# Presentation: 666436

## Legal Traceability of Time Signals

- Judah Levine
- Time and Frequency Division
- Jlevine@boulder.nist.gov
- (303) 497 3903

**Notes:**
Mailing Address:
	National Institute of Standards and Technology
	Time and Frequency Division
	Mail Stop 847
	325 Broadway
	Boulder, Colorado 80303
	FAX: 303 497 6461

## Definitions of Terms

- Traceability of time signals
- Unbroken chain of calibrations between the user and the BIPM through a National Metrology Institute (NMI)
- Technical traceability
- Designed and specified by user based on end-point requirements
- Legal traceability
- Defined by external agency
- National Assoc. of Securities Dealers

**Notes:**
National Metrology Institutes (such as NIST) offer a number of different time services with different capabilities. The choice of which service to use is made by users based on their requirements. NIST provides advice and assistance and also provides special-purpose hardware or software for some of the services.

The requirements for legal traceability often specify only an accuracy and not a specific method that must be used to realize it. For example, the National Association of Securities Dealers requires that member brokers synchronize their system clocks to NIST with an uncertainty of less than 3 seconds. No specific method is required to achieve this accuracy, although most dealers use one of the NIST digital time services for this purpose.

In general, technical traceability, which depends on a technical evaluation of the method that is used for time distribution, is necessary for legal traceability, but may not be sufficient for these purposes.  In particular, while doing the “right thing” may be adequate from a technical point of view, being able to prove that you have done the right thing may be required to satisfy the requirement from a legal point of view.

## Auditing Requirements

- Details vary with the application and the required accuracy
- Local records (automatic updates)
- Real-time oversight with certificates
- Active authentication via PKI or ...
- Implementations using only broadcast signals are often not adequate
- Just doing the right thing may not be good enough

## Treaty of the Meter (1875, 1921)

- General Conference on Weights and Measures

- International Committee On Weights and Measures

- BIPM

- CCTF

- TAI

- Sub-Working Groups

**Notes:**
The original treaty of the meter was signed in 1875 and ratified by the US Senate in 1878. It was proclaimed by President Hayes. The treat was amended slightly in 1921. The amended version was ratified in 1923 and was proclaimed by President Coolidge. There are currently 48 states that subscribe to the Treaty.

The current US representative to the CIPM is Dr. Karen Brown, the Deputy Director of NIST. The next meeting of the CIPM will be in October, 2000. The CCTF and its working groups generally meet every other year, and the next meeting will probably be in the Spring of 2001.

The BIPM has additional responsibilities besides standards for time and frequency, and there are corresponding consultative committees for these different disciplines. Currently, there are a total of 10 such committees. 

International time coordination was initially the responsibility of the Bureau International de l’Heure (BIH), which was established in 1913. The treaty establishing it was never ratified because of the outbreak of World War I. After the war, the BIH was placed under the Time Commission of the International Atomic Union in 1920. The work was transferred to the BIPM in 1987 and the BIH ceased to exist. It was replaced by the International Earth Rotation Service, which has the responsibility for estimating astronomical time scales (UT1, etc.) and for coordinating UTC by inserting leap seconds as required.

In principle, the BIPM is supervised by the International Committee on Weights and Measures (CIPM) and the Consultative Committee on Time and Frequency reports to the CIPM as well. In practice, the CCTF and the BIPM work very closely together and the technical oversight by the CIPM tends to be something of a formality.

The Consultative Committee on Time and Frequency (CCTF) establishes a number of working groups. In the current context, the most important of these probably is the Working Group on International Atomic Time, which is concerned with the details of how TAI and UTC are calculated. The Working Group on TAI in turn has a number of standing sub-groups which deal with detailed technical issues. One of these is the Committee on GPS and GLONASS Time Transfer Standards (CGGTTS), which specifies the technical details of how national timing laboratories are to exchange data using both the GPS and GLONASS satellite systems. Note that this is not the only method that is used to exchange these data. Two-way satellite time transfer is also used, and it is supervised by a separate sub-working group.

## International Time Coordination

- Coordinated Universal Time (UTC)  is calculated by the International Bureau of Weights and Measures (BIPM) using data from a world-wide ensemble of about 250 devices (cesium standards and hydrogen masers).
- Calculation of UTC is about 1 month after the fact
- August data available about 18 September

**Notes:**
The time differences between the timing laboratories are measured using common-view GPS and two-way satellite time transfer.  The common-view method, in which the same satellite is observed simultaneously at several sites, cancels or attenuates many of the systematic errors that would otherwise limit the accuracy of the GPS system.  The BIPM publishes tracking schedules for each region to assist the timing laboratories in performing these common-view observations.

Two-way satellite time-transfer uses signals exchanged between two laboratories using transponders on commercial communications satellites. A typical exchange lasts for 5 minutes and is performed three times each week.

Other methods, such as carrier-phase GPS, are being studied for this application, but these methods are still in the experimental stages.

## UTC in Real Time: UTC(lab)

- UTC(lab) provides real-time realization of UTC
- Approx. 1 month extrapolation
- BIPM Circular T gives UTC – UTC(lab)
- Values for 29 July (most recent):
- UTC – UTC(USNO) =  9 ns
- UTC – UTC(NIST)   =  28 ns
- UTC – UTC(PTB)     = -10 ns
- UTC(lab) measured *at lab. Ref. Plane*
- Channel delay to end-user not included
_*Your mileage may vary*_

**Notes:**
PTB is the Physikalisch Technische Bundesanstalt, the German equivalent of NIST. It is located in Braunschweig, Germany.
The time differences UTC-UTC(lab) vary slowly, and the values shown above are typical. They are calculated by the BIPM using data from GPS common-view observations and two-way satellite time transfer measurements.

## Mutual Recognition Arrangements	

- Established in 1999 by CIPM
- Establishes equivalences between measurements and standards in different National Metrology Institutes
- Establishes Regional Metrology Organizations (RMO) to carry out key comparisons between laboratories in a region
- NORAMET is a prototype RMO
- System formalizes long-standing informal arrangements and is still evolving

**Notes:**
NORAMET, The North and Central American Metrology Cooperative was established in 1994 to facilitate cooperation between the various standards laboratories of the region and to support the key comparisons and Mutual Recognition Arrangements as defined by the CIPM. The principal representatives are from NIST in the US, NRC (National Research Council) and SRC (Standards Research Council)  in Canada and CENAM (National Center for Metrology) in Mexico.
For more details on the Mutual Recognition Arrangements and the Regional Metrology Organizations, consult the BIPM web page at www.bipm.fr.

## Technical ambiguities due to leap seconds

- Time tags during a leap second:
- 			UTC
- Day N 	  23:59:58
- Day N	  23:59:59
- Day N	  23:59:60
- Day N+1  00:00:00

**Notes:**
The GPS system does not incorporate leap seconds into its time scale beyond the 19 seconds that had been added to UTC when the GPS time scale was defined. Thus GPS time advances with no discontinuity during the leap second.

The legal name for the UTC leap second is 23:59:60, but that name has no correspondence in the time scales of most digital systems, which keep time as the number of seconds since some epoch (1 January 1970, for example). These systems realize the leap second by effectively stopping the system clock for 1 second when they occur. The result is that there are two distinct physical seconds which have the same clock reading.

## Technical ambiguities due to leap seconds

- Time tags during a leap second:
- 			UTC		GPS
- Day N 	  23:59:58	G
- Day N	  23:59:59	G+1s
- Day N	  23:59:60 	G+2s
- Day N+1  00:00:00	G+3s

**Notes:**
The GPS system does not incorporate leap seconds into its time scale beyond the 19 seconds that had been added to UTC when the GPS time scale was defined. Thus GPS time advances with no discontinuity during the leap second.

The legal name for the UTC leap second is 23:59:60, but that name has no correspondence in the time scales of most digital systems, which keep time as the number of seconds since some epoch (1 January 1970, for example). These systems realize the leap second by effectively stopping the system clock for 1 second when they occur. The result is that there are two distinct physical seconds which have the same clock reading.

## Technical ambiguities due to leap seconds

- Time tags during a leap second:
- 			UTC		GPS 		Computers
- Day N 	  23:59:58	G		C
- Day N	  23:59:59	G+1s		C+1s
- Day N	  23:59:60 	G+2s		C+1s
- Day N+1  00:00:00	G+3s		C+2s

**Notes:**
The GPS system does not incorporate leap seconds into its time scale beyond the 19 seconds that had been added to UTC when the GPS time scale was defined. Thus GPS time advances with no discontinuity during the leap second.

The legal name for the UTC leap second is 23:59:60, but that name has no correspondence in the time scales of most digital systems, which keep time as the number of seconds since some epoch (1 January 1970, for example). These systems realize the leap second by effectively stopping the system clock for 1 second when they occur. The result is that there are two distinct physical seconds which have the same clock reading.

## Technical ambiguities due to leap seconds

- Time tags during a leap second:
- 			UTC		GPS 		Computers
- Day N 	  23:59:58	G		C
- Day N	  23:59:59	G+1s		C+1s
- Day N	  23:59:60 	G+2s		C+1s
- Day N+1  00:00:00	G+3s		C+2s
- Time intervals across leap seconds also differ among systems

**Notes:**
The GPS system does not incorporate leap seconds into its time scale beyond the 19 seconds that had been added to UTC when the GPS time scale was defined. Thus GPS time advances with no discontinuity during the leap second.

The legal name for the UTC leap second is 23:59:60, but that name has no correspondence in the time scales of most digital systems, which keep time as the number of seconds since some epoch (1 January 1970, for example). These systems realize the leap second by effectively stopping the system clock for 1 second when they occur. The result is that there are two distinct physical seconds which have the same clock reading.

## Legal complications below 1 s

- US legal time is mean solar time, almost all services transmit coordinated universal time (UTC)
- Difference can be ±0.9 s
- Difference varies slowly ~ 0.1 s per month
- Correction is transmitted by NIST radio and telephone services; also on the web
- Transmitted correction has resolution of 0.1 s

**Notes:**
The legal definition of US time is specified as mean solar time in the US Code section on time zones and daylight saving time. See Title 15 United States Code, Chapter 6,  Sub-chapter IX, section 261.

In addition to the legal definition in terms of mean solar time, there are additional complications introduced by daylight saving time and by the local time zones. These ambiguities can be particularly troublesome during the Fall transition date that marks the end of daylight saving time because the hour from 1 am – 2 am occurs twice on the day, and both of those intervals may be referred to as “local standard time.” 

There are additional complexities in Indiana, where adjacent counties can have different time zones and daylight saving time rules.

## Using GPS to get UTC

- GPS “tick” extracted from code data
- Derived from clock on SV
- Corrections:
- 	Transit time; ionosphere; troposphere;
- 	Receiver delays; output tick offset; multipath delays;	SV clock model to compute GPS time; leap seconds, ...
- Easy to do 1 ms; _**very**_ hard to do 10 ns.

## The last part: GPS to UTC

- GPS – UTC(USNO) from SV message
- GPS – NMI from NMI data
- Common View:
-     (local – GPS) – (NMI – GPS)= local – NMI
- Requires agreements on SV tracking, averaging methods, data exchange
- UTC(lab) – UTC from Circular T
- Traceability may be ambiguous

**Notes:**
From the technical point of view, these methods provide different trade-offs between ease of use and accuracy.

The first method is the easiest to use, but it depends on the accuracy of the extrapolated difference between GPS time and UTC(USNO) as broadcast by the satellites.

The second method effectively eliminates the GPS system by correcting its time signals based on observations made by a National Metrology Institute. However, these corrections are time varying, and are also a function of how the data are acquired. There may be small offsets due to these effects.

The third method is the most complicated but it has the best potential accuracy, since it uses simultaneous observations of the same satellites by the client site and the NMI. In addition, both sites use standard tracking schedules and averaging methods. The BIPM has published standards for data averaging and reporting and all NMIs adhere to them.