ITRF2008-P: SOME EVALUATION ELEMENTS AND IMPACT ON IGS RF PRODUCTS

1. ITRF2008-P:SOME EVALUATION ELEMENTSAND IMPACT ON IGS RF PRODUCTS ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010

2. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 Outline 1 - ITRF2008-P vs. IGS05 • Direct comparison • Comparison with IGS weekly frames • Estimation of satellite APCOs using both frames 2 - Impact of the RF change on IGS weekly and cumulative solutions 3 - IGS08 definition

3. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - ITRF2008-P vs. IGS05 • Comparison is not straightforward since ITRF2008-P and IGS05 are based on different discontinuity lists. • Methodology: • Match IGS05 points (CODE+PT+SOLN) to ITRF2008-P points • 116 points over 132 could be matched. (ftp://igs-rf.ign.fr/pub/ITRF2008P/ITRF2008_IGS05.txt) • 16 points have an ITRF2008-P discontinuity during their IGS05 time span. • Estimate 14 transformation parameters using the 116 matched points • Identify and remove outliers • 3 points were removed: NYA1, NYAL, PIMO.(Their IGS05 velocities are known to be not very accurate). • Re-estimate 14 transformation parameters

4. 1 - ITRF2008-P vs. IGS05 Note: Transformation parameters and residuals are computed at epoch 2005.0. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010

5. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - ITRF2008-P vs. IGS05: Velocity residuals

6. 1 - ITRF2008-P vs. IGS05: Position residuals ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010

7. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - ITRF2008-P vs. IGS05 • Velocity fields show non-negligible differences. (slide 5) • Position agreement is acceptable at t = 2005.0,but becomes quickly worse when moving away from t = 2005.0. (See previous slide and this table.)

8. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - IGS05/ITRF2008-P vs. igs10P1576 • To check that ITRF2008-P velocities are more up-to-date than IGS05 velocities, both frames were compared with the latest IGS weekly combined frame (igs10P1576.snx). • Methodology: • The 116 matched points were extracted from ITRF2008-P ⇒ « IGS08-P ». • Both IGS05 and « IGS08-P » positions were propagated to week 1576. • Both propagated frames were compared with igs10P1576.snx. • Note: The so called « IGS08-P » has nothing to do with the future IGS08. It was only built for comparison with IGS05.

9. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - IGS05/ITRF2008-P vs. igs10P1576 IGS05 - igs10P1576 residuals « IGS08-P » - igs10P1576 residuals ⇒ ITRF2008-P velocities are indeed more up-to-date than IGS05 ones.Residuals are globally ~twice less at t = 2010.23 (week 1576).

10. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - IGS05/ITRF2008-P vs. IGS weekly frames • Similar comparisons were made with IGS weekly combined frames from week 887. • Repro1 products were used for weeks 887-1536. • Operational products were used for weeks 1537-1576. • Weekly RMS are plotted on the right. • Agreement is globally better with ITRF2008 than with IGS05, especially: • At the end of the time span(This confirms that ITRF2008 velocities are more up-to-date.) • In the Up component(That was expected because IGS05 was derived from ITRF2005 which was computed from relative PCO solutions.On the other hand, ITRF2008 was directly computed from absolute PCO solutions.)

11. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - IGS05/ITRF2008-P to estimate z-APCOs(Preliminary results) • Satellite Antenna Phase Center Offsets (APCOs) time series have been computed from co1 solutions by constraining the Terrestrial Reference Frame scale to IGS05 scale or to ITRF2008-P scale. • For each satellite, the scattering of the IGS05 based time series and of the ITRF2008-P based time series were compared. • The plot shows the “IGS05 – ITRF2008-P” scattering differences. ⇒ APCOs estimated using ITRF2008-P are less scattered. Satellite ID G0xx Difference of z-APCO scattering depending on the TRF used for the frame alignment: IGS05 solution – ITRF2008-P solution

12. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 1 - ITRF2008-P vs. IGS05: Summary • Helmert parameters: • Only TX, TZ and scale are significant (+3 mm, +5 mm, -1 ppb). • Rates are negligible. • Velocities: • Agreement at 0.5 mm/y (North, East), 1.6 mm/y (Up) • Velocity differences lead to substantial position differences when moving away from t = 2005.0. • But ITRF2008-P velocities seem more reliable. Nothing surprising since: • ITRF2008-P includes 12.5 years of GNSS data until 2009.5 (10 years until 2006.0 for ITRF2005) • ITRF2008-P is based on homogeneously reprocessed GNSS data. • Satellite APCOs: • Preliminary analyzes show that APCOs derived from ITRF2008-P have a better repeatability than those derived from IGS05.

13. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 2 - Impact of the RF change onIGS weekly combined solutions • IGN strategy for combining AC solutions is not « RF dependent »: • AC solutions are not aligned to RF before combination. • Instead, « AC ↔ combined » transformation parameters are estimated during the combination. • The alignment to RF is then made at the level of the combined solution. ⇒ The switch to IGS08 will simply result in a Helmert transformation of the IGS weekly combined frames compared to current products.

14. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 2 - Impact of the RF change onIGS cumulative solution • The switch to IGS08 will be more delicate for the IGS cumulative solution, since it is not based on the same discontinuity list as ITRF2008. • Our suggestion is to replace the current cumulative solution by a new one based on: • Repro1 weekly combined frames (weeks 887-1536) +Operational weekly combined frames (weeks 1537-present) • The ITRF2008 discontinuity list • This would improve the IGS cumulative solution and would make it easily « alignable » to ITRF2008. • Another improvement should be considered for the IGS cumulative solution:constraining velocities to be equal before and after non-earthquake discontinuities.

15. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 3 - IGS08 • IGS08 will be a subset of the ITRF2008 GNSS network. • As suggested by Jim Ray, we could also define a smaller, homogeneously distributed « IGS08 core network »: • The core network would be used for global frame alignments, in order to minimize errors due to network inhomogeneity. • But a denser network is required by many users, in particular for the alignment of regional frames. • A first station selection is ongoing at IGN. It should be made available to ACs, ACC, IC,… in May.

16. ITRF2008-P: Some evaluation elements and impact on IGS RF products Paul Rebischung, Bruno Garayt, 16 April 2010 3 - IGS08 • Taking a frozen extract from ITRF2008 as IGS Reference Frame has however some drawbacks: • New discontinuities will inevitably make the number of available RF stations decrease. • It may become difficult to keep a homogeneously distributed core network. • It is maybe time to think of regular updates of the IGS Reference Frame: • The regular addition of recent data would improve the quality/accuracy of IGS08. • With more data, new stations showing good performance could also be added. • Opinions and suggestions are welcomed!