The SIM Network: Improved Time Coordination for North, Central, and South America Michael A. Lombardi National Institute of Standards and Technology (NIST), USA

n n What SIM is n n What the SIM Network is n n Common-View GPS Measurements n n Innovations of SIM Network when compared to previous CV systems n n SIM Network Measurement Results n n 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 nThe 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). nNational 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. nThe BIPM expects RMOs to review the quality systems of NMIs, and their calibration and measurement capabilities (CMCs). RMOs should also: u Help the NMIs of small and developing countries maintain standards at the level of accuracy needed to support their economies. u Organize regional comparisons to supplement the BIPM key comparisons.

Information about SIM nSIM 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. nOAS accounts for roughly 14% of the world’s population (more than 920 million people), and roughly 27% of its land mass. nAbout 2/3 of the OAS population resides in the United States, Mexico, and Brazil. nTwelve SIM nations (mostly islands) have populations of less than 1 million. nEven 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. nSIM has organized metrology working groups (MWGs) in 11 different areas, including time and frequency. The SIM Network is operated by the T&F MWG.

SIM Network Design Goals n Our design goals were: u To establish cooperation and communication between the SIM time and frequency labs now and in the future. u 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. u 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. u To make measurements with uncertainties that are small enough to characterize the best standards in the SIM region. u To report measurement results in near real-time, without the processing delays of the BIPM Circular-T. u 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. n Measurements are made at sites A and B that compare the received GPS signal to the local time standard. n Two data sets are recorded (one at each site): u Clock A - S u Clock B - S n 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 (d SA and d SB ) 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) + (d SA – d SB )

The SIM Measurement System n n Simple design makes it easy and inexpensive for SIM labs to compare their standards. It includes: u u 8-channel GPS receiver (C/A code, L1 band) u u Time interval counter with 30 ps resolution u u Rack-mount PC and flat panel display u u Pinwheel type antenna u u Applies broadcast ionospheric (MDIO) corrections n n Data are not stored in CGGTTS format. The receiver measures all visible satellites and stores 1-minute and 10-minute REFGPS averages. n n All systems are connected to the Internet, and send their files to a web server every 10 minutes. n n 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. n n Systems are installed in 10 of the 34 SIM nations. n n All units are built and calibrated at NIST n n 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)

TIME AND FREQUENCY METROLOGY WORKING GROUP Working 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 Date added to SIM Network BIPM MRA Signatory? T&F Standard Contributes to TAI/UTC? United States2005YesTime Scale and Primary StandardYes Mexico04/2005YesTime Scale (one maser and four cesiums) Yes Canada05/2005YesTime Scale (three masers and four cesiums) Yes Panama10/2005YesCesiumYes Brazil09/2006YesTime Scale (six cesiums) Yes Costa Rica01/2007YesQuartz, Cesium available soonNo Colombia02/2007NoCesiumNo Argentina07/2007YesCesiumNMI plans to contribute soon, military does now Guatemala08/2007NoRubidium on orderNo Jamaica12/2007YesCesiumNo Uruguay2008YesRubidiumNo Paraguay2008NoNone, OAS will provide RubidiumNo ChileASAPYes?NMI does not, but geodetic observatory does PeruASAPNo? Trinidad / TobagoASAPYes?No Saint LuciaASAPNo?

Rubidium Frequency Standard nMany 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. nThe selected device is a low cost (about $3000 USD) rubidium device with six configurable outputs (10 MHz, 5 MHz, or 1 pps). nWe 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 MethodTracks per dayTrack LengthSatellitesMinutes of data per day CGGTTS * single-channel CGGTTS * multi-channel typical9360 SIM maximum11520

SIM systems were designed to be easy to install and use n n Once the SIM lab receives the system, they: u u Mount and survey the GPS antenna (antenna survey software is included). u u Connect a 1 pps signal from their time standard to the system. u u Connect the system to the Internet using an Ethernet cable.

tf.nist.gov/sim

Reporting results to participating SIM laboratories n n Measurement results can be viewed using any Java-enabled web browser. Our web-based software does the following: u u 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. u u 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). u u Calculates the Allan deviation and time deviation. u u Makes 10 minute, 1 hour, and 1 day averages available in tabular form. u u Up to 200 days of data can be retrieved at once. All old data remains available, nothing is ever deleted. u u The time difference between any two laboratories can be viewed by all laboratories in the network. New results are available every 10 minutes. u u Results can be processed as “classic” common-view or all-in-view.

“Classic” Common-View n where u TD is the average time difference between the clocks at sites A and B u N is the number of satellites tracked by the multi-channel GPS receivers u REFGPS i (A) is the series of satellite tracks recorded at site A u REFGPS i (B) is the series of satellite tracks recorded at site B u CV is the number of satellites simultaneously visible at both sites n 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:

n The ONRJ – NIST baseline is currently the longest in the SIM network. The two laboratories are separated by km (surface distance is ~9500 km) and are on opposite sides of the equator. n Long Baselines represent problems for common-view because: u 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. u 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 n 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 REFGPS i (B) and REFGPS i (B) data series recorded at both sites are calculated, and the time difference TD equals the difference between the two averages: n 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. n Used by BIPM for TAI calculations since September 1, 2006.

BaselineLength (km) Average CV satellites Period (05/27/07 to 07/25/07) MJD to 54306, 60 d Time Deviation (ns) τ = ~10 minτ = 1 d RTCVRTAVRTCVRTAV WWV – NIST CNM – NIST CNM – CNMP CNMP – NRC CNMP – NIST CNMP – ONRJ CNM – ONRJ NRC – ONRJ ONRJ – NIST WWV = Colorado CNM = Mexico CNMP = PanamaNRC = Canada

Summary of Processing Methods n Real-Time Common-View (RTCV) u Uses broadcast MDIO correction, built-in to SIM network u Uses 10-minute tracks n Real-Time All-in-View (RTAV) u Uses broadcast MDIO correction, built-in to SIM network u Uses 10-minute tracks n Post-Processed All-in-View (PPAV) u Applies MSIO correction u Uses 13-minute tracks in 16-minute segments u Used by BIPM to compute UTC, called the CGGTTS format u Not used by SIM network, but data can possibly be converted to this format from the one-minute tracks

BIPM Circular T ( nPublished monthly, it contains the official results of international time comparisons. nFive 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. nThe 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.

Type B Uncertainty Component ExplanationEstimated 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 UncertaintySoftware limits the resolution of entered delay values to 0.1 ns ns SIM Network Uncertainty Analysis n Uncertainties are expressed using a method complaint with the ISO GUM standard. n We use the time deviation (TDEV) at an averaging time of 1 day as our Type A uncertainty (1.5 ns in this example). n Type B uncertainties are summarized in the table. n Combined standard uncertainty (k = 2) is < 15 nanoseconds for time, and < 1  for frequency after 1 day of averaging.

Summary of uncertainties (in nanoseconds) for various baselines in the SIM Network (June – August 2007) CNM  CNMP NIST  CNM CNM  NRCNIST  CNMP NRC  CNMPNIST  NRC NIST- ONRJ NRC - ONRJ Baseline (km) Mean Frequency Offset (parts in ) Mean Time Offset U A, σ x (  ) U B, Calibration U B, Coordinates U B, Environment U B, Multipath U B, Ionosphere U B, Ref. delay 0.5 U B, Resolution 0.05 U C, k = TIME AND FREQUENCY METROLOGY WORKING GROUP Working 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 n Improved time coordination. u The CENAM, NIST, NRC, and ONRJ time scales are now nearly always within ±50 ns of each other. n Better time standards are being maintained at many of the SIM labs. n Increased awareness of the importance of time and frequency. u 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.). n Improved status for NMIs. u Companies in SIM countries are likely to use their local NMI as a source of traceable frequency measurements. n A more visible official timekeeper. u Some SIM labs are now trying to become the official timekeepers in their respective countries.

Summary n The SIM network began operation in June 2005 with three participants. Ten NMIs now participate. Participation should eventually extend to at least 16 laboratories. n 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. n 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.