From 8efa9776fef5e5df320496277582a90e54f73061 Mon Sep 17 00:00:00 2001 From: "Eric S. Raymond" Date: Wed, 9 Dec 2015 05:56:36 -0500 Subject: Correct HOWTos on WWVB modulation change. --- www/time-service-intro.txt | 23 ++++++++++++++--------- 1 file changed, 14 insertions(+), 9 deletions(-) diff --git a/www/time-service-intro.txt b/www/time-service-intro.txt index b441ba65..63edd46b 100644 --- a/www/time-service-intro.txt +++ b/www/time-service-intro.txt @@ -2,7 +2,7 @@ :description: A primer on precision time sources and services. :keywords: time, UTC, atomic clock, GPS, NTP Eric S. Raymond -v1.4, 2015-07-11 +v1.5, 2015-12-08 This document is mastered in asciidoc format. If you are reading it in HTML, you can find the original at the GPSD project website. @@ -70,9 +70,9 @@ National time standards are also propagated through time radios. In the U.S., digital time signal is provided by WWVB in Colorado, driven directly by the NIST (National Institute of Standards and Technology) master civil time standard. Propagation delay to U.S. receivers -varies but is constant by location and no more than 200 ms; being +varies but is constant by location and no more than 20 ms; being fixed, this can be easily compensated out. The jitter added to NIST -time by the WWVB transmission system is about 6 ns <>, for a +time by the WWVB transmission system is about 6 ns <>, for a total jitter relative to UTC of about 26 ns. Signal-processing jitter in the receiver may degrade this somewhat. @@ -233,14 +233,14 @@ weak-signal performance in the dramatic way GPSes and GPS clocks have; they remain finicky and nearly as vulnerable to siting and skyview problems as the GPSes of decades ago. -Furthermore, the U.S. radio-clock industry was impacted when WWVB -changed its modulation scheme in 2013. This didn't affect -consumer-grade "atomic" clocks, which resynchronize once a day +Furthermore, the U.S. radio-clock industry was impacted when <> +changed its modulation scheme at 2012-10-29T15:00:00Z. This didn't +affect consumer-grade "atomic" clocks, which resynchronize once a day and don't use the fine details of the signal, but it obsolesced all the high-end equipment that conditioned on shorter time scales. At least one major timing-systems vendor (Spectracom) bailed out of the -time-radio market entirely, and it is not clear there are any -high-end vendors at all left in the U.S. +time-radio market entirely, and it is not clear there are any high-end +vendors at all left in the U.S. == Summary == @@ -283,7 +283,7 @@ http://www.finra.org/sites/default/files/notice_doc_file_ref/Notice_Regulatory_1 - [[[RFC-5905]]] http://www.ietf.org/rfc/rfc5905.txt[Network Time Protocol Version 4: Protocol and Algorithms Specification] -- [[[WWVB]]] http://tf.nist.gov/timefreq/pubs/bulletin/pdf/1999OCT_TF_BULLETIN.pdf[NIST +- [[[WWVBPRECION]]] http://tf.nist.gov/timefreq/pubs/bulletin/pdf/1999OCT_TF_BULLETIN.pdf[NIST Time and Frequency Bulletin NISTIR 5082-10] - [[[BIPM-T]]] ftp://ftp2.bipm.org/pub/tai/publication/cirt/[See latest Circular T from BIPM] @@ -307,6 +307,8 @@ Clock via Modem] - [[[GPSD-HOWTO]]] link:gpsd-time-service-howto.html[GPSD Time Service HOWTO] +- [[WWVB]] http://www.nist.gov/pml/div688/grp40/wwvb.cfm[NIST Radio Station WWVB] + == History == v1.0, 2015-03-10:: @@ -325,4 +327,7 @@ v1.3, 2015-03-16:: v1.4, 2015-07-11:: Text polishing, note upcoming change in FINRA, more about GPSDO precision. +v1.5, 2015-12-10:: + Fix typo in WWVB delay figure and date of modulation change. + //end -- cgit v1.2.1