TheInternational Terrestrial Reference System (ITRS) describes procedures for creatingreference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that aphysical standard might be described as a set of procedures for creating arealization of that standard. The ITRS defines ageocentric system of coordinates using theSI system of measurement.
AnInternational Terrestrial Reference Frame (ITRF) is a realization of the ITRS. Its origin is at the center of mass of the whole earth including the oceans and atmosphere. New ITRF solutions are produced every few years, using the latest mathematical and surveying techniques to attempt to realize the ITRS as precisely as possible. Due toexperimental error, any given ITRF will differ very slightly from any other realization of the ITRF. The difference between the latest as of 2006WGS 84 (frame realisation G1150) and the latest ITRF2000 is only a few centimeters and RMS difference of one centimeter per component.[1] ITRFs areEarth-centered, Earth-fixed (ECEF) reference frames.
The ITRS and ITRF solutions are maintained by theInternational Earth Rotation and Reference Systems Service (IERS). Practical navigation systems are in general referenced to a specific ITRF solution, or to their own coordinate systems which are then referenced to an ITRF solution. For example, theGalileo Terrestrial Reference Frame (GTRF) is used for theGalileo navigation system; currently defined as ITRF2005 by theEuropean Space Agency.[2]
The ITRF realizations developed from the ITRS since 1991 include the following versions:[3]
| Name | Ref. epoch | EPSG code | Notes |
|---|---|---|---|
| ITRF91 | 1988.0 | 4913 79038991 | |
| ITRF92 | 1988.0 | 4914 79048992 | First realization of the ITRS |
| ITRF93 | 1988.0 | 4915 79058993 | |
| ITRF94 | 1993.0 | 4916 79068994 | |
| ITRF96 | 1997.0 | 4917 79078995 | |
| ITRF97 | 1997.0 | 4918 79088996 | |
| ITRF2000 | 1997.0 | 4919 79098997 | First solution that combines unconstrained space geodesy solutions free from anytectonic plate motion model.[4] From this version onwards, the motion of the tectonic plate is represented in the solution for each station as a velocity vector. Previous ITRFs only continued the initial positions, using a motion model to fill in the velocity. |
| ITRF2005 | 2000.0 | 4896 79108998 | Constructed with input data under the form of time series of station positions and Earth Orientation Parameters.[5] This version introduces extra parameters to describe the year-periodic motion of the stations:A (amplitude) andφ (phase) per-axis. This sort of seasonal variation has an amplitude of around 1 cm and is attributed to non-tidal loading effects (e.g. the shifting weight of water). |
| ITRF2008 | 2005.0 | 5332 7911 8999 | Includes tropospheric modeling and improved solution methods.[6] |
| ITRF2014 | 2010.0 | 7789 7912 9000 | Generated with an enhanced modeling of nonlinear station motions.[7] Specifically:
|
| ITRF2020 | 2015.0 | 9988 99899990 | [8] |
GNSS systems:[2]
National systems:
The GPS reference epoch was moved from 2000.0 to 2001.0 in G1150 due to the magnitude 7.9 Denali Fault earthquake in Alaska in November 2002. Still in 2022 ITRF2020 was released, yet GPS was only using G2139 in its antennas, which was aligned to ITRF2014 (IGb14) (though at epoch 2016.0, not reference epoch 2010.0).[9] On 7 January 2024 move to IGS20 happened, so WGS 84 is now aligned with ITRF2020, including PSD (post-seismic deformation), also called G2296.
On the other hand GLONASS is using PZ-90.11, which is close to ITRF2008 at epoch 2011.0 and is using 2010.0 epoch (that means when you use reference transformation to PZ-90.11 you will get January 2010 date).