1. The IGS contribution to ITRF2014 Paul Rebischung, Bruno Garayt, Zuheir Altamimi, Xavier Collilieux th IUGG General Assembly, Prague, 28 June 2015

2. Outline Daily IGS repro2 SINEX combinations
Combined IGS repro2 dataset
Origin/scale of daily combined solutions
Modeling of station position time series
Discontinuities
Post-seismic deformations
Long-term stacking (input to ITRF2014)
Spectral analysis of residual time series

3. Daily IGS repro2 SINEX combinations
Smoothed daily WRMS of the « AC – combined » station position residuals
AC contributions to the repro2 campaign
Overall inter-AC agreement:
Station positions: ≈ 1.5 mm (horizontal), ≈ 4 mm (vertical)
Geocenter: ≈ 2-5 mm (X, Y), ≈ 5-10 mm (Z)
Terrestrial scale: Excellent (< 1 mm; < 0.1 mm/yr)
EOPs: ≈ μas (pole coordinates), ≈ μas/d (pole rates), ≈ μs/d (LOD)

4. Combined IGS repro2 dataset
7714 daily SINEX files
Daily station positions, geocenter coordinates and EOPs
January 2, → February 14, 2015
1848 stations
Stations included in the daily repro2 combined solutions. The size and color of each dot is function of the number of days n each station is present.
Histograms of the lengths (top) and numbers of data points (bottom) of the repro2 station position time series

5. Geocenter motion (1/2) (X) (Y) (Z)
― Daily repro2 combined geocenter coordinates
― Smoothed repro2 combined geocenter coordinates
― Smoothed geocenter coordinates derived from the SLR contribution to ITRF2014 (All time series shown in the figure are detrended.)
Smoothed periodograms of the: ― repro2
― SLR-derived geocenter time series shown in the left figures.
Spectral power in mm2.

6. Geocenter motion (2/2) Non-negligible offsets and rates wrt ITRF2008
Offset at [mm]
Rate [mm/yr]
Annual amp [mm]
Annual phi [deg]
Semi-ann amp [mm]
Semi-ann phi [deg]
WRMS [mm]
XGC
1.6
0.28
1.5 / 2.6
41 / 48
0.9 / 0.8
271 / 282
3.5 / 2.9
YGC
2.6
-0.40
3.6 / 2.8
310 / 320
0.2 / 0.3
336 / 159
ZGC
7.0
-0.18
3.8 / 5.9
181 / 26
1.3 / 1.4
223 / 202
6.4 / 5.2
Results from trend + annual + semi-annual fits to: ― the repro2 combined geocenter time series
― geocenter time series derived from the SLR contribution to ITRF2014
Non-negligible offsets and rates wrt ITRF2008
Annual geocenter motion:
Under-estimated along X
Over-estimated along Y
Out-of-phase with SLR along Z

7. Terrestrial scale Repro2 scale factor time series:
― Scale factors estimated between the daily repro combined solutions and IGb08
― Scale factors derived from a loading model (ECMWF+GLDAS+ECCO2;
Smoothed periodograms [mm2] of the: ― repro2
― loading model-derived scale factor time series shown in the left figure.
Annual amp [mm]
Annual phi [deg]
Semi-ann amp [mm]
Semi-ann phi [deg]
WRMS [mm]
1.4 / 0.8
-114 / -103
0.5 / 0.1
118 / 138
0.8 / 0.5
Results from trend+annual+semi-annual fits to the: ― repro2
― loading model-derived scale factor time series shown in the left figure.
Repro2 scale factor time series:
Mostly constituted of annual and semi-annual variations
Shows little evidence of draconitics
≈ Half can be explained by loading + network effect
Contribution of GNSS to the ITRF2014 temporal scale variations?

8. Discontinuity detection
Visual detection with help of:
List of equipment changes
Catalogue of co-seismic displacements (updated from Métivier et al., 2014)
Analyzed 1176 stations with repro2 series > 700 days
2090 discontinuities detected (≈ 1.8 per station)
― antenna change
― receiver change
― earthquake
― 04:361:03530 – Sumatra Earthquake (9.0)
― 07:330:00000 – Antenna Change
― 12:102:31117 – Sumatra Earthquake (8.6)
Histograms of the lengths (top) and numbers of data points (bottom) of inter-discontinuity intervals
Distribution of discontinuity causes

9. Post-seismic deformation modeling
Goal: correct post-seismic deformations before stacking
For each E, N, H time series:
Test following models: exp, log, exp+exp, exp+log [+ velocity disc.]
Select best model based on Bayes’ Information Criterion (BIC)
209 models
139 stations
65 earthquakes
Stations with post-seismic models
Earthquake epicenters

10. SAMO (Samoan Islands)
log + δV δV
log + δV

11. COCO (Cocos Islands)
exp
exp
exp
log
exp
log

12. Long-term stacking (input to ITRF2014)
Innovations:
Post-seismic deformations corrected a priori
Estimation of annual + semi-annual signals
1066 stations retained (yet)
Time series > 700 days
Well-behaved residuals
Daily WRMS of stacking residuals (East, North, Up)
Provisional ITRF2014 GNSS network

13. Stacked periodograms White + flicker noise background
GPS draconitic harmonics up to the 8th
Fortnightly tidal lines:
Mf tide (13.6 d)
O1 alias (14.2 d)
M2 alias (14.8 d)
Stacked Lomb-Scargle normalized periodograms of the stacking residual time series
― East (with annual + semi-annual terms estimated)
― North/10 (with annual + semi-annual terms estimated)
― Up/ (with annual + semi-annual terms estimated)

14. Next steps Prepare next IGS Reference Frame (IGS14/igs14.atx)
Select subset of stable stations
Re-estimate satellite antenna phase center offsets
Take new ground antenna calibrations into account
Prepare next-generation IGS cumulative solution
Based on repro2 data + new discontinuity list
How to deal with post-seismic deformations operationally?
Scientific investigation:
Analyze noise content in individual station position time series
Study spatial structure of draconitic/fortnightly errors and of noise
Study AC-specific errors (through AC-specific long-term stackings)

15. Thank you for your attention!