Date: Fri, 9 Jan 1998 17:29:27 +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: <19980109172927.06125@lemis.com> In-Reply-To: <19980108232535.39313@futuresouth.com>; from Tim Tsai on Thu, Jan 08, 1998 at 11:25:35PM -0600 References: <michaelh@cet.co.jp> <199801090340.VAA13302@nospam.hiwaay.net> <19980108232535.39313@futuresouth.com>
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On Thu, Jan 08, 1998 at 11:25:35PM -0600, Tim Tsai wrote: >>> 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. 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. >> 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. 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. Greg
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