Accurate positioning is crucial to hydrographic surveying. Collecting depth soundings have no meaning unless they are associated with a position on the Earth's surface. Horizontal control is the use of a positioning system to determine the location of a sounding. The RAINIER uses the Differential Global Positioning System (DGPS) to obtain the latitude, longitude, and time of each acquired sounding.

A positioning accuracy of 95% confidence that does not exceed 5 meters plus 5 percent of depth is a required standard for the RAINIER's hydrographic surveys. The RAINIER and her launches use Ashtech GPS receivers and a positioning system called POS/MV (Position and Orientation System for Marine Vessels) to obtain positioning navigation and attitude data for each sounding.

The ship and its launches normally pick up corrected GPS signals from U.S. Coast Guard Differential GPS Beacon Stations. Sometimes, however, there are areas where these vessels cannot pick up station signals dues to remote locations or steep topography. In these situations, the RAINIER sets up "fly-away" DGPS stations.

A fly-away station is a portable unit, consisting of Ashtech GPS dual frequency receivers that obtain positioning data directly from GPS satellites. The station consists of a VHF broadcast tower, Ashtech receivers, computers for data downloads, and batteries. The station is usually set up in an open space on high ground.