The Earth is constantly changing shape.
To be understood in context, when the motion of the Earth's crust is observed, it must be referenced.
A Terrestrial Reference frame provides a set of coordinates of some points located on the Earth's surface. It can be used to
measure plate tectonics, regional subsidence or loading  and/or used to represent the Earth
when measuring its rotation in space.
This rotation is measured with respect to a frame tied to stellar objects, called a celestial reference frame.
The International Earth Rotation and Reference Systems Service (IERS) was
created in 1988 to establish and maintain a Celestial Reference Frame, the ICRF,
a Terrestrial Reference Frame, the ITRF. The
Earth Orientation Parameters (EOPs) connect these two frames together.
These frames provide a common reference to compare
observations and results from different locations .
Nowadays, four main geodetic techniques are used to compute
accurate coordinates: the
Since the tracking network equipped with the instruments of those techniques
is evolving and the period of data available increases with time, the ITRF is constantly being updated.
11 realizations of the ITRS were set up from 1988.
The latest is the ITRF2005.
All these realizations include station positions and velocities. They
model secular Earthís crust changes thatís why they can be used to compare observations from different epochs.
All the higher
frequencies of the station displacements can be accessed with the IERS conventions, chapter 7
. Continuity between the
realizations has been ensured as much as possible when adopting conventions for ITRF definitions.
The relationship linking
all these solutions is of utmost importance.
They are supplied here by the transformation parameters.
The International Terrestrial Reference System (ITRS)
is a world spatial reference system co-rotating with the Earth in its diurnal motion in space.
The IERS, in charge of providing global references to the
astronomical, geodetic and geophysical communities, supervises the
realization of the ITRS. Realizations of the ITRS are produced by
the IERS ITRS Product Center (ITRS-PC) under the name International
Terrestrial Reference Frames (ITRF).
ITRF coordinates were obtained by combination of individual TRF solutions
computed by IERS analysis centers using the observations of Space Geodesy
They all use networks of stations located on sites covering the whole
For more details:
Terrestrial Reference System and Terrestrial Refrence Frame definition. TRS & TRF
Relationship between TRS's
, Terrestrial Reference Systems.
ITRS & ITRF
: The International Reference System/Frame.