In May 2000 the U. S. Department of Defense eliminated the purposeful degradation of accuracy of the Global Positioning System (GPS) called Selective Availability (SA). This action resulted in the accuracy of GPS improving from around 50 m to better than 10 m. With this improvement in accuracy civilians are enjoying very good performance from low-cost handheld GPS receivers, although GPS is still not accurate enough for many professional uses. The limitation of accuracy of GPS is caused by a number of system errors. Some system-wide correctable errors of GPS are reduced through the use of differential GPS techniques. The elimination of SA will allow the optimization of Wide Area DGPS networks because of the change in characteristics of the remaining error sources.