WWVB time code - radio clocks
Kendrick Webster
Some people have expressed interest in WWVB, so here is some more
information including the format of the broadcast and some info about
other NIST time and frequency services (including the Automated
Computer Time Service accessable via modem).
WWVB is a service provided by the National Institute of Standards and
Technology (formerly National Bureau of Standards). Broadcasts started
in 1956. 13kW is broadcast at 60kHz out of Fort Collins, CO. The
transmitted accuracy is normally better than 1 part in 100 billion.
Due to signal propagation effects (which are much less of a problem at
60kHz than at high frequencies) the received accuracy is apprx 0.1 ms.
The WWVB time code is pulse-width modulated onto the carrier at one
pulse per second. Each pulse is generated by reducing the carrier
power 10dB at the start of the second and restoring full power 200,
500, or 800 milliseconds later to convey a binary "0", a binary "1",
or a position reference marker, respectively.
power reduced meaning
______________________________________
800 ms reference marker
500 ms logic "1"
200 ms logic "0"
The time code frame repeats once per minute. It is subdivided into
6 subframes that are 10 seconds long each. Data is sent in BCD with
the most-significant bit first.
The code frame follows:
second meaning / function notes
_________________________________________________________________________
00 reference marker
01 minutes * 40 00..59 -- minutes
02 minutes * 20
03 minutes * 10
04 unused
05 minutes * 8
06 minutes * 4
07 minutes * 2
08 minutes * 1
09 reference marker
10 unused
11 unused
12 hours * 20 00..23 -- hours, UTC
13 hours * 10
14 unused
15 hours * 8
16 hours * 4
17 hours * 2
18 hours * 1
19 reference marker
20 unused
21 unused
22 days * 200 001..366 -- day of year
23 days * 100
24 unused
25 days * 80
26 days * 40
27 days * 20
28 days * 10
29 reference marker
30 days * 8
31 days * 4
32 days * 2
33 days * 1
34 unused
35 unused
36 UT1 sign -- Add sign of UT1 correction
37 UT1 sign -- Subtract
38 UT1 sign -- Add this bit has same value as #36
39 reference marker
40 UT1 correction * 800ms UT1 difference from UTC
41 UT1 correction * 400ms
42 UT1 correction * 200ms
43 UT1 correction * 100ms
44 unused
45 year * 80 00..99 -- year
46 year * 40
47 year * 20
48 year * 10
49 reference marker
50 year * 8
51 year * 4
52 year * 2
53 year * 1
54 unused
55 leap year indicator
56 leap second warning
57 Daylight Savings Time bit -- see description below
58 Daylight Savings Time bit
59 reference marker
The 0.8s reference markers are transmitted at the end of each 10s
subframe and one is also transmitted at the beginning of the frame.
Two markers in a row only occur at the end/beginning of the frame.
UTC and UT1 are timescales. UTC is known as "Coordinated Universal
Time" and is based on atomic frequency standards and was established
by international agreement in 1972. It is governed by the
International Bureau of Weights and Measures (BIPM) in Paris, France.
UT1 is an astronomical time scale based on the rotation of the Earth.
In order to maintain UTC to within +/- 0.9 seconds of UT1, leap seconds
are occasionally inserted. A leap second is normally added every 1
or 2 years, usually on June 30 or December 31. Leap seconds are always
added at the end of the month. The leap second warning bit at 56s
is set to a logic "1" early in the month in which a leap second will be
inserted and is reset to "0" immediately following the insertion.
Bits 36 through 43 indicate the difference from UTC to UT1. If bits
36 and 38 are "1" and bit 37 is "0", the correction is added to UTC
to obtain UT1. If bits 36 and 38 are "0" and bit 37 is "1", the
correction is subtracted from UTC. The value of the correction is
sent in bits 40 through 43 and ranges from 0 to 0.9 seconds in .1s
increments.
The DST bits 57 and 58 are "0" when standard time is in effect. They
are both "1" when DST is in effect. On the day of the change from
standard to DST, bit 57 is changed from "0" to "1" at 0000 UTC. Exactly
24 hours later, bit 58 also changes from "0" to "1" at 0000 UTC. On
the day of the change from DST to standard, bit 57 goes from "1" to "0"
at 0000 UTC followed 24 hours later by bit 58. Receivers displaying
local time can automatically make the one-hour adjustment at the
appropriate local time.
UTC is referenced to the zero meridian which passes through Greenwich,
England. When local time changes from standard to DST, UTC does not
change. The offset from UTC to local time changes by 1 hour. For
example, Eastern Standard Time (EST) is -5 hours from UTC, but Eastern
Daylight Time (EDT) is only 4 hours behind UTC. Here in Colorado we're
-7 hours from UTC when on standard time and -6 hours when on daylight
savings time.
WWVB identifies itself (Station Identification) by advancing its carrier
phase by 45 degrees at 10 minutes after the hour and returning to normal
phase at 15 minutes after the hour.
Finally, note that NIST encourages users to obtain updated information
because there may be future changes to the format. (Though I'd imagine
nothing too drastic since a lot of users already depend on the service
and don't want their equipment to become obsolete).
--------------
This information was obtained from NIST Special Publication 432 "Time
and Frequency Services" for sale by the Superintendent of Documents,
U.S. Government Printing Office, Washington, D.C. 20402.
NIST is in no way responsible for any typing errors I may have made.
The wording is purely my own fault also. My personal confidence in
this info is good, however, because I have successfully decoded
WWVB transmissions and I proofread this info carefully before posting.
--------------
Some other NIST time and frequency dissemenation services:
WWV and WWVH shortwave broadcasts -- you may hear these broadcasts
via phone by dialing (303) 499-7111 (Boulder, Colorado) to hear
WWV and (808) 335-4363 (Kauai, Hawaii) to hear WWVH. These
broadcasts also cary a BCD time-code on a 100 Hz subcarrier in
addition to audio. Frequencies are 2.5, 5, 10, 15, and 20 MHz.
GOES satellite time services
NIST frequency measurement service
NIST time and frequency bulletin -- monthly bullitin about WWV, WWVH,
WWVB, GOES, Loran-C, GPS, and NIST time scales. Free subscription
to users of NIST services by writing to:
Editor, Time and Frequency Bulletin, Division 847
National Institute of Standards and Technology
325 Broadway
Boulder, CO 80303-3328
--------------
Automated Computer Time Service
This is really a pretty cool thing! Its a 300/1200 baud modem dial-up
at (303) 494-4774 which allows software to call and set your computer's
clock. The service is free except for the cost of making a long-distance
telephone call. Measurement of and compensation for phone line delay is
provided. It typically takes about 5 seconds on-line if you don't need
line-delay compensation or about 10 or 15 seconds if you do. I have
written a nice Windows 3.1 program which utilizes this service to set
your DOS and CMOS clocks. I'll send you a copy for $12 including postage
anywhere in the US. Just email me (kweb...@holly.colostate.edu) if
you're interested. If you're into writing your own software, just call
the dial-up with your terminal software and type '?' to get a description
of the service and its data format.
'njoy and happy clockbuilding!
--Ken
Booth, Thomas G
I thought it may be of interest to those reading this thread...
: From: dmoo...@interaccess.com (Dave Moorman)
: Subject: Re: Very low freq./WWVB reciever
: Date: Sat, 08 Apr 1995 20:18:39 -0500
:
: In article <3lv5dg$8...@newsbf02.news.aol.com>, egru...@aol.com
: (EGrumling) wrote:
:
: > What I would like to do is
: > build a reciever and just decode the signal into a display (to use as a
: > UTC clock).
: >
: > Any ideas...
: > E
:
: There is a company called FAR Circuits which makes circuit boards for a
: variety of projects. One is called the The Most Accurate Frequency
: Standard which is from January, February, and March, 1994, 73 magazine.
It
: uses WWVB transmissions to make a very accurate standard.
:
: The address is:
:
: FAR Circuits
: 18N640 Field Court
: Dundee,
: Illinois 60118
:
: Also, Ham Radio mag described a similar project back in the 1980's.
:
: (The usual disclaimers)
:
: Dave Moorman, K9SW
: Downers Grove, Illinois
And my hat's off to Ken for an excellent dissertation on the WWVB time code
in his previous post...considering he's effectively at "ground zero" for
receiving WWV and WWVB transmissions, he ought to know a thing or two about
'em ( ;-) ) !
TGB
\\ The opinions expressed herein are my own. //