PNT ADVISORY PANEL MEETING

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• PNT ADVISORY PANEL MEETING
• Standard Time and Frequency Applications
• and
• GPS
• 4 October 2007
• Ron Beard, Chairman
• ITU-R Working Party 7A
• B
• U.S. Naval Research Laboratory
• Washington, D.C.

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TOPICS

• ITU-Rs Role in Standard Time and Frequency
  Signal Services
• The Role of GPS in Time Scales
• Broadcast Time and Frequency Services
• The Future of UTC
• IGS/NRL Clock Products Working Group
• The Role of GPS in Telecommunications
• Telephone Networks
• Cellular Networks
• Internet Timing
• Summary

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ITU-R Study Group 7 Science Services

• Working Party 7A Time Signals and Frequency
  Standard Emissions
• Responsible for Standard Frequency and Time
  Signal (STFS) services, both terrestrial and
  satellite.
• Scope includes the dissemination, reception and
  exchange of STFS services and coordination of
  these services, including satellite techniques,
  on a worldwide basis.
• Goals are to develop and maintain ITU-R
  Recommendations in the TF Series and Handbooks
  relevant to SFTS activities, covering the
  fundamentals of the SFTS generation, measurements
  and data processing. These ITU-R Recommendations
  are of paramount importance to telecommunication
  administrations and industry, to which they are
  first directed. They also have important
  consequences for other fields, such as radio
  navigation, electric power generation, space
  technology, scientific and metrological
  activities and cover the following topics
• Terrestrial SFTS transmissions, including HF,
  VHF, UHF broadcasts television broadcasts
  microwave link coaxial and optical cables
• Space-based SFTS transmissions, including
  navigation satellites communication satellites
  meteorological satellites
• Time and frequency technology, including
  frequency standards and clocks measurement
  systems performance characterization time
  scales time codes

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Coordinated Universal Time (UTC)

• Defined by ITU-R Recommendation TF 460-6
• A Stepped Atomic Time Scale Generated and
  Maintained by the BIPM
• Supported by the IERS in determination of (UTC
  UT1)
• Incorporated by Reference into the Radio
  Regulations
• Originated as a Common Reference for
  Coordinating time signals
• Compromise between Continuous Atomic Time and
  Solar Mean Time (Universal Time)
• Universal Time provides Solar Rotation Angle from
  Prime Meridian to Local Meridian needed for
  celestial navigation

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DEFINITION OF UTC
ITU-R TF.460-6 STANDARD-FREQUENCY AND TIME-SIGNAL
EMISSIONS (1970-1974-1978-1982-1986-1997-2002) To
maintain worldwide coordination of standard
frequency and time signals Disseminate standard
frequency and time signals in conformity with the
SI second Continuing need for UT immediate
availability to an uncertainty of 0.1 second TAI
- International reference timescale of atomic
time based on SI second as realized on a rotating
geoid. Continuous scale from origin 1 Jan
1958 TAI UT2 on January 1, 1958 0 h TT
TAI 32.184 s UTC - Basis of coordinated
dissemination of standard frequency and time
signals. Maintained by the BIPM. Corresponds
exactly in rate with TAI but differs by integral
number of seconds, adjusted by insertion or
deletion of seconds to ensure agreement within
0.9 s of UT1. TAI UTC 33 s DUT1 -
Dissemination to include predicted difference UT1
UTC (values given by IERS in integral multiples
of 0.1 s)
Leaps Seconds may be introduced as the last
second of any UTC month. December and June
Preferred, March and September second choice.
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FUTURE OF THE UTC TIMESCALE

• Question ITU-R 236/7 (2001)
• 1. What are the requirements for
  globally-accepted time scales for use both in
  navigation and telecommunications systems, and
  for civil time-keeping?
• 2. What are the present and future requirements
  for the tolerance limit between UTC and UT1?
• 3. Does the current leap second procedure satisfy
  user needs, or should an alternative procedure be
  developed?
• Proposed Modifications to UTC Definition
• 1. Change tolerance of UTC UT1 to one Hour
  (500 years to accumulate)
• 2. Eliminate Leap Second
• 3. Create New Time Scale (Use of TAI not
  recommended by BIPM)

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INTERNATIONAL TIME LINKS
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BIPM Time Scale Generation
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TAI-UT1
TAI-GPST
Seconds
TAI-UTC
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SYSTEM TIME KEEPING NEEDS

• Traditional timekeeping is a post processed value
• TAI and UTC are post processed time scales
  delayed from 30 to 60 days.
• Electronic systems are filled with oscillators
  and clocks generating time and frequency data
  that must be correlated across systems and
  nations in Real-Time
• Reference Time needs to be continuous and
  available on demand (Real-Time)
• More and More systems are adopting their own
  system time. e.g., GPS TIME
• The increasing number of systems could
  potentially result in a multiplicity of system
  time scales
• UTC should be the single common Reference Time

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GPS TIME and UTC (USNO)
UTC(USNO) is generated by USNO and participates
as a contributor to BIPM/UTC. GPS users assume
UTC(USNO) is the global reference but many use
GPS Time directly The uncertainty with respect
to UTC is disregarded or not-significant for most
users GPS Time (GPST) is the system internal
continuous timescale Primarily used for
positioning and navigation Secondarily used for
disseminating time GPST offset and uncertainty
with respect to UTC
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BIPM CIRCULAR T
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IGS/BIPM Pilot Project (transitioned to Working
Group in 2003)

• GOAL Develop strategies to exploit geodetic
  techniques for improved global time/frequency
  comparisons
• Began March 1998 w/ participation of gt 35 groups
• IGS contributions
• global dual-frequency tracking network
• standards for operating geodetic stations
• efficient data delivery system
• state-of-the-art analysis groups/methods/products
  
• BIPM contributions
• high-accuracy metrological standards/measurements
• timing calibration methods
• timescale algorithms independent comparisons
• formation dissemination of UTC

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IGS CONTRIBUTING TIMING CENTERS
GPS space clocks
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IGS (NRL) Time Scales

• Two Time Scales Produced (Loosely steered to GPS
  Time)
• Rapid (IGRT)
• Final (IGST)
• Stability better than 2?10-15 /day, GPST
  stability of 2?10-14 /day
• Kalman filter implementation
• Formulated as a frequency ensemble
• Deterministic models for rates drifts
• Process noise capabilities White FM, Random Walk
  FM, Random Run FM
• Inputs from 54 H-maser, 32 Cs, 27 Rb clocks,
  25 stations at timing labs
• Dynamic weighting of clocks
• Robust outlier detection
• Modular software design
• Can support IGS move to real-time operations
• Implemented for Final Rapid clock products
• Loosely aligned to GPS Time via an LQG steering
  algorithm

Became Routinely Available in 2004.
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IGS Clock Products
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IGS TIME SCALES
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Future Timing Needs/Directions for IGS

• Upcoming timescale improvements
  (2007/08)Improved satellite clock modeling
  (prediction) Improved dissemination of UTC
• Absolute calibration techniques/capabilities at
  the sub-nanosecond level (particularly for
  antennas)
• Conventions for handling or measuring
  inter-modulation biases very relevant given
  other upcoming GNSS (e.g., Galileo)
• Intra-system biases C1-P1, P1-P2 (DCBs), F1-F2,
  etc.
• Inter-system biases GPS-Galileo, etc.
• IGS currently assumes
• Broadcast values (TGD) are tied absolutely via
  small tracking network (JPL) of calibrated AOA
  Rogue receivers

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TIMING in TELECOM PNT

• Telecommunications
• Syntonization of Data Streams and Communication
  Channels
• Understood as Frequency/Bit Rate/Clocking
• Position Navigation and Timing (PNT)
• Synchronization of Signal Generators and
  Timekeeping Systems
• Understood as Phase or Phase Offset in
  Timekeeping or Time Metrology
• Time of Day (TOD) in Telecom Terms

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TELECOM STRATUM HIERARCHY
Cesium Ensemble or GPS Receiver Rb
GPS/Rb Oscillator
Rb Oscillator Crystal Oscillator
Crystal Oscillator
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Public Switched Telephone Network (PSTN)

• Backbone for interconnection between the Networks
• Digital Implementation
• Optical Fiber Network SONET/SDH
• Distributed Architecture of PRS rather than
  centralized PRS of an Ensemble of Cesium
  Standards
• PRS now consists of tow Rubidium secondary
  standards steered to GPS Time.

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Wireless Network Architecture
PSTN PUBLIC SWITCHED TELEPHONE NETWORK
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Wireless Cell Site Air Interface frequency
tolerances

• D-AMPS (IS-136 TDMA) 0.5 parts per million
• GSM 0.05 parts per million
• CDMA 0.05 part per million

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CDMA Digital Wireless Frequency Timing
requirements

• CDMA
• 0.05 ppm
• Precise time reference required as well as
  frequency
• GSM and TDMA do not require time reference
• IS-95 (Section 7.1.5.2)
• Base stations transmit their pilot sequence
  within 3
• To meet base station requires GPS and atomic or
  high quality quartz local clock.
• Specification is 7 over a 24 hour period.

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Paging Network

• GPS Simulcast synchronization 1 microsecond
• Paging message time stamping to absolute GPS
  time

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Synchronization Timing in Wireless Networks

• GPS BITS required in MSC BSC due to SONET
  rings multiple carrier connectivity
• Cell site frequency stability associated reuse
  enhanced by GPS timing advanced clock
  technologies
• CDMA requires GPS synchronization
• Third generation (3G) wireless will most likely
  use GPS advanced clock technologies
• GPS in mobiles will be one of the location
  technologies
• Multiple service providers advanced transport
  like SONET, ATM voice over IP create sync
  islands solved only by GPS everywhere

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SUMMARY

• GPS has become the primary method of providing
  and coordinating Time and Frequency Services
  Worldwide
• The use in Telecommunications is extensive, both
  civilian and military
• PSTN Public Switched Telephone Network
• Wireless Mobile, Paging Services
• Internet, NTP Time Servers, Banking, Financial
  Transfers
• Sensor Networks (Geophysical and Remote Sensing)
• Power Generation and Distribution
• The extent of utilization is difficult to
  determine due to the ready availability of
  off-the-shelf equipment
• Increased capability provided by GPS is being
  exploited
• The maintenance of timing within GPS itself is of
  secondary priority
• GPS Availability and Capability has impacted the
  Time and Frequency Industrial Base