The differential GPS (DGPS) method is based on knowledge of the highly accurate, geodetically surveyed location of a GPS reference station, which observes GPS signals in real time and compares their ranging information to the ranges expected to be observed at its fixed point. The differences between observed ranges and predicted ranges are used to compute corrections to GPS parameters, sources of error, and/or resultant positions. Differential corrections can be broadcast to GPS users, who can apply the corrections to their received GPS signals or computed positions. Corrections can also be stored for later analysis and dissemination. Differential techniques are used in many civilian applications to eliminate the effects of SA.
Instead of using C/A-code pseudoranging information to obtain a position solution, many GPS receivers also measure the L-band carrier signals. This technique, known as carrier phase tracking, works by determining which portion of the 19 centimeter L1 and 24 centimeter L2 carrier waves are striking the antenna at a given instant in time, thus revealing the phase of the received signals. With subsequent signal processing that includes squaring or cross-correlating the carrier waves, carrier phase tracking can produce very precise measurements, sometimes as good as 1 to 5 millimeters. Thus, the technique is valuable for high-performance applications. The difficulty with using carrier phase tracking is determining the exact number of carrier wave cycles along the path from the GPS satellites to the receiver's antenna, also known as ambiguity resolution. For static positioning, this can be accomplished by various techniques that include knowing the approximate location of the antenna and simultaneously tracking signals from all satellites in view of the antenna. The process is more difficult for real-time dynamic positioning but can still be used. Because GPS receivers using this technique give civilian users access to both the L1 and L2 frequencies without accessing the codes transmitted on L1 and L2, they are often referred to as “codeless” receivers.
A variety of federal, state, county, and public sector organizations and their counterparts in other countries are establishing or planning to use networks of GPS reference stations, utilizing both differential and carrier phase tracking techniques, for either real-time navigation or post-processed positioning. The use of GPS networks for research in the Earth and oceanic sciences has been well established for a number of years. For example, the National Aeronautics and Space Administration (NASA) and other organizations from various nations have established the International GPS Service for Geodynamics, a network of more than 140 continuously operating reference stations, data centers, and analysis centers that collectively support geophysical and geodetic research, such as the measurement of active tectonic processes, ice sheet movements, changes in sea level, and variations in the Earth's rotation.
Ground-based GPS networks and receivers on board low Earth orbiting (LEO) satellites are also being used to sense the atmosphere by measuring the delay encountered as GPS signals pass through the troposphere and the ionosphere. Water vapor measurements made with GPS-based remote sensing may be important for weather forecasting and research on global climate change.