system and thereby defines the ITRF origin and contributes to the ITRF scale. GPS contributes the large number of sites that define the ITRF (contributing to its density) and contributes to precise monitoring of polar motion. GPS, DORIS, and SLR are used to position space-orbiting platforms in the ITRF, and GPS is used to position instruments on the Earth’s land and sea surfaces (for example, tide gauges and buoys). Locating instruments for two or more techniques near each other at certain ITRF sites (a practice called “co-location”) enables connectivity between these techniques.
None of the space geodesy techniques alone is capable of providing all the necessary parameters for ITRF definition (origin, scale, and orientation). Although satellite techniques are sensitive to the center of mass of the entire Earth system (a natural ITRF origin and the point around which a satellite orbits), the VLBI technique is not (its ITRF origin is arbitrarily defined through mathematical calculations). The scale is dependent on the modeling of some physical parameters (such as troposphere or ionospheric refraction), and the absolute ITRF orientation (unobservable by any technique) is conventionally defined through specific mathematical constraints, typically to try to realize no-net or zero-average rotation with respect to the bulk of the Earth’s mass. Multi-technique combinations are therefore essential for the ITRF determination.
The most critical ITRF parameters of interest to mean sea level studies in particular, and other investigations in general, are the origin and scale and their long-term stability. For example, any scale bias in the ITRF definition propagates directly to the height component of the stations, and any scale and/or origin bias will directly map to the mean sea level estimation (Beckley et al., 2007). Although SLR currently provides the most accurate realization of the Earth’s long-term center-of-mass (the geo-