2. Measurement systems; sampler of applications

Lecture 2Measurement SystemsGEOS 655Tectonic Geodesy

VLBI and SLRVLBI – Very Long BaselineInterferometrySLR – Satellite Laser Ranging

The VLBI CorrelatorLooked like a movie-version computer command center.Today the correlator is done in software, and data can be digital streams

Quasar Radio Emissions Recorded• Quasar = “Quasi-stellarobject”• Each quasar emits a uniqueand time-varying radio signal• Radio emissions from quasarsrecorded– Several telescopes record samequasar at same time– Then re-orient and record adifferent quasar– Repeat• Recordings sent to specialcomputer called a correlator

Signals Filtered, Down-converted

Cross-correlation• Signals cross-correlated (spectral), 3 observables:– Phase delay (ambiguous but very precise)– Group delay (unambiguous)– Phase delay rate of change (unambiguous)

Very Long Baseline InterferometryVLBIGeometric Delayand rateδgS B )BaselineEarth surfaceδg B.S≡ τ ( t)= + Δτ( tc

Observation Equationδg B.S≡ τ ( t)= + Δτ( t)c• Measurements to different quasars meandifferent direction vectors S.– Measure different components of B• Tau represents other path delays

Very Long Baseline InterferometryVLBIGeometric Delayand rateδgS B )BaselineEarth surfaceδg B.S≡ τ ( t)= + Δτ( tc

A Sample VLBI Baseline• Plot shows evolution ofbaseline length– Westford (Massachusetts)– Wettzell (southern Germany)• Note improvement inscatter over time• Baseline length is invariantto rotations of network =most precise measure

VLBI Site Velocities Circa 1995• No sites in formerSoviet Union, most ofAfrica• Poor southernhemisphere coverage• But sufficient to provethat plate motions over1980-1990 agreedwith NUVEL-1 modelToday there are a few more sites, but not many more – very expensive

VLBI in Alaska!• NASA developedsome Mobile VLBIsystems, and evenbrought one to Alaska– Smaller dish (5m)– All fit in one trucktrailer– Shipped across Alaskain connex containers!

LunarSatelliteLaser RangingLLRSLRMeasuring Time PropagationMoonEarth

SLR Principles• Emit laser pulse; start clock• Photon detector stops clock on returnedpulse– Shorter-duration pulse = higher precision• No need for simultaneous observations,high-accuracy timing, just an accurate“stopwatch”• Many pulses sent for each orbit

SLR Principles

SLR Principles• In practice, a week’s data is used todetermine a weekly average position. Untilthe 2000s they used monthly solutions.• SLR satellites have very stable orbits, canestimate a single orbit for >10 years ofobservations• SLR very sensitive to geocenter, large-scalegravity field

SLR Compared to Plate ModelEarly 1990s results

Geodesy and NUVEL-1A• Geodetic plate motion rates in the 1980s wereabout 5-6% slower than the NUVEL-1 platemotion model predicted, but all in the samedirection.• The cause was an error in the magnetic timescale.When the timescale was updated in the early1990s, the anomaly used for NUVEL-1s magneticprofiles was found to be about 6% older.• The plate motion model was recalibrated andgeodesy and geology were found to agree.

An Update on this Theme• Nazca–South Americamotion has sloweddown over the last 15Ma• Geodesy agrees withthe most recentgeologic time (~1million year average)Norabuena et al.

VLBI and SLR• Originally designed for measurement of crustalmotion• Today’s uses:– SLR: determination of geocenter– SLR: changes in earth’s gravity field (J2, etc).– SLR: tracking of some satellites– VLBI: earth rotation rate– VLBI: earth orientation in space– both: contribute to stable definition of reference frame• IAG services provide weekly solutions for ITRF– IVS for VLBI– ILRS for SLR

International Association of GeodesySpace Geodesy Services• International Earth Rotation and Reference SystemsService (IERS) (1988)• Intern. GNSS Service (IGS) (1994)• Intern. Laser Ranging Service (ILRS) (1998)• Intern. VLBI Service (IVS) (1999)• Intern. DORIS Service (IDS) (2003)• And more …• Unifying project: Global Geodetic Observing System(GGOS)http://www.iag-aig.org/

International Association of GeodesyOther Services• International Gravimetric Service• International Gravity Field Service• International Center for Earth Tide• Permanent Service for Mean Sea Level• Time Section of the International Bureau ofWeights and Measures• IAG Bibliographic Service http://www.iag-aig.org/

Global Positioning SystemGPSSatelliteEarthGPS AntennaNavigation Message sent by eachsatellite:- Orbit parameters- Clock correctionsGPS Measurements:- Pseudorange- Phase

IGS Current Network

IGS = International GNSS Service• IGS coordinates operation of global satellitetracking network• IGS provides– Daily and weekly coordinate solutions– Daily and weekly precise orbit products– Estimates of earth orientation/rotation• IGS Central Bureau– http://www.igs.org run by JPL• Several projects (ionosphere estimates, GPS attide gauges, precise time transfer, etc.)

IGS Orbit Quality• Orbit quality improved from 20 cm in 1993to 2-3 cm since 2004, approaching 1-2 cm.

Rapid Orbit Quality (1 day)• Accuracy today < 2-3 cm.

Rapid Orbit Quality (3 hours)• Accuracy today < 5 cm.

The IGS as an Official ServiceFull exploitation of signal• Exceptionally high TEC values observed by IGS on October 29, 2003

The IGS as an Official ServiceFull exploitation of signal• Mean TEC was high, but not extraordinary on Oct 29, 2003.

Getting IGS Data and Products• 8 IGS Analysis Centers– 4 in USA: NGS, JPL/NASA, UC San Diego, MIT• 4 IGS Global Data Centers: Raw data, orbits,clocks, troposphere, ionosphere– UC San Diego (SOPAC, sopac.ucsd.edu)– NASA/CDDIS (cddis.gsfc.nasa.gov)– Institute Geographique National, France(igs.ensg.ign.fr)– Korean Astronomical and Space Institute(gdc.kasi.re.kr)