Current status of ITRF Co-location Sites
Zuheir Altamimi
IGN-France
ITRF Product Center
Co-locations
• Co-location sites are vital for multitechnique combination (ITRF)
– Number & Distribution of co-location sites
– Quality of local ties
– They impact the combined frame
parameters (Origin, Scale, Orientation)
Current Co-locations (1999 on)
(8)
(59)
(16)
(2)
Missing Ties
(2)
SLR/GPS
DORIS/GPS (1)
Brewster
Ottawa
Krimea
URUM
KUNM
SAKAL
MIZU
Haleakala
VLBI/GPS
(4)
VLBI/SLR
(1)
Missing Ties
• There are missing ties for point(s) in sites
having multiple points from one
technique
• Ties after EQ are not available, e.g.
– Arequipa (SLR, GPS, DORIS)
– Fairbanks (VLBI, GPS, DORIS)
• Ties after EQ should be used with care,
esp. in case of co-seismic effect !
Dubious/Discrepant Ties
Disagreement btw Terrestrial & Space Geodesy vectors
– Hard to discriminate between the two
– ~ 15 - 20 % of the total currently available
ties are discrepant  1 cm
(preliminary ITRF2004 analysis)
Co-locations
A new difficulty
• Most of IGS/GPS stations experience jumps
(discontinuities) in the time series of station
positions
• Most of IGS/ILRS Co-location sites are
concerned
• A few SLR station discontinuities are detectable
• Having only one tie: to which segment should
the tie be applied ???
• Careful combination analysis is needed
A Co-location site with Discontinuities
X
GPS
SLR
Tie
Time
Current VLBI-SLR Co-locations (1999 on)
(9)
Inhomogeneous distribution of VLBI&SLR Networks and their Co-locations
ITRF Scale in danger !!!
Current VLBI-GPS Co-locations (1999 on)
(36)
Current VLBI-DORIS Co-locations (1999 on)
(9)
Poor number and distribution of VLBI and DORIS Co-locations
Current SLR-GPS Co-locations (1999 on)
(25)
Current SLR-DORIS Co-locations (1999 on)
(7)
Only 7 SLR-DORIS Co-locations
Current DORIS-GPS Co-locations (1999 on)
(37)
Conclusions
• Poor number&Distr. of SLR & VLBI Co-locations
• Networks configuration needs improvemnt
– GGOS WG on Networks, Communications &
Infrastructure
• To improve tie quality, ideally repeated surveys are
needed (yearly, each 2 years)
• Current Ties are available at:
http://itrf.ensg.ign.fr/ITRF_solutions/2004/doc/ITRF2004-localties.tab