All data collected during geophysical surveys must be accurately located. It is necessary to establish positions for features so that they can be mapped and correlated to features from different surveys. We rely on Differential GPS (Global Positioning System) navigation during survey operations to provide these accurate positions.
The GPS system consists of 24 to 27 satellites that orbit the earth every twelve hours in six different planes. They orbit at an altitude of approximately 20,000 km with an inclination of 55 degrees. At any time, a minimum of four satellites are above the horizon from any spot on earth. The satellite broadcasts a signal that contains the position of the satellite and the precise time the signal was transmitted. The position of the satellite is transmitted in a data message that is superimposed on a code that serves as a timing reference. The satellite uses an atomic clock to maintain synchronization of all the satellites in the constellation. The receiver compares the time of broadcast encoded in the transmission with the time of reception measured by an internal clock, thereby measuring time-of-flight to the satellite. Several such measurements can be made at the same time to different satellites, allowing a continual fix to be generated in real time (Dana, 1994).
The basic accuracy of a GPS fix is 5-7 meters. Shipboard operations use differential corrections that increase the accuracy to 2 3 meters. The differential corrections are broadcast from U.S. Coast Guard stations and are received by special antennas that are integrated into the GPS receivers. Starting in 2003, nearshore geophysical surveys conducted by the WHSC routinely use RTK (Real Time Kinematic) techniques to further increase the accuracy to 23 centimeters horizontal accuracy.
Dana, Peter H., 1994, The Geographer’s Craft Project, Department of Geography, The University of Colorado at Boulder.