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 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. 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.