# Presentation: 484289

## 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.

**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**
**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**

## 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

## 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

## DEFINITION OF UTC 

**(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)

- DEFINITION OF UTC

**Leaps Seconds may be introduced as the last second of any UTC month.**
**December and June Preferred, March and September second choice.**

## 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)

## INTERNATIONAL TIME LINKS

## BIPM Time Scale Generation

## TAI-UT1

**TAI-UT1**

**TAI-UTC**

**TAI-GPST**

**Seconds**

## 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

## GPS TIME and UTC (USNO)

- 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

**GPS TIME and UTC (USNO)**

## BIPM CIRCULAR T 

## 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 > 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

## IGS CONTRIBUTING TIMING CENTERS

| IGS 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 (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.  *

## IGS Clock Products

|  |  |  |  |  |
| --- | --- | --- | --- | --- |
| GPS SATELLITE & STATION CLOCKS | ACCURACY | LATENCY | UPDATES | SAMPLE INTERVAL |
| Broadcast |  ~7ns |   real time | -- | daily |
| Ultra-Rapid (observed) | ~0.2 ns | real 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. |  |  |  |  |

## IGS TIME SCALES

## 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), *Φ**1**-**Φ**2*, etc.
- Inter-system biases: GPS-Galileo, etc.
- IGS currently assumes
- Broadcast values (*T*GD) are tied absolutely via small tracking network (JPL) of calibrated AOA Rogue receivers

## “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**

## TELECOM STRATUM HIERARCHY

**Cesium Ensemble**
**or**
**GPS Receiver + Rb**

**GPS/Rb Oscillator**

**Rb Oscillator**
**Crystal Oscillator**

**Crystal Oscillator**

## 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.

## Wireless Network Architecture

**Wireless Network Architecture**

**PSTN = PUBLIC SWITCHED TELEPHONE NETWORK**

## 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

##    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 Wireless**
**Frequency & Timing requirements**

## Paging Network

**Paging Network**

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

## 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

- 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**

## 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