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GPS and World Metrology
BIPM/NMIJ-AIST Workshop/SymposiumMay 17-21, 2005
Dr. Demetrios MatsakisU.S. Naval Observatory
GPS = Global Positioning System
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)
• Global Navigation Satellite Systems (GNSS)– GLONASS– GALILEO
• Compatibility and Interoperability with GPS
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GPS receivers measure distance from satellites A, B, and C by thepseudoranges a, b, and c. Pseudoranges are measured as travel times, and converted to distances.If the receiver is at known position and calibrated, time can be obtained from observing one satellite.If the receiver’s time is known and its timing delays are calibrated, its antenna’s position is at the intersection of spheres centered on the satellites with radii a, b, and c.If the receiver’s time and position are not known, they can be inferred from observations of four satellites - but the time offset must be calibrated.
Time is at the Core of GPS
A
B
C
ab
c
GPtS = Global Positioning and Timing System
Carrier Phase of GPS Signal
• Does not carry direct timing information• But it provides necessary and sufficient
information for many applications• A hundred times more precise
– 700 ps carrier wavelength << chip lengths
IGS: International GNSS Service
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 box of sushi?
Geodesy and GPS
• Earthquakes• Polar Motion• Continental Drift• Length of Day
Schematic Illustration Of The Forces That Perturb The Earth’s Rotation
Source: Thomas Gold, Nature
Sea level
loadingMelting of ice
Atmosphericloading
Groundw
ater
Electromagnetic coupling
Core phrenology
Plate tectonics
OceanCurrents
Viscoustorques
Win
ds
Earth-
quakes
Lunar &
Solar
Gravit
ation
alAttr
actio
n
CORS Array in USA
THE TSUNAMI
Position of North Pole, 1984-2002
Source: http://giub.geod.uni-bonn.de/vlbi/IVS-AC/combi-all/start.html
Variations In Length Of Day (pre-GPS)se
c. o
f tim
e /d a
y 0.003
0.001
0.002
0.00045000 45500 46000 46500 47000 47500 48000
MJD 19901982
The Earth turns faster in the northern winter
Length of Day – modern data
Length of Day
decade fluctuations
Long-term trend
Weather and GPS
IGS formal errors are .6 mm pwv
Ionosphere and GPS
2-hour snapshots, 0-85 TEC http://www.cx.unibe.ch/aiu
Relativity and GPS
• Will not work without relativistic corrections– Tested every day, and every night– Gravitational red shift > Time Dilation >>0
• One-way test of isotropy of speed of light– Michelson-Morely experiment was round-trip– Analysis by Petit and Wolf of the BIPM
Atomic Time and GPS
• International Atomic Time (TAI)– Insensitive to Earth’s rotational slowdown
• Coordinated Universal Time (UTC)– Leapseconds inserted – 1 day = 24 *60*60 International Seconds (SI)
• Days of leap seconds have 86401 SI seconds
• Clocks from >50 participating laboratories• Asia playing a growing role
GPS and UTC(USNO)
GPS COMMON-VIEW
Common-View Limitations
Linking Clocks Around the World
International SynchronizationBefore and After GPS
Real-time Time TransferUTC(USNO)-USNO(AMC), ns, real-time
Two Way Satellite Time and Frequency Transfer (TWSTFT)
USNO Two-Way Satellite Time Transfer Earth Terminals
GPS and TWSTFT Sensitivity
Type Time stability1000 s
Timestability
24 hr
Time Accuracy
24 hr
Frequencyaccuracy
24 hr
GPS Direct Broadcast(GPSDO-single freq.)
5 to 10 ns * 2 ns 3 to 10 ns 4x10-14
GPS Common-View(2500 km baseline)
5 ns 1 ns 1 to 5 ns 2x10-14
GPS Carrier-Phase 20 ps 0.1 ns 1 to 3 ns 2x10-15
TWSTFT <0.1 ns 0.1 to .2 ns 1 ns 2-4x10-15
TWSTFT vs. GPS Time Transfer Noise
• Precision (15 minutes averaging)1. 10 ps -- carrier phase TWSTFT (??, Unproven)2. 20 ps -- carrier phase GPS3. 100 ps -- TWSTFT4. 5 ns - GPS Common View
• Accuracy, limited by systematic errors1. 1 ns -- all TWSTFT methods2. 3 ns -- all GPS methods
TWSTFT vs. GPSAdministrative Costs
• Data analysis1. TWSTFT is simple and real-time2. Carrier phase GPS requires mathematician
• Others will reduce your data for free• Almost real-time (several hours latency)
• Hardware1. GPS hardware less expensive, lower maintenance2. TWSTFT requires electricians and technicians3. TWSTFT must rent satellite time
Prediction: Time Transfer in 2010(date of predictions: 2000)
1 Direct GPS: 1 ns2 Post-processed carrier-phase GPS: 10 ps
accuracy3 Real-time carrier phase GPS almost as good,
using predicted orbit and reference clock data4 Carrier-phase based Two Way Satellite Time
Transfer will be at 1 ps level5 Optical time-transfer technology will be
advancing to operational levels
Prediction: State-of-the-Clock-Art in 2010(date of predictions: 2000)
1 UTC will be computed hourly using:– Masers for hours to weeks– Trapped-ion for days to months– Atomic fountains for days to years
2 Exciting clocks, just becoming operational, will include– Optical Frequency Standards– Space-based trapped-ion and beam clocks
Predictions For End-User, 2010(date of predictions: 2000)
• Trend towards cheaper clocks, with greater reliance on time transfer to GPS, etc.
• With the removal of SA and future upgrades, rubidium clocks steered to GPS may be replaced by crystals steered to GPS.
• Improvements to middle-end clocks may languish,though they will never, ever stop.
Conclusions
• GPS has changed metrology forever• GPS, GALILEO, and their enhancements
such as QZSS will continue to transform metrology
