decade. Determining local vertical rates at such a level of accuracy remains challenging, however, partly because of challenges involved in maintaining the stability of the reference frame and maintaining frequent leveling (at least annually) between the GNSS/GPS antenna and the tide gauge in places where these are not closely co-located. Regular leveling surveys are often neglected over time, particularly in places where the distance between the two instruments is large. Where this distance is more than one kilometer leveling ties error can be large and become a significant part of the error budget. Thus, except for sites with well-established local or regional stability, GNSS/GPS stations more than one kilometer from the tide gauge cannot really be considered as co-located. In many cases the GNSS/GPS station may need to be located on or very close to the tide gauge to represent its vertical motion accurately
Recommendation: The Committee recommends that the United States support the TIGA initiative and similar efforts to accurately determine the vertical motion of reference tide gauges.
Artificial satellites are an essential component of the precise geodetic infrastructure. These satellites provide the link between the global reference frame and the end user of geodetic products. Indeed, without the satellite component there would not be an ITRF. The artificial satellites can be broadly divided according to the measurements they provide (Table 4.1). GNSS/GPS satellites are used for precise point positioning anywhere on or near Earth’s surface (including low-Earth orbit). Satellite altimetry (radar or laser) provides elevation profiles of ocean, land, and ice surfaces at accuracies needed to monitor, for example, global mean sea level or polar ice sheet changes. Inter-