due to tectonic plate dynamics. The “orientation” essentially refers to the definition of the zero point for longitude and latitude. Unlike origin and scale, which are determined directly by the geodetic observations, we are free to choose any point on Earth as zero longitude or zero latitude. We try to maintain consistency with the historical Greenwich meridian for zero longitude and Earth’s equator as zero latitude, which is perpendicular to the Earth’s spin axis, but these directions cannot be fixed due to plate tectonic motion and polar motion. Therefore, by convention the orientation of the ITRF is designed to maintain consistency with previous reference frames and specified by the requirement of no-net or zero-average rotation with respect to horizontal plate motion of the Earth’s surface. This is difficult to realize in practice due to the limited number of sites distributed on the various continental plates, which are all moving above Earth’s mantle and core.
Continuous, long-term geodetic observations are crucial if the ITRF is to account correctly for the complex movements of points on the surface of the Earth, so that we can characterize and model these movements precisely. In the absence of technique-specific systematic errors, and if all geophysical processes are accurately accounted for in the geodetic analysis, the ITRF properties should be stable over time (that is, they should not exhibit any drift or discontinuities over the time-span of the geodetic observations; see Box 5.1). Any deficiencies in the accuracy or continuity of the ITRF will limit the quality of science that it can support.
The stability and accuracy of the ITRF over long time periods is a primary limiting factor for understanding sea level change, land subsidence, crustal deformation, and ice sheet dynamics. Of these, a quantifying long-term change in sea level imposes the most stringent observation requirements. The ITRF constitutes the foundation connecting observations in space, time, and evolving technology, and provides the framework in which global and regional observations of sea level change can be understood and properly interpreted. A stable ITRF is required if sea level measurements at sub-millimeter accuracy made today are to be meaningfully compared with measurements made a decade from now. The ITRF also can be extended to regional and local studies in order to link multidisciplinary observations and ensure long-term consistency, precision, and accuracy (see Box 5.1). For the ITRF to accurately quantify long-term sea level change, the ITRF must be both accurate and accessible at the 1-millimeter level, with a stability of 0.1 millimeters per year.
Given that existing reference frames have not achieved this level of accuracy and stability, it is not surprising that one of the largest sources of error in the global characterization of long-term sea level variation is uncertainty in the ITRF. For example, a 2-millimeter-per-year error in the relative velocity between the Earth’s mean surface and the Earth system’s center of mass can result in an error as large as 0.4 millimeters per year in the determination of mean global sea level variation using satellite altimetry (see Table 3.1). The effect on measuring local or regional sea level can be even larger. A scale rate error of 0.1 parts per billion per year would cause an apparent sea level change of 0.6 millimeters per year. To put this in context, the mass loss from the Greenland ice sheet is estimated to be on the order of 200 gigatons per year on average over the last few years, corresponding to approximately 0.7 millimeters per year of rise in global mean sea level; Antarctica is losing a similar amount of ice (Velicogna, 2009). Thus, the uncertainty in the observation of sea level change due to errors in the ITRF is currently almost at the same level as the contribution of either ice sheet to sea level rise. Furthermore, there is evidence that the rate at which ice sheets lose mass is increasing by approximately 30 gigatons per year (corresponding to approximately 0.1 millimeters per year increase in sea level rate) (Velicogna, 2009). Improving the ITRF is, therefore, of paramount importance for the study of global sea level rise and its possible acceleration.