International Terrestrial Reference System and Frame

File:ITRF-Fundamental-u.Satellitenstationen,46 po.jpg

ITRF reference stations

The International Terrestrial Reference System (ITRS) describes procedures for creating reference frames suitable for use with measurements on or near the Earth's surface. This is done in much the same way that a physical standard might be described as a set of procedures for creating a realization of that standard. The ITRS defines a geocentric system of coordinates using the SI system of measurement.

An International 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 to experimental error, any given ITRF will differ very slightly from any other realization of the ITRF. The difference between the latest WGS 84 and the latest ITRF (as of 2006) is only a few centimeters.

The ITRS and ITRF solutions are maintained by the International 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, the Galileo Terrestrial Reference Frame (GTRF) is used for the Galileo navigation system; currently defined as ITRF2005 by the European Space Agency.

Versions

The ITRF realizations developed from the ITRS since 1992 include the following versions:

Name	Epoch	EPSG code	Notes
ITRF92	1988.0	4914	First realization of the ITRS
ITRF93	1988.0	4915
ITRF94	1993.0	4916
ITRF96	1997.0	4917
ITRF97	1997.0	4918
ITRF2000	1997.0	4919	First solution that combines unconstrained space geodesy solutions free from any tectonic plate motion model 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 contined the initial positions, using a motion model to fill in the velocity.
ITRF2005	2000.0	4896	Constructed with input data under the form of time series of station positions and Earth Orientation Parameters. This version introduces extra parameters to describe the year-periodic motion of the stations: A (amplitude) and φ (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	Includes tropospheric modeling and improved solution methods.
ITRF2014	2010.0	7789	Generated with an enhanced modeling of nonlinear station motions. Specifically: a semiannual component is added to the traditional annual periodic model; smooth parametric fits are to model post-seismic deformation as opposed to the traditional approach using piecewise linear functions.

Users

GNSS systems:

• Galileo Terrestrial Reference Frame (GTRF), ITRF2005; own implementation using IGS sites.
• GPS WGS 84 of 2013, ITRF2008; International GNSS Service (IGS) implementation.
• BeiDou Coordinate System, China Terrestrial Reference Frame (CTRF) 2000 = ITRF97 at epoch 2000.0; own implementation.
• GLONASS PZ-90.11 is nominally its own system, but is quite close to ITRF and uses many of the same techniques.

National systems:

• United States: WGS 84 (see above); domestic use is mainly based on NAD 83 instead.
• China: CTRF 2000 per above.

See also

• ECEF
• Geodetic system
• International Celestial Reference System and Frame
• Terrestrial reference frame
• World Geodetic System

External links

• What is ITRF?
• Terrestrial reference systems and frames (chapter 4 of IERS Conventions 2010)