The International Terrestrial Reference Frame (ITRF) is the most accurate and widely adopted global geodetic reference frame, providing the standard coordinate system for expressing positions on Earth’s surface with centimeter-level or better accuracy. Maintained by the International Earth Rotation and Reference Systems Service (IERS), ITRF serves as the foundational reference for scientific applications, GNSS satellite orbits, and high-precision positioning services worldwide.

Each ITRF realization (e.g., ITRF2014, ITRF2020) is defined by the coordinates and velocities of hundreds of geodetic stations distributed globally, determined through the combination of four space geodetic techniques: GNSS, Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). This multi-technique approach enables accurate determination of the reference frame origin (Earth’s center of mass), scale, and orientation, achieving accuracies that no single technique could attain alone.

ITRF is fundamentally time-dependent because Earth’s surface moves continuously due to plate tectonics. Station coordinates are published at a specific reference epoch (e.g., 2015.0 for ITRF2020), along with velocity vectors describing each station’s motion. Users must apply these velocities to transform coordinates to any other epoch, a critical consideration when combining data collected at different times. In tectonically active regions like California or Japan, failing to account for epoch differences can introduce errors of centimeters to meters over just a few years.

For GNSS positioning applications, understanding the relationship between ITRF and other reference frames is essential. Precise GNSS products from the IGS are provided in ITRF, and high-accuracy positioning services typically deliver coordinates in current ITRF. However, practical applications often require coordinates in regional datums (NAD83, ETRS89, GDA2020) that may be tied to specific ITRF realizations or have different velocity models. Proper coordinate transformations between ITRF and these regional frames are necessary to achieve consistent, accurate positioning aligned with local survey control and mapping products.