Time synchronization in North America

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Time synchronization in North America can be achieved with many different methods, some of which require only a telephone, while others require expensive, sensitive, and rare electronic equipment. In the United States, the United States Naval Observatory provides the standard of time, called UTC(USNO), for the United States military and the Global Positioning System, [1] while the National Institute of Standards and Technology provides the standard of time for civil purposes in the United States, called UTC(NIST).

Contents

ITU-R Standard Frequency and Time Signals

A standard frequency and time signal service is a station that operates on or immediately adjacent to 2.5 MHz, 5 MHz, 10 MHz, 15 MHz, 20 MHz, and 25 MHz, as specified by Article 5 of the ITU Radio Regulations (edition 2012). [2] The US service is provided by radio stations WWV (Colorado) and WWVH (Hawaii).

The methods below provide either Coordinated Universal Time (UTC), which is defined by Recommendation ITU-R TF.460, [3] or the official U.S. implementation of UTC, officially labeled UTC (NIST).

Internet time sources

Several different time synchronization protocols exist on the Internet, including:

ProtocolRFCsAccuracyPortsTime servers
Daytime Protocol RFC 868 Nearest secondTCP port 13
Time Protocol (a.k.a. NETDATE) RFC 868 Nearest secondTCP/UPD part 37
NTP RFC 1305, RFC 5905
SNTP RFC 1769, RFC 2030
ICMP TIMESTAMP RFC 0792, page 16
IP TIMESTAMP option RFC 0791, 3.1, page 16

GPS time synchronization

GPS receiver requirements

Utility frequency

In 2009 the Federal Energy Regulatory Commission made time error correction (TEC) of the power grid frequency mandatory. [9] While TEC does not provide full synchronization (date and time) and synchronization is lost in case of a power outage it provides an inexpensive way to maintain high long term accuracy of synchronous clocks found in most household appliances. Once the initial time is set the power grid will typically maintain the accuracy within 10 seconds relative to UTC by adjusting frequency. [10]

All time sources

Several different organizations provide publicly accessible recorded voice time sources, including the NIST Telephone Time of Day Service, see speaking clock. Other sources include GPS, terrestrial radio time signals, and internet services, as listed below.

North American Time Synchronization Sources
Time SignalTime ProvidedAccuracySourceSignal FormatHardware RequirementsLinux/Unix SoftwareWindows Software
GPS UTC(USNO)1575.42 MHz NMEA 0183 sentences, 1PPS signal.Minimum: GPS receiver that works with one's chosen software; this requires some combination of GPGGA, GPRMC, GPZDA, GPGSA, and GPGSV, sentences.

Ideal: GPS receiver (OCXO/DOCXO disciplined clock preferred) with NMEA 0183 output on RS-232 or USB, with GPZDA sentence sent at least once a second, and 1PPS signal on DCD, which includes at least these devices:

  • Gpsd plus ntpd with the GPSD NG client driver [12]
WWVB UTC(NIST)60 kHz AM IRIG "H" PWM time code with PM BPSK signal overlaid. [18] Radio-controlled clock:
  • NIST list of receivers [19]
  • AC-100-WWVB Time Receiver
  • AC-500-MSF Time Receiver
  • ClockWatch Radio Sync [20]
  • F6CTE's CLOCK [15]
WWV 2.5, 5, 10, 15, and 20 MHz AMVoice with modified IRIG-H format time code on 100 Hz sub-carrier (CCIR code)
  • HF radio and antenna (plus software if automatic updating of computer time is desired)
  • TrueTime TL-3 WWV Receiver
  • ntpd with Radio WWV Audio Demodulator/Decoder (driver can tune ICOM HF radios via C-IV)
  • COAA's Radio Clock [21]
  • F6CTE's CLOCK [15]
WWVH 5, 10, and 15 MHz AMVoice with modified IRIG-H format time code on 100 Hz sub-carrier (CCIR code)
  • HF radio and antenna (plus software if automatic updating of computer time is desired)
  • TrueTime TL-3 WWV Receiver
CHU 3.33 MHz, 7.85 MHz, 14.67 MHz Bell 103 modem tones, decodable by most computer modems
  • ntpd with Radio CHU Audio Demodulator/Decoder driver (driver can tune ICOM HF radios via C-IV)
Daytime ProtocolUTC (variant depends on server's time source)
  • ClockWatch Pro for Windows [22]
Time Protocol
Network Time Protocol pool.ntp.org Computer with NTP client that syncs at least once an hour. ntpd, sntp, ntpdate
NTPSec
Precision Time Protocol
  • Domain Time II [25]
NIST Telephone Time of Day Service [26] UTC(NIST)
  • +1-303-499-7111
  • +1-808-335-4363
Voice announcement with sync pips.Telephone connection, earn/aManual sync only.
NIST Automated Computer Time Service (ACTS) [27]
  • +1-303-494-4774
  • +1-808-335-4721
Windows computer with dialup modem.
  • ntpd with NIST/USNO/PTB Modem Time Services driver
  • ClockWatch Pro for Windows [22]
USNO Master Clock modem time [28] +1-202-762-1594Computer with Bell 212A or CCITT V.22 compatible modem
US Naval Observatory time serviceUTC(USNO)
  • +1-202-762-1401
  • +1-202-762-1069 (Washington, D.C.)
  • +1-719-567-6742 (Colorado Springs)
earn/a

See also

Related Research Articles

<span class="mw-page-title-main">Global Positioning System</span> American satellite-based radionavigation service

The Global Positioning System (GPS), originally Navstar GPS, is a satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of the global navigation satellite systems (GNSS) that provides geolocation and time information to a GPS receiver anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellites. It does not require the user to transmit any data, and operates independently of any telephonic or Internet reception, though these technologies can enhance the usefulness of the GPS positioning information. It provides critical positioning capabilities to military, civil, and commercial users around the world. Although the United States government created, controls and maintains the GPS system, it is freely accessible to anyone with a GPS receiver.

<span class="mw-page-title-main">Leap second</span> Extra second inserted to keep civil time in sync with the Earths rotation

A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972 and since then 27 leap seconds have been added to UTC.

Time and frequency transfer is a scheme where multiple sites share a precise reference time or frequency. The technique is commonly used for creating and distributing standard time scales such as International Atomic Time (TAI). Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomena with each other, as well as cell phone towers coordinating handoffs as a phone moves from one cell to another.

<span class="mw-page-title-main">Radio clock</span> Type of clock which self-synchronizes its time using dedicated radio transmitters

A radio clock or radio-controlled clock (RCC), and often (incorrectly) referred to as an atomic clock is a type of quartz clock or watch that is automatically synchronized to a time code transmitted by a radio transmitter connected to a time standard such as an atomic clock. Such a clock may be synchronized to the time sent by a single transmitter, such as many national or regional time transmitters, or may use the multiple transmitters used by satellite navigation systems such as Global Positioning System. Such systems may be used to automatically set clocks or for any purpose where accurate time is needed. RC clocks may include any feature available for a clock, such as alarm function, display of ambient temperature and humidity, broadcast radio reception, etc.

<span class="mw-page-title-main">WWV (radio station)</span> U.S. government shortwave radio station broadcasting time signals

WWV is a shortwave radio station, located near Fort Collins, Colorado. It has broadcast a continuous time signal since 1945, and implements United States government frequency standards, with transmitters operating on 2.5, 5, 10, 15, and 20 MHz. WWV is operated by the U.S. National Institute of Standards and Technology (NIST), under the oversight of its Time and Frequency Division, which is part of NIST's Physical Measurement Laboratory based in Gaithersburg, Maryland.

<span class="mw-page-title-main">WWVH</span> Radio time signal station in Kekaha, Hawaii, United States

WWVH is the callsign of the U.S. National Institute of Standards and Technology's shortwave radio time signal station located at the Barking Sands Missile Range, in Kekaha, on the island of Kauai in the state of Hawaii.

WWVB is a time signal radio station near Fort Collins, Colorado and is operated by the National Institute of Standards and Technology (NIST). Most radio-controlled clocks in North America use WWVB's transmissions to set the correct time. The 70 kW ERP signal transmitted from WWVB is a continuous 60 kHz carrier wave, the frequency of which is derived from a set of atomic clocks located at the transmitter site, yielding a frequency uncertainty of less than 1 part in 1012. A one-bit-per-second time code, which is based on the IRIG "H" time code format and derived from the same set of atomic clocks, is then modulated onto the carrier wave using pulse-width modulation and amplitude-shift keying. A single complete frame of time code begins at the start of each minute, lasts one minute, and conveys the year, day of year, hour, minute, and other information as of the beginning of the minute.

JJY is the call sign of a low frequency time signal radio station located in Japan.

<span class="mw-page-title-main">DCF77</span> German time signal radio station

DCF77 is a German longwave time signal and standard-frequency radio station. It started service as a standard-frequency station on 1 January 1959. In June 1973 date and time information was added. Its primary and backup transmitter are located at 50°0′56″N9°00′39″E in Mainflingen, about 25 km south-east of Frankfurt am Main, Germany. The transmitter generates a nominal power of 50 kW, of which about 30 to 35 kW can be radiated via a T-antenna.

<span class="mw-page-title-main">Real-time clock</span> Circuit in a computer that maintains accurate time

A real-time clock (RTC) is an electronic device that measures the passage of time.

Clock synchronization is a topic in computer science and engineering that aims to coordinate otherwise independent clocks. Even when initially set accurately, real clocks will differ after some amount of time due to clock drift, caused by clocks counting time at slightly different rates. There are several problems that occur as a result of clock rate differences and several solutions, some being more acceptable than others in certain contexts.

<span class="mw-page-title-main">Satellite navigation</span> Use of satellite signals for geo-spatial positioning

A satellite navigation or satnav system is a system that uses satellites to provide autonomous geo-spatial positioning. It allows satellite navigation devices to determine their location to high precision using time signals transmitted along a line of sight by radio from satellites. The system can be used for providing position, navigation or for tracking the position of something fitted with a receiver. The signals also allow the electronic receiver to calculate the current local time to a high precision, which allows time synchronisation. These uses are collectively known as Positioning, Navigation and Timing (PNT). Satnav systems operate independently of any telephonic or internet reception, though these technologies can enhance the usefulness of the positioning information generated.

<span class="mw-page-title-main">Real-time kinematic positioning</span> Satellite navigation technique used to enhance the precision of position data

Real-time kinematic positioning (RTK) is the application of surveying to correct for common errors in current satellite navigation (GNSS) systems. It uses measurements of the phase of the signal's carrier wave in addition to the information content of the signal and relies on a single reference station or interpolated virtual station to provide real-time corrections, providing up to centimetre-level accuracy. With reference to GPS in particular, the system is commonly referred to as carrier-phase enhancement, or CPGPS. It has applications in land survey, hydrographic survey, and in unmanned aerial vehicle navigation.

A pulse per second is an electrical signal that has a width of less than one second and a sharply rising or abruptly falling edge that accurately repeats once per second. PPS signals are output by radio beacons, frequency standards, other types of precision oscillators and some GPS receivers. Precision clocks are sometimes manufactured by interfacing a PPS signal generator to processing equipment that aligns the PPS signal to the UTC second and converts it to a useful display. Atomic clocks usually have an external PPS output, although internally they may operate at 9,192,631,770 Hz. PPS signals have an accuracy ranging from a 12 picoseconds to a few microseconds per second, or 2.0 nanoseconds to a few milliseconds per day based on the resolution and accuracy of the device generating the signal.

<span class="mw-page-title-main">Atomic clock</span> Extremely accurate clock

An atomic clock is a clock that measures time by monitoring the resonant frequency of atoms. It is based on atoms having different energy levels. Electron states in an atom are associated with different energy levels, and in transitions between such states they interact with a very specific frequency of electromagnetic radiation. This phenomenon serves as the basis for the International System of Units' (SI) definition of a second:

The second, symbol s, is the SI unit of time. It is defined by taking the fixed numerical value of the caesium frequency, , the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom, to be 9192631770 when expressed in the unit Hz, which is equal to s−1.

Crystal oscillators can be manufactured for oscillation over a wide range of frequencies, from a few kilohertz up to several hundred megahertz. Many applications call for a crystal oscillator frequency conveniently related to some other desired frequency, so hundreds of standard crystal frequencies are made in large quantities and stocked by electronics distributors. Using frequency dividers, frequency multipliers and phase locked loop circuits, it is practical to derive a wide range of frequencies from one reference frequency.

Orolia, formerly Spectracom, is a global manufacturer of precision time and frequency instruments and network-centric equipment used in a wide range of industries.

<span class="mw-page-title-main">Error analysis for the Global Positioning System</span> Detail of the global positioning system

The error analysis for the Global Positioning System is important for understanding how GPS works, and for knowing what magnitude of error should be expected. The GPS makes corrections for receiver clock errors and other effects but there are still residual errors which are not corrected. GPS receiver position is computed based on data received from the satellites. Errors depend on geometric dilution of precision and the sources listed in the table below.

Two independent clocks, once synchronized, will walk away from one another without limit. To have them display the same time it would be necessary to re-synchronize them at regular intervals. The period between synchronizations is referred to as holdover and performance under holdover relies on the quality of the reference oscillator, the PLL design, and the correction mechanisms employed.

<span class="mw-page-title-main">GPS disciplined oscillator</span>

A GPS clock, or GPS disciplined oscillator (GPSDO), is a combination of a GPS receiver and a high-quality, stable oscillator such as a quartz or rubidium oscillator whose output is controlled to agree with the signals broadcast by GPS or other GNSS satellites. GPSDOs work well as a source of timing because the satellite time signals must be accurate in order to provide positional accuracy for GPS in navigation. These signals are accurate to nanoseconds and provide a good reference for timing applications.

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