The SIM Common-View GPS Comparison Network, Traceability, and Legal Metrology

1. The SIM Common-View GPS Comparison Network, Traceability, and Legal Metrology

2. International Comparisons are made via Common View GPS • Two users, A and B, compare their time standard to the same GPS satellite at the same time. • Two data sets are recorded (one at each site): • Clock A - GPS • Clock B - GPS • Data is exchanged. Subtract A from B to get the difference between the two clocks. • Common errors cancel, including GPS time errors. • Common-view GPS is the primary method used for the international computation of UTC.

3. BIPM Circular T (www.bipm.org)

4. SIM Comparisons • SIM is the Systema Interamericano de Metrologia (Interamerican Metrology System) with 34 member nations • Real-Time common-view measurements, unlike post processed Circular-T, results are updated every 10 minutes • Comparisons already in progress between Canada, Mexico, and United States. CENAMEP will be added soon. All national time and frequency labs (over 12 labs) in North, Central, and South America are expected to be included as funding becomes available. • Unilateral, no single laboratory serves as the hub.

5. SIM Common-View GPS System • Collects data from all satellites in view • Stores data (10-min averages) in 32 x 144 matrix • Uploads to Internet for on-the-fly processing every 10 minutes. • Collects about 400 minutes of data from each satellite per day • Web-based software (CGI scripts) plots the composite and individual tracks recorded at both sites, plus does the common-view data reduction • Web-based software calculates average frequency and time offset, plus Allan deviation and time deviation. • If a SIM laboratory is maintaining a national time and frequency standard, this system allows them to continously compare it, 24 hours per day, 7 days a week, to the standard maintained by all other participating SIM laboratories.

6. Requirements for Joining Network • A dedicated Internet connection (always on with a static IP address) • A 1 pps timing reference • A 5 or 10 MHz reference for use as the time interval counter time base • For laboratories with no standard we are investigating supplying rubidium oscillators • Current members of the network are Mexico, Canada, Panama, and the United States • When/if funding becomes available through OAS, we want to supply every SIM laboratory that wants to participate and that meets the requirements a system • If you work for a SIM NMI and are not on the SIM email list, please send me email at lombardi@nist.gov • The list will allow you to communicate with Mauricio Lopez, the chair of the SIM time and frequency working group. It will also allow you to communicate with Jean-Simon, Carlos Donado, myself, and all other members of the time and frequency working group.

7. Time Interval Method

9. Counter Self Test

15. Common-View, Common-Clock Calibration

24. Uncertainty Analysis • TDEV serves as Type A uncertainty. • Type B uncertainties are summarized in Table. • Combined standard uncertainty is near 15 nanoseconds for NORAMET baselines, less than 1  10-13 for frequency.

26. Traceability

27. The property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties. ISO definition of traceability

28. Establishing traceability to national and international standards provides evidence that measurements are being made correctly Traceable measurements are essential elements of: quality control systems trade agreements allowing compatible products to be manufactured all over the world The Importance of Traceability

29. For calibration laboratories, the program for calibration of equipment shall be designed and operated so as to ensure that calibrations and measurements made by the laboratory are traceable to the SI (Systeme International) units of measurement. ISO Guide 17025 - 5.6.2.1.1

30. A calibration laboratory establishes traceability of its own measurement standards and measuring instruments to the SI by means of an unbroken chain of calibrations or comparisons linking them to relevant primary standards of the SI units of measurement. The link to SI units may be achieved by reference to national measurement standards. National measurement standards may be primary standards, which are primary realizations of the SI units or agreed representations of SI units based on fundamental physical constants, or they may be secondary standards which are standards calibrated by another national metrology institute. 5.6.2.1.1 (continued)

31. The traceability chain is a series of comparisons between the device under test to a reference. The final comparison in the chain is made using the International System (SI) units as a reference. Each comparison is a link in the chain. The uncertainty of each comparison (link) must be known and documented. National metrology institutes (NMIs) provide the ultimate measurement references for their countries. The intent of all NMIs is to realize the SI units as closely as possible. Although the goal is to establish traceability to the SI, this is often done by comparing to an NMI that in turn compares its references to the SI. The Traceability Chain

32. The Traceability Pyramid

33. SIM laboratories can link back to the SI by: Contributing to UTC as listed on the BIPM’s Circular-T document, and/or By participating in international comparisons via the SIM network with other laboratories that contribute to UTC. This link back to the SI is necessary to complete the traceability chain. Once a NMI’s primary standard is traceable to the SI, other laboratories in their country can establish traceability by using services provided by their NMI. If a NMI has services with low enough measurement uncertainties to meet the needs of their nation’s customers, there is no need for customers in their country to go elsewhere (Germany, United States, etc.) to establish traceability. Linking to the SI

34. In-House Calibrations NMI monitoring of GPS Publication of the time and frequency offset between UTC(k) and GPS could allow customers to link back to UTC(k) through the reception of GPS Services Time Signal Radio Stations (HF, LF, or satellite) Network Time Protocol (NTP) service for computer time-of-day synchronization is a convenient way to distribute UTC(k) Time-of-day service (voice over telephone or Internet, or web clock via Internet) Ways to establish traceability to the SI through a SIM NMI

35. Traceability chain for signals controlled by a NMI

36. Internet Network Time Protocol (NTP) • The recognized standard for time synchronization via the Internet, defined by RFC-1305 (see www.ntp.org) • Typically accuracy of synchronization ranges from 1 to 100 milliseconds, useful for time stamp purposes, but not nearly good enough for telecommunication networks. For example, CDMA needs time-of-day with about 1000 times the accuracy that NTP can provide. • NTP servers listen for a NTP request on port 123, and respond by sending a udp/ip data packet in the NTP format. The data packet includes a 64-bit timestamp containing the time in UTC seconds since January 1, 1900 with a resolution of 200 ps. The full resolution is meaningless due to the noisy Internet medium. • A client can access several servers to get the best results, averaging down the path noise and delays obtained from each.

37. NTP is based on a hierarchy of servers with stratum layers, but there are no specifications for stratum accuracy. Instead: Stratum-0 is a reference clock synchronized to UTC with little or no delay, such as a GPSDO or a national time scale, such as UTC(NIST). There are no Stratum-0 servers, only clocks. A Stratum-1 server is the highest form of NTP server. It is directly connected to a Stratum-0 clock. There are a relatively small number of Stratum-1 NTP servers available for unlimited public access. A list is available at: http://ntp.isc.org A Stratum-2 server synchronizes over the network to a Stratum-1 server. A Stratum-3 server synchronizes over the network to a Stratum-2 server, and so on. NTP Servers

38. Traceability chain for signals monitored by a NMI (such as GPS)

39. Legal and Technical Requirements for Time and Frequency Metrology

40. All computer system clocks and mechanical time stamping devices must be synchronized to within three seconds of the National Institute of Standards and Technology (NIST) atomic clock. Any time provider may be used for synchronization, however, all clocks and time stamping devices must remain accurate within a three-second tolerance of the NIST clock. This tolerance includes all of the following: The difference between the NIST standard and a time provider’s clock; Transmission delay from the source; and The amount of drift of the member firm’s clock. For example, if the time provider’s clock is accurate to within one second of the NIST standard, the maximum allowable drift for any computer system or mechanical clock is two seconds. The U.S. Stock Market (NASDAQ) uses NIST time as a reference

41. Synchronization Requirements for the Electric Power Industry

42. Commercial timing devices are often checked with field standard stopwatches. Most modern stopwatches are controlled by quartz oscillators, and they typically meet or exceed the performance of a quartz wristwatch (see above). However, they are often calibrated against audio timing signals from NIST radio station WWV or a similar source, and the uncertainty of the calibration is often limited by the human reaction time involved in starting and stopping the timer. Therefore, the legally required measurement uncertainty is typically about 0.01% or 0.02% (1 or 2 parts in 104). NIST Handbook 44 specifies 15 s for a 24 hour interval, or 0.017%. Some states and municipalities have their own laws. For example, the state of Pennsylvania code states that an electronic stopwatch shall comply with the following standards: (i) The common crystal frequency shall be 32,768 Hz with a measured frequency within plus or minus 3 Hz, or approximately .01% of the standard frequency. (ii) The stopwatch shall be accurate to the equivalent of plus or minus 9 seconds per 24-hour period. Field Standard Stopwatches

43. U.S. Requirements for Commercial Timing Devices

44. The Federal Communications Commission (FCC) in the United States provides the following rules for carrier frequency departure (frequency offset) for broadcasting stations. These rules are contained in the section 73.1545 of the United States Federal Register. AM radio stations ( 20 Hz), or 2  10-5 at 1 MHz FM radio stations ( 2000 Hz), or 2  10-5 at 100 MHz TV stations, video and audio carriers ( 1000 Hz), or ~1  10-5 for channel 5 International Broadcast Stations, 0.0015%, or 1.5  10-5 These requirements (roughly 1 part per 100,000) are technically easy to meet. However, broadcasters still require their equipment to be periodically checked against a known standard. Carrier Frequency Departure Tolerances for Radio and Television Stations

45. Available to users in countries operating under the guidance of the International telecommunications Union (ITU). E1 has a total of 32 channels 64 kbps channels - 30 for voice and 2 for framing and signaling at a digital bit stream of 2.048 Mbps - compared to the 24 voice channels and 1.544 Mbps of the North American T1 standard. E2 = 120 channels, 8.448 Mbps E3 = 480 channels, 34.368 Mbps E4 = 1920 channels, 139.268 Mbps E5 = 7680 channels, 565.148 Mbps E1 Specification for Telephone Voice Channels

46. Cycle Slips • A single voice channel sends data at a 64 kbps rate, and is sampled 8000 times per second, or every 125 microseconds. When the time error exceeds half of this amount, or 62.5 microseconds, a cycle slip occurs. Half the period is used to indicate the worst case, i.e., that the two clocks are moving in opposite directions. • Cycle slips occur when one or more network clocks runs at a frequency that is not within tolerance. This results in the loss of data, and sometimes in a dropped call.

47. Stratum Timing Requirements for Primary Reference Source or Clock (PRS or PRC)

48. Stratum Timing Requirements for Primary Reference Source or Clock (PRS or PRC)

49. CDMA Cellular Phone Signals • CDMA digitizes data and speads it over the entire bandwidth it has available. Multiple calls are overlaid over each other on the channel, with a unique code assigned to each. This is similar to the spread spectrum technique that GPS uses to distinguish between satellites. • CDMA normally complies with the TIA/EIA IS-95 standard that uses GPS time as base station time. Thus, nearly all CDMA base stations contain GPS receivers. • Timing requirement is 10 µs, even if GPS is unavailable for up to 8 hours. During normal operation, base stations are synchronized to within 1 µs. • Time code contains local time, no time zone correction is necessary. • Frequency requirement is 5  10-8 for transmitter carrier frequency. • The time-of-day is usually right on a CDMA phone. • There are more than 100,000 CDMA base stations equipped with GPS in the United States. • There are two main types of IS-95 CDMA systems: • Advanced Mobile Phone System (AMPS) with forward link signals using frequencies from 869 to 894 MHz. • Personal Communications Systems (PCS) with forward link signals using frequencies from 1930 to 1990 MHz. PCS phones often fall back and use the analog AMPS band when digital service is unavailable.

50. CDMA Base Stations