at 05:29:27PM +1030 Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk 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