National Time Standards - PowerPoint PPT Presentation

1 / 12
About This Presentation
Title:

National Time Standards

Description:

Congress fixes Weights and Measures. Constitution, Article I, Section 8 ... General Conference on Weights and Measures (CGPM) -- 'Mise en pratique' ... – PowerPoint PPT presentation

Number of Views:15
Avg rating:3.0/5.0
Slides: 13
Provided by: juda151
Category:

less

Transcript and Presenter's Notes

Title: National Time Standards


1
National Time Standards
  • Judah Levine
  • Time and Frequency Division
  • National Institute of Standards and Technology
  • Boulder, Colorado

2
History of US standards
  • Congress fixes Weights and Measures
  • Constitution, Article I, Section 8
  • Weights and measures to Treasury
  • 14 June 1836 (5 Stat. 133)
  • NBS Organic act 3 March 1901 (31 Stat. 1449)
  • NBS to Dept. of Commerce in 1903

3
The Metric Convention
  • Signed in Paris on 20 May 1875
  • established BIPM in Sevres
  • General Conference on Weights and Measures (CGPM)
    -- Mise en pratique
  • International Committee on Weights and Measures
    (CIPM) -- VP from NIST
  • Consultative Committee for T F
  • working group on atomic time

4
International Atomic Time (TAI)
  • Computed monthly by BIPM using data from a
    world-wide ensemble of 250 commercial frequency
    standards
  • computation is about one month after the fact
  • Additional data from primary frequency standards
    (NIST, PTB, ...) used to maintain long-term
    stability

5
Coordinated Universal Time (UTC)
  • Derived from TAI by addition of leap seconds as
    needed (every 1-1.5 yr)
  • maintains synchronization between atomic time
    scales and rotation rate of the earth
  • next leap second at end of 1998
  • Only available after the fact (since derived from
    TAI)
  • Is not realized in a physical clock

6
US estimates of UTC UTC(NIST) and UTC(USNO)
  • Derived in real-time from local clock ensemble
    using averaging procedure
  • Steered towards UTC using small rate changes
    applied monthly(NIST) or irregularly as needed
    (USNO).
  • Differences are slowly varying and are on the
    order of nanoseconds.
  • not always true outside of US

7
Time Services
  • NIST radio services
  • WWV, WWVH, WWVB
  • Same message on dial-up talking clock
  • ACTS
  • dial-up digital time service
  • Internet services
  • NTP, daytime, time, ...
  • Authenticated service (proposed)

8
Traceability and accuracy
  • All NIST-operated servers are directly traceable
    to UTC(NIST).
  • Accuracy on the order of milliseconds or better
    (limited by jitter in channel delay).
  • Monitored and controlled in real-time from NIST
    facilities
  • Performance of other servers varies
  • NTP servers often use WWVB or GPS

9
Global Positioning System(GPS) Time
  • 24 satellites, generally at least 4 visible at
    all times at any location
  • Each satellite transmits complex message
    including time, position, etc.
  • Performance of each satellite is observed by DoD
    monitor stations
  • satellite messages are updated periodically based
    on tracking information

10
GPS time scale
  • SV time -- based on on-board clock. Estimate of
    offset from system time is part of message.
  • GPS system time -- computed by controllers based
    on GPS observations
  • does not include leap seconds, but message has
    this value
  • steered to (not equal to) UTC(USNO)
  • estimate of offset is in message

11
  • Intentional degradation of the signal
  • Selective availability and anti-spoofing
  • Above effects usually lt 1 ?s and can be ignored,
    but ...
  • Common view using a single satellite
  • (You - satellite) - (Lab - satellite)
  • NIST, USNO and BIPM publish data on time offsets
    of all satellites.
  • Many cheap (hand-held) GPS receivers display the
    wrong time! (seconds off).

12
Typical Traceability Requirement
  • National Association of Securities Dealers (NASD)
  • Order Audit Trail System (OATS)
  • Rule 6953
  • NASD Notice 98-33
  • OATS Reporting Technical Specifications, Chapter
    2
  • Clocks traceable to NIST within 3s
Write a Comment
User Comments (0)
About PowerShow.com