Geodesy is the science of measuring and monitoring the size and shape of the Earth and the location of points on its surface. NOAA's National Geodetic Survey (NGS) is responsible for the development and maintenance of the National Spatial Reference System (NSRS) that is used for positioning, navigation, mapping and charting. Positioning techniques are used to build roads, bridges, buildings, as well as monitor larger structures such as dams and suspension bridges.
NGS immediately recognized the potential of the Global Positioning System (GPS), for applications far more accurate than originally intended. Following the launch of the first GPS satellites, system engineers expected that the positional accuracy for civilian uses would be about 70 meters (230 feet). While many scientists worldwide lauded a positioning system with 10-meter accuracy, scientists at NGS, as early as 1979, were publishing papers on how to use the GPS signals for improving positioning accuracy.
NGS and other academic institutions worldwide, began research to develop highly accurate uses of GPS, known as “differential carrier phase positioning.” At first, this capability was limited solely to determining positions of two GPS receivers relative to one another, when both sat stationary for hours. Even this limited application was a revolutionary improvement over traditional surveying. No line-of-sight was required, and distances of hundreds of kilometers could be surveyed in hours, rather than the weeks or months required by traditional survey methods.
Within a few years, NGS scientists began developing a “kinematic” method of precisely locating a moving receiver, opening the door for the development of new scientific endeavors. Disciplines including airborne gravity, magnetics, radar, photogrammetry, and LIDAR owe their existence to this precise positioning method. Kinematic processing provides position accuracies within centimeters even with one of the GPS receivers moving. This new survey technique revolutionized land surveying as well, greatly multiplying surveyors’ productivity over traditional methods. Kinematic GPS is now in use throughout the world.
Continuously Operating Reference Stations
In a field of study that is thousands of years old, GPS represents a quantum leap in geodesy. As advanced as GPS technology is, most commercially available GPS receivers are only accurate within several meters. Considering that the Earth is almost 25,000 miles in circumference, the difference of a few meters may not seem important. This level of accuracy may be adequate for a hiker in the woods or someone driving a car. But there are many scientific, military, and engineering activities that require much higher levels of positioning accuracy—often to within a few centimeters or less! To provide measurements at this level of accuracy, NGS developed the Continuously Operating Reference Stations (CORS) network. CORS is a network of hundreds of stationary, permanently operating GPS receivers throughout the United States. Working 24 hours a day, seven days a week, CORS stations continuously receive GPS radio signals and integrate their positional data into the National Spatial Reference System. This data is then distributed over the Internet. After logging onto the CORS website, users can download GPS data for many post-processing applications and determine the accuracy of their coordinates to the centimeter. This system has been especially useful in assessing the integrity of buildings and bridges in areas that are geologically active or have been impacted by natural disasters such as hurricanes or floods.