center) for the ITRF origin, estimates of the geocenter location (and its variations owing to seasonal mass redistribution on the Earth’s surface, an important geophysical signal in itself) still need to be improved for all the geodetic techniques. Because the ITRF relies on SLR to define its origin and on SLR and VLBI for its scale, the importance of these two techniques for ITRF accuracy and stability over time should not be underestimated. Hence, the problems of scale and origin stability that can particularly affect GNSS/GPS techniques can be overcome by careful alignment to the ITRF, which in turn requires sufficient overlap in networks at co-located sites. Unfortunately, the current SLR and VLBI networks and their co-locations are already poorly distributed and are decreasing over time, posing a threat for the long-term stability of the ITRF. For example, the analysis of the ITRF of 2005 and the pre-2008 analysis showed that the poorly distributed SLR and VLBI networks and scale bias up to 1 part per billion (corresponding to 6 millimeters) and a scale drift up to 0.1 part per billion per year (0.6 millimeters per year). This drift is considerably larger than the science requirement (less than 0.1 millimeters per year) to measure sea level change (see Table 3.1).
Thus, the ITRF is based on information derived from a combination of multiple geodetic techniques. As described in Chapter 4, however, each technique has its own unique targets; VLBI observes quasars, SLR ranges to selected laser geodetic satellites, and GNSS/GPS depends on the navigation satellites. Though this may change in the future, no technique currently contributing to the ITRF has a direct connection to any other technique. Each realizes its own internally consistent set of coordinates, but it is only through local ties at co-located sites that a completely resolved reference frame is realized. As a result, the ITRF quality will suffer from any network degradation over time because it is heavily dependent on the network configuration. The current configuration of co-located sites (in particular, sites with three and four co-located techniques) is far from optimal. The following sections describe the current configuration of co-location sites, including their quality, number, and distribution.
A co-location site is defined by the presence of two or more geodetic instruments occupying simultaneously or subsequently very close locations. These locations must be precisely surveyed in three dimensions, using either classical geodetic methods (usually angles, distances, and leveling measurements between instrument reference points or geodetic markers) or GNSS/GPS (Altamimi, 2005). The national agencies that operate geodetic instruments generally perform least-squares adjustments of local surveys to yield the local ties that connect co-located instrument reference points. Geodetic markers are unambiguous reference points for which geodetic coordinates can be determined. Markers can be either a well-defined physical point anchored in a geodetic monument (such as a pillar or pole) or an instrument reference point (for example, the intersection of axes of an SLR telescope or VLBI antenna, or a GNSS/GPS or DORIS antenna reference point).
Inter-marker distance and accuracy of the local tie are the two main criteria that must be considered for the definition of a co-location site (Altamimi, 2005). Given the need for local tie vectors to be precise at the 1-millimeter level, and considering the increase in atmospheric refraction as a function of increased station separation, the distances between geodetic markers at co-location sites should not exceed 1 kilometer. In addition, repeat surveys of the marker “footprint” are necessary for long-term local tie stability. The current reality, however, is sub-optimal. The poor geographic distribution and insufficient number of co-location sites forces geodesists, for the purpose of the ITRF determination, to consider stations to be co-located even when separated by up to 30 kilometers (for example, the Tidbinbilla/Orroral complex site in Australia). In terms of accuracy, the typical uncertainty of the local ties used for the current ITRF is 2–5 millimeters (sometimes larger than 5 millimeters for the less precise ties). With the increased precision available from geodetic techniques, a precision of 1 millimeter or better should be the goal of all new local tie surveys.