1 High Precision Applications of Global Navigation Satellite Systems Jake Griffiths IGS Analysis Coordinator NOAA/NGS Brief introduction to GNSS About

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High Precision Applications of Global Navigation Satellite Systems

Jake GriffithsIGS Analysis Coordinator

NOAA/NGS

• Brief introduction to GNSS• About the International GNSS Service (IGS)• IGS core products

– what, when and how?– current quality state and limiting errors

• Plans for 2nd reprocessing and next reference frame• Ongoing challenges

2013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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Main Global Navigation Satellite Systems• U.S. – Global Positioning System (GPS)

– currently 32 active satellite vehicles (30 healthy) in orbit• latest launch (GPS IIF) successful, under on-orbit testing

• Russia – Globalnaya Navigatsionnaya Sputnikovaya Sistem (GLONASS)– currently 29 active vehicles (24 healthy) in orbit

• 4 spares• 1 in test mode

• Europe – Galileo– to be inter-operable with GPS and GLONASS– currently 4 active vehicles in orbit

• initial operating capability (IOC; 18 satellites) expected by ~2015• final operating capability (FOC; 30 satellites) expected by ~2020

• China – Beidou– currently 15 active vehicles in orbit

• regional satellite system—5 geost. Earth orbit (GEO), 5 incl. geosync. orbit (IGSO)• plus global satellite system—30 medium Earth orbit (MEO)


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How a GNSS Works• Satellites in MEO

– vehicle altitudes ~20,000 km

• Transmit L-band radio signals (e.g., L1,L2,L5)– GPS: carrier waves modulated

by C/A and P codes; other GNSS are similar

• Ground antenna+receiver pairs track transmit signals– geodetic grade equip collects

raw observations for precise positioning, navigation and timing applications

• Service supporting high- precision GNSS apps?– International GNSS Service

(IGS)


animation source: wikipedia.orgSource: unavco.org

Source: boeing.comGPS IIF

Source: unavco.org

GODE

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What is the IGS?• An International Association of Geodesy (IAG) Technique Service• Voluntary federation of >200 worldwide agencies aimed at providing

the highest quality GNSS data and products in support of:– Earth science research and education– other high-precision applications

• Organization:– Governing Board (Chair, U. Hugentobler)– Central Bureau (sponsored by NASA, managed by JPL)– Tracking Network (Coordinator, R. Khachikyan)– Data Centers (Chair, C. Noll)– Infrastructure Committee (Chair, I. Romero)– Analysis Centers (ACs) & Analysis Center Coordinator (ACC)– Working Groups, Pilot Projects, Product Coordinators– Associate Members & representatives from other IAG Services

• Other IAG Technique Services?– ILRS (SLR), IVS (VLBI) and IDS (DORIS)


(more details at igs.org)

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IGS GNSS Tracking Network


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Series ID Latency Issue times(UTC)

Data spans(UTC) Remarks

Ultra-Rapid(predicted half)

IGU real-time@ 03:00, 09:00, 15:00, 21:00

+24 hr @00:00, 06:00,12:00, 18:00

● for real-time apps● GPS & GLONASS● issued with prior IGA

Ultra-Rapid(observed half)

IGA 3 - 9 hr@ 03:00, 09:00,15:00, 21:00

-24 hr @00:00, 06:00,12:00, 18:00

● for near real-time apps● GPS & GLONASS● issued with following IGU

Rapid IGR 17 - 41 hr @17:00 daily

±12 hr @12:00

● for near-definitive, rapid apps● GPS only

Final IGS 12 - 19 d weekly eachThursday

±12 hr @12:00 for7 d

● for definitive apps● GPS & GLONASS

IGS Core Product Series


orbits, clocks, polar motion & LOD (ERPs), and station positions (Finals only)

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Outline for How IGS Core Products are Derived


Analysis Center (AC) Products

-Latest IERS and IGS conventions generally adopted-Adjust all obs model parameters

- Ultra-rapid and Rapid tightly constrained to a priori datum

- Finals uses no-net-rotation (NNR) constraint over a priori coordinates of core set of RF stations

-Finals realizes AC daily quasi-instantaneous “fiducial-free” frame w.r.t. a priori datum

IGS AC Coordinator (NOAA/NGS)

J. Griffiths and K. Choi

- weighted average of AC products- Rapid and Final clocks are aligned to IGS timescale

IGS RF WG Chair (IGN)B. Garayt, A. Duret and P.

Rebischung

a priori datum (IGS08/IGb08)

satellite orbits & clocks (SP3), receiver clocks (CLK), tropo

delays (TRO), and polar motion & LOD (ERP) + daily

station positions (SNX)

Combined Orbits, Clocks, and ERPs (Rapid & Ultra-rapid only)

AC SINEXrotations

DORIS

SLR

VLBIInternational Terrestrial Reference Frame (ITRF)

AC SNX files(Finals only)

AC SP3, CLK & ERP files

Combined daily station positions and ERPs, stacked for long-term estimates and RF maintenance

Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS).

IGS TRFprods

Final (IGS) orbits (GPS, GLO), clocks (SV, Rx), ERPs, and

TRF prods

Rapid (IGR) orbits (GPS), clocks (SV,

Rx), ERPs

Ultra-rapid (IGU) orbits (GPS, GLO), clocks (SV), ERPs

IGS Core Products

Main analysis difference between IGU/IGR & IGS is constraints on a

priori RF station positions at AC level

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Current Analysis Centers


Center Name Final(IGS)

Rapid(IGR)

Ultra(IGU)

cod Centre for Orbit Determination in Europe, Bern, Switzerland

emr Natural Resources Canada (NRCan), Ottawa, Canada

esa European Space Agency, European Space Operations Center (ESOC), Darmstadt, Germany

gfz GeoForschungsZentrum, Potsdam, Germany

gop Geodetic Observatory Pecny, Czech Republic

grg CNES Groupe de Recherche de Geodesie Spatiale (GRGS), Toulouse, France

jpl Jet Propulsion Laboratory, Pasadena, USA

ngs National Oceanic and Atmospheric Administration (NOAA), Silver Spring, USA

sio Scripps Institution of Oceanography, La Jolla, USA

mit Massachusetts Institute of Technology, Boston, USA

usn U.S. Naval Observatory, Washington, D.C., USA

whu Wuhan University, Wuhan, China

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• >3.6 million file downloads per month• 5 biggest users of CDDIS/IGS files:

– U.S. 64.3%, Indonesia 19.3%, Canada 1.64%, Sweden 1.57%, Belgium 1.16%

• Details 1/2012 thru 6/2012 …

Product GNSS Total Hits SP3(%)

ERP(%)

CLK(%)

SNX(%)

SUM(%)

Ultra-rapid GPS 11,711,506( 4 * 2,927,877 daily)

93.7 3.1 3.2

Final (IGS) GPS 1,359,656 60.7 6.8 24.8 5.8 2.0

Rapid GPS 887,986 65.6 8.7 16.9 6.4

Final (IGL) GLO 225,515 99.1 0.3 0.6

Ultra-rapid(IGV)

GPS & GLO 223,562 95.0 5.0

Popularity of Core Products- download statistics @ NASA/CDDIS (06/2010 thru 06/2012) -

Courtesy: C. Noll (NASA/CDDIS)


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Core Product AccuraciesSeries Product Types Accuracies Output Intervals

Ultra-Rapid(predicted half)

● GPS orbits ~ 5 cm (1D) 15 min● GLONASS orbits ~10 cm (1D) 15 min● GPS SV clocks ~3 ns RMS / ~1.5 ns Sdev 15 min● ERPs: PM + dLOD ~250 µas / ~50 µs 6 hr

Ultra-Rapid(observed half)

● GPS orbits ~ 3 cm (1D) 15 min● GLONASS orbits ~5 cm (1D) 15 min● GPS SV clocks ~150 ps RMS / ~50 ps Sdev 15 min● ERPs: PM + dLOD <50 µas / ~10 µs 6 hr

Rapid ● GPS orbits ~2.5 cm (1D) 15 min● GPS SV & station clocks ~75 ps RMS / ~25 ps Sdev 5 min● ERPs: PM + dLOD <40 µas / ~10 µs daily

Final

● GPS orbits <2.5 cm (1D) 15 min● GLONASS orbits <5 cm (1D) 15 min● GPS SV & station clocks ~75 ps RMS / ~20 ps SDev 30 s (SVs) + 5 min● ERPs: PM + dLOD <30 µas / ~10 µs daily● Terrestrial frames ~2.5 mm N&E / ~6 mm U daily


5 cm (1D) orbit error = ~0.4 cm (3D) position error over 1000 km baseline (Beser & Parkinson, 1982)

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• Harmonic errors– Griffiths and Ray (2012, GPS Solut.) showed that defects in IERS sub-daily EOP tidal

model are major error source• probably main source of pervasive harmonic signals in all products

• In addition, at 2012 IGS Workshop J. Ray et al. showed that:– systematic rotations are another leading error

• they effect all core products (maybe clocks too??)

– over ~annual scales, Final products appear rotationally less stable than Rapids• appears to affect IGS polar motion• also seems to affect X- & Y- rotational stability of IGS orbit and PPP results

– and suggested:• may be due to inadequate intra-AC self-consistency in Finals

– situation could improve (inadvertently) in switch to daily SINEX integrations• but quasi-rigorous combination method should be re-examined• because further study of long-term dynamical stability of IGS products would be limited

till these issues are resolved

Limiting Errors in IGS Products

More at acc.igs.org/orbits/igs12-rot-errs.pdf


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Limiting Errors in IGS Products



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% o

f GPS

Sta

tions

Frequency (cycles per year)

dE

dN

dU

(figure from X. Collilieux et al., 2011)

Harmonic Errors: Background (1/2)• GPS-sun geometry repeat period

– “draconitic” year = 351.2 d – 1st & 2nd harmonics overlay

seasonal signals

• IGS station coordinates (2006, 2008)– in all dNEU components– up to at least 6th harmonic

• later found in all parameters:– “geocenter” variations– polar motion rates (esp 5th & 7th)– LOD (esp 6th)– orbit discontinuities (esp 3rd)

• strong fortnightly signals alsocommon


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Harmonic Errors: Background (2/2)• 1) local multipath effect at stations

– station-satellite geometry repeats every sidereal day, approximately– 2 GPS orbital periods during 1 Earth inertial revolution

• actual GPS repeat period = (1 solar day - ~245 s)• sidereal period (K1) = (1 solar day - 235.9 s)

– for 24-hr sampling (e.g., data analysis), alias period → GPS draconitic year

• 2) mismodeling effect in satellite orbits– empirical solar radiation parameters intrinsically linked to orbital period– but no precise mechanism proposed yet

• subsequent slides examine the impact of errors in a priori IERS model for sub-daily tidal EOP variations on GPS orbits– EOP tide errors at ~12 hr couple directly into GPS orbit parameters– EOP tide errors at ~24 hr may couple into other estimates– sub-daily EOP total magnitudes are ~1 mas = 13 cm shift @ GPS altitude– IERS model is known to have visible errors, which could reach the 10 to 20% level


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Harmonic Errors: Sub-daily Alias and Draconitic (1/3)

• Simulated impact ofsub-daily EOP tidalerrors on IGS orbits– generated “fake”

model by changingadmittances by up to20%—assumed errorsderived fromcomparing IERS modelto test model from R.Ray (NASA/GSFC)

– process ~3 years ofGPS orbits with IERS& “fake” models• difference conventional & EOP-test orbits @ 15 min intervals• compute spectra of differences for each SV, stack & smooth• compare spectral differences: input model errors vs. orbital response

Frequency (cycles per day)

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long-period errors absorbedmostly by ERPs, not orbits

short- period errors go into orbits


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Harmonic Errors: Sub-daily Alias and Draconitic (2/3)• Compare simulated EOP signatures with IGS Orbits

– basic problem is a limited independent “truth” (via SLR) for IGS orbits• but can compute discontinuities between daily orbit sets• doing so aliases sub-daily differences into longer-period signals• to compare, also compute EOP-induced orbit differences once daily

• IGS ORBIT JUMPS– fit orbits for each day with

BERNE (6+9) SRP orbit model– parameterize fit as

plus 3 SRPs per SV component– fit 96 SP3 orbit positions for each SV as pseudo-observations for Day A– propagate fit forward to 23:52:30 for Day A– repeat for Day B & propagate backwards to 23:52:30 of day before– compute IGS orbit jumps at 23:52:30

• SIMULATED EOP SIGNATURES– difference conventional & EOP-test orbits at 23:45:00 only

• Compute IGS orbit jumps over ~5.6 yr, test orbits over ~2.8 yr

Z ,Y ,X ,Z ,Y ,X


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Harmonic Errors: Sub-daily Alias and Draconitic (3/3)• Offset peaks in ~14, ~9 and ~7 d bands due to simple daily sampling of

input errors


~1.0 cm white noise floor

10/√3 cm = ~5.8 cm (1D) annual errors

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Harmonic Errors: Summary• Harmonics of 351 d pervasive in all IGS products• Simulated orbital response to IERS sub-daily EOP tide model errors

– compared conventional orbits to EOP-test orbits at 15 min intervals

• Beating of sub-daily EOP tides causes spectral differences at other periods– long-period errors go into PM & LOD– short-period errors go mostly into orbits– bump in background noise at 2 cpd -> resonance with GPS orbital period

• Compared IGS orbit discontinuities to EOP-test orbit differences at 23:45:00– 24 h sampling causes sub-daily EOP tide errors to alias at ~14, ~9 and ~7 d bands ->

peaks offset from expected periods– peaks at several (mostly odd) harmonics of 351 d

• IERS diurnal & semi-diurnal tide model errors are probably main source for pervasive sub-daily alias and several draconitic errors in IGS orbits


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Further Elaboration on Limiting Errors



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Switch to Daily TRFs in Finals• Finals now based on daily SINEX (terrestrial frame) integrations

– prior to GPS Wk 1702 (19 Aug 2012)• products based on weekly SINEX—AC orbits pre-aligned using weekly-averaged AC SINEX

rotations and daily AC PM-x and PM-y deviations from combined ERPs• daily AC SINEX rotations now used to pre-align AC orbits—ERPs rots. no longer used

– higher scatter in combined orbits, ERPs and station positions• but less than sqrt(7) expected for random error• and smaller than other existing systematic errors

– did not resolve rotational instability of Finals– mitigates impacts of unmodeled non-tidal atmospheric loading effects on IGS

products– increased temporal resolution in station position time series

• needed for continued study of non-tidal crustal loading models and impacts to IGS products

– since exposed previously unknown sensitivity of GPS-derived ERP estimates to GLONASS orbit mismodeling• sensitivity is time-correlated with GLONASS eclipse seasons• CODE/ESA currently studying this effect


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• Long-standing (since 2000) error in using AC SINEX rotations for AC Final orbit pre-alignment– prior to GPS Wk 1702 (19 Aug 2012), AC X- and Y- SINEX rotations were applied with

incorrect sign convention• improved RX & RY in PPP using IGS by up to ~0.035 mas (~4.4 mm @ equator) in RMS• but systematic errors remain in RZ—clear ~60d signal (harmonic errors in AC clocks?)

• Note: since Wk 1650, Final PPP using IGR (acc.igs.org/index_igsacc_ppp.html) gives: RX=-0.016 (RMS=0.041) RY=0.015 (RMS=0.039) RZ=-0.004 (RMS=0.022)

– IGS RX & RY better than IGR for now– IGS RZ now biased w.r.t. IGR, and has higher scatter

…and Correcting a Coding Error in Combo Software


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Rotations of Current Final orbits (AC minus IGS)• Scatter of all AC rotations

decreased markedly starting at Wk 1702– no impact in switch to daily

SNX– primarily from fixing combo

software

• Since revealed ESA self-consistency issues– poorly aligned to IGS frame– residual distortion between TRF

and their orbits—see RX & RY– corrected on Wk 1732

• Now RY of IGR (violet) is biased– ESA consistency issues in IGR

IGx08IGS05

fixed AC orbit pre-alignment

ESA fixed TRF issue

- weekly means -


1 mas = ~ 13 cm @ GPS altitude

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IGx08IGS05


ESA fixed TRF issue

WRMS of AC Orbit Residuals Since IG1- AC solutions minus IGS Final , after pre-alignment -

• Inter-AC agreement approaches ~1 cm– switch to daily TRFs seems to have improved AC agreement for now– ESA dominates; EMR and JPL improved slightly to ~18 mm WRMS since IGx08– IGS Final has ~4 mm WRMS difference with IGR—which prods are more precise?


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Time [GPS Wk; April 22, 2012 thru May 12, 2013]

• w.r.t. IGS frame, IGR consistently more precise in all 3 components…– probably due to combination of errors in AC Final clocks– but could be from difference between IGR and IGS analysis approach

IGS vs IGR – More From PPP using Final Products- Mean station RMS after Helmert transformation to IGS frame -


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Other Known Systematic Errors• Ongoing efforts to address:

– limitations of empirical solar radiation pressure (SRP) models• toward physical-based models (IGS Orbit Dynamics WG)• Rodriguez-Solano et al. (2009, 2011, 2012) SRP model w/ handling of eclipses (2013)

– quality of non-tidal loading models and effects on IGS products• IERS Study (http://geophy.uni.lu/ggfc-nonoperational/uwa-call-data.html)• effects are negligible on secular frame• loading can be modeled at stacking level with equivalent results

– time variations of low-degree terms in geopotential field• impacts on orbits: ~7 mm RMS (Melachroinos et al., AGU 2012)• effect on ~annual signal in IGS station position time series?• conventional model under development

– tidal displacements at stations• ocean pole tide (JPL and EMR) & S1-S2 tidal atm loading model (pending update)

– improved satellite attitude modeling (mostly benefits satellite clocks)– modeling higher-order ionosphere effects

• most ACs working to implement 2nd-order correction

• Unclear which of these developments will be ready for IG22013 NASA/GSFC Summer Seminar Series -- 12 June 2013

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IGS 2nd Reprocessing and ITRF2013


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How will IG2 Differ from IG1 & Current Operations?- more details at http://acc.igs.org/reprocess2.html -

• Longer data span (~1994 thru mid-2013)– IG2 + operational prods thru 2013 -> IGS contribution to ITRF2013

• Updated models, frames & methodologies– IERS 2010 Conventions generally adopted– NGA stations data w/ new antenna calibrations (for improved ITRF <-> WGS 84 tie)?– IGb08.SNX/igs08.atx framework (improved a priori datum)– combined products based on AC 1d TRF integrations

• with corrected approach for applying AC SINEX rotations to AC orbits• no non-tidal atmospheric loading at obs level

– 2nd-order iono corrections & S1-S2 atm. loading displacements @ stations– Earth-reflected radiation pressure (albedo) modeling (most ACs still to adopt)

• reduce ~2.5 cm radial bias w.r.t. SLR [e.g. Urschl et al., 2007; Zeibart et al., 2007]• plus antenna thrusting [e.g., Rodriguez-Solano et al., 2009, 2011, 2012]

– satellite attitude modeling by all clock ACs

• Sub-daily alias and draconitic errors will remain• Final preps and initial processing by late June? Finalize in November?• Expect to deliver SINEX files for ITRF2013 by early 2014


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Expected AC and IG2 Products- more details at http://acc.igs.org/reprocess2.html -

• Daily GPS orbits & satellite clocks (in IGST?)– 15-minute intervals (SP3c format)

• Daily satellite & tracking station clocks (in IGST?)– 5-minute intervals (clock RINEX format)

• Daily Earth rotation parameters (ERPs)– from SINEX & classic orbit combinations (IGS erp format)– x & y coordinates of pole– rate-of-change of x & y pole coordinates (should not be used due to sensitivity to

sub-daily tidal errors)– excess length-of-day (LOD)

• Weekly (IG2 only) & daily terrestrial coordinate frames with ERPs– with full variance-covariance matrix (SINEX format)

• May also provide (TBD)– daily GLONASS orbits & satellite clocks– 30-second GPS clocks (in IGST?)– ionosphere maps, tropospheric zenith delay estimates– new bias products


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Who will Contribute to IG2?- more details at http://acc.igs.org/reprocess2.html -

• All IGS Final product Analysis Centers:‐– CODE/AIUB – Switzerland – JPL – USA– EMR/NRCan – Canada – MIT – USA– ESA/ESOC – Germany – NGS/NOAA – USA– CNES/GRGS – Toulouse, France – SIO – USA – GFZ – Potsdam, Germany

• Plus 1 reprocessing Center– ULR – University of La Rochelle TIGA (tide gauges), France– PDR – Potsdam-Dresden Reprocessing group (in IG1, but will not be in IG2)

• Plus 1 Center contributing to TRF only:– GFZ TIGA – Potsdam, Germany


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IGS05

Expected Performance of IG2?- WRMS of AC repro1 orbits wrt IG1 -

By late 2007, inter-AC agreement bi-modal, approaching ~1.5 cm


Large scatter for some ACs in early IG1—expected to be improved in IG2 contributions

Time [GPS Wk; Dec. 26, 1993 thru Nov. 11, 2011]Courtesy of G. Gendt (GFZ Potsdam)

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WRMS of AC Orbit Residuals Since IG1- AC solutions minus IGS Final , after pre-alignment -

• If current performance is any indication– could approach 1 cm inter-AC agreement for much of IG2


inter-AC agreement reaches ~1.0 cm

IGx08IGS05


ESA fixed TRF issue

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Expected Performance of IG2 TRFs?- RMS of Recent AC TRFs wrt IGS -

• Improvement in precision expected from:– horizontal tropo gradients estimated

by all ACs– 2nd order iono corrections– Earth-reflected radiation pressure

(albedo) modeling

• Improvement in accuracy expected from:– igs08.atx (depends on antenna type)

• Switch to daily AC TRFs:– should not impact quality of weekly

combined TRFs (input to ITRF2013)– but will provide increased resolution

of non-tidal displacements

WR

MS

w

.r.t

.

co

mb

inat

ion

Courtesy: P. Rebischung (IGN/LAREG)


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• Contribution to the ITRF2013 scale rate?– satellite PCOs will be included in combination & stacking of IG2 TRFs.– assumption that PCOs are constant → “intrinsic GNSS scale rate”

• No contribution to the ITRF origin yet– remaining unmodeled orbital forces– origins of IG2 TRFs likely not reliable enough

• Some systematic errors still a challenge!– main source: antenna calibrations

• > 1 cm errors revealed at stationswith uncalibrated radomes

• few mm errors likely at stationswith “converted” antenna calibrations

– will cause trouble in use of local tiesfor ITRF2013 colocation sites• consider to exclude in next ITRF

Courtesy: P. Rebischung (IGN/LAREG)

IG2 contribution to ITRF2013


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Other Challenges: Mostly Network Issues(not addressed by IG2)


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• 28/92 ( 30%) multi-technique sites have an uncalibrated radome– nearly half (13/28) operated by JPL

Uncalibrated Radomes


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Uncalibrated Radomes: Impact on ITRF (1/2)– including all co-location sites

• systematic VLBI <-> SLR scale discrepancy

Courtesy: Z. Altamimi (IGN/LAREG)


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Uncalibrated Radomes: Impact on ITRF (1/2)– when GNSS co-located sites with uncalibrated radomes are excluded

• VLBI <-> SLR scale difference amplified by 0.2 ppb (network effect + calibration errors)

Courtesy: Z. Altamimi (IGN/LAREG)


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Loss of Core RF Stations (1/2)– core RF network

• optimal spatial distribution• mitigate network effects in IGS SINEX combination (from X. Collilieux Ph.D. work)


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• Decrease in number of core RF stations– mostly due to anthropogenic impacts (antenna changes, etc.)– some displaced by earthquakes

• IGS08 -> IGb08 update on 7 Oct 2012– recovered sites with linear velocities before/after positional discontinuity

• Overall (linear) rate of loss = ~0.13 sta/wk since end date of ITRF2008– <IGb08: rate = ~0.16 sta/wk– >IGb08: rate = ~0.22 sta/wk

• Today– best case: 71 core stations– actual: ~54

• Need for thorough studyof impacts on stability ofIGS reference frame

• Station operators should limit disruptions, esp. at co-location sites

100% dataavailability

actual dataavailability

Loss of Core RF Stations (2/2)


Courtesy: K. Choi (NOAA/NGS)

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Summary


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Conclusions: IGS Errors• Current IGS products are of high accuracy and precision

– GPS orbits• overall <2.5 cm (1D)• errors now dominated by Z- frame rotation scatter and possibly AC clock errors

– X- & Y- frame rotations of Final orbits improved by ~0.035 mas (~4.4 mm @ GPS)• RMS scatter of AC orbits up to 1.6 cm• sub-daily alias and draconitic errors from IERS diurnal/semi-diurnal tides

– ERPs• PM-x & PM-y: <30 as • dLOD: ~10 s

– terrestrial frames• ~2 mm N&E• ~5 mm U

• But Rapid products still slightly more precise than Finals– discrepancies have been reduced, but needs to be further study– may be due to combination of errors in AC Final clocks?

• Because IGS products are of high quality, can measure subtle signals


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Conclusions: Repro2• Latest models, frames & methods to have largest impact since IG1

– IERS 2010 Conventions– IGb08/igs08.atx framework– Earth-reflected radiation pressure (albedo) modeling– sub-daily alias & draconitic errors will remain

• To result in full history of IG2 products (1994 to mid-2013)– daily products:

• GPS orbits & SV clocks (SP3c) @ 15 min intervals• GPS SV and station clocks (clock RINEX) @ 5 min intervals• Earth Rotation Parameters (IGS ERP)• terrestrial coordinate frames (IERS SINEX)

– expected delivery for ITRF2013 -> early 2014

• And possibly some ancillary products– GLONASS orbits & clocks– 30-second SV & station clocks– bias products


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Conclusions: More Repro2 and Other Challenges• IG2 quality should approach current IGS prods

– quality for later (~2000 -> present) IG2 products will be best– early IG2 probably better than IG1 equivalents, but not as good as later IG2

• Ongoing Challenges– uncalibrated radomes at co-location sites

• one recently available at SMST!! (co-located w/ SLR; unavail. for ITRF2008)

– positional discontinuities at RF stations• 50% of IGS stations have discontinuities: harmful in co-location sites• GNSS/IGS is the link between the 3 other techniques in ITRF

– loss of core RF stations• anthropogenic site disturbances (incl. many equip. changes)• data loss, and earthquakes & other physical processes

– known biases and other systematic errors• harmonic and sub-daily alias errors in all IGS products• site-specific errors [e.g., Wetzell observations by Steigenberger et al., REFAG2010]


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Questions?


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Extra Slides


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• M2 aliases into PM-x and PM-y; O1 aliases into LOD• 1st draconitic harmonic enters PM-x & LOD



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Spectrum of Daily ERP Differences due to sub-daily EOP Tidal Model “Errors”

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Harmonic Errors: Sub-daily Alias and Draconitic• Simulated impact of

sub-daily EOP tidalerrors on IGS orbits– generated “fake”




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Harmonic Errors: Sub-daily Alias and Draconitic• Simulated impact of

sub-daily EOP tidalerrors on IGS orbits– generated “fake”




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bump in background power – resonance of ~2 cpd sub-daily tide errors and GPS orbital period?


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Harmonic Errors: Sub-daily Alias and Draconitic (3/3)• Aliasing of sub-daily errors responsible for some harmonics of 351 d

– peaks at other harmonics likely caused by other errors


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1st, 3rd, 4th, & 10th harmonics also caused by sub-daily EOP errors

other harmonics -- aliasing of other errors


~1.0 cm white noise floor

10/√3 cm = ~5.8 cm (1D) annual errors

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• at diurnal period, EOP model errors absorbed into orbits, esp cross- & along-track

05


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tidal lines excited above background orbit noise

unexpected peak in cross-track – probably a beat effect

Spectra of Orbital Responses tosub-daily EOP Errors – Near 1 cpd


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• at semi-diurnal period, EOP model errors absorbed mostly into orbit radial (via Kepler’s 3rd law)

06


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• background power is lower• errors absorbed in all three components


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• same near 4 cpd


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COMPARISON OF EXPECTED AC DATA USAGECOMPARISON OF EXPECTED AC DATA USAGEANALYSIS CENTER

SYSTEM OBS TYPE ORBIT DATA ARC LENGTH

DATA RATE

ELEVATION CUTOFF

ELEVATION INVERSE WGTS

CODE GPS + GLO DbDiff (weak redundant)

24 h 3 min 3 deg 1/cos2(z)

EMR GPS + GLO UnDiff 24 h 5 min 10 deg none

ESA GPS + GLO UnDiff 24 h 5 min 10 deg 1/sin2 (e)

GFZ (& GTZ)

GPS + ?GLO?

UnDiff ?? 24 h ?? 5 min 7 deg 1/2sin(e)for e < 30 deg

GRG GPS + GLO UnDiff 3 + 24 + 3 h 15 min 10 deg none

JPL GPS UnDiff 3 + 24 + 3 h 5 min 7 deg none

MIT GPS DbDiff (weak redundant)

24 h(SRPs constr.— 9d noise model)

2 min 10 deg a2 + (b2/sin2(e))a,b from site residuals

NGS GPS DbDiff (redundant)

24 h 30 s 10 deg [5 + (2/sin(e)) cm]2

SIO GPS DbDiff (weak redundant)

24 h 2 min 10 deg a2 + (b2/sin2(e))a,b from site residuals

ULR GPS DbDiff (weak redundant)

24 h 3 min 10 deg a2 + (b2/sin2(e))a,b from site residuals


COMPARISON OF EXPECTED AC SATELLITE DYNAMICSCOMPARISON OF EXPECTED AC SATELLITE DYNAMICSANALYSIS CENTER

NUTATION & EOPs

SRP PARAMS

VELOCITY BRKs

ATTITUDE SHADOW ZONES

EARTH ALBEDO

CODE IAU 2000AR06; BuA ERPs

D,Y,B scales; B 1/rev

every 12 hr + constraints

nominal yaw rates used

E+M: umbra & penumbra

impld.—turned off

EMR IAU 2000AR06; BuA ERPs

X,Y,Z scales stochastic

none yaw rates estimated

E: umbra & penumbra

applied

ESA IAU 2000; BuA ERPs


none; Along, Along 1/rev

accelerations



applied + IR

GFZ (& GTZ) IAU 2000; GFZ ERPs

D,Y scales @ 12:00 + constraints

yaw rates estimated


applied + AT

GRG IAU 2000; IERS C04 & BuA ERPs

D,Y scales; X & D 1/rev

stoch. impulse during ecl.

yaw rates estimated


applied + IR

JPL IAU 2000AR06; IERS C04

X,Y,Z scales stochastic

none yaw rates estimated


applied

MIT IAU 2000; BuA ERPs

D,Y,B scales; B(D,Y) 1/rev

none; 1/rev constraints



applied

NGS IAU 2000; BuA ERPs


@ 12:00 + constraints

none; del eclipse data


applied + AT

SIO IAU 2000; BuA ERPs

D,Y,B scales; D,Y,B 1/rev

none; 1/rev constraints



applied

ULR IAU 2000; BuA ERPs

D,Y,B scales; D,Y,B 1/rev

none nominal yaw rates used


applied


COMPARISON OF EXPECTED AC TIDAL MODELSCOMPARISON OF EXPECTED AC TIDAL MODELSANALYSIS CENTER

SOLID EARTH EARTH POLE

OCEAN LOAD

OCEAN POLE

OCEAN CMC

sub-daily EOPs

CODE IERS 2010;dehanttideinel.f

eqn 23a/b mean pole

FES2004;hardisp.f

none sites & SP3 IERS 2010;subd nutation

EMR IERS 2010 eqn 23a/b mean pole

FES2004;hardisp.f

IERS 2010 sites & SP3 IERS 2010

ESA IERS 2010;dehanttideinel.f

eqn 23a/b mean pole

FES2004;hardisp.f

none sites & SP3 IERS 2010 & PMsdnut.for

GFZ (& GTZ) IERS 2010 eqn 23a/b mean pole

FES2004 none sites & SP3 IERS 2010;PMsdnut.for

GRG IERS 2010 eqn 23a/b mean pole

FES2004 none sites & SP3 IERS 2010

JPL IERS 2010 eqn 23a/b mean pole

FES2004;hardisp.f

IERS 2010 sites & SP3 IERS 2010

MIT IERS 2010 eqn 23a/b mean pole


NGS IERS 2010;dehanttideinel.f

eqn 23a/b mean pole

FES2004;hardisp.f

none sites & SP3 IERS 2010 & PMsdnut.for

SIO IERS 2010 eqn 23a/b mean pole


ULR IERS 2010 eqn 23a/b mean pole



COMPARISON OF EXPECTED AC GRAVITY FORCE MODELSCOMPARISON OF EXPECTED AC GRAVITY FORCE MODELSANALYSIS CENTER

GRAVITY FIELD EARTH TIDES

EARTH POLE

OCEAN TIDES

OCEAN POLE

RELATIVITY EFFECTS

CODE EGM2008; C21/S21 due to PM

IERS 2010 IERS 2010 IERS 2010 – FES2004

none dynamic corr &bending applied

EMR EGM2008 IERS 2010 IERS 2010 IERS 2010 – FES2004

none no dynamic corr;bending applied

ESA EIGEN-GL05C IERS 2010 IERS 2010 IERS 2010 – FES2004


GFZ (& GTZ)

JGM3; C21/S21 due to PM


none no dynamic corr &bending applied

GRG EIGEN GL04S; C21/S21 due to PM

IERS2010 IERS 2010 IERS 2010 – FES2004

none dynamic corr; bending applied

JPL EGM2008; C21/S21 due to PM; C20, C30, C40


Desai & Yuan

IERS 2010; eqn 6.23a

dynamic corr &bending applied

MIT EGM2008; C21/S21 due to PM

IERS 1992; Eanes Love #

none none none no dynamic corr;bending applied

NGS EGM2008 IERS 2010 IERS 2010 IERS 2010 – FES2004


SIO EGM2008; C21/S21 due to PM



ULR EGM2008; C21/S21 due to PM




NGS 2nd Reprocessing

GPS Final products

GPS Rapid products

GPS Ultra-Rapid

products

CORS Data Analysis

CORS Network/Data Support

GPS orbits, Earth Orientation

Parameters

GPS Metadata Maintenance

OPUS-S OPUS-RS OPUS-NET(NGS internal)

IGS Analysis Center Coordination (ACC)

NGS Goal 1: Support the Users of the National

Spatial Reference System

NGS Goal 2: Modernize and Improve the National Spatial Reference System

CORS coordinates

Through IGS

Products

CORS Branch Task Flow Map

Orbit Models

CORS Solution

OPUS-DBNGSIDB

Experimental

NSRS Realization

(next page) IGS & ITRF


International Collaboration


- Adjust all obs model parameters in a minimally constrained (no-net rotation; NNR) solution - Realizes an NGS global frame w.r.t. a priori datum (IGS08) using latest IERS and IGS conventions

Align NGS-derived frame to IGS2013

International Terrestrial Reference Frame (ITRF)

Adjust passive network to NAD

83 (2013)

IGS AC and RF Coordinators

Flowchart for NSRS Realizationa priori datum (IGS08)

Contribute NGS

Finals to IGS

GPS orbits, Earth Orientation Parameters, IGS Station Positions


- Tie CORS to global network and NGS Repro2 orbits and ERPs at normal equation level using NNR

CORS coordinates

Finals Orbit, Clock, ERP and SINEX Combinations

Final products from other IGS Analysis Centers

Daily IGS SINEX files to ITRF

DORIS

SLR

VLBI

Combination of solutions from the four space geodetic techniques (GPS, VLBI, SLR, DORIS).

Stack SINEXfiles

usingCATREF

Realizes NGS-derived secular frame

ITRF2013

CORS in NGS-derived global frame

Obtain NAD 83 coords via successive14-parameter

transformationsNAD 83 (2013)

Load NAD 83 (2013) coords into NGSIDB

NAD 83 (2013) Coordinates

NGS Strategic Goals

Goal 1: Support the Users of the National Spatial

Reference System

Goal 2: Modernize and Improve the National

Spatial Reference System

NGS CORS+global SINEX

IGS Realization of ITRF2013IGS2013 (station coordinates, satellite antenna calibrations)

Documents

1 High Precision Applications of Global Navigation Satellite Systems Jake Griffiths IGS Analysis Coordinator NOAA/NGS Brief introduction to GNSS About