RADIOCOMMUNICATIONSTUDY GROUPS

PNT ADVISORY PANEL MEETING

Standard Time and Frequency Standard Time and Frequency ApplicationsApplications

andand

GPSGPS

4 October 2007

Ron Beard, Chairman ITU-R Working Party 7A

B

U.S. Naval Research LaboratoryWashington, D.C.

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TOPICSTOPICS

ITU-R’s Role in Standard Time and Frequency Signal Services

The Role of GPS in Time ScalesBroadcast Time and Frequency ServicesThe Future of UTC

IGS/NRL Clock Products Working Group

The Role of GPS in TelecommunicationsTelephone NetworksCellular NetworksInternet Timing

Summary

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ITU-R Study Group 7 Science ServicesITU-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)Coordinated Universal Time (UTC)

Defined by ITU-R Recommendation TF 460-6A Stepped Atomic Time Scale Generated and Maintained by the BIPMSupported 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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ITU-R TF.460-6 STANDARD-FREQUENCY AND TIME-SIGNAL ITU-R TF.460-6 STANDARD-FREQUENCY AND TIME-SIGNAL EMISSIONSEMISSIONS

(1970-1974-1978-1982-1986-1997-2002)(1970-1974-1978-1982-1986-1997-2002)

To maintain worldwide coordination of standard frequency and time signalsDisseminate standard frequency and time signals in conformity with the SI secondContinuing 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 hTT = 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)

DEFINITION OF UTCDEFINITION OF UTC

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 TIMESCALEFUTURE OF THE UTC TIMESCALEQuestion 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 LINKSINTERNATIONAL TIME LINKS

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BIPM Time Scale GenerationBIPM Time Scale Generation

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TAI-UT1

TAI-UTC

TAI-GPST

Secon

ds

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SYSTEM TIME KEEPING NEEDSSYSTEM TIME KEEPING NEEDS

Traditional timekeeping is a post processed valueTAI 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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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 navigationSecondarily used for disseminating time

GPST offset and uncertainty with respect to UTC

GPS TIME and UTC (USNO)GPS TIME and UTC (USNO)

14 s ( )UTC GPST GPST

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BIPM CIRCULAR T BIPM CIRCULAR T

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IGS/BIPM Pilot Project IGS/BIPM Pilot Project (transitioned to Working Group in 2003)(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 > 35 groups

IGS contributions:global dual-frequency tracking networkstandards for operating geodetic stationsefficient data delivery systemstate-of-the-art analysis groups/methods/products

BIPM contributions:high-accuracy metrological standards/measurementstiming calibration methodstimescale algorithms & independent comparisonsformation & dissemination of UTC

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IGS CONTRIBUTING TIMING CENTERSIGS CONTRIBUTING TIMING CENTERSIGS Site Time Lab Freq. Std. Location

AMC2 AMC H-Maser Colorado Springs, CO USA

BOR1 AOS Cesium Borowiec, Poland

BRUS ORB H-Maser Brussels, Belgium

IENG IEN Cesium Torino, Italy

KGN0 CRL Cesium Koganei, Japan

MDVJ VNIIM H-Maser Mendeleevo, Russia

MIZU NAO Cesium Mizusawa, Japan

NISU NIST H-Maser Boulder, CO USA

NPLD NPL H-Maser Teddington, UK

NRC1 NRC H-Maser Ottawa, Canada

NRC2 NRC H-Maser Ottawa, Canada

OBE2 DLR Rubidium Oberpfaffenhofen, Germany

OPMT OP H-Maser Paris, France

PENC SGO Rubidium Penc, Hungary

PTBB PTB H-Maser Braunschweig, Germany

SFER ROA Cesium San Fernando, Spain

SPT0 SP Cesium Boras, Sweden

SYDN NMI Cesium Sydney, Australia

TLSE CNES Cesium Toulouse, France

TWTF TL Cesium Taoyuan, Taiwan

USNO USNO H-Maser Washington, DC USA

USN3 USNO H-Maser Washington, DC USA

WAB2 CH H-Maser Bern, Switzerland

WTZA IFAG H-Maser Wettzell, Germany

WTZR IFAG H-Maser Wettzell, Germany

+ GPS space clocks …

IGS High Performance Clocks

rubidiums (27)

masers (54)cesiums (32)

time lab stations (25)

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IGS (NRL) Time ScalesIGS (NRL) Time Scales

Two Time Scales Produced (Loosely steered to GPS Time)Rapid (IGRT)Final (IGST)Stability better than 210-15 /day, GPST stability of ~ 210-14 /day

Kalman filter implementation Formulated as a frequency ensembleDeterministic models for rates & driftsProcess noise capabilities: White FM, Random Walk FM, Random Run FMInputs from ~ 54 H-maser, 32 Cs, & 27 Rb clocks, ~25 stations at timing

labs

Dynamic weighting of clocksRobust outlier detectionModular software designCan support IGS move to real-time operationsImplemented for Final & Rapid clock productsLoosely aligned to GPS Time via an LQG steering algorithm

Became Routinely Available in 2004. Became Routinely Available in 2004.

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IGS Clock ProductsIGS Clock Products

GPS SATELLITE & STATION CLOCKS

ACCURACY LATENCY UPDATES SAMPLE INTERVAL

Broadcast ~7ns real time -- daily

Ultra-Rapid (observed) ~0.2 nsreal time 4 times daily 15 min

Ultra-Rapid (predicted) ~5 ns

Rapid 0.1 ns 17 hours daily 5 min

Final 0.1 ns ~13 days weekly 5 min

NOTE: IGS accuracy limit based on comparisons with satellite laser ranging results.

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IGS TIME SCALESIGS TIME SCALES

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Future Timing Needs/Directions for IGS 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), Φ1-Φ2, 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 i

i

DCB 0

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““TIMING” in TELECOM & PNTTIMING” in TELECOM & PNT

Telecommunications “Syntonization” of Data Streams and Communication ChannelsUnderstood as Frequency/Bit Rate/Clocking

Position Navigation and Timing (PNT)“Synchronization” of Signal Generators and Timekeeping

SystemsUnderstood as Phase or Phase Offset in Timekeeping or Time

MetrologyTime of Day (TOD) in Telecom Terms

Bit

Bits/s

Read Message Bits

Bits/s

Time Difference

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TELECOM STELECOM STRATUMTRATUM H HIERARCHYIERARCHY

PRIMARY REFERENCE STANDARD

DIGITAL SWITCHING CLOCK

DSC

CHANNEL BANK END USER MUX

STRATUM 1

STRATUM 2

STRATUM 3

STRATUM 4

SYNCHRONOUS NETWORK

FREE RUNNING ACCURACY

DIGITAL SWITCHING CLOCKDIGITAL SWITCHING CLOCK

111 10

81.6 10

64.6 10

632 10

Cesium EnsembleorGPS Receiver + Rb

GPS/Rb Oscillator

Rb OscillatorCrystal Oscillator

Crystal Oscillator

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Public Switched Telephone Network Public Switched Telephone Network (PSTN)(PSTN)

Backbone for interconnection between the NetworksDigital 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

PSTN

BASESTATION

CONTROLLER

BASESTATION

CONTROLLER

BASESTATION

CONTROLLER

BASESTATION

CONTROLLER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

BASESTATION

TRANSCEIVER

MOBILESWITCHING

CENTER

MOBILESWITCHING

CENTER

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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• 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.

CDMA Digital WirelessFrequency & Timing requirements

s

s

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Paging Network

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

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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

Synchronization & Timing in Wireless Networks

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SUMMARYSUMMARY

GPS has become the primary method of providing and coordinating Time and Frequency Services Worldwide

The use in Telecommunications is extensive, both civilian and militaryPSTN Public Switched Telephone NetworkWireless Mobile, Paging Services Internet, NTP Time Servers, Banking, Financial TransfersSensor 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 exploitedThe maintenance of timing within GPS itself is of secondary priorityGPS Availability and Capability has impacted the Time and Frequency

Industrial Base