From mboxrd@z Thu Jan 1 00:00:00 1970 X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me X-Spam-Level: X-Spam-Status: No, score=-1.9 required=5.0 tests=BAYES_00 autolearn=unavailable autolearn_force=no version=3.4.4 Path: eternal-september.org!reader01.eternal-september.org!reader02.eternal-september.org!news.eternal-september.org!mx02.eternal-september.org!feeder.eternal-september.org!news.glorb.com!Xl.tags.giganews.com!border1.nntp.dca1.giganews.com!nntp.giganews.com!buffer1.nntp.dca1.giganews.com!local2.nntp.dca.giganews.com!nntp.earthlink.com!news.earthlink.com.POSTED!not-for-mail NNTP-Posting-Date: Tue, 25 Nov 2014 12:15:55 -0600 From: Dennis Lee Bieber Newsgroups: comp.lang.ada Subject: Re: how to analyze clock drift Date: Tue, 25 Nov 2014 13:16:04 -0500 Organization: IISS Elusive Unicorn Message-ID: References: <87k32oi7r8.fsf@debian.uxu> <98h17atrhtl9kitthjf8ukt1f7rk1ribvc@4ax.com> <8761e54qt2.fsf@debian.uxu> <119jk3v83ilwp$.94ppz78taoc4.dlg@40tude.net> <81qc6fcqnm1t$.52r91xazm7qo$.dlg@40tude.net> X-Newsreader: Forte Agent 6.00/32.1186 X-No-Archive: YES MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Usenet-Provider: http://www.giganews.com NNTP-Posting-Host: 108.79.223.64 X-Trace: sv3-uARc1M92FQEHYMSayHfkdwiBx7E4LRbKw6oNr0UlCXPsdzOVUjG8jF0bhz4uk+7CfOdrCB5xFPdUU44!yZz2vOVL8XPqGtcPx6uLsZoojJ0jI9yvbTD2nkKYd3un6i1EOvlBNcEcQgyH/f1vIfb04UF23ZOw!gJ4INmhiBJNLJ4euH9ASvVFsJfU= X-Abuse-and-DMCA-Info: Please be sure to forward a copy of ALL headers X-Abuse-and-DMCA-Info: Otherwise we will be unable to process your complaint properly X-Postfilter: 1.3.40 X-Original-Bytes: 6028 Xref: news.eternal-september.org comp.lang.ada:23715 Date: 2014-11-25T13:16:04-05:00 List-Id: On Tue, 25 Nov 2014 06:04:28 -0800 (PST), brbarkstrom@gmail.com declaimed the following: > >The third reference in my previous post mentions software NIST provides >that give time signals from NIST in several formates. One format is the >Network Time Protocol (RFC-1395), where "The NIST servers listen for a NTP request on port 123, and respond by sending a udp/ip data packet in the NTP >format. The data packet includes a 64-bit timestamp containing the time in >UTC seconds since January 1, 1900 with a resolution of 200 ps." I think >that should probably be sufficient for the 3 ns accuracy desired in determining >clock drift. > Note that part of the NTP protocol (or receiving computer implementations) also incorporates lots of stuff to determine correction factors for the receiving computer and the latencies in the network. As a result, it is not as precise as you may want it to be. http://en.wikipedia.org/wiki/Network_Time_Protocol """ NTP is intended to synchronize all participating computers to within a few milliseconds of Coordinated Universal Time """ and """ NTP can usually maintain time to within tens of milliseconds over the public Internet, and can achieve better than one millisecond accuracy in local area networks under ideal conditions. """ Note that: milliseconds NTP is used to synchronize wall-clock time between computers by bouncing packets between them, but does not provide a fixed/stable clock signal itself. >I suspect that it might also be possible to get time from a gps-equipped >smartphone. Since the GPS satellites maintain atomic time and are carefully >cross-checking with ground stations, they are probably a potential source >of data for this problem. > Yes, but ... The time at the receiver is adjusted by fitting the delays from the satellites... One reason you need four satellites for a decent fix... you have to fit a local time along with distances from the satellites to determine location. They are a source for standard time these days... Consumer GPS and phones likely have a simple quartz clock for normal time-keeping that gets updated when ever a GPS fix is performed. For a computer lab, a standard time base is something like this http://www.arbiter.com/catalog/product/model-1084a-b-c-gps-satellite-precision-time-clock-%2840ns%29.php but again it is only meant to synchronize the wall clock time of disparate computers [the equivalent of setting your watch while listening to a time announcement on a radio]. You have to move up to http://www.arbiter.com/catalog/product/model-1083b.php to get standardized frequency outputs which can be used for clock differencing. >More exotic solutions might be uncovered from a bit of further research. >For example, the astronomers doing Very Long Baseline Interferometry need >to do remote time synchronization of high accuracy. I don't know the >methods they use, but maybe they have something that could be turned into >a useful tool. > Probably boxes like the above 1083b or 1084a -- depending upon whether they need a civil time-stamp or a reference frequency (you'd need the latter to calibrate a receiver, for example -- and wouldn't be using NTP with its latencies; rather you'd be using a distribution amplifier and lots of carefully measured coax so that the signal gets delivered to all end points at the same time). >Finally, after thinking about your response a bit, I think the average time >delay between a WWWV station in the US and a receiver in the EU would be >fairly constant -- except for variations due to changes in the index of >refraction and reflected signals bounced off the ionosphere. The constant >part of the delay becomes an offset in a linear regression data reduction. >Of course, this is minor quibble with your comment. > If you are in the EU, you shouldn't be using WWVB (WWV is an AM voice/tick signal [though there is a BCD subcarrier]; WWVB is a digital signal for automated setting of clocks). The UK has MSF on the same 60kHz as WWVB, and Japan has JJY. {I suspect those are the stations my watch handles as all three are on the same frequency and just need to decode the strongest signal -- the Citizen Skyhawk will identify which US/EU/JP was used on the last synchronization}. http://en.wikipedia.org/wiki/Time_from_NPL http://en.wikipedia.org/wiki/WWVB -- Wulfraed Dennis Lee Bieber AF6VN wlfraed@ix.netcom.com HTTP://wlfraed.home.netcom.com/