1/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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

2/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France Outline The IGS terrestrial frame combinations: –Presentation –Methodology –Results –Prospects IGS08: –Definition –Availability over time –IGS08 core network

3/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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 and velocities –Updated each week NRCAN since 1999 IGN since 2010

4/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France TF combination: Network

5/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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

6/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France TF combination: Results W. avg. of AC residualsStd. 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

7/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France

8/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France TF combination: How to progress? Need to understand the discrepancies between AC frames ⇒Maps of the « AC – igs » residuals

9/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France COD EMR ESA GFZ GRG JPL MIT NGS SIO Mean SH fits of « AC-igs » residuals over weeks

10/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France TF combination: How to progress? Study the « AC distorsions » over 1994 – 2010 (using repro1) Replace the cumulative solution by a new one based on repro1 Study « AC – cumulative solution » residuals in the frequency domain Study the geocenter discrepancies

11/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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 stations back to 1994 (for repro2).

12/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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

13/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08: Result of the selection 241 stations Global coverage But density heterogeneities

14/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08: Availability over time

15/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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 »

16/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08 core network

17/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France Thanks for your attention!

18/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France Backup slides

19/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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

20/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France RF combination: Methodology Paradox of the combined geocenter: –Rigorous geocenter combination ⇒ No translation parameters –But… –In practice, AC solutions are brought to the same frame before 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.

21/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08: Selection criteria Distribution of GNSS stations among IGS stations in ITRF2008

22/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08: Selection criteria Distribution of antenna calibration types among IGS stations in ITRF2008

23/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France IGS08: Availability over time

24/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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

25/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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.

26/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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 IGS IGS08 core East NorthHeight x(t) = In cos(ωt) + Out sin(ωt)

27/17 REFAG Symposium 6 October 2010 – Marne-la-Vallée, France 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)