For differential correction to work, fixes recorded by the mobile receiver must be synchronized with fixes recorded by the base station (or stations). You can provide your own base station, or use correction signals produced from reference stations maintained by the U.S. Federal Aviation Administration, the U.S. Coast Guard, or other public agencies or private subscription services. Given the necessary equipment and available signals, synchronization can take place immediately ("real-time") or after the fact ("post-processing"). First let's consider real-time differential.
WAAS-enabled receivers are an inexpensive example of real-time differential correction. "WAAS" stands for Wide Area Augmentation System, a collection of about 25 base stations set up to improve GPS positioning at U.S. airport runways to the point that GPS can be used to help land airplanes (U.S. Federal Aviation Administration, 2007c). WAAS base stations transmit their measurements to a master station, where corrections are calculated and then uplinked to two geosynchronous satellites (19 are planned). The WAAS satellite then broadcasts differentially-corrected signals at the same frequency as GPS signals. WAAS signals compensate for positioning errors measured at WAAS base stations, as well as clock error corrections and regional estimates of upper-atmosphere errors (Yeazel, 2003). WAAS-enabled receivers devote one or two channels to WAAS signals, and are able to process the WAAS corrections. The WAAS network was designed to provide approximately 7-meter accuracy uniformly throughout its U.S. service area.
DGPS: The U.S. Coast Guard has developed a similar system, called the Differential Global Positioning Service. The DGPS network includes some 80 broadcast sites, each of which includes a survey-grade base station and a "radiobeacon" transmitter that broadcasts correction signals at 285-325 kHz (just below the AM radio band). DGPS-capable GPS receivers include a connection to a radio receiver that can tune in to one or more selected "beacons." Designed for navigation at sea near U.S. coasts, DGPS provides accuracies no worse than 10 meters. Stephanie Brown (personal communication, Fall 2003) reported that where she works in Georgia, "with a good satellite constellation overhead, [DGPS accuracy] is typically 4.5 to 8 feet."
Survey-grade real-time differential correction can be achieved using a technique called real-time kinematic (RTK) GPS. According to surveyor Laverne Hanley (personal communication, Fall 2000), "real-time kinematic requires a radio frequency link between a base station and the rover. I have achieved better than centimeter accuracy this way, although the instrumentation is touchy and requires great skill on the part of the operator. Several times I found that I had great GPS geometry, but had lost my link to the base station. The opposite has also happened, where I wanted to record positions and had a radio link back to the base station, but the GPS geometry was bad."