TIME IN THE SPACE AGE
DAVID W. ALLAN Allan's Time Inc.
NEIL ASHBY University of Colorado &
CLIFF HODGE National Physical Laboratory
Precise timing is at the heart of much of modern society
more so than most people may realize. In this evolving Information Age,
precise timing is used to manage information flow through many network
nodes so that it is reliable robust and inexpensive.
Unprecedented navigation accuracy is now right at our fingertips using
hand-held receivers that are part of the Global Positioning System, which
is based on a synchronized atomic clock system.
Literally billions of timing devices go into service each year. Understanding
how they all play together is an intriguing story and adds to an appreciation
of "what makes things tick." What is time based on? How do we know we have
set a clock correctly? Where is the best clock in the world? How can we
get the "right time"?
Practical precise timing in the modern era came with the invention
of the quartz crystal oscillator and quartz-crystal filters in the I 920s.
Atomic clocks were invented in the '40s, and since then their accuracy
has on average doubled every two years-interestingly, the same rate of
improvement as for computer memory density.
The most accurate measurement known to humanity now is time-related-the
duration of a second. The ultimate international reference for accurate
time and frequency is Coordinated Universal Time or UTC (Temps Universal
Coordunne' in French), which receives its time and frequency input from
timing centers around the globe.
In 1970 the Coordinated Universal Time system was devised by an international
advisory group of technical experts within the International Telecommunication
Union (ITU). The ITU felt it was best to designate a single abbreviation
for use in all languages in order to minimize confusion. Since unanimous
agreement could not be achieved on using either the English word order,
CUI or the French word order, TUC, a compromise of using neither, UTC,
was adopted. Currently, the best accuracy for the determination of the
second for UTC is equivalent to +1 second in 10 million years.
Because time and frequency can be measured so accurately, they are
often used to determine other fundamental quantities, such as the volt,
ampere, ohm, and meter. For example, in 1983 the General Conference of
Weights and Measures (Conference Cenerale des Poids et Mesures), in Paris,
redefined the meter as "the length of the path traveled by light in vacuum
during a time interval of 1/299 792 458 of a second." It is expected that
all of the base units in metrology for the support of technological development
will eventually be traceable back to the second.
Perhaps there is no better example of how precise timing is used for
accurate navigation, positioning, and time transfer than
the Global Positioning System (GPS). The system features a set of 24
orbiting satellites, each with a synchronized atomic clock on board, which
transmit signals carrying precise time tags. At a GPS receiver, the computer
on board uses a precision quartz-crystal clock to calculate the time of
arrival, and hence the time of flight, of the signals from each of the
Since the signals travel at the speed of light, and the transmission
times are known, the receiver's computer can turn the time of flight into
a very accurate estimate of the distance to each satellite. In this process,
the high accuracy of the GPS atomic clocks is transferred to the clock
inside the receiver. The result: easy access to highly accurate position
and time measurements, which a multitude of users are capitalizing on.
In fact, the number and variety of uses of GPS have exploded, transforming
almost every endeavor in which accurate position and time are needed. For
example, the January 1998 issue of GPS World magazine highlights a receiver
survey that lists over 400 kinds of receivers available from more than
70 companies. In 1996, well over a half-million GPS receivers were marketed
by Japan alone, many of them for vehicle tracking. In addition, many types
of massive engineering and cbmmunications systems are now being developed
that largely depend on GPS. Many countries, for instance, are planning
to use GPS with augmentation from other systems for navigation and control
of aircraft. ecl3
Atomic clocks in space in lock-step with Coordinated Universal Time disseminate
precisely the right time round the world
What is time?
No unique "true" time derivable from natural phenomena exists. So, by international
agreement, Coordinated Universal Time was set up as the world's official
time-by definition. The word "coordinated" means that UTC is generated
through the cooperation of about 50 nations that are now signatories of
the Convention of the Metre, which was first established by agreement of
17 nations in 1875. Timing data is supplied to the Bureau International
des Poids et Mesures, near Paris, by responsible timing laboratories and
observatories around the world. In turn, timing experts at the bureau combine
that data into an official time scale, designated as UTC.
Timing and navigation almost always go hand in hand. Accurate longitude
determination was a critical maritime issue during the 16th and 17th centuries.
Because the earth rotates, the accurate determination of longitude requires
determination of the time-that is, how much the earth has turned up to
the moment of measurement. It was known as early as 1530 that having a
good clock on board ship, were it possible, would be a tremendous aid to
In 1675 the observatory at Greenwich, England, was established to conduct
the astronomical observations needed by ships at sea to determine their
longitude. During the 17th century, several nations offered prizes for
an acceptable method of making this determination. In 1714 the British
Parliament offered a prize of 20000 pounds to ships sailing from England
to the West Indies for the measurement of the longitude of the journey-accurate
to within 30 nautical miles. This goal translates into a clock that can
keep time to better than 3 seconds per day. The difficulty was the sea,
whose motion precludes the use of pendulum- based devices.
Coming to the rescue was clockmaker John Harrison, who through his
persistence and genius, proved that a seaworthy chronometer could be built
that could determine accurate longitude. Harrison's chronometers overcame
harsh environmental problems and worked to within about a second per day.
The ship's log of Captain James Cook, who used a copy of a Harrison chronometer
while exploring the South Pacific from 1772 to 1775, expresses great praise
for the advantage gained in using the clock: "our never failing guide,
In 1884 a treaty agreed to by 25 countries established Greenwich Mean
Time (GMT) as the world's official time and the meridian through Greenwich
Observatory as the prime meridian [Fig. 1]. GMT, the local time at the
observatory, was determined using astronomical measurements. Each nation
then adjusted the time for its own time zone(s) by adding or subtracting
the right number of hours.
As clocks have improved and as needs have changed, Greenwich Mean Time
has evolved into UTC, which relies on atomic rather than astronomical clocks.
Although UTC is the correct time by definition and convention, there are
many sources of time. The question, "What time is it?" should be changed
to, "Whose time is it? - especially if there is an interest in the precision
and accuracy of the source and how well that source ties in to UTC.
Understanding how Coordinated Universal Time.is generated requires
some grasp of clocks and timekeeping techniques. Almost any clock may be
considered as a two- part device [Fig. 2]. One part is an oscillating device
for determining the length of the second or some other desired time interval.
This is usually referred to as the clock's frequency standard, which oscillates
at some rate determined by physical laws.
Historically, the pendulum was the classic time interval standard (although
Galileo used his own pulse in many experiments). A pendulum's frequency
depends on its length and the gravitational acceleration at the pendulum's
location. An ideal pendulum that swings through its lowest point once per
second will have a length of approximately 1 meter. A pendulum's frequency
will, however, depend on its environment since the pendulum's length will
change when the temperature changes.
Currently, the typical wristwatch has as its frequency standard a quartz-crystal
oscillator with a frequency of 32 768 Hz. This number of oscillations is
convenient for the associated digital electronic circuit, because if the
number is divided by 2'~, which is easy for a digital chip divider, the
result is one pulse per second.
An atomic clock supplies a much more accurate frequency than a pendulum
or a quartz crystal.The atomic clock uses as its reference the frequency
associated with a quantum transition between two energy levels in an atom.
For a given quantum transition, the photons emitted or absorbed have a
unique frequency proportional to the energy difference between the two
energy levels, with little variability around this value. The achievement
of atomic clock metrology is to harness the frequency of these photons
while affecting the natural atomic resonance only slightly.
The current official definition of the second is much more elegant
than that based on the environmentally sensitive pendulum. At the 13th
General Conference of Weights and Measures (Resolution 1, 1967), the second
was defined as "the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels of the
ground state of the cesium-133 atom." In other words, one official second
elapses when the defined number of cycles transpiring for the electromagnetic
signal associated with the photon are either given off or absorbed by this
The second part of a clock is a counter that keeps track of the number
of ticks or clock cycles that have occurred since the clock was set. This
part is represented, for example, by the gears and clock face in a pendulum
clock, or by an accumulating register and a display in an atomic clock.
After being set initially, the clock provides its estimate of the correct
In principle, if a clock were set perfectly and if its frequency or
rate remained perfect, it would keep the correct time indefinitely. In
practice, this is impossible for several reasons. One is that the clock
cannot be set perfectly. Another is that random and systematic variations
are intrinsic to any oscillator, and when these random variations are averaged,
the result is often not well behaved. Also, time on a clock depends on
its position and motion (there are relativistie effects). And finally,
but invariably, environmental changes cause the clock's frequency to vary
from the ideal. So, if a clock is measured with sufficient precision, its
reading will almost never agree with UTC.
To compensate for some of these timing instabilities, modern
techniques can be used to couple a clock with sensors and a microprocessor.
The quartz crystal oscillator provides a cost-effective means of achieving
reasonably good clock stability, and can be very effective if properly
interfaced with a computing capability and set of sensors.
A quartet of quality measures
Four useful measures help to describe a clock's quality: frequency accuracy,
frequency stability, time accuracy, and time stability. These measures
are not all independent. Frequency (or rate) accuracy measures how well
the clock can realize the defined length of the second.
One yardstick that is commonly used is the change in the error of a
clock's time divided by the elapsed time over which the change occurred.
The goal of Harrison's chronometers was to have a frequency accuracy
of better than 3 seconds per day Modern primary frequency standards have
frequency accuracies of about a nanosecond per day.
Frequency stability, on the other hand, measures the change in frequency
from one time interval to the next. A particular value of this time interval,
T, is chosen, called the averaging time. The frequency stability is then
ascertained for different values of T. A clock can have a high frequency
error and still be very stable, that is, the frequency or rate error stays
about the same. For example, a clock may have a rate inaccuracy and gain
I second a day, but if that rate remains exactly the same, it would have
perfect frequency stability, and consequently perfect time predictability.
Time accuracy means how well a clock agrees with UTC. Often, what is
needed is time accuracy at several locations in a system or network, as
with the GPS. The GPS time broadcast by each of the satellites must be
synchronous with that of all the other satel - lites in the constellation,
to within a small number of nanoseconds, in order for the system to work
properly. This is accomplished by having atomic clocks on board the satellites
and then synchronizing the clocks.
Time stability is correlated with frequency stability since the time
error of a clock is proportional to the integral of its frequency error.
But time stability is often useful as a measure of change with respect
to some uniform flow of time in a time measurement system or in a time-distribution
or dissemination system. If the time or frequency errors of a clock can
be estimated, then compensating corrections can be made.
If the time and rate (frequency) of a clock with good stability are
calibrated against some better clock, the former clock can provide an estimate
of the better clock's reading, should the better one not be immediately
available. In a hierarchy of calibrations, the best clocks and frequency
standards are often referred to as "primary."
In determining the accuracy of some primary frequency standards, all
factors affecting the accuracy are evaluated to obtain a best estimate
of the second. Often this procedure precludes these standards from running
continuously, as is needed for clock operation. In such a case, a secondary
clock that has been calibrated in rate by a primary frequency standard
is used. The technique-used extensively in the generation of UTC-allows
the secondary clock to perpetuate an estimate of the time of the primary
frequency standard as if it had been operating as a clock on a continuous
An important modern timekeeping concept focuses on the power df properly
used clock ensembles, that is, sets of many clocks. Historically, astronomical
time was based on the movements of one earth, one moon, one sun, one solar
system, and one celestial sphere of moving stars and planets. Even though
there was only one of each, reliability was not an issue,- the earth was
not expected to stop spinningi. With the introduction of atomic clocks,
many timekeepers, each of which could generate its own estimate of time,
came into existence. The concept of a clock ensemble has great merit for
reliability since one clock could quit working and for improved average
From measurements of the time difference between just two clocks, it
is impossible to tell which one is deviating. Three independent clocks
allow an independent estimate of the stability of each, but four are needed
in case one quits. Common systematic errors can be removed by comparison
with primary frequency standards. Once the individual clock stabilities
are known, optimum weights can be assigned to each of the clocks.
What's more, it has been shown that if the clocks are well-characterized
and optimum weights are given to them in a properly developed combining
algorithm, the weighted ensemble can perform better than the best clock
in the set. If one clock fails or has a bad reading, it can be detected
and rejected so that it does not perturb the ensemble's output. These ensembles
can generate a highly reliable and stable real-time output. The clock ensemble
at the National Institute of Standards and Technology (NIST), in Boulder,
Cob., has been running in such an ensemble mode since 1968.
Distortions in timekeeping
Deviations in the observed time of a clock can be
caused by measurement noise internal clock deviations, and external environmental
perturbations affecting the clock. Measurement noise arises from imperfections
in the system that is used to observe the clock readings. Internal clock
deviations can arise from deviations either in the clock's frequency standard
or in the counting mechanism that keeps~track of the hours, minutes, seconds,
and so on. Environmental effects can arise from variations in such factors
as local temperature and barometric pressure.
To illustrate how proper clock characterization and ensembling can
overcome some of these problems, the authors compared an ensemble of three
cheap stop- watches with the UTC time scale generated by NIST for a period
of 145 days [see "In praise of cheap stopwatches," opposite]. In this experiment
the measurement noise arose principally from uncertainties in the authors'
ability to trip a camera shutter at a precise moment. Environmental effects,
due to seasonal variations in temperature during the observing period,
could be clearly discerned, and temperature variations could be assessed
by comparing the residual errors of two of the three clocks. Also measured
were frequency inaccuracies and frequency drift, and appropriate corrections
were applied to each of the clock readings.
After removal of such systematic and environmental effects, the improvement
in predictability was well over a factor of a thousand, giving a time predictability
of the ensemble of about I second per year- for just $181
Because most of the users of UTC want official time to tie to earth time
(earth's rotation on its axis gives a time scale called UTI), a dilemma
arises. People interested in precision time and frequency want the most
uniform and accurate time possible. Yet, because the planet speeds up and
slows down milliseconds in a day, earth time is not useful for precision
metrology. For that reason, it was decided to make UTC a compromise time
Changes due to instabilities in the earth's spin rate are accommodated
by employing leap seconds in UTC, while on the other hand, the length of
the UTC second is kept as close as possible to the definition based on
the cesium atom. UTC was set to be synchronous with UTI at 0000 hours on
1 January 1958. Until 1972, frequency steps and 0.1-second time steps were
used to chase the instabilities in earth time. From 1972 through
1996, there were 20 leap seconds.
As mentioned earlier, UTC is generated near Paris at the Bureau International
des Poids et Mesures. There, data from about 230 clocks from 60 laboratories
around the world is collected. Each clock has its rate and stability measured
and is tested for abnorrrial behavior. Each clock also receives a weighting
factor corresponding to its individual stability. This procedure provides
uniformity (stability) in combining data from the entire set of clocks.
Reliability is thus achieved by the sheer number of clocks involved. The
entire process to generate UTC takes about one month.
Currently, the length of the second is being determined by evaluations
from 10 laboratory cesium-beam primary frequency standards. To have the
UTC second agree as closely as possible to the definition and to obtain
an overall world-best estimate, a weighted combination of these primary
frequency standards is taken according to their individual accuracies.
The time scale generated by combining this set of primary frequency standards
from around the world is called International Atomic Time (TAI, or Temps
Atomique International). TAI is Coordinated Universal Time without leap
While the sotirce of the second for International Atomic Time is derived
from the primary frequency standards throughout the world, the flywheel
that remembers the calibrations provided by these standards is the ensemble
of the 230 or so contributing clocks.
Many laboratories need good estimates of UTC before it is available
from the international bureau of weights and measures. There are 47 timing
centers around th~ world that generate their own current estimate of UTC,
for use in their respectivt countries. These are called UTC(k)s, with the
"k" denoting the particular timing center. For example, the UTC estimate
generated by the United States Naval Observatory's Master Clock in Washington
D.C., is UTC(USNO MC). The Nava Observatory has responsibility for supply
mg time and frequency to the Departmen of Defense. Another government group
NIST, generates UTC(NIST) and has res ponsihility for determining the length
0 the second for the United States. Both organizations also supply time
for other U.S. government entities and their time scales usually agree
to within about 20 ns.
By international agreement, all timing centers worldwide strive to
keep their UTC(k) within 100 ns of UTC.
Currently, theses predictions are within approximately 10 ns.
Each of the 230 clocks that contribute to Coordinated Universal Time
reports its data with respect to the UTC(k) available to that clock. Each
UTC(k) including a clock at the Paris Observatory is measured against GPS
time. When these time differences are substracted, each UTC(k) is known
in comparison to the Paris Observatory clock using GPS time as a transfer
standard, which drops out in the substraction.
The French bureau then sends out a monthly bulletin, reporting the
time differences of each of the UTC(k)s with respect to UTC for the previous
This bulletin is like a report card, telling each timing center
how good its prediction was. When it is received, it tells a center "officially"
what time it was!
Advantages with satellites
Measuring and comparing time and frequency of clocks remote from each other
can be accomplished in a variety of ways and with a variety of accuracies
and stabilities. Usually, the better the accuracy and stability, the more
expensive the system. Satellite techniques, in general, have demonstrated
many advantages over terrestrial techniques, outdoing them in accuracy,
integrity, availability, continuity, coverage, and perhaps, more importantly,
With the advent of the GPS, great progress has been made in transporting
time and frequency The atomic clocks on board the satellites are like portable
clocks in the sky, continuously available anywhere near earth through inexpensive
receivers. Accuracies better than I us with respect to coordinated time
are made readily available by the proper use of GPS timing receivers.
The GPS has become the world's principal supplier of accurate time.
It is used extensively both as a source of time and as a means of transferring
time from one location to another. Three kinds of time are available from
the system; GPS time, UTC as estimated and produced by the USNO, and the
times from each free-running GPS satellite's atomic clock.
The Master Control Station at Falcon Air Force Base near Colorado Springs,
Colo., gathers the GPS satellites' data from five monitor stations located
around the globe. A software program estimates the clock time error, frequency
error, frequency drift, and orbit for each of the satellites. This information
is uploaded to each satellite so that it can broadcast estimated errors
as correction terms, in real time, as part of the data message. The process
provides GPS with time consistency across the constellation to within a
small number of nanoseconds, and accurate position determination of the
satellites to within a few meters.
Any discontinuity in GPS time would throw receivers out of lock, so
the system cannot tolerate the introduction of leap seconds. GPS time must
be kept within 1µs of UTC(USNO MC), ignoring the leap seconds. Typically,
the steering performance is within 20 ns. In 1910, when the Department
of Defense started keeping time on the satellites, its system time was
synchronized with UTC. Since then, GPS minus International Atomic Time
has been very close to 19 seconds, while UTC has been delayed many leap
To provide an estimate of Coordinated Universal Time derivable from
a positionmg satellite signal, a set of UTC corrections is also supplied
as part of the broadcast signal. These are the time difference estimate
between GPS time and the UTC(USNO MC) modulo I second. With these corrections
a receiver can, in principle, calculate an estimate of UTC(USNO MC). Except
for the purposeful degradation of the GPS signal, called selective availability,
by the Department of Defense for security reasons, this calculation may
have an accuracy of about 10 ns. Since the USNO predictions have been successful
to within about 10 ns, combining these two independent error sources yields
a real-time potential uncertainty (ignoring selective availability) level
of about 14 ns.
Handling relativity effects
The great accuracies now being attained with clocks
require that relativistic effects be included in comparing the times and
frequencies of clocks at different primary frequency standards laboratories.
Relativistic effects are also important for navigation using the GPS. Basically,
relativity effects on clocks are of three types; slowing of the rate of
a moving clock, frequency shifts due to differences of gravitational potential,
and the inconsistencies of synchronization in a rotating reference frame,
such as on the spinning earth.
The synchronization problem can be handled by introducing an inertial
reference frame that has its origin at the earth's center, which is not
rotating. Since the speed of light is constant in this earth-centered inertial
frame, a hypothetical, but self-consistent, set of synchronized clocks
could be distributed throughout the frame. Relativistic corrections could
then be applied to real clocks so that their readings agree with the hypothetical
clocks at each of their locations in the earth-centered inertial frame.
The resulting self-consistent time system in the inertial frame can then
be used by earth- fixed observers. Called "coordinate time," this approach
is used for UTC.
Mean sea level is approximately a surface of constant total gravitational
potential, called the geoid, in the earth-fixed rotating frame. Conveniently,
several relativistic effects combine so that ideal clocks at rest on the
rotating geoid all tick at the same rate. This rate defines the SI second
for UTC as currently given by the cesium atom.
Corrections must be applied to primary frequency standards laboratories
to compensate for their distances from the geoid. The primary frequency
standard for the United States, NIST-7, runs fast with respect to an ideal
clock at the geoid by about 15.55 ns per day, which is significant, given
its current accuracy of about O.4 ns per day.
As for slowing the rate of a moving clock, the Global Positioning System
has at least eight relativistic effects that must be accounted for. The
slowing of a moving clock means that GPS satellite clocks lag behind clocks
on the geoid by 7.11 us per day. Gravitational frequency shifts cause GPS
satellite clocks to run fast relative to clocks at the geoid by 45.7 us
per day. The net effect on atomic clocks in a GPS orbit is that they run
fast by 38.59 us per day. GPS satellite clock frequencies are corrected
downward by this amount before launch, in order for the broadcast signal
frequencies to be correct when received.
Also, since GPS orbits are not perfectly circular, additional relativistic
effects arise from second-order Doppler shifts, and from gravitational
frequency shifts, as the satellite moves toward and away from the earth.
The system's receiver software must correct for relativistic effects due
to this eccentricity, sometimes by as much as several dozens of nanoseconds.
When the system's transmitter and receiver clocks are corrected in this
way, they will be self-consistently synchronized. GPS time is thus another
example of coordinate time.
Accuracy and stability of UTC
In recent years, the accuracy and stabi lity of UTC have been been dramatically
improved. These advances are primarily due to the 1991 introduction of
the Hewlett- Packard model 5071A cesium-beam clock. Besides being about
10 times more accurate than any other commercial frequency standard, this
clock also has less sensitivity to the environment-by more than a factur
of 10. The result is a clock with an exceptionally long-term stability~usually
averaging well below I ns per day. From 1991 to 1996, the stability of
UTC improved about tenfold.
With these same clocks, timing centers are nowadays able to do a much
better job of predicting coordinated time. Because of the outstanding performance
of the HP 5071As, the weight assigned them is nearly three-fourths that
for the entire UTC ensemble~ven though their total number is less than
half of all the contributors to coordinated time [Fig. 3].
Moreover, in the last few years, emerging technologies such as laser
energy-state pumping and detection have brought about additional improvements
in cesium primary frequency standard accuracies. Other advances are laser
cooling down to near absolute zero using photon pressure, followed by a
photon pressure pulse producing a controlled low-velocity atomic fountain.
Improvement in accuracy of nearly a factor of 10 has already been achieved.
Though it does not exist in nature, an ideal clock can be conceptualized.
If three or more independent clocks are compared, the individual instabilities
of each with respect to this ideal clock may be estimated. A clock's instabilities
are directly related to its predictability [Fig. 4].
Since UTC is not directly available as a clock, real-time approximations
to it are made available from 47 timing centers around the world. Several
of these timing centers have means of broadcasting the time from their
UTC(k)s. Aside from prediction errors, another problem in accessmg the
different UTC(k)s arises from instabilities in the methods employed for
transmitting and making available this standard time and frequency information.
Considerable effort has gone into measuring the instabilities of various
time and frequency transfer techniques. When the frequency stability of
different methods of time and frequency dissemination is measured, it is
found that essentially all techniques improve with increased averaging
One of the best time transfer techniques is GPS common-view, which
provides a fractional frequency stability of about 10.14 with one day of
averaging. To explain the technique, suppose the time of a clock in Boulder,
Cob., is being communicated to the Paris Observatory. At a predetermined
instant, the clock in Boulder measures its time difference with respect
to GPS time, B-C. At the same instant, the GPS receiver in Paris, listening
to the same satellite, measures its time difference with respect to GPS
time, P-G. These data are exchanged and subtracted, yielding the time difference,
B-P. With all the delay corrections properly accounted for, the accuracy
of this technique has been shown to be a few nanoseconds. The effects of
selective availability drop out in the subtraction, as do many other common-mode
Even though the GPS common-view technique presently limits the short-term
stability of UTC, its introduction in the early '80s refined the accuracy
and stability of time transfer by more than a factor of 20. It has been
designed into special GPS timing receivers, for automatic operation, and
is currently the principal operational means of communicating the times
of the contributing clocks to the Bureau International des Poids et Mesures
for generation of UTC.
In recent years, the use of GPS as a source of UTC has increased rapidly.
Although 1µs accuracies are achieved with minimal effort and little
cost, there are many applications that require better timing, most of which
need a real-time output, such as for telecommunications or for synchronizing
a power grid. However, the degradation in the GPS signal caused by selective
availability can cause peak-to-peak variations of hundreds of nanoseconds.
The current GPS common-view technique, which avoids most of the selective
availability degradation, only gives time differences between two clocks
after the fact.
Anovel approach was developed a few years ago in which the nature of
the instabilities of selective availability were studied. Understanding
how greatly these instabilities differed in character from the instabilities
in precision clocks allowed real-time filters to be designed for timing
receivers in fixed locations. Using, for example, the HP 5071A clock with
an appropriate filter essentially eliminates the selective availability
in real time. lfclocks based on quartz crystal 9scillators or rubidium
gas-cell frequency standards are used instead, a good level of select availability
filtering is still achievable. Products based on such filtering concepts
have become very popular and useful for precision network timing.
Airline safety is a key issue internationally While
navigation and timing for all of avionics has taken a giant step forward
with the availability of GPS, that system by itself does not provi
deI the accuracy integrity, availability, and a continuity of service that
is needed for civil aviation. But some exciting new concepts for improvements
with augmentations of GPS are being planned.
One involves the Global Navigation Satellite System (GNSS), a satellite-delivered
global navigation and positioning system that meets civilian users' requirements
in a cost-effective way. Precise, accurate timing- including Coordinated
Universal Time-is built into its designs, and atomic clocks are at the
heart of its success.
In the offing is a set of geostationary satellites that will
be used to augrnent the signals already available from GPS and Glonass,
Russia's Global Navigation Satellite System. These new satellites are being
sponsored by Inmarsat, the London-based international telecommunications
Also for civilian use, the U.S. government plans to implement a new
GPS signal on the Block hF GPS satellites, scheduled for replenishment
launches around the year 2008. This signal would provide civilian users
with an accurate measurement of time delays due to the ionosphere, which
is now one of the biggest uncertainties in precision airline navigation.
But, because of the congestion of the l~2-GHz region of the electromagnetic
spectrum, securing an allocation for this new frequency is by no means
Other improvements in satellite position prediction and clock accuracy
are under way. For instance, intersatellite links are being implemented
with the GPS.
Block IIR program, which should increase the time and position accuracy
available from the GPS. The Block IIR satellites are next in the launch
What's more, the Federal Aviation Administration is in the process
of developing and implementing a GPS Wide-Area Augmentation System (WAAS)
for all U.S. air traffic, involving GPS-like signals from geostationary
satellites. WAAS could bring about large fuel savings, increased in-flight
safety, and all-weather precision landings. Ultimately, with future local
augmentations near airports, the goal of such programs for position determination
for precision landings is 0.8 meters.
Asimilar augmentation is being implemented in Europe, which cannot
be covered at an adequate level by transmissions from the WAAS geostationary
satellites. Egnos, the European Geostationary Navigation Overlay Service,
will augment the existing GPS and Glonass signals with transponders on
geostationary satellites. Sometimes also called GNSS- 1, the first-generation
Global Navigation Satellite System, Egnos will ultimately support five
levels of service over different service areas. It is designed to provide
the best performance over Europe, while guaranteeing a minimum level of
service with global earth coverage. Initial operation is expected in 1999
for levels 1, 2 and 3, with the launch of two satellites. Full operation
is scheduled for 2002 at all five service levels.
The Russian authorities appear to be committed to making Glonass available
for civil aviation and acceptable to the international community. A study
is under way, under the auspices of the Tacis Telrus project, to identify
options for cooperation between Western Europe and Russian industry involving
Clonass and other related navigation systems.
Besides being a provider of CNSS services at the international level
with WAAS and Egnos, Inmarsat is proposing to add a dual-frequency navigation
function to its Intermediate Circular Orbit communications satellites,
which are due to be launched around 1999~2000. The proposed system, the
International Satellite Navigation Service (ISNS), would offer both an
overlay service and an independent dual-frequency navigation signal, which
will refine the accuracy in accounting for the timing signal's propagation
delay. ISNS is expected to be capable of providing nonprecision approach
capability worldwide without differential ground augmentation.
For users in the Asia Pacific region, the Japanese Space Agency plans
to launch two geostationary satellites for relaying navigation, integrity,
ranging, and differential messages. The service will mainly cover the Pacific
Ocean, which is at present poorly served by the lnmarsat-3 overlay system.
The satellites, called MTSATs and already in the development stage, will
also provide communications and weather capabilities to users.
The initial MTSAT satellite is scheduled for launch in 1999, with a
replacement satellite in 2005. This service alone will not meet requirements
for primary means of navigation, but will probably provide a component
to a future Asian wide-area system. The Indonesian government is planning
to launch two additional geostationary satellites with navigation payloads,
to complement MTSAT in the South-Eastern Asia region. The first satellite
is expected to be launched in 2000.
In Europe, the European Space Agency has made much progress in its
development of the wholly civil GNSS-2. Following an initial GNSS-2 Mission
Analysis study of system architectures in 1995, the European agency is
now concentrating on such key technical issues as satellite orbit configurations.
The program is aimed at demonstrating a flight experiment by 1999.
This ambitious project is expected to be funded through public and
private initiatives, supported by a Europe that is keen to see implementation
of a developmental system as soon as possible. The plan is for GNSS-2 to
achieve full operation around 2007-2010 and remain operational until at
least the year 2025.
Ultimately, the decision to fabricate the next-generation GNSS will
be made at political and institutional levels. Who needs it? Who will pay
for it? Ultimately, only through international cooperation and coordination
can there be a truly seamless, international, next-generation GNSS. The
European Space Agency is currently engaged in discussions with Russia,
Japan, and the United States to ensure that the project has a common goal
and to minimize the duplication of effort. Without the cooperation of all
partners, the cost of providing such a system would be prohibitive.
For example, the GPS and Glonass are not currently synchronized and
use different coordinate reference frames. In 1996 the Russian Federation
agreed to improve synchronization of its time scales with UTC. This will
become an important factor for one-way time-dissemination for GNSS- I where
the time scales for the three systems will need to be coordinated. There
is also the additional problem that Clonass includes leap seconds and GPS
At present, the GPS coordinate reference frame complies to within a
few tens of centimeters with the International Terrestrial Reference Frame.
From the last report of the Russian authorities, getting the Glonass reference
frame to also comply poses a challenge and may not happen for some time.
Because the next~generation CNSS user market will be automotive as
well as avionic, the accuracy requirements will be much stricter-lO cm
is one of the long- term goals, for collision avoidance on smart highways.
Light travels 30 cm in 1 ns. Already methods that could accomplish this
incredible goal have been proposed and are being studied, but propagation
delay inaccuracies pose one of the biggest problems.
Finally, the issue of who controls the next-generation GNSS is complex,
and beyond the scope of this article. Some of the challenges ahead include
encryption of the signal, which would need agreement from all parties owing
to the global naturc of the system. Ensuring a seamless system will also
be difficult. And for safety-critical applications, responsibility and
liability for the operation of the system is currently an active area of
UTC prospects and problems
Since UTC is the common reference for all national
time scales, and recent advancements have boosted its accuracy and stability,
all the timing needs throughout the world are essentially being satisfied
through the current methods of generation and distribution. The prospect
of making UTC a real-timeservice is a burden that will fall on each nation.
But, now that GPS has international coverage, this burden is minimal.
Presently, GPS can be used to obtain an estimate of the UTC(USNO MC)
clock. If the time output of a good multi- channel C/A (clear access) code
GPS receiver is averaged for one day against a sufficiently stable local
clock, such as a cesium standard, the resulting estimate of UTC(USNO MC)
will be within 20 ns about 95 percent of the time. With that kind of performance,
the GPS broadcast correction should be within 30 ns of UTC about 95 percent
of the time.
Typically the frequency excursions of UTC(USNO MC) through GPS average
below 10 ns per day. As for the GPS clock ensemble, it has a long-term
frequency stability of about 2 ns per day. The frequency stability of UTC
(USNO MC) is about 0.2 ns per day, and its rate is a predicted-forward
estimate of Coordinated Universal ~me, well within I ns per day.
In the future, there should continue to be much improvement in the
accuracy of the coordinated time second as well as that of the SI second
generated by the primary standards, which are being constructed now with
anticipated accuracies of 0.01 ns per day. These improved accuracies will
benefit the precision user community within the limitations of the methods
of communicating time and frequency.
But there are two other developments that could greatly improve the
usefulness of UTC: making it a real-time service, such as Creenwich Mean
Tme used to be, and decreasing the measurement noise in the time and frequency
transfer techniques. The measurement noise associated with communicating
time from one location to another is of such a nature that the performance
of state-of-the-art clocks cannot be utilized at a distance. The noise
also degrades the short-term stability of coordinated time.
The fact that UTC is not available in real time means that none of
the users having real-time synchronization needs can use official world
time. Still the number of such applications is increasing rapidly-for example,
telecommunications network nodes need highly accurate synchronization~o
alternative solutions are needed for synchronizing timing networks.
The traditional GPS common-view technique is used to transfer the times
and frequencies of most of the standards contributing to the generation
of International Atomic Time and UTC This procedure removes most of the
effects of selective availability and employs one-day averages, but requires
a large amount of time for processing data from many laboratories before
the comparisons are published.
Improvements are anticipated by enhancing GPS receiver stabilities
and accuracies and by bettering the characterization of atmospheric delays.
One technique that has the potential to provide short-term performance
in real-time is the GPS advanced common-view (ACV) technique, which uses
more than one GPS satellite at a time. Another is the two-way satellite
time and frequency transfer technique, which exhibits excellent short-term
stability-limited by only 200 or 300 picoseconds of white-noise phase modulation
taken over 1-second averages.
However, environmental effects on the transmit and receive equipment
can cause degradation of the longer-term performance. The enhanced GPS
technique is a systems approach in which the attributes of the reference
clock are best utilized to filter the effects of the GPS selective availability
degradation, to obtain a real-time estimate of UTC.
Both GPS common view and GPS advanced common view use the broadcast
time code. An alternative approach using the system's carrier phase for
transferring time and frequency information between two remote sites shows
promise, but more research is needed.
The long-term accuracy and stability of UTC can take advantage of the
best clocks in the world and are currently doing so. This situation is
possible because over a long enough time, the measurement noise can be
averaged away. The problem is that the more clocks improve, the more time
is needed to average away the measurement noise. The result is a need for
both real-time access to UTC and better methods of communicating time and
frequency between clocks and to the user community.
These difficulties are not insurmountable and reasonable solutions
have been proposed. As clocks improve along with methods for communicating
time and frequency, available accuracies will also improve, providing additional
benefits to an ever-growing community of users of precise time.
To probe further
A comprehensive treatment of the Global Positioning System (GPS), including
its background and history can be found in "Global Positioning System:
Theory and Applications, Vols. I and II," eds. B. W. Parkinson and J. J.
Spilker Jr., in Progress in
Astronautics and Aeronautics (American Institute of Aeronautics and
Astronautics, 370 L'Enfant Promenade, Washington, DC
2002~2518 1996; ISBN 1-56347-106-X, 1~56347-107-8).
A more detailed discussiori of the topics addressed in this article
is given in "The SPACE AGE Science of Timekeeping," Hewlett-Packard Application
Note 1289, by D. W. Allan, N. Ashby, and C. Hodge, available from Jerrie
Kerins, Hewlett-Packard Corp., Santa Clara Division, Box 58059, Santa Clara,
Several annual congresses publish proceedings that can provide much
additional information on time, frequency, and navigation. Among these
are Proceedings of the Institute of Navigation-GPS-93, IONGPS-94, ION-GPS-95,
ION-GPS-96, and IONGPS-97 (1800 Diagonal Rd., Suite 4801, Alexandria, VA
22314);Proceedings of the European Frequency and lime Forum (Institution
of Electrical Engineers, London); Precise Time and Time Interval Applications
and Planning Meetings, U.S. Naval Observatory (NASA Center for Aerospace
Information, Springfield, Va.); National Technical Information Service-NTIS,
distributor; and Proceedings of the Annual Symposium on Frequency Control
(sponsored by the IEEE, published by NTIS).
Articles on the Global Positioning System and its augmentations can
be found in the periodical GPS World., Box 6139, Duluth, MN 55806~139;
800-346-0085 or218-723- 9477; Web, wvvw.gpsworld.coml.
Among the Internet sites of relevance to GPS, with links to related
topics, are http://physics/nist.gov/Genint/time.html
A monograph from the National Institute of Standards and Technology
(N 1ST) that is also very useful is "NIST Technical Note 1337, Characterization
of Clocks and Oscillators (1990)," U.S. Government Printing Office, Washington,
About the authors
David W. Allan worked at the U.S. National Institute of Standards and Technology
(NIST) from 1960 until 1992, focusing on time and frequency research, development,
and dissemination. He is currently in Africa on sabbatical from his position
as president of Allan's Time Inc., Fountain Green, Utah, a time and frequency
metrology company. His contributions include the development of an internationally
adopted method of characterizing the performance of clocks, known as the
Neil Ashby is a professor of physics at the University of Colorado,
where he joined the faculty in 1962. He also consults for NIST. His research
formed the basis for relativistic corrections being properly included in
the Global Positioning System.
Clifford C. Hodge is currently a project leader at the National Physical
Laboratory in Teddington, Middlesex, England, where he is involved in developing
the international Global Navigation Satellite System (GNSS). He previously
was a resident research associate at NASA's Jet Propulsion Laboratory in
IEEE Spectrum editor: Tekla S. Perry