Geoscience Reference
In-Depth Information
(DMA) has proposed to replace the value of
G
in the WGS 84 by the standard
IERS value and to refine the coordinates of the GPS tracking stations. The
revised WGS 84, valid since January 2, 1994, has been given the designation
WGS 84 (G 730), where the 'G' indicates that the respective coordinates
used were obtained through GPS and the following number 730 indicates
the GPS week number when DMA has implemented the refined system.
In 1996, the U.S. National Imagery and Mapping Agency (NIMA) - the
successor of DMA - has implemented a revised version of the frame denoted
as WGS 84 (G 873) and being valid since September 29, 1996. The frame is
realized by monitor stations with refined coordinates. The associated ellip-
soid and its gravity field are now defined by the four parameters
a, f, GM, ω,
which are slightly different compared to the respective ITRF values, e.g., the
current WGS 84 (G 873) frame and the ITRF97 show insignificant systematic
differences of less than 2 cm. Hence, they are virtually identical.
Note that the refinements applied to the WGS 84 reference frame have
reduced the uncertainties in the coordinates of the frame, the uncertainty
of the gravitational model, and the uncertainty of the geoid undulations;
however,
they have not changed the WGS 84 coordinate system in the sense
of definition !
More general, the relationship between the WGS 84 and the ITRF is
characterized by two statements: (1) WGS 84 and ITRF are consistent; (2)
the differences between WGS 84 and ITRF are in the centimeter range world-
wide (National Imagery and Mapping Agency 2000).
However, if a transformation between reference frames is required, this
is accomplished by a datum transformation (see Sect. 5.7).
Numerical values for the WGS 84 (reference frame)
As mentioned at the very beginning of Sect. 2.11, the reference ellipsoid and
its gravity field are completely determined by four constants. The current
defining parameters for WGS 84 are listed in Table 2.3.
Table 2.3. Defining parameters of the WGS 84
Parameter and value
Description
a
= 6 378 137 m
semimajor axis of the ellipsoid
f
=1
/
298
.
257 223 563
flattening of the ellipsoid
10
8
m
3
s
−
2
GM
= 3 986 004
.
418
·
geocentric gravitational constant of
the earth (including the atmosphere)
10
−
11
rad s
−
1
ω
= 7 292 115
·
angular velocity of the earth
