Accuracy of OPUS solutions for 1- to 4-h observing sessions

We processed 30 consecu- tive days of Global Positioning Sys- tem (GPS) data using the On-line Positioning Users Service (OPUS) provided by the National Geodetic Survey (NGS) to determine how the accuracy of derived three-dimen- sional positional coordinates de- pends on the length of the observing session T, for T ranging from 1 h to 4 h. We selected five Continuously Operating Reference Stations (CORS), distributed widely across the USA, and processed the GPS data for each with corresponding data from three of its nearby CORS. Our results support the current OPUS policy that recommends using a minimum of 2 h of static GPS data. In particular, 2 h of data yiel- ded a root mean square error of 0.8, 2.1, and 3.4 cm in the north, east, and up components of the derived positional coordinates, respectively. Results drastically improve for solutions containing 3 h or more of GPS data.