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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. TOPICS. ITU-R’s Role in Standard Time and Frequency Signal Services The Role of GPS in Time Scales - PowerPoint PPT Presentation
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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.
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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.
RADIOCOMMUNICATIONSTUDY GROUPS
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)
RADIOCOMMUNICATIONSTUDY GROUPS
INTERNATIONAL TIME LINKSINTERNATIONAL TIME LINKS
RADIOCOMMUNICATIONSTUDY GROUPS
BIPM Time Scale GenerationBIPM Time Scale Generation
RADIOCOMMUNICATIONSTUDY GROUPS
TAI-UT1
TAI-UTC
TAI-GPST
Secon
ds
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
BIPM CIRCULAR T BIPM CIRCULAR T
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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)
RADIOCOMMUNICATIONSTUDY GROUPS
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.
RADIOCOMMUNICATIONSTUDY GROUPS
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.
RADIOCOMMUNICATIONSTUDY GROUPS
IGS TIME SCALESIGS TIME SCALES
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
““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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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.
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
Paging Network
• GPS Simulcast synchronization: ±1 microsecond • Paging message time stamping to absolute GPS time
RADIOCOMMUNICATIONSTUDY GROUPS
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
RADIOCOMMUNICATIONSTUDY GROUPS
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