The International Terrestrial Reference Frame (ITRF) “yy” is a realization of the International Terrestrial Reference System (ITRS), where the “yy” represents the year of the solutions. Since its fi rst realization in 1988, there have been many variations and additions in use of data types and changes in computational strategy.

International Earth Rotational Service (IERS), successor to Bureau International de L’ Heure (BIH), Paris, France, has so far performed 10 solutions for the years 88, 89, 90, 91, 92, 93, 94, 96, 97, and 00. But, in the 10 solutions, IERS has realized only three reference frames for the three Reference Epochs (RE):

Note:

1. “ITRFyy” without the reference epoch (nn.0) does NOT completely identify the “ITRF”.

2. ITRF88 to ITRF93 were realized for the same RE (88.0) and thus, these six solutions realize the same “ITRF”. There is no geodetic validity in a 7- parameter transformation between them.

3. ITRFyy (nn.0) does not constitute a geodetic system by itself. It is an accurate realization of the Earth’s “TRF” for the specifi ed reference epoch. This is available for defi ning and establishing national, regional,
continental, and/or world geodetic system.

4. When the ITRFyy (nn.0) is used to defi ne and establish a geodetic system, the RE “nn.0” does not change, as the defining “RF” remains the same; only the time epoch of the GPS network adjustment is attributed to the newly adjusted geodetic system.

Important Consideration: The three IERS realized “ITRF (88.0, ITRF (93.0), and ITRF (97.0)” are defined consistent to the BIH realized Conventional Terrestrial System (CTS), Epoch 84.0. But, the users should note that these four “RFs” have different origin and orientation.

Clarifi cations:

1. The origin is at the Earth’s center of mass.

2. The reference meridian is a new realization. It is not to be associated to the old longitude of the telescope of the Greenwich Observatory.

3. The ITRFyy (nn.0) is NOT based on the International Association of Geodesy (IAG) defi ned Geodetic Reference System (GRS) 1980.

4. ITRFyy is NOT a vertical datum.