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For the Virginia based broadcast radio station, please see WWVB-FM.
WWVB antenna and support towers.
WWVB is a NIST time signal radio station near Fort Collins, Colorado, co-located with WWV. WWVB is the station that radio-controlled clocks throughout North America use to synchronize themselves. The signal transmitted from WWVB is a continuous 60 kHz carrier wave, derived from a set of atomic clocks located at the transmitter site. A 1 bit-per-second time code, which is based on the IRIG “H” format of time code and derived from the same set of atomic clocks, is then modulated onto the carrier wave using a technique described as pulse width modulation followed by amplitude-shift keying. The time in this code is given in UTC, which the radio-controlled clocks then have to convert to their own local time. A single complete frame of time code lasts one minute.
Contents
1 Antennas
2 Modulation Format
3 Propagation
4 Antenna re-use with former WWVL
5 WWVB east plans
6 See also
7 References
8 External links
//
Antennas
WWVB antenna coordinates (WGS84)
North
4040?50.6?N 10503?01.7?W? / ?40.680722 105.050472? / 40.680722; -105.050472
South
4040?28.9?N 10502?42.3?W? / ?40.674694 105.045083? / 40.674694; -105.045083
Coordinates: 4040?41?N 10502?49?W? / ?40.67806 105.04694? / 40.67806; -105.04694
There are two identical antennas used to radiate the WWVB signal. Both antennas are 122 meters tall, and their centers are separated by 857 meters. The physical configuration of each antenna is a diamond-shaped “top loaded monopole”, consisting of several cables spread on a flat plane from the top of their support towers, and a vertical cable that connects the top plane to a “helix house” on the ground. Each helix house contains a dual fixed-variable inductor system, which is used to keep the antenna system at its maximum radiating efficiency. The amount of cable used in each antenna is supposed to approach an optimum length of one-quarter wavelength, which for 60 kHz is almost 1.25 km.
Modulation Format
At the start of each UTC second, the WWVB 60 kHz carrier, which has a normal power of 70 kW, is reduced in power by 17 dB to 1.4 kW. Before July 12, 2005, when WWVB’s maximum ERP was 50 kW, the power reduction was 10 dB, resulting in a 5 kW signal. The type of bit transmitted on each second is determined by when the carrier wave is returned to normal power within that second. If the carrier power is returned to normal in one-fifth of a second, or 0.2 s, from when it was reduced, the bit is a zero. If the carrier power is returned to normal in one half-second, or 0.5 s, the bit is a one. If the carrier power is returned to normal in four-fifths of a second, or 0.8 s, the bit is a marker bit. Marker bits are sent during seconds 0, 9, 19, 29, 39, 49 and 59 of each minute; the other 53 seconds are binary time code data. (Unused bits are transmitted as binary 0.) Thus, the start of the second of two consecutive marker bits indicates the top of the minute, as well as serves as the on-time marker for the next frame of time code. A marker bit is also sent during leap seconds, so in this exceptional event, three consecutive marker bits will be transmitted.
WWVB also, as a method of station identification, advances the phase of its carrier wave by 45 at ten minutes past the hour, and returns to normal (a ?45 shift) five minutes later. This phase step is equivalent to “cutting and pasting” 1/8 of a 60 kHz carrier cycle, or approximately 2.08 ?s.
Bit
Weight
Meaning
Bit
Weight
Meaning
:00
FRM
Frame reference marker bit
:30
8
Day of year (continued)
:01
40
Minutes
:31
4
:02
20
:32
2
:03
10
:33
1
:04
0
:34
0
Unused, always 0.
:05
8
:35
0
:06
4
:36
+
DUT1 sign.If +, both bits 36 and 38 are set.If ?, bit 37 is set.
:07
2
:37
?
:08
1
:38
+
:09
P1
Marker bit
:39
P4
Marker bit
:10
0
Unused, always 0.
:40
0.8
DUT1 value (00.9 s).DUT1 = UT1?UTC.
:11
0
:41
0.4
:12
20
Hours
:42
0.2
:13
10
:43
0.1
:14
0
:44
0
Unused, always 0.
:15
8
:45
80
Year
:16
4
:46
40
:17
2
:47
20
:18
1
:48
10
:19
P2
Marker bit
:49
P5
:20
0
Unused, always 0.
:50
8
:21
0
:51
4
:22
200
Day of year1=January 1365=December 31366=December 31, leap year
…(and so on)
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