Drivers, priorities, and best practices for maintenance and expansion of the global network of GNSS stations co-located with tide gauges

Geocentric vertical motion measured by GNSS stations enhances the interpretation and utility of relative sea level measurements from tide gauges (TG). In particular, GNSS-derived rates of vertical motion allow for separation of sterodynamic contributions to local sea level change from geodetic and tectonic contributions. Not only is this separation essential for accurate coastal impact assessments during coming decades, but the global set of colocated GNSS-TG pairs (cGNSS) contributes to ongoing efforts to improve reconstructions of 20th century global mean sea level and is essential for identifying drifts and biases in satellite altimetry. Despite the benefits of colocated systems, the number of cGNSS stations with direct vertical ties to tide gauge benchmarks remains limited. Expansion of the cGNSS network is warranted, but observing system requirements for the joint analysis of GNSS and TG data vary depending on application and location. Here, we review and update the quantitative basis for guidance from the Global Sea Level Observing System (GLOSS) on the expansion and maintenance of the global cGNSS network. Topics include prioritization of TG sites to receive new cGNSS, the need for intercomparison and assessment of differing GNSS solutions at TG sites, and best practices for installation. These overarching principals will be supplemented with specific examples of challenges and successes in the installation and maintenance of colocated GNSS-TG pairs during an ongoing partnership between the University of Hawaii Sea Level Center and the Pacific GPS facility.