the sim network: improved time coordination for north, central, and south america michael a....
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The SIM Network: Improved Time Coordination for North, Central, and
South America
Michael A. LombardiNational Institute of Standards and Technology (NIST), USA
What SIM is
What the SIM Network is
Common-View GPS Measurements
Innovations of SIM Network when compared to previous CV systems
SIM Network Measurement Results
Benefits of SIM Network
Outline
SIM is the Interamerican Metrology System, one of the world’s five major Regional Metrology
Organizations (RMOs) recognized by the BIPM
The purpose of RMOs
The International Bureau of Weights and Measures (BIPM) works to ensure the worldwide uniformity of measurements and their traceability to the International System of Units (SI).
National Metrology Institutes (NMIs) that sign the BIPM Mutual Recognition Arrangement (MRA) agree to use the same units of measurement and to compare their standards internationally. This allows the measurements made in one country to be accepted and trusted in other countries, which is important for international trade.
The BIPM expects RMOs to review the quality systems of NMIs, and their calibration and measurement capabilities (CMCs). RMOs should also:
Help the NMIs of small and developing countries maintain standards at the level of accuracy needed to support their economies.
Organize regional comparisons to supplement the BIPM key comparisons.
Information about SIM SIM consists of NMIs located in the 34 member nations of the
Organization of American States (OAS), which extends throughout North, Central, and South America, and the Caribbean region.
OAS accounts for roughly 14% of the world’s population (more than 920 million people), and roughly 27% of its land mass.
About 2/3 of the OAS population resides in the United States, Mexico, and Brazil.
Twelve SIM nations (mostly islands) have populations of less than 1 million.
Even though NIST is a member, SIM is not as well established in the world timekeeping arena as EUROMET or APMP. However, participation from the Americas is on the rise and probably has more potential for future expansion than any other region.
SIM has organized metrology working groups (MWGs) in 11 different areas, including time and frequency. The SIM Network is operated by the T&F MWG.
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Trinidad and TobagoJamaicaPanamaUruguay
Costa RicaNicaraguaParaguay
El SalvadorHonduras
HaitiBolivia
Dominican RepublicGuatemala
EcuadorChile
VenezuelaCanada
ArgentinaColombia
Population of SIM Nations (excludes United States, Brazil, and Mexico, and countries with < 1 million)
SIM Network Design Goals Our design goals were:
To establish cooperation and communication between the SIM time and frequency labs now and in the future.
To provide the smaller SIM laboratories not involved in other international comparisons (those who do not appear on the Circular-T) with a convenient way to compare their standards to the rest of the world so that they can establish measurement traceability to the SI units of time and frequency.
To make the required equipment low cost and easy to install, operate, and use, because resources at SIM laboratories are limited and staff sizes are small.
To make measurements with uncertainties that are small enough to characterize the best standards in the SIM region.
To report measurement results in near real-time, without the processing delays of the BIPM Circular-T.
To build a democratic network that did not favor any single laboratory or nation, and to allow all members to view the results of all comparisons.
SIM Network comparisons are made via common-view GPS measurements
The common-view method involves a GPS satellite (S), and two receiving sites (A and B). Each site has a GPS receiver, a local time standard, and a time interval counter.
Measurements are made at sites A and B that compare the received GPS signal to the local time standard.
Two data sets are recorded (one at each site): Clock A - S Clock B - S
The two data sets are then exchanged and subtracted from each other to find the difference between Clocks A and B. Delays that are common to both paths (dSA and dSB) cancel, but delays that are not common to both paths contribute uncertainty to the measurement. The equation for the measurement is:
(Clock A – S) – (Clock B – S) =(Clock A – Clock B) + (dSA – dSB)
The SIM Measurement System Simple design makes it easy and inexpensive for SIM
labs to compare their standards. It includes: 8-channel GPS receiver (C/A code, L1 band) Time interval counter with 30 ps resolution Rack-mount PC and flat panel display Pinwheel type antenna Applies broadcast ionospheric (MDIO) corrections
Data are not stored in CGGTTS format. The receiver measures all visible satellites and stores 1-minute and 10-minute REFGPS averages.
All systems are connected to the Internet, and send their files to a web server every 10 minutes.
The web server processes data “on the fly” in near real-time. Results can be viewed on the web in either common-view or all-in-view format.
Systems are installed in 10 of the 34 SIM nations.
All units are built and calibrated at NIST
Systems are paid for by either OAS or the participating NMI and become the property of the NMI.
SIM Receiver
Calibrations
SIM systems are calibrated at NIST prior to shipment. Calibrations are performed using the common-view, common-clock method. The SIM laboratory installs the same antenna cable and antenna that were used during the calibration.
Calibrations last for 10 days. The time deviation (Type A uncertainty) of the calibration is less than 0.2 ns after one day of averaging. The combined uncertainty is estimated at 4 ns, because a variety of factors can introduce a systematic offset.
Surveyed Antenna Poles (~ 6 m from reference, 20 cm uncertainty)
235 consecutive 10-day calibrations of same SIM unit(range = ~1 ns, average = 0.3 ns, TDEV at 1 day = 0.2 ns)
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TIME AND FREQUENCY METROLOGY WORKING GROUPWorking to support time and frequency metrology throughout the Americas
United States, 2005
Mexico, 2005
Canada, 2005
Panama, 2005
Brazil, 2006
Costa Rica, 2007
Colombia, 2007
Argentina, 2007
Guatemala, 2007
Jamaica, 2007
Uruguay
Paraguay
Peru
Saint Lucia
Trinidad & Tobago
Chile
Country Country Date added to Date added to SIM NetworkSIM Network
BIPM MRA BIPM MRA Signatory?Signatory?
T&F StandardT&F Standard Contributes to TAI/UTC?Contributes to TAI/UTC?
United States 2005 Yes Time Scale and Primary Standard Yes
Mexico 04/2005 Yes Time Scale (one maser and four cesiums)
Yes
Canada 05/2005 Yes Time Scale(three masers and four cesiums)
Yes
Panama 10/2005 Yes Cesium Yes
Brazil 09/2006 Yes Time Scale(six cesiums)
Yes
Costa Rica 01/2007 Yes Quartz, Cesium available soon No
Colombia 02/2007 No Cesium No
Argentina 07/2007 Yes Cesium NMI plans to contribute soon, military does now
Guatemala 08/2007 No Rubidium on order No
Jamaica 12/2007 Yes Cesium No
Uruguay 2008 Yes Rubidium No
Paraguay 2008 No None, OAS will provide Rubidium No
Chile ASAP Yes ? NMI does not, but geodetic observatory does
Peru ASAP No ? No
Trinidad / Tobago ASAP Yes ? No
Saint Lucia ASAP No ? No
Rubidium Frequency Standard
Many of the potential SIM labs do not have a frequency standard, so the MWG plans to provide a low-cost rubidium standard to those labs, if OAS can provide the funding.
The selected device is a low cost (about $3000 USD) rubidium device with six configurable outputs (10 MHz, 5 MHz, or 1 pps).
We hope to develop software to manually or automatically adjust the rubidium frequency. In automatic mode, this software will pull common-view GPS data from the Internet and then implement a frequency locked loop that steers the rubidium to agree with the remote time scale.
Simple format collects more data without the need for a tracking schedule
The SIM Network has some advantages over traditional common-view systems
* Consultative GPS and GLONASS Time Transfer Sub-committee
The data exchange is handled automatically via the Internet, so results are made available in near real-time
The BIPM results are typically from 2 to 7 weeks old at the time of publication The SIM results are updated every 10 minutes
Method Tracks per day Track Length Satellites Minutes of data per day
CGGTTS *
single-channel
48 13 1 624
CGGTTS *
multi-channel
90 13 8 typical 9360
SIM 144 10 8 maximum 11520
SIM systems were designed to be easy to install and use
Once the SIM lab receives the system, they:
Mount and survey the GPS antenna (antenna survey software is included).
Connect a 1 pps signal from their time standard to the system.
Connect the system to the Internet using an Ethernet cable.
tf.nist.gov/sim
Reporting results to participating SIM laboratories
Measurement results can be viewed using any Java-enabled web browser. Our web-based software does the following:
Plots the one-way GPS data (average of all satellites and tracks for each individual satellite) as recorded at each site relative to the local standard.
Plots the time and frequency difference between NMIs using the common-view method (common-view data are averaged across all satellites and are also shown for each individual satellite).
Calculates the Allan deviation and time deviation.
Makes 10 minute, 1 hour, and 1 day averages available in tabular form.
Up to 200 days of data can be retrieved at once. All old data remains available, nothing is ever deleted.
The time difference between any two laboratories can be viewed by all laboratories in the network. New results are available every 10 minutes.
Results can be processed as “classic” common-view or all-in-view.
“Classic” Common-View
where TD is the average time difference between the clocks at sites A and B N is the number of satellites tracked by the multi-channel GPS receivers REFGPSi(A) is the series of satellite tracks recorded at site A REFGPSi(B) is the series of satellite tracks recorded at site B CV is the number of satellites simultaneously visible at both sites
CV
BREFGPSAREFGPSTD
i
N
ii ))()((
1
As applied by the SIM network, this technique aligns and differences data from the individual satellite tracks, and discards data from satellites that are not in common view at both sites. The basic equation is:
The ONRJ – NIST baseline is currently the longest in the SIM network. The two laboratories are separated by 8623.5 km (surface distance is ~9500 km) and are on opposite sides of the equator.
Long Baselines represent problems for common-view because:
Over long baselines, the few satellites that are in common view at both sites are at low elevation angles. This makes them more susceptible to ionospheric delay correction errors.
Over very long baselines, no satellites will be in common-view at both sites.
ONRJ – NIST baseline
The SIM systems at NIST and ONRJ track an average of 7.3 and 7.4 satellites, respectively. However, only 1.4 satellites are simultaneously in view at both sites.
ONRJ to NIST baseline, 60-day run, 10 degree mask angle
All-in-View To allow for situations when few if any satellites are in common view, the SIM network software can
also present measurement results using the "all-in-view" method where the satellite tracks are not aligned and no tracks are discarded. Instead, the averages of the REFGPSi(B) and REFGPSi(B) data series recorded at both sites are calculated, and the time difference TD equals the difference between the two averages:
)()( BREFGPSAREFGPSTD ii
The all-in-view method allows comparisons to be made between two clocks located anywhere on Earth, regardless of the length of the baseline. None of the satellites used in the comparison are required to be in common-view at both sites.
Used by BIPM for TAI calculations since September 1, 2006.
Baseline Length (km)
Average CV
satellites
Period (05/27/07 to 07/25/07)MJD 54247 to 54306, 60 d
Time Deviation (ns)
τ = ~10 min τ = 1 d
RTCV RTAV RTCV RTAV
WWV – NIST 78.2 7.2 0.78 0.76 0.72 0.72
CNM – NIST 2198.9 5.1 0.97 1.11 1.27 1.29
CNM – CNMP 2544.0 5.3 1.30 1.30 1.88 1.94
CNMP – NRC 3989.0 4.7 1.06 1.19 1.54 1.59
CNMP – NIST 4194.9 4.4 1.03 1.10 1.49 1.53
CNMP – ONRJ 5153.1 3.5 1.50 1.09 1.96 1.90
CNM – ONRJ 7351.1 2.2 2.03 1.11 1.71 1.69
NRC – ONRJ 7681.2 2.1 2.33 1.00 1.49 1.51
ONRJ – NIST 8623.5 1.4 2.12 0.88 1.62 1.46
WWV = Colorado CNM = Mexico CNMP = Panama NRC = Canada
Summary of Processing Methods
Real-Time Common-View (RTCV) Uses broadcast MDIO correction, built-in to SIM network Uses 10-minute tracks
Real-Time All-in-View (RTAV) Uses broadcast MDIO correction, built-in to SIM network Uses 10-minute tracks
Post-Processed All-in-View (PPAV) Applies MSIO correction Uses 13-minute tracks in 16-minute segments Used by BIPM to compute UTC, called the CGGTTS format Not used by SIM network, but data can possibly be converted to this format
from the one-minute tracks
BIPM Circular T (www.bipm.org) Published monthly, it contains the official results of international time comparisons.
Five labs in the SIM network have their standards listed on the Circular-T. The Circular-T numbers are post processed and obtained with completely independent receiving equipment.
The real-time numbers obtained through the SIM network are in good agreement with the Circular-T numbers, well within our stated uncertainties. This helps validate our results.
UTC(CNM) - UTC(NIST)
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SIM Network Data obtained in real-time
Post processed Circular-T data
UTC(ONRJ) - UTC(NIST) via ~8600 km baseline
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SIM Network Data obtained in real time
Post processed Circular T data
UTC(NIST) - UTC(NRC)
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MSIO link (NIST Novatel/NRC TTR-5)
Circular T
SIM Network link
Time Deviation of UTC(NIST) - UTC(NRC) from 1/1/07 to 9/12/07
1.0E-10
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1.0E+04 1.0E+05 1.0E+06 1.0E+07
Averaging time (s)
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MSIO link (NIST Novatel Rx / NRC TTR-5)
SIM Network link
Type BUncertainty Component
Explanation Estimated Uncertainty
Calibration of SIM unit at NIST
Absolute accuracy of delay calibration is limited to about 4 ns.
4 ns
Environmental variations
Receiver delays can change due to temperature or voltage fluctuations from antenna cables or power supplies.
3 ns
Antenna Coordinates Error
Assumes that antenna position (x, y, z) is known to within 1 m.
3 ns
Propagation delay changes due to multipath
Multipath is caused by GPS signals being reflected from surfaces near the antenna.
2 ns
Propagation delay changes due to ionospheric conditions
The SIM system uses the ionospheric corrections broadcast by the satellites, and does not apply measured ionospheric delay corrections. This uncertainty represents the typical difference between the modeled and measured correction.
2 ns
Cable delay measurements made by the SIM laboratory.
Usually done with a time interval counter and is subject to small errors.
0.5 ns
Resolution Uncertainty Software limits the resolution of entered delay values to 0.1 ns.
0.05 ns
UbUakc
U 22
SIM Network Uncertainty Analysis
Uncertainties are expressed using a method complaint with the ISO GUM standard.
We use the time deviation (TDEV) at an averaging time of 1 day as our Type A uncertainty (1.5 ns in this example).
Type B uncertainties are summarized in the table.
Combined standard uncertainty (k = 2) is < 15 nanoseconds for time, and < 1 10-13 for frequency after 1 day of averaging.
ns 3.132525.4225.22 c
U
Summary of uncertainties (in nanoseconds) for various baselines in the SIM Network
(June – August 2007)
CNM CNMP
NIST CNM
CNM NRC NIST CNMP
NRC CNMP NIST NRC
NIST-ONRJ
NRC - ONRJ
Baseline (km) 2544.0 2198.9 3520.7 4194.9 3989.0 2471.3 8623.5 7681.2
Mean Frequency Offset(parts in 1014)
1.2 0.3 0.9 1.6 3.6 0.5 3.6 3.3
Mean Time Offset 109.6 15.7 -45.6 125.4 -155.7 -30.2 25.0 -53.7
UA, σx() 2.2 1.2 1.3 1.8 1.8 0.8 1.8 1.7
UB, Calibration 4 4 4 4 4 4 4 4
UB, Coordinates 3 3 3 3 3 3 3 3
UB, Environment 3 3 3 3 3 3 3 3
UB, Multipath 2 2 2 2 2 2 2 2
UB, Ionosphere 2 1.5 2.5 3 3 1.5 3.5 3.5
UB, Ref. delay 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5
UB, Resolution 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05
UC, k = 2 13.7 13.0 13.6 14.2 14.2 12.8 14.7 14.6
TIME AND FREQUENCY METROLOGY WORKING GROUPWorking to support time and frequency metrology throughout the Americas
Using MDIO instead of MSIO introduces an ~3 ns shift in the mean time offset (Type B uncertainty), but the
stability is similar
Benefits to the SIM Region
Improved time coordination. The CENAM, NIST, NRC, and ONRJ time scales are now nearly always within ±50 ns of
each other.
Better time standards are being maintained at many of the SIM labs.
Increased awareness of the importance of time and frequency. SIM labs are introducing new calibration services and improving existing services to better
support local industry. New time services are also being introduced (NTP servers, web clocks, etc.).
Improved status for NMIs. Companies in SIM countries are likely to use their local NMI as a source of traceable
frequency measurements.
A more visible official timekeeper. Some SIM labs are now trying to become the official timekeepers in their respective
countries.
Summary
The SIM network began operation in June 2005 with three participants. Ten NMIs now participate. Participation should eventually extend to at least 16 laboratories.
The SIM network is advancing the state of time coordination and time and frequency metrology throughout the SIM region. It provides NMIs with a convenient way to compare their standards and to establish continuous traceability to the SI.
The SIM network produces measurement results that agree closely with those published in the BIPM’s Circular-T, but that have the distinct advantage of being available in near real-time.