1/17 refag symposium 6 october 2010 – marne-la-vallée, france recent results from the igs...

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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Recent Results from the IGS

Terrestrial Frame Combinations__________________________________________________________________________________________________

Elaboration of IGS08

Paul Rebischung, Bruno Garayt

Session 2:Strenghts, weaknesses, modelling standards

and processing strategies of space geodetic techniques

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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Outline

• The IGS terrestrial frame combinations:– Presentation– Methodology– Results– Prospects

• IGS08:– Definition– Availability over time– IGS08 core network

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: Products

• Weekly combined solution:– Combination of 9 AC solutions

– Station positions

– ERP (XPO, XPOR, YPO, YPOR, LOD)

– « Apparent geocenter »

• Cumulative solution:– Stacking of the weekly solutions

– Station positions at 2005.0 and velocities

– Updated each week

NRCAN since 1999

IGN since 2010

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: Network

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: Methodology

• Preparation of AC solutions:– Remove constraints → Unconstrained, singular normal matrices– Fix some parameters (UT, satellite PCOs)– Solve normal equations by adding minimal constraints

• Iterative combinations of AC solutions– Estimated parameters:

• Combined station coordinates and ERP• « AC → combined » transformation parameters

– Internal constraints ⇒ Combined solution is in a mean frame.– Rejection of outliers (5 cm / 5 σ)– Update of AC variance factors

• Final combination with transformation parameters fixed

• Explicitation of combined geocenter

• Alignment to IGS05

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: ResultsW. avg. of AC residuals Std. dev. of AC residuals

East

North

Up

Change in the combination method

Global level of agreement between AC solutions:

• 1-2 mm in horizontal• 2-5 mm in vertical

Excellent agreement with ‘mig’:

< 0.5 mm in horizontal< 1 mm in vertical⇒ Independent validation of the combination

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: How to progress?

• Need to understand the discrepancies between AC frames

⇒ Maps of the « AC – igs » residuals

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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COD

EMR

ESA

GFZ

GRG

JPL

MIT

NGS

SIO

Mean SH fits of « AC-igs » residuals over weeks 1548-1599

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: How to progress?

• Study the « AC distorsions » over 1994 – 2010(using repro1)

• Replace the cumulative solution by a new onebased on repro1

• Study « AC – cumulative solution » residuals in the frequency domain

• Study the geocenter discrepancies

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08

• IGS realization of ITRF2008:– Will become the Reference Frame of IGS products.

⇒ Consistency and stability of IGS products will critically depend on IGS08.

– Will be used by many GNSS users to access ITRF2008.

• Initially intended to be a subset of “good” GNSS stations from ITRF2008– But because of new antenna robot calibrations included in

igs08.atx,small shifts will be applied to the ITRF2008 coordinates of some stations.

• Unlike IGS05, IGS08 should include RF stationsback to 1994 (for repro2).

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Selection criteria

• Geographical location

• “ITRF2008 quality”:– Data span (5 years at least)– Maximal time spans between two discontinuities (3 years at least)– Number of discontinuities (5 at most)– Absence of velocity discontinuities– Standard deviation of last velocity (0.3 mm/y at most)– Residual time series (RMS + visual inspection)

• Equipment:– Type of antenna calibration– GPS+GLONASS capability– Presence of an external atomic clock– Co-location with other techniques

• Belonging to IGS05

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Result of the selection

• 241 stations

• Global coverage

• But densityheterogeneities

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Availability over time

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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Aliasing of loading signals

• Weekly solutions are aligned to IGSxx.

• Helmert parameters absorb part of the loading signals. ⇒ Station annual signals are reduced/amplified and/or shifted.

• This aliasing can be reduced by using a well distributed RF (Collilieux, 2010).

⇒ « IGS08 core network »

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08 core network

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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Thanks for your attention!

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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Backup slides

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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TF combination: Methodology

• Variance component estimation:

– A priori: variance factors from previous week

– After each iteration:

• : weighted squared sum of solution s residuals

• : redundancy factor of solution s

• : number of observations from solution s

• : design matrix of solution s

• : combined covariance matrix

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REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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RF combination: Methodology

• Paradox of the combined geocenter:– Rigorous geocenter combination ⇒ No translation parameters

– But…

– In practice, AC solutions are brought to the same framebefore geocenter is combined.

⇒ Estimated precision is too optimistic (usually [1 mm ; 1 mm ;3 mm]).

– To be used with care!

– There is still a long way before GNSS can contribute to the ITRF origin.

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Selection criteria

Distribution of GNSS stations among IGS stations in ITRF2008

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Selection criteria

Distribution of antenna calibration types among IGS stations in ITRF2008

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08: Availability over time

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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New antenna calibrations

• New robot calibrations by Geo++ available for igs08.atx

• But repro1 (input for ITRF2008) computed with old calibrations

• To ensure consistency between IGS08 & igs08.atx:– Drop affected stations from IGS08

– Apply shifts to the ITRF2008 coordinates of these stations

Same problem as the transition from ITRF2005, relative calibrations to IGS05, absolute calibrations

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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New antenna calibrations

• To estimate coordinate shifts due to calibration changes,– PPP tests were performed at IGN, ESA and CNES.

– ACs provided parallel solutions using the new calibrations.

• Good global agreement– But for certain antenna types, fixing ambiguities or not seems to have

an impact on the estimated shifts.

• When possible, IGN PPP shifts will be applied.

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08 core network

• Simulations performed by Xavier Collilieux:

Differences at the annual frequency between the true values and the residuals of the 7-parameter transformation:

⇒ Annual signals are better retrieved if « IGS08 core » is used instead of IGS08.

In (mm) Out (mm) In (mm) Out (mm) In (mm) Out (mm)

Full IGS08 0.3 0.3 0.4 0.2 1.0 1.3

IGS08 core 0.1 0.1 0.2 0.1 0.6 1.1

East North Height

x(t) = In cos(ωt) + Out sin(ωt)

REFAG Symposium6 October 2010 – Marne-la-Vallée, France

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IGS08 core network

• Simulated translations and scale factor:

Full IGS08

IGS08 core

Loading model including: - Atmospheric loading - Non-tidal ocean loading - Continental water loading(T. van Dam, University of Luxembourg)