The “Real” Definition of “ITRF”

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:

1. Time evolution in orientation of the reference frame will create no residual global rotation with regards to the crust
2. Z-axis = The direction of the IERS Reference Pole (IRP) corresponds to the BIH CTP, Epoch 1984.0, with an uncertainty of 0.005″
3. 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:

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.