Quality assessment of GPS reprocessed

1
Quality assessment of GPS reprocessed
Terrestrial Reference Frame
X Collilieux 1 Z Altamimi 1 L.
Métivier 1 T. van Dam 2
1 IGN/LAREG and GRGS 2 University of Luxembourg
Acknowledgment J. Ray (NGS)
2
Outline

• Data
• GPS Frame origin and scale. Investigation of
  their temporal behaviors
• Station position time series
• GPS TRF assessment

Data Origin Scale Station position
time series TRF assessment
3
Data
AC Epoch Nb of stations IG1 combined 1997.0-200
9.5 570 GFZ 1998.0-2008.0 248 ESA 1997.0-2009.0
358 COD 1997.0-2009.0 224 MIT 1998.0-2008.0 74
8
Data Origin Scale Station position
time series TRF assessment
mi1
gf1
co1
es1
IGS
Nb of stations per week
4
GPS translation and scale (1/5)
Comparison with ITRF2008 preliminary solution
(SLR origin)
mi1
Data Origin Scale Station position
time series TRF assessment
5
GPS translation and scale (1/5)
Comparison with ITRF2008 preliminary solution
(SLR origin)
mi1
gf1
Data Origin Scale Station position
time series TRF assessment
6
GPS translation and scale (1/5)
Comparison with ITRF2008 preliminary solution
(SLR origin)
mi1
gf1
co1
Data Origin Scale Station position
time series TRF assessment
7
GPS translation and scale (1/5)
Comparison with ITRF2008 preliminary solution
(SLR origin)
mi1
gf1
co1
Data Origin Scale Station position
time series TRF assessment
es1
8
GPS translation and scale (2/5)
W.r.t. ITRF2008 prel.
Offset (t2000) (mm) Offset (t2000) (mm) Drift (mm/yr) Drift (mm/yr)
Min Max Min Max
TX -3.7 0.2 -0.2 0.2 -0.60 0.05 -0.12 0.04
TY -5.4 0.2 -2.8 0.2 -0.06 0.04 0.91 0.06
TZ -13.5 0.5 -10.1 0.4 0.34 0.09 0.78 0.13
Scale 8.5 0.1 10.2 0.1 -0.38 0.01 -0.24 0.01
Data Origin Scale Station position
time series TRF assessment

• GPS scale is based on ITRF2005 scale through GPS
  satellite antenna offsets.
• This non-zero scale offset is explained by
• ITRF2008P and ITRF2005 scale difference
• Adoption of absolute phase center variations

9
GPS translation and scale (3/5)
Seasonal variations of the apparent geocenter
motion
mi1
gf1
co1
es1
Data Origin Scale Station position
time series TRF assessment
SLR
Good fit on Y component. Still inconsistent X and
Z variations
10
GPS translation and scale (4/5)
Comparison of scale variations with a loading
model
6 h
Loading model
12 h


1 month
Loading effects
Some examples
GPS data (ITRF2008)
Loading model
Data Origin Scale Station position
time series TRF assessment
Bad fit
Good fit
11
Station position time series Scale issue
Comparison of scale variations with a loading
model
Load
mi1
mm
gf1
co1
es1
Data Origin Scale Station position
time series TRF assessment
Difference with the loading model

• Cause of these variations
• Deficiency in the loading model
• Draconitic period (harmonics detected in the
  power spectra)
• Thermal effects

mm
As the scale behavior is stable do not estimate
scale non-linear temporal variations to avoid
loosing loading signal
12
Station position time series - Spectral map (1/2)
Repeatability
Data Origin Scale Station position
time series TRF assessment
mi1
co1
gf1
es1
ITRF2008
ITRF2005
13
Station position time series - Spectral map (2/2)

• 5 latest years of IGS combined analysed for
  spectral content.

• For the whole period, no difference between Acs,
  but the 6th harmonic more visible in the North

Proportation of stations which detect a given
frequency

• Is the power of the 4th harmonic of the
  draconitic frequency lower in ITRF2008 data?

Data Origin Scale Station position
time series TRF assessment

• Comparison of the amplitude of the 4th harmonic
  signal between
• ITRF2005 residuals (2001-2006)
• ITRF2008 residuals (2004-2009)

IG1 reprocessed
IGS ITRF2005
14
Station position time series - Comparison with a
loading model (2/3)
Amplitude of the annual signal
Vertical
? 43
Period 1997.0-2006.0
Station considered
ITRF2005 GPS (IGS)
Data Origin Scale Station position
time series TRF assessment
? 56
GPS annual signal ampl.
ITRF2008 GPS (IG1)
Loading model annual signal ampl.
15
Comparison with a loading model (3/4)
Amplitude of the annual signal
Vertical
? 43
? 17
? -13
ITRF2005 GPS (IGS)
Data Origin Scale Station position
time series TRF assessment
? 56
? 26
? -5
ITRF2008 GPS (IG1)
Horizontal
16
Comparison with a loading model (4/4)
Phase of the annual signal histogramme of the
difference of phase between GPS and loading model
ITRF2005 GPS (IGS)
Data Origin Scale Station position
time series TRF assessment
ITRF2008 GPS (IG1)
Significant improvement in the height only
17
Station position time series - Annual signal and
AC solutions
Data Origin Scale Station position
time series TRF assessment
mi1
es1
gf1
co1
18
Agreement with IVS VLBI reprocessed solution
Comparison with newly reprocessed VLBI (IVS)
solution
Height local tie residual improvement
Made on GPS-VLBI co-location sites
Data Origin Scale Station position
time series TRF assessment
----------------------
----------------------- WRMS Positions
Velocities 2D h
2D h t2000.0 mm
mm/yr ------------------------------------
------------------------------------ ITRF2005
0.3 1.6 0.1
0.5 ITRF2008 P 0.1 0.8 0.1
0.4 ---------------------------------------------
---------------------------
If VLBI positions and velocities are fixed to
their estimated values,
19
Conclusion
Origin

• GPS frame origins differ from SLR origin by less
  than 1 mm/yr for each component
• The GPS translation non-linear variations are
  still biased compared to SLR and loading model.

Scale

• The scale behavior of analysis GPS reprocessed
  solution is stable.
• 31 to 40 WRMS reduction when corrected by a
  loading model

Station position time series

• The repeatability of GPS position time series is
  smaller, especially in vertical, than previous
  non-homegenously reprocessed solution
• The estimated annual variations are closer to
  loading model than in the past. Still larger
  power in horizontal for GPS.
• Harmonic of the draconitic frequency still
  visible. Their power is comparable than previous
  GPS solutions.

Reference positions and velocities

• GPS station long term positions and velocties
  are closer to VLBI results