USNO and GPS Time

Workshop on Time and Frequency Services with GalileoDecember 5, 2005

Dr. Demetrios MatsakisU.S. Naval Observatory

A Legal Note

As related to policy, any ideas or opinions expressed in this presentation should be considered to be those of the speaker, and not those of the U.S. Naval Observatory, the U.S. Department of Defense, or the government of the United States of America.

GPS = Global Positioning System

GPS - an Idea That Grew

• Conceived as a military system• Planned for civilian use after KAL007 disaster• Selective Availability zeroed in 2000• Carrier Phase Technology• Second and third frequencies for civilians• More and better satellites

An Idea that Reproduced

• GPS Enhancements– Wide Area Augmentation System (WAAS)– European Geostationary Navigation Overlay

Service (EGNOS) – Quasi Zenith Satellite System (QZSS)– Indian and Chinese enhancements planned

• Global Navigation Satellite Systems (GNSS)– GLONASS– GALILEO

• Compatibility and Interoperability with GPS

GPtS = Global Positioning and Timing System

Most Error Sources are Related to Time

• Multi-path • Ionosphere delay• Troposphere delay• Equipment delay

– Temperature effects– Humidity effects

• Satellite clock errors• Antenna position errors• Satellite orbit errors

Every Error Source can be a Measurement Opportunity

•Position Measurements•Geodesy Measurements•Ionosphere Measurements•Weather measurements•Frequency Measurements•Time Measurements

Position Measurements• Every modern surveyor uses a GPS receiver

– WGS84/ICTRF coordinates– Translates coordinates to national standards

• Someday there will be GPS/GNSS systems in– Every car– Every cell phone– Every child’s clothing– Every carton of Löwenbräu

Why Safety of Life Needs Robustness

Benefits of Combining GNSS Systems• Extra precision

– Outlier detection– Model-fitting

• Better models help identify outliers

• Extra robustness– Urban canyons, Building Interiors

• Multipath• Signal loss

– Interference of all types• Unintentional• Unavoidable• Intentional

GPS Time• Kalman Filter Inputs

– GPS satellite clocks– GPS ground station clocks

• Was 5 sites, now 18– At very end: UTC(USNO)-GPS Time from day before

• Kalman Filter Outputs– GPS Time– Satellite Orbits– (for broadcast) UTC(USNO)-GPS Time

• Computed every 15 minutes– But satellites are uploaded about once a day– So some satellites broadcast day-old information about

their clocks and orbits

GPS Kalman Filter• Estimates clocks, satellite orbits, nuisance parameters• Does not yet estimate troposphere

– Variations map strongly into site clock estimate• No Master Clock

– Composite Clock– Every clock, after corrections, realizes GPS time

• No external measure of truth in filter itself – USNO time input comes at very end, through steering

• Reference: Brown, ION-GPS, 1991

Unsteered GPS Composite Clock

Simulated bydeconvolving real data

50600 50800 51000 51200 51400 51600 51800

-15

-10

-5

0

MJD (days)

Tim

e D

iffer

ence

(μs)

The solution: UTC(USNO)• GPS Composite clock steered to UTC(USNO)• Daily uploads GPS-UTC(USNO)• GPS steered in acceleration, not frequency

– Add +10-19 /s or –10-19 /s to acceleration of every clock– Bang-bang steering– Minimizes frequency changes between daily uploads– GPS-III may have cross-links

• Single upload to change all satellite clock messages

• Picture and details on next two viewgraphs

Bang-Bang Examples: 3 Cases

UTC(USNO) and GPS

USNO Clock Ensemble• Frequency Standards

– 73 High-Performance Cesiums– 20 Cavity-Tuned Masers

• Distribution– 19 environmental chambers– three buildings – Washington, DC and Colorado Springs, Co

• Improved Standards– Rubidium Fountain, prototype to be operational by 2006– JPL Trapped-Ion Mercury Standard

USNO’s Main Clock Vault

USNO Atomic Fountain

Cold atom collection and launch

Microwave cavity

Free flight region

Optical detection

Goal: Six Rubidium Fountains by 2008

USNO’s GPS Antenna Array

Two Way Satellite Time and Frequency Transfer (TWSTFT)

USNO Two-Way Satellite Time Transfer Earth Terminals

Construction Beginning This Month

Conclusions

• GPS Time should be as stable as possible• GPS Kalman Filter is an excellent algorithm• USNO Master Clock is a key part of that

algorithm