Gravity field of the earth - Physical Geodesy - page 84

Reference frames: WGS 84 and ITRF
Now we need the distinction between definition and realization . When using
the term “coordinate system” or “reference system”, then this implies the
definition only; however, when using the term “coordinate frame”, then a
realization is implied (Mueller 1985). So far, we have only given a definition
of the WGS 84; therefore, we ought to denote this as WGS 84 CTRS. Now
we consider a realization and, therefore, use the term “coordinate frame”.
Following closely National Imagery and Mapping Agency (2000) and
Hofmann-Wellenhof et al. (2001: Sect. 3.2.1), an example of a terrestrial
reference frame is - on the basis of the previous definition - the WGS 84
reference frame (often simply denoted as WGS 84 - as we will also do).
Associated to this frame is a geocentric ellipsoid of revolution, originally de-
fined by the four parameters (1) semimajor axis a , (2) normalized second
degree zonal gravitational coecient C 20 , (3) truncated angular velocity of
the earth ω , and (4) earth's gravitational constant G . This frame has been
used for GPS since 1987.
Another example for a terrestrial reference frame is the one produced
by the IERS and is called International Terrestrial Reference Frame (ITRF)
(McCarthy 1996). The definition of the axes is analogous to the WGS 84, i.e.,
the Z -axis is defined by the IERS Reference Pole (IRP) and the X -axis lies
in the IERS Reference Meridian (IRM); however, the realization differs! The
ITRF is realized by a number of terrestrial sites where temporal effects (plate
tectonics, tidal effects) are also taken into account. Thus, ITRF is regularly
updated (almost every year) and the acronym is supplemented by the last
two digits of the last year whose data were used in the formation of the
frame, e.g., ITRF89, ITRF90, ITRF91, ITRF92, ITRF93, ITRF94, ITRF95,
ITRF96, ITRF97, or the full designation of the year, e.g., ITRF2000.
The comparison of the original WGS 84 and ITRF revealed remarkable
differences (Malys and Slater 1994):
1. The WGS 84 was established through Doppler observations from the
TRANSIT satellite system, while ITRF is based on Satellite Laser
Ranging (SLR) and Very Long Baseline Interferometry (VLBI) obser-
vations. The accuracy of the TRANSIT reference stations was esti-
mated to be in the range of 1 to 2 meters, while the accuracy of the
ITRF reference stations is at the centimeter level.
2. The numerical values for the original defining parameters differ from
those in the ITRF. The only significant difference, however, was in
the earth's gravitational constant G WGS − G ITRF =0 . 582 · 10 8 m 3 s − 2 ,
which resulted in measurable differences in the satellite orbits.
On the basis of this information, the former U.S. Defense Mapping Agency