# Time and Frequency Activities at the  U.S. Naval Observatory

Authors: Demetrios Matsakis
Journal: International Journal of Navigation and Observation
 (2008)

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## Abstract

The US Naval Observatory (USNO) has provided timing for the navy since 1830 and, in cooperation with other institutions, has also provided timing for the United States and the international community. Its Master Clock (MC) is the source of UTC(USNO), the USNO's realization of Coordinated Universal Time (UTC), which has stayed within 5 nanoseconds RMS of UTC since 1999. The data used to generate UTC(USNO) are based upon 73 cesium and 21 hydrogen maser frequency standards in three buildings at two sites. The USNO disseminates time via voice, telephone modem, LORAN, Network Time Protocol (NTP), GPS, and two-way satellite time transfer (TWSTT). This paper describes some of the changes being made to meet the future needs for precision, accuracy, and robustness.


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## Body

## 1. Introduction

The determination of time has been
a crucial factor in navigation for centuries.
Many well-known observatories, such as Greenwich, the Observatory of Paris, and the
USNO were founded in part to measure the time for seafaring purposes. With the advent of radio navigation, the
need for precise time was pushed to the nanosecond level because the light
travel time, when converted to distance at the approximate rate of passing 1
meter in 3 nanoseconds, was used for triangulation.  This determination requires knowledge or
inference of the relevant clocks’ times to this level.  Increasingly, cm-level positioning goals as
well as coherent interferometry applications lead to future timing requirements
at the picosecond  level.In order to serve navigational
needs of GPS, as well were time and frequency requirements of other users and
systems, the USNO has specialized in real-time, robust timekeeping.  This paper describes how time is generated at
the USNO and transferred to the users.


## 2. Time Generation

The most important part of the USNO Time Service Department is its staff, which
currently consists of 27 positions.  Of
these, the largest group, almost half the staff, is directly involved in time
transfer.  The rest are fairly evenly
divided between those who service the clocks, those who monitor them, and those
who are working to develop new ones.The core stability of USNO time
is based upon the clock ensemble.  We
currently have 69 HP5071 cesium clocks made by
Hewlett-Packard/Agilent/Symmetricom, 4 cesium CsIII-EP clocks made by
Datum/Symmetricom, and 24 cavity-tuned “Sigma-Tau/Datum/Symmetricom” hydrogen
maser clocks, which are located in two Washington DC buildings and at the USNO Alternate
Master Clock (AMC), located at Schriever Air Force Base in Colorado.  The clocks used for the USNO timescale are
kept in 19 environmental chambers, whose temperatures are kept constant to
within 0.1 degree C and whose relative humidities (for all masers and most
cesiums) are kept constant to within 1%.
The timescale is based only upon the Washington DC clocks.  On June 7, 2007, 70 standards were weighted
in the timescale computations.The clock
outputs are sent to the measurement systems using cables that are phase-stable
and of low temperature coefficient and where possible all the connectors are
SMA (screw-on).  The operational system
is based upon switches and counters that compare each clock against each of
three master clocks once per hour and store the data on multiple computers,
each of which generates a timescale and is capable of controlling the master
clocks.  The measurement noise is about 25
picoseconds (ps) RMS, which is less than the variation of a cesium clock over
an hour.  Because the masers only vary by
about 5 picoseconds over an hour, we also measure them using a system to
generate comparisons every 20 seconds, with a measurement noise of 2 picoseconds.  For robustness, the low-noise system measures
each maser two ways, with different master clocks as references.  All clock data, and time transfer data are
gathered by redundant parallel computer systems that are protected by a firewall
and backed up nightly on magnetic tape.Before
averaging data to form a timescale, real-time and postprocessed clock editing
is accomplished by analyzing deviations in terms of frequency and time; all the
clocks are detrended against the average of the best detrended cesiums [1].
A maser average represents the most precise average in the short term,
and the detrending ensures that it is equivalent to the cesium average over
periods exceeding a few months.  A.1 is
the USNO’s operational timescale; it is dynamic in the sense that it weights
recent maser and cesium data by their inverse Allan variance at an averaging
time (tau) equal to the age of the data.
Both A.1 and the maser mean are available on the web pages.UTC(USNO)
is created by frequency-steering the A.1 timescale to UTC using a steering
strategy called “gentle steering” [2–4],
which minimizes the control effort used to achieve the desired goal, although
at times the steers are so small that they are simply inserted.  To realize UTC(USNO) physically, we use the
one pulse per second (1-PPS) output of a frequency divider fed by a 5 MHz
signal from an Auxiliary Output Generator (AOG).  The AOG creates its output from the signal of
a cavity-tuned maser steered to a timescale that is itself steered to UTC [2–5].
The MC has a backup maser and an AOG in the same environmental
chamber.  On 29 October 2004, we changed
the steering method so that state estimation and steering are achieved hourly
with a Kalman filter with a gain function as described in [6].  A second master clock (mc),
duplicating the MC, is located in an adjacent chamber.  In a different building, we have the same
arrangement for a third mc, which is steered to the MC.  Its backup AOG is steered to a mean
timescale, based only on clocks in that building, which is itself steered to
the MC.An
important part of operations is the USNO Alternate Master Clock (AMC), located
at Schriever AFB in Colorado,
adjacent to the GPS Master Control Station.
The AMC’s mc is kept in close communication with the MC through use of two-way
satellite time transfer (TWSTT) and modern steering theory [7].  The difference is often
less than 1 nanosecond (ns).  In 2005, we
installed the hardware for replacement and upgrade of the switched and
low-noise measurements systems, the DC backup power systems, and the computer
infrastructure.  We have not yet
integrated the three masers and 12 cesiums at the AMC into the USNO’s
Washington DC timescale, but it remains a possibility that carrier-phase TWSTT
or GPS techniques can be made reliable and accurate enough to attempt this.The
operational unsteered timescale (A.1) is based upon averaging only the better
clocks, which are first detrended using past performance.  As a result of a study conducted in 2000 [8], we have widened the definition of
a “good clock,” and we are recharacterizing the clocks less frequently.  We are also continuing to work on developing
algorithms to combine optimally the short-term precision of the masers with the
longer-term precision of the cesiums and the accuracy of International Atomic
Time (TAI) itself.  It is planned to
implement an algorithm that steers the MC hourly and tightly to a timescale
based only upon masers, which is steered to a cesium-only timescale that itself
is steered to UTC using the information in the Circular T [6, 9].  The steered
cesium-only timescale would either be based upon the Percival Algorithm [1, 10], a Kalman-filter, or an ARIMA
algorithm.  As an alternative variation,
individual masers could be steered to the cesium-only timescale before being
averaged to create the maser-only timescale.


## 3. Stability of UTC(USNO)

Figure1 shows how then UTC(USNO)
has compared to UTC and also how its fractional frequency has compared to the
unsteered maser mean, relative to an overall constant offset.Interplay between the
time and fractional frequency stability of the USNO Master Clock, from February,
1997 to the present.
(a)(b)The top plot of Figure1 is UTC-UTC(USNO)
from the International Bureau of Weights and Measure’s (BIPM’s) Circular
T.  The lower plot shows the fractional frequency
of the Master Clock referenced to the maser mean, after a constant has been
removed.  The rising curve previous to
MJD 51 000 is due to the graduated introduction of the 1.7 × 10−14 blackbody correction applied to the primary frequency standards to which EAL is
steered to form UTC.  The steering time
constant for the time deviations between the Master Clock and the mean was
halved to 25 days on MJD 51 050.
Beginning about 51 900, the mean has usually been steered so as to
remove only half the predicted difference with UTC each month.  Less aggressive clock characterization was
implemented at around 52 275.  Hourly
steers were implemented on 53 307. Vertical lines indicate the times of these
changes.  UTC(USNO) has stayed within 5
nanoseconds RMS of UTC for 5 years.Most of our users need and desire access to only
UTC(USNO), which is accessible via GPS and other time transfer modes.  Other users are interested in UTC, and for
those we make predictions of UTC-UTC(USNO) available on the web pages.  The web pages also provide the information
needed for users who are interested in using the MC to measure absolute
frequency.  For those users interested
mostly in frequency stability, we have made available the difference between
the MC and the maser mean using anonymous ftp.The long-term stability of the Master Clock is set by
steering to UTC. The exceptional
stability of the USNO’s unsteered mean can also be used to attempt to diagnose
issues involving the long-term stability of UTC itself.  The dense purple line in Figure2 shows the fractional
frequency difference between our unsteered cesium average and EAL, which is the
unsteered timescale generated by the BIPM that is steered to primary frequency
standards so to create UTC.  The large contribution
of USNO-DC cesiums to UTC masks some of the difference between the USNO cesiums
and the clocks of other institutions; however, that effect has been removed by
increasing the difference 25%. Also plotted are the unsteered cesium average fractional frequency against the SI second as
measured by primary frequency standards at National Institute of Standards and
Technology (NIST) and PTB.  Initially, it
appeared that the HP5071 beam  tubes had a very small frequency drift, however
since MJD 52500 the pattern has become less clear.Figure 2
Fractional
frequency of unsteered average of USNO-DC cesiums against that of EAL and of
primary frequency standards.  The
frequencies have been shifted in the vertical direction for display, and the
difference with the cesium average has been scaled to remove the contribution
of USNO-DC cesiums to EAL.In order to improve timescale operations, the USNO has a
staff of four developing rubidium-based atomic fountains [11].  Figure 3 shows the performance of the
prototype fountain over a 40-day period, while housed in a room subject to
several-degree temperature variations.Figure 3
Performance of rubidium fountain against a USNO maser mean, as measured by the
total deviation statistic.  The straight
line segment is a fit to the inverse square-root curve expected for white
frequency noise.


## 4. Time Transfer

Requests sent to the Washington site were not responded to. Table1 shows how many times the USNO was
queried by various time-transfer systems in the past year.  The fastest-growing service is the Internet
service Network Time Protocol (NTP).
Until recently, the number of individual requests doubled every year
since the program was initiated.  The
billions of requests correspond to at least several million users.  Unfortunately, in late 2004 the NTP load
reached 5000 queries per second at the Washington DC site, which saturated the internet connections [12]. Due to this saturation, perhaps a third of the NTP requests
sent to the Washington
site was not responded to.  In August
2005, the Defense Information Services Agency (DISA) provided higher-bandwidth internet
access, and the query rate increased to 6000 packet requests/second.  Although the query rate has remained near
this level since then, such upgrades of internet capacity may prove
insufficient to cope with the projected growth.Table 1
Yearly access rate of low-precision time distribution services.
Telephone voice-announcer800 000Leitch clock system90 000Telephone modem200 000Web server850
millionNetwork time protocol (NTP)200
billion (see text)As an example of NTP Time transfer accuracy,
Figure4 shows the error between our AMC and Washington facilities, which are
separated by about 2500 km.Figure 4
Observed error in NTP Time transfer between
USNO-DC and USNO-AMC.  Blue plot shows
.1-day averages when the 10% of the data exceeding .4 milliseconds error are removed.  Red dots are simple .1-day averages of all
the data, of which 5% exceed .5 milliseconds deviation.Greater precision is
required for two services for which the USNO is the timing reference: GPS and
LORAN.  USNO monitors LORAN at its
Washington DC site.  With some assistance from the USNO, the US
Coast Guard has developed its time of transmission monitoring (TOTM) system so
it can steer using data taken near the point of transmission using UTC(USNO)
via GPS.  Direct USNO monitoring at its
three points of reception is used as a backup and crude check [13], and the USNO is pursing a
collaborative effort with the Loran Support Unit (LSU) to test an Enhanced
Loran (ELORAN) receiver system.GPS is an
extremely important vehicle for distributing UTC(USNO).  This is achieved by a daily upload of GPS
data to the Second Space Operations Squadron (2SOPS), where the Master Control
Station uses the information to steer GPS Time to UTC(USNO) and to predict the
difference between GPS Time and UTC(USNO) in subframe 4, page 18 of the broadcast
navigation message.  GPS Time itself was
designed for use in navigational solutions and is not adjusted for leap
seconds.  As shown in Figure5, users can
achieve tighter access to UTC(USNO) by applying the broadcast
corrections.  For subdaily measurements,
it is a good idea, if possible, to examine the age of each satellite’s data so
that the most recent correction can be applied.Recent daily averages of UTC(USNO)
minus GPS Time and UTC minus GPS’s delivered prediction of UTC(USNO).
(a)(b)Figure6 shows the RMS stability
of GPS Time and that of GPS’s delivered prediction of UTC(USNO) as a function
of averaging period.  Note that the RMS
corresponds to the component of the “Type A” (random) component of a user’s
achievable uncertainty.Figure 6
The precision of GPS Time and of
GPS’s delivered prediction of
UTC(USNO), using  TTR-12 data since
12JUL02, measured by the attainable external precision (RMS, mean not removed)
as a function of averaging time, and referenced to UTC(USNO).  Improved performance in accessing UTC(USNO)
could be realized if only the most recently updated navigation messages are
used. The accuracy attainable over a given averaging time also depends upon the
calibration of the user’s receivers.Figure7 shows the GPS
Time RMS frequency accuracy, as measured through the reference UTC(USNO), along
with the frequency stability as measured by the Allan deviation (ADEV).  The ADEV is shown for comparison; however,
there is little justification for its use, since the measured quantity is
stationary.  In this case, the RMS is not
only unbiased—it is the most widely accepted estimator of
the true deviation.  Improved performance
with respect to the predictions of the USNO Master Clock’s frequency can be
realized if the most recently updated navigation messages are used in the data
reduction.Figure 7
RMS fractional frequency accuracy
(external precision) and the fractional frequency stability, as measured by the
Allan deviation, of GPS Time and for GPS’s delivered prediction of UTC(USNO),
using TTR-12 data since 7FEB05. Reference frequency is that of UTC(USNO).Since 9 July 2002, the
official GPS precise positioning service (PPS) monitor data have been taken
with the TTR-12 GPS receivers, which are all-in-view and dual-frequency [14].
The standard setup includes temperature-stable cables and flat-passband,
low-temperature-sensitivity antennas.  Our
single-frequency standard positioning service (SPS) receivers are now the BIPM-standard
“TTS” units, and we are calibrating and evaluating temperature-stabilizing
circuits.  Operational antennas are
installed on a 4-meter-tall structure built to reduce multipath by locating GPS
antennas higher than the existing structures on the roof.Although
not directly required by frequency transfer users, all users ultimately benefit
from calibrating a time transfer system, because repeated calibrations are the
best way to verify long-term precision.
For this reason, we are working with the U.S. Naval Research Laboratory
(NRL), the BIPM, and others to establish absolute calibration of GPS receivers [15].
Although we are always trying to do better, bandpass dependencies,
subtle impedance-matching issues, power-level effects, and even multipath
within anechoic test chambers could preclude significant reduction of 2.5 nanoseconds
1-sigma errors at the L1 and L2 frequencies, as reported in [16].
Since this error is largely uncorrelated between the two GPS
frequencies, the error in ionosphere-corrected data becomes 6.4 nanoseconds.  Experimental verification by side-by-side
comparison contributes an additional square root of two.  For this reason, relative calibration, by
means of traveling GPS receivers, is a better operational technique, as long as care is taken that there are no systematic multipath differences
between antennas.  We strongly support
the BIPM’s relative calibration efforts for geodetic GPS receivers, and in
particular are looking forward to comparisons with the multipath-free TWSTT
calibrations.In 2003, the Wide-Area
Augmentation System (WAAS) became operational.
The USNO has been collecting data on WAAS network time (WNT). Daily
averages generated by averaging WNT with WAAS-corrected time from GPS
satellites are very similar to WNT-only averages. WNT obtained by narrow-beam
antenna may be the optimal solution for a nonnavigational user for whom
interference is a problem or jamming may be a threat.The USNO
has been participating in discussions involving the interoperability of GPS,
Galileo, QZSS, and GLONASS.  In December
of 2006, a Galileo monitor station was installed, and detailed plans have been
made to monitor the GPS/Galileo timing offset (GGTO) [17] in parallel and in concert with the Galileo precise timing
facilities (GPTF).  The GGTO will be
measured by direct comparison of the received satellite timing, and by the use
of TWSTT to measure the 1-pps offset between the time signals at the USNO and
GPTF.  The GGTO will eventually be
broadcasted by both GPS and Galileo, for use in generating combined position
and timing solutions.  To exchange
similar information with the QZSS system, plans are underway to establish a
TWSTT station in Hawaii.With the
use of multiple GNSS systems, problems involving receiver and satellite biases
will become more significant.  These have
been shown to be related to the complex pattern of delay variations across the
filtered passband, and correlator spacing.
In principle, every satellite would have a different bias for every
receiver/satellite combination [18].  USNO has analyzed how calibration errors
associated with the timing group delay (TGD) bias measurements of GPS result in
a noticeable offset in GPS Time versus
UTC, as measured in the BIPM’s Circular T (see Figure 8) [19].Figure 8
UTC-GPS as reported in the Circular T, and
UTC-GPS inferred by subtracting UTC(USNO)-GPS from UTC-UTC(USNO).  UTC(USNO)-GPS can be obtained from the
satellite broadcasts, as in Figure5
and is also measured directly at the USNO.The most accurate means of operational long-distance time
transfer is TWSTT [20–22], and the
USNO has strongly supported the BIPM’s switch to TWSTT for TAI generation.  We routinely calibrate and recalibrate the TWSTT at 20 sites each year, and in
particular we maintain the calibration of the transatlantic link with the Physikalisch-Technische Bundesanstalt (PTB) through comparisons with
observations at a second TWSTT frequency [23] and with the carrier-phase GPS receivers whose IGS
designations are USNO, USN3, and PTBB. For improved precision, we have made
some efforts to develop carrier-phase TWSTT [24].  For improved
robustness, we have begun constructing loop-back setups at the USNO, moved
electronics indoors where possible, and developed temperature-stabilizing
equipment to test on some of the outdoor electronics packages.The Time Service Department of
the USNO has also actively pursued development of GPS carrier-phase time
transfer, in cooperation with the International GPS Service (IGS).  With assistance from the Jet Propulsion
Laboratory (JPL), the USNO developed continuous filtering of timing data and
showed that it can be used to greatly reduce the day-boundary discontinuities
in independent daily solutions without introducing long-term systematic
variations [22].  Working with the manufacturer, the USNO has
helped to develop a modification for the TurboRogue/Benchmark receivers, which
preserve timing information through receiver resets.  Using IGS data, the USNO has developed a
timescale that is now an IGS product [25].  The USNO is currently contributing to
real-time carrier-phase systems run by JPL/NASA [26] and the Canadian real-time NRCan networks [27].  The current operational
USNO receiver models are subject to apparently spontaneous calibration
variations at the 1-nanosecond level [28].  In 2007, we will be experimenting with three
different receiver models based upon newer technology.The continuous real-time sampling
by highly precise systems was increased in 2006 when the USNO-DC became a
full-fledged GPS monitor site, in cooperation with the National
Geospatial-Intelligence Agency (NGA).  The
NGA is installing improved GPS receivers, which would make possible an
alternate means of providing time directly to GPS, both at the Washington
site and at
the AMC.


## 5. Measures to Secure the Robustness of the Master Clock

The most comm on source of nonrobustness is the occasional
failure of the environmental chambers.
In order to minimize such variations, and to house the fountain clocks,
we are constructing
and equipping a new clock building (see Figure9).  The building has redundant environmental
controls designed to keep the entire building constant to within 0.1 degree and 3% relative
humidity even when an HVAC unit is taken offline for maintenance.  The clocks themselves will be kept on vibrationaly
isolated piers.  The instrument racks
will be standardized at all USNO locations.Figure 9
New clock building, November 2006.The clocks
in all Washington DC buildings
are protected by an electrical power system whose design includes multiple
parallel and independent pathways, each of which is capable of supplying the
full electrical power needs of the Master Clock.  The components of each pathway are
automatically interchangeable, and the entire system is supplemented by local
batteries at the clocks that can sustain performance long enough for staff to
arrive and affect most possible repairs.
Although we have never experienced a complete failure of this system,
most of the components have failed at least once.  These failures and periodic testing give some
confidence in the robustness of the system.The comm on design in all the operations and improvements
is reliance upon multiple parallel redundant systems continuously operated and
monitored.  Such a scheme can be no more
reliable than the monitoring process.
For this reason, we have also ordered the parts to create a system
wherein we will have two fully real-time interchangeable and redundant computer
systems in two different buildings.  Each
would be capable of carrying the full load of operations and sensing when the
other has failed so it can instantly take control.  Each computer could access data continuously
being stored in either of two mirrored disk arrays in the two buildings, and
each of those disk arrays has redundant storage systems so that three
components would have to fail before data are lost.  In addition, we do a daily tape backup of all
data, and maintain a restrictive firewall policy.  Other measures too have been taken.


## 6. Disclaimer

Although some manufacturers are
identified for the purpose of scientific clarity, the USNO does not endorse any
commercial product nor does the USNO permit any use of this document for
marketing or advertising.  We further
caution the reader that the equipment quality described here may not be
characteristic of similar equipment maintained at other lab oratories,
nor of equipment currently marketed by any commercial vendor.



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