The Last Word on Time
Previously, we have taken a look at how cesium clocks, the most-accurate time and frequency references available to us, work. We also saw that the National Institute of Standards and Technology (NIST) has implemented the most-accurate primary time reference yet, the cesium fountain clock, which uses cutting-edge science and technology like laser cooling, none of which has anything to do with software! How can broadcasters and others take advantage of the highly precise time and frequency references that NIST has to offer? NIST promulgates precise time and frequency references in a number of ways.
Time and frequency information are broadcast on WWVB from Ft. Collins, Colo., at a carrier frequency of 60 kHz, with a power of 50 kW. This VLF signal, available in most of North America, provides a precise time reference for many uses – including consumer clocks and wristwatches that are equipped to receive a VLF signal. It is still used by most, if not all, the television networks as the NIST-traceable reference for their secondary time and frequency standards.
NIST continually monitors the received WWVB signals at Boulder, Colo., comparing the phase of the received signal to the primary standard. An archive of the accumulated daily phase shift for WWVB relative to NIST is available on the NIST Web site. No voice announcements are transmitted on WWVB. A timecode with which the 60 kHz carrier frequency is synchronized is broadcast continuously at a rate of 1 bit per second, using pulse width modulation. The carrier power is reduced 10 dB at the beginning of each second, so that the leading edge of every negative-going pulse is on time.
Full power is restored 0.2 second later for a binary "0," 0.5 second later for a binary "1," or 0.8 second later to denote a position marker. The timecode uses the binary coded decimal (BCD) format, in which binary digits represent decimal numbers. The BCD timecode contains the year, day, hour, minute, second and flags for Daylight Saving Time, leap years, and leap seconds.
The frequency uncertainty of the WWVB signal is less than one part in 1012, and with path delay removed, WWVB can provide UTC with an uncertainty of less than 100 microseconds. The path delay variations are quite small at the 60 kHz carrier frequency.
LORAN-C, a VLF radio navigation system operated by the U.S. Coast Guard, uses a carrier frequency of 100 kHz, referenced to cesium standards. Because the VLF ground-wave signal is quite stable and easy to receive, LORAN-C receivers are often used as frequency references. The NIST monitoring station at Boulder continually monitors the signals from three different LORAN-C transmitters, compares the phase of the received signals to the NIST primary standard and publishes the accumulated daily phase shift in an archive.
NIST broadcasts time and frequency information from two shortwave stations, WWV and WWVH, using multiple transmitters on various frequencies. WWV is located in Ft. Collins, Colo., and WWVH is located in Hawaii. Both WWV and WWVH broadcast on 2.5, 5, 10 and 15 MHz, and WWV additionally broadcasts on 20 MHz.
Shortwave reception conditions vary greatly depending on the time of day, time of year, carrier frequency, etc., so multiple frequencies are used to transmit the same information, assuring that there is a good chance to receive the signals on at least one frequency at any given time or place. Like WWVB, WWV and WWVH transmit Coordinated Universal Time (UTC), which is a 24-hour version of what we used to know as Greenwich Mean Time. There is a voice announcement about 7.5 seconds before each minute stating what the time will be at the minute.
ON THE PULSE
An audible pulse is transmitted every second except the 29th and the 59th seconds of each minute. The first pulse of each hour is an 800-millisecond burst of a 1,500 Hz sine wave. The first pulse of each minute is an 800-millisecond burst of either a 1,000 Hz sine wave from WWV or a 1,200 Hz sine wave from WWVH. The remaining seconds pulses are 5-millisecond bursts of either a 1,000 Hz sine wave or a 1,200 Hz sine wave, depending on the location. These brief bursts sound like clock ticks.
Each second’s pulse is preceded by 10 milliseconds of silence and followed by 25 milliseconds of silence. Between the second’s pulses, standard frequencies are transmitted during many, but not all, minutes. Most minutes have 500 (WWV) or 600 (WWVH) Hz tones between the second’s pulses.
Once per hour (except the first hour of each day) a 440 Hz tone is used in minute 1 (WWVH) or minute 2 (WWV) of the hour. 440 Hz might be recognized as the musical note A above middle C. The BCD time information that is broadcast on WWVB is also broadcast on the shortwave signals on a 100 Hz subcarrier, which may itself be used as an accurate audio frequency reference.
In addition to all this information, voice announcements are made giving geophysical alerts, marine storm warnings and Global Positioning System (GPS) status reports. The Global Positioning System is a worldwide radio navigation system using signals broadcast from a constellation of satellites.
The GPS frequencies are derived from onboard cesium standards. NIST continually monitors the GPS signals, comparing the frequency standard on each satellite to the NIST primary standard, publishing an archive containing the frequency uncertainty of each satellite. Thus, GPS receivers can also provide traceable time and frequency references.
Finally, NIST Network Time Service (NTS) allows synchronization of computer clocks via the Internet, providing the computer clock with time that is directly traceable to UTC (NIST). NTS responds to time requests from any Internet client in three protocols: DAYTIME, TIME, and NTP. The DAYTIME protocol is widely used by small computers with operating systems like DOS.
The time server monitors port 13, and responds to time requests in either tcp/ip or udp/ip formats. The timecode format is similar to the one NIST uses in its dial-up Automated Computer Time Service (ACTS), and consists of the following parts: the Modified Julian Date (MJD), which is a count of the number of days since January 1, 4713 B.C., followed by the year, month and day in two-digit numbers, the UTC time in hours, minutes and seconds, and a code that tracks Daylight Savings Time.
These are followed by codes indicating leap seconds, the health of the server, the number of milliseconds that the timecode is advanced to partially compensate for network delays (currently 50 ms), the UTC/NIST label and an on-time marker.
The TIME protocol returns a 32-bit unformatted binary number that represents the time in UTC seconds since January 1, 1900. Time protocol requests are monitored on port 37, and responded to in either tcp/ip or udp/ip formats. This is a very simple protocol; therefore, some information cannot be conveyed, such as Daylight Savings Time status and server health.
A COMPLICATED PROTOCOL
NTP is the most complex and sophisticated of the protocols, and is used in large computers and workstations. The NTP client software runs continuously in the background, and can be configured to query several servers and average the results. NIST servers listen for NTP requests on port 123, and respond with a udp/ip data packet in the NTP format. The data packet contains a 64-bit time stamp containing the time in UTC seconds since January 1, 1900, with a resolution of 200 picoseconds.
NTS client software is available free-of-charge from the NIST Web site. For computers not connected to the Internet, ACTS, referred to above, permits a dial-up modem to obtain UTC. As you can see, there are many ways to find out the exact time.
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