<h4>2.5 ITRS and other terrestrial reference systems</h4>

\subsection{Relation of the ITRS to other terrestrial reference systems}

It is worthwhile to look at the relation of the ITRF to other global frames.
For practical purposes, some global systems were introduced 
on national level. 
Of particular interest here is the World Geodetic System
(WGS), which is still widely used in nonscientific applications.
A first World Geodetic Reference System 
was in 1960 introduced by the U.S. Department of
Defense (DoD) as WGS60 and later updated through WGS66 and WGS72. 

In 1984, the DoD introduced the 
World Geodetic System 1984 (WGS84). The first materialization
of the WGS84 was based on observations
from the U.S. Navy Navigation Satellite System (Doppler Transit). 
This materialization of the WGS84 was achieved by aligning 
as closed as possible the DoD reference frame NSWC-9Z2 to the 
BIH Conventional Terrestrial System (BTS) at the epoch 1984.0.
The latler was realized by the adopted coordinates of a globally
distributed set of tracking stations with an estimated accuracy
of 1-2 meters. 

In January 1987, the U.S. Defense Mapping Agency (DMA)
started using the WGS84 for the computation of the precise
ephemerides of the TRANSIT satellites. 
Using these ephemerides, the coordinates of the ten DoD GPS monitoring stations
were determined by Doppler tracking.
Until recently, GPS broadcast orbits were generated from GPS tracking 
data from these stations, fixing the Doppler derived coordinates of these stations
(i.e.~neglecting any station movement, including tectonic plate motions).

Based on broadcast ephemeries, GPS receivers provide coordinates in 
the WGS84, and for many
practical purposes such as air navigation, the WGS84 is 
in use\footnote{see www.wgs84.com/default.htm for
an example}. Therefore, it is worthwhile to consider the current
relation between WGS84 and the more accurate ITRF. In 
1994, the DoD made an attempt to align WGS84 
with ITRF. For that, new coordinates for the
ten DoD tracking stations were determined at the epoch 1994.0
used GPS tracking data
collected at these sites 
together with  a subset of the IGS tracking stations,
with the ITRF91 coordinates of the later stations being
held fixed in the process. This refined WGS84 realization
is denoted as WGS84 (G730), with the 'G' indicating that the
frame is GPS derived and '730' denoting the GPS week number 
when the new coordinates where implemented by DMA in their orbit
processing. Moreover, the original WGS84 GM value was replaced by the 
value given in the IERS 1992
standards 
\cite[see][the value there is $3986004.418 \cdot 10^8$ m$^3$/s$^2$]{mccarthy92}.
The introduction of this new frame for GPS resulted in more precise ephemeris
in the GPS broadcast messages.
 
For the original realization of the WGS84, transformation parameters
between the ITRF and WGS84 are available for the ITRF90.
Based on a comparison of ITRF and WGS84 (G730) and the later
WGS84 (G873), it is found that these new WGS84 realizations
are coincident with ITRF at about the 0.1 meter level. For these
realizations there are no official transformation parameters
available. This means that one can consider ITRF coordinates also
to be expressed in WGS84 at the 10 cm level.

The importance of the ITRF as the most accurate global reference frame is increasingly
acknowledged outside the scientific community. For example, the European Commission and
the Government of the United States of America recently agreed to align the
reference systems of Galileo and GPS as close as possible to the ITRS, in
order to ensure the interoperability of the two GNSS \cite[]{europeancom2004}.