Since 1988, the International Earth Rotation Service (IERS) has realized 11 International
Terrestrial Reference Frame (ITRF), viz., ITRF88 to ITRF05. While studying the geodetic details
about the latest realization, an insight into the "real" de? nition of an "ITRF" was discovered.
To
create no residual global rotation with regards
to the crust in time evolution in orientation,
IERS in the first ITRF88 retained the BIH Conventional
Terrestrial System (CTS) and its Conventional
Terrestrial Pole (CTP) 1984.0. Since then, it
has realized the same "Pole". However, IERS changed
the name "CTP" to International Reference Pole
(IRP).
Historical Start
The Earth's first Terrestrial Reference
Frame (TRF) was called Conventional
International Origin (CIO) 1905 where the
mean orientation of the Z-axis was defi ned
by International Latitude Service from
six years of observations between 1900 to
1905 (Note: There is NO other "CIO" ever
defi ned). The records are not clear whether
ILO ever provided a specifi c defi nition
for the X-axis and/or the zero meridian.
BIH Conventional Terrestrial System (CTS)
Then, Bureau International de L'Heure (BIH)
provided the next most complete 3-D TRF, known
as CTS, Epoch 1984.0 (Figure 1).
The full defi nition of CTS 84.0 is:
Origin = Earth's Center of Mass
Z-axis = The direction of the BIH Conventional
Terrestrial Pole (CTP), Epoch 1984.0
X-axis = Intersection of the BIH Zero Meridian
and the plane containing the Earth's origin
"O" and perpendicular to the Z-axis or the Equatorial
plane
Y-axis = The third axis, which is perpendicular
to the Z- and X-axis, lies in the Equatorial
plane, and points towards East.
The above three axes de. ne a 3-D geocentric right-handed coordinate system. (In the "right-handed" system, the right thumb points towards the
Pole, index . nger aligns along the
X-axis, and middle . nger along the Y-axis or points towards East.)
International Terrestrial
Reference Frame (ITRF)
The International Earth Rotation
Service (IERS) succeeded BIH in
1988. It then realized the fi rst TRF and
released under the name as ITRF88
(or ITRFyy). Here, the "yy" denotes
the "year" up to which the data sets
have been used in the realization.
IERS released six more ITRF89,
ITRF90, ITRF91, ITRF92, ITRF93, and
ITRF94 over the next six years. Then,
with a break for 1995, IERS released
ITRF96 and ITRF97. There after, the
next two are ITRF2000 and ITRF2005.
Note: IERS did not and still does not
release or identify specifi c information
about the Reference Epoch (RE) associated with each of its ITRF solution
from 1988 to 2005.
Definition of ITRFyy
The IERS Tech Notes 21
(McCarthy, 96) states:
Time evolution in orientation of
the reference frame will create
no residual global rotation
with regards to the crust
Z-axis = The direction of
the IERS Reference Pole
(IRP) corresponds to the BIH
CTP, Epoch 1984.0, with
an uncertainty of 0.005"
X-axis = IERS Reference
Meridian (IRM) is coincident
with the BIH Zero Meridian,
Epoch 1984.0, with an
uncertainty of 0.005".
Thus, the above defining specifications
clearly and categorically laid down
that the first seven realizations,
viz., ITRF88 to ITRF94, are ONLY
the IERS updated realizations of
the BIH defined "CTS 1984.0".
Note: IERS retained the 3-D "ZXY"
coordinate frame (Figure 1) intact
and kept the "correspondence and
coincidence" with the BIH CTS
84.0 for all its 11 realizations.
The Associated Reference
Epoch "RE"
It was around mid-2000, information
about the associated "RE" pertaining
to the ITRF88 to ITRF97 was obtained
for the first time from IERS (Altamimi,
00). Later on, the "RE" information
for ITRF00 and ITRF05 was obtained
from IERS website. The specifics are:
ITRFyy
Associated "RE"
ITRF88, ITRF89, ITRF90,
ITRF91, ITRF92, ITRF93
1988.0
ITRF94, ITRF96
1993.0
ITRF97, ITRF00
1997.0
ITRF05
2000.0
Contribution of "RE"
For ITRF88 (88.0), IERS provides the
coordinates and velocities (X, X') for
the IGS stations at the RE = 88.0, which
in turn enabled a user to "realize" the
BIH CTS 84.0. And, the set (X, X') for
ITRF89 (88.0) would be the same as of
the previous year, but for a few additional
stations and realized with "additional"
data sets. In case of ITRF94 (93.0), the
set (X, X') would be updated for 5-
year time difference between the two
associated "RE", viz., 88.0 and 93.0.
Similarly, the latest ITRF05 (00.0)
now provides a new "updated" set of
(X, X') with respect to an new "RE =
00.0" for realizing BIH CTS 84.0.
Confirmation
The transformation parameters between
ITRF00 and ITRF05 (www.IERS.
org) confi rm that the two "TRF" have
the same origin, orientation, and scale
and the latest ITRF05 has successfully
realized the BIH CTS 84.0.
Note: Transformation parameters
between any two ITRFs have no
geodetic signifi cance for any practical
and/or non-scientifi c application(s).
Important Clarifications
Any ITRF does not constitute
a geodetic system. It is adopted
to defi ne the system.
ITRF is neither a horizontal
nor a vertical datum.
Conclusion
All 11 realizations, viz., ITRF88 to
ITRF05, provide users essentially four
sets of (X, X') for four "RE", viz.,
88.93.0, 97.0, and 00.0, "leading"
users to the BIH CTS 84.0.
References
Altamimi, Z., 2000. "Personal e-mail exchanges".
McCarthy, D., 1996. "IERS Conventions,
1996", IERS Tech Notes 21.
Muneendra
Kumar, PhD
Chief Geodesist (Retired), US National Geospatial-Intelligence
Agency, munismk@yahoo.com
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