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
International Atomic Time (abbreviated TAI, from its French name ) is a high-precision atomic coordinate time standard based on the notional passage of proper time on Earth's geoid. TAI is a weighted average of the time kept by over 450 atomic ...
(TAI).
Time transfer solves problems such as astronomical observatories correlating observed flashes or other phenomena with each other, as well as
cell phone tower
A cell site, cell tower, or cellular base station is a cellular-enabled mobile device site where antennas and electronic communications equipment are placed (typically on a radio mast, tower, or other raised structure) to create a cell, or adjac ...
s coordinating handoffs as a phone moves from one
cell
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
to another.
Multiple techniques have been developed, often transferring reference clock
synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
from one point to another, often over long distances. Accuracy approaching one
nanosecond
A nanosecond (ns) is a unit of time in the International System of Units (SI) equal to one billionth of a second, that is, of a second, or 10 seconds.
The term combines the SI prefix ''nano-'' indicating a 1 billionth submultiple of an SI unit ( ...
worldwide is economically practical for many applications.
Radio-based navigation systems are frequently used as time transfer systems.
In some cases, multiple measurements are made over a period of time, and exact
time synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
is determined retrospectively. In particular, time synchronization has been accomplished by using pairs of
radio telescope
A radio telescope is a specialized antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency ...
s to listen to a
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
, with the time transfer accomplished by comparing time offsets of the received pulsar signal.
Examples
Examples of time and frequency transfer techniques include:
*Simultaneous observation methods:
** Simultaneous
carrier
Carrier may refer to:
Entertainment
* ''Carrier'' (album), a 2013 album by The Dodos
* ''Carrier'' (board game), a South Pacific World War II board game
* ''Carrier'' (TV series), a ten-part documentary miniseries that aired on PBS in April 20 ...
phase
Phase or phases may refer to:
Science
*State of matter, or phase, one of the distinct forms in which matter can exist
*Phase (matter), a region of space throughout which all physical properties are essentially uniform
* Phase space, a mathematic ...
observation of
GNSS
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 (longitude, latitude, and altitude/elevation) to high pre ...
signals (such as
GPS
The Global Positioning System (GPS), originally Navstar GPS, is a Radionavigation-satellite service, satellite-based radionavigation system owned by the United States government and operated by the United States Space Force. It is one of t ...
)
** Time transfer by simultaneous observation of
TV transmissions
** Time transfer by simultaneous observation of
radio astronomy
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...
signals
*Two-way transfer methods:
**
Two-way satellite time and frequency transfer
*Network methods:
**
Network Time Protocol
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable- latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in c ...
**
Precision Time Protocol
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems. ...
One-way
In a one-way time transfer system, one end transmits its current time over some communication channel to one or more receivers.
The receivers will, at reception, decode the message, and either just report the time, or adjust a local clock which can provide hold-over time reports in between the reception of messages. The advantage of one-way systems is that they can be technically simple and serve many receivers, as the transmitter is unaware of the receivers.
The principal drawback of the one-way time transfer system is that
propagation delays of the communication channel remain uncompensated except in some advanced systems. Examples of a one-way time transfer system are the clock on a church or town building and the ringing of their time-indication bells;
time balls,
radio clock
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 st ...
signals such as
LORAN
LORAN, short for long range navigation, was a hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee system but operated at lower frequencies in order to provide an improved range u ...
,
DCF77 and
MSF; and finally the
Global Positioning System
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 sy ...
which uses multiple one-way time transfers from different satellites, with positional information and other advanced means of delay compensations to allow receiver compensation of time and position information in real time.
Two-way
In a two-way time transfer system, the two peers will both transmit, and will also receive each other's messages, thus performing two one-way time transfers to determine the difference between the remote clock and the local clock.
The sum of these time differences is the
round-trip delay between the two nodes. It is often assumed that this delay is evenly distributed between the directions between the peers. Under this assumption, half the round-trip delay is the propagation delay to be compensated. A drawback is that the two-way propagation delay must be measured and used to calculate a delay correction. That function can be implemented in the reference source, in which case the source capacity limits the number of clients that can be served, or by software in each client. The
NIST
The National Institute of Standards and Technology (NIST) is an agency of the United States Department of Commerce whose mission is to promote American innovation and industrial competitiveness. NIST's activities are organized into physical sci ...
provides a time reference service to computer users on the Internet, based on Java applets loaded by each client. The
two-way satellite time and frequency transfer (TWSTFT) system being used in comparison among some time laboratories uses a satellite for a common link between the laboratories. The
Network Time Protocol
The Network Time Protocol (NTP) is a networking protocol for clock synchronization between computer systems over packet-switched, variable- latency data networks. In operation since before 1985, NTP is one of the oldest Internet protocols in c ...
uses packet-based messages over an IP network.
Common view
The time difference between two clocks may be determined by simultaneously comparing each clock to a common reference signal that may be received at both sites. As long as both end stations receive the same satellite signal at the same time, the accuracy of the signal source is not important. The nature of the received signal is not important, although widely available timing and navigation systems such as GPS or LORAN are convenient.
The accuracy of time transferred in this way is typically 1–10 ns.
GNSS
Since the advent of GPS and other
satellite navigation
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 (longitude, latitude, and altitude/elevation) to high pr ...
systems, highly precise, yet affordable timing is available from many commercial
GNSS receiver
A satellite navigation device (satnav device) is a user equipment that uses one or more of several global navigation satellite systems (GNSS) to calculate the device's geographical position and provide navigational advice.
Depending on the s ...
s. Its initial system design expected general timing precision better than 340 nanoseconds using low-grade "coarse mode" and 200 ns in precision mode. A GPS receiver functions by precisely measuring the transit time of signals received from several satellites. These distances combined geometrically with precise orbital information identify the location of the receiver. Precise timing is fundamental to an accurate GPS location. The time from an
atomic 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 betwee ...
onboard each satellite is encoded into the radio signal; the receiver determines how much later it received the signal than it was sent. To do this, a local clock is corrected to the GPS atomic clock time by solving for three dimensions and time based on four or more satellite signals. Improvements in algorithms lead many modern low-cost GPS receivers to achieve better than 10-meter accuracy, which implies a timing accuracy of about 30 ns. GPS-based laboratory time references routinely achieve 10 ns precision.
See also
*
International Earth Rotation and Reference Systems Service
The International Earth Rotation and Reference Systems Service (IERS), formerly the International Earth Rotation Service, is the body responsible for maintaining global time and reference frame standards, notably through its Earth Orientation Pa ...
*
Precision Time Protocol
The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network. On a local area network, it achieves clock accuracy in the sub-microsecond range, making it suitable for measurement and control systems. ...
*
Synchronization
Synchronization is the coordination of events to operate a system in unison. For example, the conductor of an orchestra keeps the orchestra synchronized or ''in time''. Systems that operate with all parts in synchrony are said to be synchronou ...
*
Time and frequency metrology
*
Time signal
A time signal is a visible, audible, mechanical, or electronic signal used as a reference to determine the time of day.
Church bells or voices announcing hours of prayer gave way to automatically operated chimes on public clocks; however, audi ...
*
Time synchronization in North America 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 Observ ...
References
{{DEFAULTSORT:Time Transfer
Telecommunications techniques
Synchronization