USNO and GPS Time - DLR Portal · USNO and GPS Time Workshop on Time and Frequency Services with...
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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 - 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
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
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
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 Atomic Fountain
Cold atom collection and launch
Microwave cavity
Free flight region
Optical detection
Goal: Six Rubidium Fountains by 2008