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=-0.3 required=5.0 tests=BAYES_00, REPLYTO_WITHOUT_TO_CC autolearn=no 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!aioe.org!.POSTED!not-for-mail From: "Dmitry A. Kazakov" Newsgroups: comp.lang.ada Subject: Re: how to analyze clock drift Date: Mon, 24 Nov 2014 22:03:19 +0100 Organization: cbb software GmbH Message-ID: <81qc6fcqnm1t$.52r91xazm7qo$.dlg@40tude.net> References: <188uppnlnvqgq$.1kjz3jnhjxqji.dlg@40tude.net> <87fvdd38qi.fsf@debian.uxu> <87a93l35dm.fsf@debian.uxu> <9t7t6al8bmifd9krh6koiegttgsvcovadg@4ax.com> <87d28h1cj9.fsf@debian.uxu> <3apu6ap126abi6oalch9vpre20hjij2uon@4ax.com> <87k32oi7r8.fsf@debian.uxu> <98h17atrhtl9kitthjf8ukt1f7rk1ribvc@4ax.com> <8761e54qt2.fsf@debian.uxu> <119jk3v83ilwp$.94ppz78taoc4.dlg@40tude.net> Reply-To: mailbox@dmitry-kazakov.de NNTP-Posting-Host: wfRpp7ltpEWhI2na6kgpfA.user.speranza.aioe.org Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Complaints-To: abuse@aioe.org User-Agent: 40tude_Dialog/2.0.15.1 X-Notice: Filtered by postfilter v. 0.8.2 Xref: news.eternal-september.org comp.lang.ada:23700 Date: 2014-11-24T22:03:19+01:00 List-Id: On Mon, 24 Nov 2014 12:30:53 -0800 (PST), brbarkstrom@gmail.com wrote: > If you want to get time standard information, you can start with the > very short background piece from Wikipedia: > > http://en.wikipedia.org/wiki/Standard_time_and_frequency_signal_service The earth radius Re=6_371_000 m. Pi * Re / c where c is the speed of light 299_792_458 m/s is a rough estimation of the delay the signal takes traveling from the US to EU, ignoring refraction, interference, amplifiers, encoders/decoders etc. This is catastrophic 67 ms. It could be improved by statistical processing to, maybe, 10 ms or so. Now compare that with the resolution of a typical real-time clock, which is >3 ns! Add here times required to sample the signal, to pass it through the system layers, and you will understand how poor the thing is for any time measurement (except maybe for the continental shift times (:-)). Fortunately, neither global time signals nor NTP is needed for time measurements, clock drift included. -- Regards, Dmitry A. Kazakov http://www.dmitry-kazakov.de