| Local ties are differential coordinates between tracking reference
points in co-location sites were two or more geodetic instruments
are operating. They are a key element in the ITRF combination allowing
to connect the different techniques together and to construct the ITRF.
We first recall the ITRF combination strategy, focussing on the usage
of local ties, their measurement epochs and their uncertainties.
In the same way as for space geodesy technique solutions, local ties
are introduced in the ITRF combination with proper variances to account
for their uncertainties, and taking into account the epochs of their
measurements. We then assess their quality by comparing their respective
baseline components to those estimated by space geodesy solutions
incorporated in the ITRF2008 combination. We evaluate their impact on
the ITRF2008 combination results, and attempt to isolate those co-
location sites were new surveys are needed. |