Date: Thu, 8 Jan 1998 23:25:35 -0600 From: Tim Tsai <tim@futuresouth.com> To: David Kelly <dkelly@hiwaay.net> Cc: FreeBSD Hackers <Hackers@FreeBSD.ORG> Subject: Re: GPS for xntpd Stratum 1 servers Message-ID: <19980108232535.39313@futuresouth.com> In-Reply-To: <199801090340.VAA13302@nospam.hiwaay.net>; from David Kelly on Thu, Jan 08, 1998 at 09:40:48PM -0600 References: <michaelh@cet.co.jp> <199801090340.VAA13302@nospam.hiwaay.net>
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> > On Wed, 7 Jan 1998, David Kelly wrote: > > > If you are on the 15th floor of a 30 floor building then you are going > > > to have troubles with the GPS in any case. Any view out the window will > > > have less than half the sky visible. Sometimes there may not be enough > > > satelites visible for a lock. > > > > Yeah, Mike Smith suggested RS-232 <==> Fiber <==> RS-232 to the roof, but > > unfortunately doing that costs an arm and a leg in this concrete jungle. > > Er, maybe just a leg these days. 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. If you can extend a small pole (say 3 feet) out the window then it would be more than enough. I've stuck GPS antenna mounted on a broom stick out of hotel windows enough times to know this works. :-) > So whatcha do is take that old 386sx16 and put it on ethernet, up on the > roof with the GPS, and let it be the timeserver. 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. Tim
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