The first and original version
of the "WGS 84", defined by
a special committee of the
Defense Mapping Agency
(DMA), was released in September
1987. As this task of updating the WGS
72 was concurrent with development
of the North American Datum (NAD)
1983, the committee members always had many in-depth discussions with the members of the special committee of the National Geodetic Survey (NGS). This approach ensured the; correct geodetic definition both for
WGS 84 and NAD 83. Around 1992,
it was decided by DMA that, in
future update(s) of the "WGS 84" for
accuracy enhancement, the academia
and other satellite geodesy experts
would be associated. However, that scientific
participation
was not
followed and
three subsequent
updates were
carried out without
in-depth discussions of satellite
geodetic theory and/or correct
statistical evaluation. The non-scientific
procedure(s) allowed definition
deficiencies to creep in. This paper
outlines the geodetic details of the
three updated versions of 1994, 1996,
and 2001 and brings out in "open"
the definition deficiencies in the
current version WGS 84 (G1150),
which otherwise will remain hidden
within the National Geospatial-
Intelligence Agency (NGA).
The correctly defined "WGS 84", the
coordinate system used in GPS, is a
critical requirement for the geodetic
integrity and accurate GPS positioning.
1984 "Original" Definition
The WGS 84 was originally defined
with BIH Conventional Terrestrial
System (CTS) for Reference Epoch
"RE (84.0)". The main satellite
data sets used were from the
Navy Navigation satellite System
(NNSS). At the time of release in
1987, the accuracy achieved was
in the order of ± 1 - 2 meter and as
such the tidal effects, as specified
in the International Association
of Geodesy (IAG) Resolution 16
of 1983 were not considered.
The "Three" WGS 84 Updates
1994 "WGS 84 (G730)"
This version was updated with the International Earth Rotation Service
(IERS) realized International Terrestrial
Reference Frame (ITRF) 19921, RE
(88.0). During this update, NGA moved
the RE (88.0) of the defining ITRF to
RE (94.0), which is incorrect. For this
"change", DMA geodesists did not
have the capability and expertise. And,
they did not have the authority to override
IERS. Note: With a new origin
and orientation of its three axes, WGS
84 (G730) is geodetically a different
coordinate system than the original
WGS 84. For mapping, the two could
be considered the same. 1 First six ITRF
solutions, viz., ITRF 1988, ITRF 1989,
ITRF 1990, ITRF 1991, ITRF 1992,
and ITRF 1993, were realized for the
RE (88.0). As the ITRF 1993 was based
on all the data sets available up to the
end of year 1993 and thus realized in
1994, it would not have been possible
for DMA to define the WGS 84 (G730),
which was realized using the GPS data
for the week starting 2 January 1994.
1996 "WGS 84 (G873)"
At the time of this update, the ITRF
1994, RE (93.0) was used (Note:
ITRF96 (93.0) was not available). But,
DMA geodesists again incorrectly
moved the epoch of the defining
"RF" to RE (97.0). And, for geodetic
application, they created the third
WGS 84. In addition, ignoring IAG
Resolution No. 16 of 1983 and
bypassing IERS Conventions (IERS,
96), which recommend the "Zero-tide"
model, National Imagery and Mapping
Agency (NIMA) geodesists adopted an
"arbitrary" practice to use "Tide-free"
model. Note: According to IERS, the
positions in the "Tide-free" environ
are non-realistic and not observable.
2001 Current "WGS 84 (G1150)"
During the updating of this version,
the ITRF00, RE (97.0) was used.
But, like the 1994 and 1996 versions,
NIMA geodesists incorrectly moved
the RE of the defining RF from (97.0)
to (01.0), They also kept the 1996
practice for "Tide-free" model, even
after being alerted that the world's
eminent geodesists support the "Zerotide"
of the IAG' standing Resolution
No. 16 of 1983. Furthermore, during the adjustment of the GPS tracking
stations network, about 65% stations
were held fixed (Note: An objection by
the first author was not even discussed).
This over constrained adjustment is
statistically incorrect and not acceptable.
Note: For geodetic positioning, this
is the fourth version of WGS 84.
The "Version" Identifiers
The "G730", "G873", and "G1150"
indicate the GPS-week, of which
the data sets were used to realize the
three updates. As these "identifiers"
do NOT specifically identify any
definite time epoch, they do NOT
have any geodetic significance.
Important "Contrast" To Note
In SIRGAS 2000, the "RE" of the
defining ITRF has NOT been "moved".
Analytical Conclusion
The current "WGS 84 (G1150)"
is incorrectly defined, does not
comply with IAG Resolution No.
16 of 1983, and its time epoch is
not definitive. Furthermore, the
adjustment of the GPS tracking station
network is statistically incorrect.
Reference
IERS, 96 IERS Conventions,
Tech Note 21, July 1996.
Geodesists without
Boundaries
To provide analysis, interpretation,
opinion, advice, and/or consultation
concerning Geodetic System and
Datums, Maps, Charts, GPS and
Marine Positioning, Geospatial
Information System, gravity,
and International Boundaries, an
independent experts group is formed
“Geodesists without boundaries”
under the leadership of Dr Muneendra
Kumar, Nat. Geospatial-intelligence
Agency (Retired), USA. The other
members of the groups are Prof Erik
W Grafarend, University of Stuttgart
(Retired), Germany; Prof James P
Reilly, New Mexico State University
(Retired), USA; Dr. Manohar G Arur,
Survey of India (Retired), India;
Dr Siva Gopal, Battelle Memorial
Institute (Retired), USA. For details
contact, Dr Muneendra Kumar, munidmk@kmapsystems2005.info
Muneendra Kumar,
PhD, Nat. Geospatialintelligence
Agency
(Retd), USA, munidmk@
kmapsy
James P Reilly, PhD,
New Mexico State
University (Retd), USA, jpreilly@zianet.com
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