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The SIM Time Network and its role in the time and frequency laboratory Michael A. Lombardi National Institute of Standards and Technology (NIST), USA

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The purpose of rmos

The SIM Time Network and its role in the time and frequency laboratoryMichael A. LombardiNational Institute of Standards and Technology (NIST),

The purpose of rmos

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 purpose of RMOs world’s five major Regional Metrology Organizations (RMOs) recognized by the BIPM

  • 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). 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:

    • Organize regional comparisons to supplement the BIPM key comparisons so that more nations can establish traceability to the SI. This is the role of the SIMTN.

Information about sim
Information about SIM world’s five major Regional Metrology Organizations (RMOs) recognized by the BIPM

  • 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 13% of the world’s population (about 910 million people as of 2009), and roughly 27% of its land mass. SIM is the largest RMO in terms of land area.

  • About 2 out of 3 people in the SIM region live in the United States, Brazil, or Mexico (roughly 617 million people).

  • Eleven SIM nations (mostly islands) have less than 1 million people.

  • SIM has organized metrology working groups (MWGs) in 11 different areas, including time and frequency. The SIM Time Network is operated by the T&F MWG.

Sim time network design goals
SIM Time Network Design Goals world’s five major Regional Metrology Organizations (RMOs) recognized by the BIPM

  • Our design goals were:

    • To establish cooperation and communication between the SIM time and frequency labs now and in the future.

    • To build a network that allowed all SIM NMIs to compare their time standards to those of the rest of the world.

    • To utilize equipment that was low cost and easy to install, operate, and use, because SIM NMIs typically have small staffs and limited resources.

    • To be capable of measuring the best standards in the SIM region. This meant that the measurement uncertainties had to be as small, or nearly as small, as those of the BIPM key comparisons.

    • To report measurement results in near real-time, without the processing delays of the BIPM key comparisons.

    • To build a democratic network that favored no single laboratory or nation, and to allow all members to view the results of all comparisons.

The purpose of rmos

United States, 2005 world’s five major Regional Metrology Organizations (RMOs) recognized by the BIPM

Mexico, 2005

Canada, 2005

Panama, 2005

Brazil, 2006

Costa Rica, 2007

Colombia, 2007

Argentina, 2007

Guatemala, 2007

Jamaica, 2007

Uruguay, 2008

Paraguay, 2008

Peru, 2009

Trididad & Tobago, 2009

Chile, 2010

Saint Lucia, 2010


Working to support time and frequency metrology throughout the Americas

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.

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.

The SIM Measurement System

The purpose of rmos compare their standards. It includes:

Reporting results to participating sim laboratories
Reporting results to participating SIM laboratories compare their standards. It includes:

  • 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.

The purpose of rmos

Benefits to the SIM Region compare their standards. It includes:

  • Improved time coordination.

  • Better time standards are being maintained at many of the SIM labs.

  • Increased awareness of the importance of time and frequency.

    • Some 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 have become or are trying to become the official timekeepers in their countries.

Selecting a time and frequency standard rubidium cesium gpsdo or ensemble time scale
Selecting a Time and Frequency Standard: compare their standards. It includes: Rubidium, Cesium, GPSDO, or ensemble time scale

Choices in frequency standards

Cesium Oscillators compare their standards. It includes:

Used to define the second, one of the seven base units of the SI

A true intrinsic standard

Costly, probably $35 000 to $75 000 USD

Rubidium Oscillators

Low cost (often under $5 000 USD) but need to be adjusted often to compensate for aging and frequency drift

GPS Disciplined Oscillators

A quartz or rubidium oscillator continuously steered to agree with signals from the GPS satellites. Cannot be adjusted, but doesn’t need it.

Price ranges from about $1 000 to $15 000 USD

Are they an acceptable choice as a primary frequency standard?

Choices in Frequency Standards

Rubidium oscillators
Rubidium Oscillators compare their standards. It includes:

  • The least expensive atomic oscillator

  • Typically costs about 1/10 as much as a cesium, but its unadjusted accuracy is typically about 1000 times worse

    • $3,500 rubidium might be accurate to a few parts in 1010 after warm up

    • $35,000 cesium is likely to be accurate to within a few parts in 1013

  • They need to be adjusted periodically

    • Frequency change (due to aging) can exceed 1 x 10-11 per month, or 1 x 10-10 per year

  • Not acceptable for BIPM key comparisons

Cesium oscillators
Cesium Oscillators compare their standards. It includes:

  • A true primary standard for a cal lab, since the SI second is defined based on cesium

  • Maintenance cost is high, since the beam tube (which is often more than half the cost of the oscillator) typically needs replacement after about 10 years

  • Can operate for many weeks, months, or years without requiring adjustment, maintaining average frequency of less than 1 × 10-13 if properly maintained.

  • Still needs to be checked against a secondary standard to make sure that it is working - a failed cesium becomes an OCXO.

Gps disciplined oscillators gpsdo
GPS Disciplined Oscillators (GPSDO) compare their standards. It includes:

  • A self-calibrating standard

  • Care must be taken to ensure that GPS signals are being received and that the GPSDO is working properly

  • Performance varies, but even the worst units will be accurate to better than 1 × 10-12 over a one day interval

  • Great performance for the money, but not adjustable and not accepted in BIPM key comparisons

  • A good thing to have, however, as a backup or secondary standard

Ensemble time scales
Ensemble Time Scales compare their standards. It includes:

  • Require multiple cesium oscillators.

  • Very expensive, but a time scale will be more stable than any of its individual clocks. It will also keep going if one of the clocks fails.

  • Requires measurement hardware, phase steppers, synthesizers, etc.

  • CENAM, NIST, NRC, and ONRJ now have ensemble time scales and can help other SIM labs that want to start one.

The purpose of rmos

UTC(NIST) Time Scale compare their standards. It includes:

The NIST Time Scale consists of an ensemble of commercial clocks, currently six hydrogen masers and four cesium beam standards.

The weighted average of these clocks generates continuous signals from a high resolution frequency synthesizer that is locked to a hydrogen maser. Both 5 MHz (frequency) and 1 pps (time) signals are generated.

The clock ensemble is periodically calibrated using the NIST-F1 primary standard.

Of course, a time scale can be built with fewer clocks and without hydrogen masers.. However, at least three cesiums are required. The time scales at CENAM and ONRJ have built or redesigned in recent years and provide excellent performance.

Selecting measurement equipment
Selecting Measurement Equipment compare their standards. It includes:

Essential equipment for a frequency measurement laboratory
Essential Equipment for a Frequency Measurement Laboratory compare their standards. It includes:

  • Essential Equipment

    • Primary Frequency Standard

    • Secondary Frequency Standard (if you don’t have a multi-clock time scale)

    • Distribution Amplifier

    • Oscilloscope

    • Universal Counter

    • Signal Generator

Distribution amplifier
Distribution Amplifier compare their standards. It includes:

  • The frequency signals (typically 10 MHz sine waves) from the lab’s primary standard can be distributed throughout the work area using a distribution amplifier.

  • Signals from the distribution amplifier should be used as the external time base for all instrumentation in the laboratory (counters and signal generators, for example). This will ensure that the lab’s measuring instruments have the same frequency accuracy as the primary standard.

Oscilloscope compare their standards. It includes:

  • Allows the viewing of waveforms and pulses, an indispensable device for the time and frequency lab.

  • Can be used to measure frequency, but that should only be done if a counter is not available.

Key oscilloscope specifications
Key Oscilloscope Specifications compare their standards. It includes:

  • Bandwidth

    • 20 MHz (low end) to 70 GHz (high end)

    • A 100 MHz unit is adequate for a timing lab

  • Time base range

    • Best scopes can scale from about 50 ps to 50 s per division (10 divisions)

    • A unit that scales from 5 ns to 50 s per division is adequate for a timing lab

  • Frequency Counter Resolution

    • Typically about 1 x 10-4 (1 kHz resolution at 10 MHz), not useful for any serious measurement, but provides quick check of frequency

  • Time Interval Resolution

    • Proportional to length of the interval

    • Typically about 1 x 10-4 (1 µs for 10 ms interval)

  • Number of channels

    • Ranges from 2 to 8 plus external trigger

    • 2 channel handles pattern drift method

  • t function is very useful for time interval measurements

Universal counter
Universal Counter compare their standards. It includes:

  • Counts frequency, time interval, period, and events (totalize mode). Some universal counters include other functions like phase, peak voltage, rise/fall time, etc.

  • Probably the most important instrument in a time and frequency lab, useful in nearly all areas of T&F metrology.

Universal counter specs frequency
Universal Counter Specs (Frequency) compare their standards. It includes:

  • Frequency Range

    • 100 MHz (low end) to 50 GHz (high end)

    • For most purposes, a unit that can count up to 100 MHz is adequate.

  • Resolution (number of digits)

    • 7 digits ($200 counters) to 13 digits

    • 8-digit counter has 1 Hz resolution at 10 MHz:

      • 10 000 000 Hz

      • This allows it to detect frequency changes as small as 1 x 10-7

    • 12-digit counter has 100 µs resolution at 10 MHz:

      • 10 000 000 000 0 Hz

      • This allows it to detect frequency changes as small as 1 x 10-11

      • 12-digit counters are relatively cheap, and recommended

  • Any selected counter must be able to accept an external time base.

Universal counter specs time interval
Universal Counter Specs (Time Interval) compare their standards. It includes:

  • Time Interval Resolution

    • 100 ns (low end, period of 10 MHz time base) to 20 ps (high end)

    • Relatively low cost counters have 150 ps resolution, which is more than adequate. Smallest frequency offset which can be resolved at 1 second is 150 ps / 1 second, or 1.5 x 10-10.

  • Time Interval Accuracy and Range

    • Typically, no better than 1 ns for even the best counters, but it is not proportional to the length of the interval like it is for oscilloscopes.

    • A good counter can can measure intervals from 1 ns to about 100 000 seconds. The best oscilloscopes can measure shorter intervals, but it is unlikely that cal labs will need to do this.

Signal generator
Signal Generator compare their standards. It includes:

  • Generates signals at user selectable frequencies and amplitudes

    • Some units (called function generators) generate a variety of different waveforms, other units generate just sinewaves and/or squarewaves

Signal generator uses
Signal Generator Uses compare their standards. It includes:

  • Extremely useful tool for generating signals to test measurement systems. Also useful for repairing equipment.

    • Can generate frequencies with small offsets from nominal

    • Can generate a needed frequency locked to house reference, when no standalone oscillator exists

    • Can generate test signals needed to calibrate stopwatch calibrator or other systems

Key signal generator specifications
Key Signal Generator Specifications compare their standards. It includes:

  • Frequency Range

    • 1 MHz or less (low end) to 100 GHz or more (high end)

    • A device that goes a little above 10 MHz is adequate.

  • Amplitude Range

    • 0.01 to 10 V peak-to-peak (50 ohms) is typical and will handle most lab functions

  • Resolution

    • Resolution tends to be lower on devices with the largest frequency range, for example a device that goes to 1 GHz probably will only have 1 or 10 Hz resolution.

    • 1 µHz resolution is about as good as it gets, and is desirable for cal lab purposes. It allows generating a 10 MHz signal with a 1 x 10-13 frequency offset. Some units extend 1 µHz resolution out to about 80 MHz. With a unit like that you can generate a signal with a 1.25 x 10-14 frequency offset.

  • External time base is mandatory

Optional equipment for a frequency measurement laboratory recommended for most labs
Optional Equipment for a Frequency Measurement Laboratory compare their standards. It includes:(recommended for most labs)

  • Optional Equipment

    • An automated phase comparison system

      • Homebrew (PC and time interval counter, for example)

      • Commercially available phase comparator (heterodyne system, for example)

      • NIST Frequency Measurement System

    • Frequency Dividers and Multipliers

    • A stopwatch calibrator

    • Data Analysis Software (homebrew, Stable32, Excel, etc.)

Basic model of a phase comparator for measuring frequency

PC compare their standards. It includes:

Data Acquisition System

Basic Model of a Phase Comparator for Measuring Frequency

Device Under Test

Reference Frequency


f  f0


Oscilloscope, frequency counter,

time interval counter,

Dual Mixer Time Difference System

Homebrew system
Homebrew System compare their standards. It includes:

  • Several types of systems are possible, but the most practical would probably consist of a time interval counter, a PC, and software

    • Software must be written to collect a series of measurements from the counter (Basic, C, Labview, etc.)

    • Data can be analyzed using your own software, Stable 32, or Excel. You can download Francisco’s software for free from the SIM web site.

    • Frequency dividers must be built or purchased to divide the standard reference and DUT signals to a common low frequency, usually 1 Hz

Time interval method
Time Interval Method compare their standards. It includes:

Dual mixer time difference system dmtd
Dual-Mixer Time Difference System (DMTD) compare their standards. It includes:

Commercial dmtd system
Commercial DMTD System compare their standards. It includes:

  • Measures frequencies from 1 MHz to 20 MHz

  • Computes Allan deviations from 0.01 s to 106 s

  • Produces phase and frequency plots

  • Resolution is about 100 femtoseconds (0.1 ps), about 200 times better than the best time interval counters.

Stopwatch calibrator
Stopwatch Calibrator compare their standards. It includes:

  • A handy instrument to have if you laboratory calibrates stop watches and timers

  • Several models are available, costing between about $2,000 and $3,500

  • They reduce the time required to perform calibrations

Software compare their standards. It includes:

  • The laboratory should have software available to perform basic time and frequency functions, such as:

    • Estimate average frequency accuracy from phase data

    • Compute Allan deviation

    • Produce graphs for calibration reports

    • Perform uncertainty analysis

  • Software can written in-house with any programming language and many functions are easy enough to automate with Excel. One commercially available software package is Stable32 ( The free software written by Francisco Jimenez can be downloaded here:

Joining the bipm key comparisons
Joining the BIPM key comparisons compare their standards. It includes:

The purpose of rmos

The SIM Time Network collects more data than the CGGTTS* format used by the BIPM, but is not compatible

  • Simple format collects more data without the need for a tracking schedule.

* Consultative GPS and GLONASS Time Transfer Sub-committee

The cggtts common view data format

GPS RCVR: NBS10 format used by the BIPM, but is not compatible


MJD= 51658 YR=00 MONTH=04 DAY=24 HMS=14:47:20 (UT)


REV DATE = 2000-04-03

RCVR = NBS10....................

CH = 01

IMS = 99999


X = -1288398.27 m

Y = -4721698.10 m

Z = +4078625.68 m


COMMENTS = NO COMMENTS..............

INT DLY = 53.0 ns

CAB DLY = 0199.9 ns

REF DLY = 0066.7 ns


CKSUM = 74


hhmmss s .1dg .1dg .1ns .1ps/s .1ns .1ps/s .1ns .1ns.1ps/s.1ns.1ps/s

3 08 51655 105800 780 380 760 -1058301 -1131 -571 -1098 415 163 107 +2 76 +0 02

8 32 51655 111400 780 319 2933 -7071115 -3061 -246 -3082 290 074 125 -20 85 -9 34

13 28 51655 113000 780 415 3083 +6965884 -30 -94 -241 625 019 100 -12 71 -7 FB

3 74 51655 114600 780 296 530 -1058331 +929 -503 +962 470 163 133 +19 92 +24 17

31 08 51655 121800 780 498 706 -7572 -400 -197 -390 470 180 87 +4 99 +14 DD

13 32 51655 123400 780 569 2693 +6966345 +171 -440 -40 424 011 79 +0 90 +9 F0

18 68 51655 125000 780 279 1829 -341335 +18 -132 +22 698 182 141 +35 152 +44 16

31 74 51655 132200 780 283 472 -7436 +2669 -73 +2678 441 206 139 +29 190 +36 24

The CGGTTS Common-view Data Format

Steps required in order to appear on the bipm circular t and contribute to utc
Steps required in order to appear on the BIPM Circular-T and contribute to UTC

  • You need to have a cesium oscillator

  • You need to have a CGGTTS compatible GPS receiver

  • Your country must be a signatory of the CIPM MRA

  • You must contact the BIPM and provide information on the name/address of the laboratory, clocks (model, serial number), time transfer equipment in the laboratory, and any other relevant information. They will then assign an acronym and a code to your laboratory, and a code to each clock.

  • You must submit a data file once per month (by the 5th day of the month) by FTP

Bipm compatible common view receivers

There are a few receivers that you can buy. The cost is usually between $15,000 and $35,000 USD.

AOS TTS-2 (single frequency)

AOS TTS-3 and TTS-4 (dual frequency)

Dicom GTR50 (dual frequency)

Novatel (dual frequency)

PolaRx2eTR (dual frequency)

BIPM-Compatible Common-view Receivers

Three possibilities for the future
Three possibilities for the future usually between $15,000 and $35,000 USD.

  • SIM system with conversion software (used by INTI in Argentina but not 100% compatible)

  • Make the SIM system simultaneously produce both the SIMTN and the CGGTTS formats

    • This is difficult because some information that is necessary for CGGTTS compatibility cannot easily be extracted from the SIM receivers.

  • Design a new CGGTTS receiver for SIM. This has been discussed at NIST and CENAM.

    • The currently available receivers cost between $15,000 and $35,000 USD.

    • A single frequency receiver could probably be designed and sold for $10,000 USD (perhaps less).

    • A dual frequency receiver will cost more because of the high price of the parts.

    • Please let us know if there is any interest.