Establishment of Geodetic Coordinate Systems - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 358

techniques such as VLBI, SLR, LLR, GPS (since 1991), and DORIS (since 1994;
Doppler orbitography and radio-positioning integrated by satellite). IERS regularly
provides annual IERF solutions, which are published in the IERS annual reports.
The available ITRF solutions include ITRF0, ITRF88, ITRF89, ITRF90, ITRF91,
ITRF92, ITRF93, ITRF94, ITRF96, ITRF97, ITRF2000, ITRF2005, and

ITRF2008. The basic steps to compute ITRF are as follows:
First, each individual SSC solution provided by the analysis centers is reduced to
a reference epoch t 0 using their respective station velocity models; then, the method
of combined adjustment yields ITRF station coordinates and seven transformation
parameters for each SSC with respect to combined solutions of ITRF. The veloc-
ities of ITRF stations can be calculated by two methods: one is similar to calculat-
ing station coordinates except that its model is deduced from a coordinate
transformation formula; the other is obtained by differentiating coordinates at two
different epochs.
To represent the ITRF station coordinates by geodetic coordinates, IERS
recommended the fundamental geodetic constants of the universal GRS (Geodetic
Reference System). The currently adopted GRS80 was

recommended by

IUGG1979 and its ellipsoid parameters (see Table 4.1 ).

ITRF and IGS

With the establishment of the International Global Navigation Satellite System
Service (IGS), ITRF and GPS are more closely interrelated (Altamimi and
Collilieux 2009). IGS is in close collaboration with ITRF. On the one hand, ITRF
provides IGS with an absolute long-term datum; IGS, on the other hand, provides
global GPS observation data and helps to improve ITRF solutions.
Assuming that the reference epoch of a GPS campaign is T c , we are allowed to
make use of the observational data and precise ephemeris of this campaign to
calculate the station coordinates. We might encounter two kinds of precise ephem-
eris; one is based on the WGS84 coordinate system while the other is based on ITRF
(such as IGS precise ephemeris). The station coordinates obtained by using the
precise ephemeris with different coordinates will end up in different coordinate
systems.
When using the IGS precise ephemeris (whose reference frame is assumed to be
ITRFyy), we should adopt the coordinates of given points in the ITRFyy reference
frame (at reference epoch T 0 ), and reduce the coordinates from T 0 to the epoch of
observation T c based on the velocities of the given points in ITRFyy. If necessary,
we can impose constraints on the reduced coordinates of the given points and, in
this case, the coordinates of the unknown points obtained belong to the ITRF
reference frame adopted by the IGS precise ephemeris.