These deliberate errors are no longer being introduced, but overcoming them turned out to be quite simple in practice. Differential GPS (DGPS) corrects for measurement errors by comparing the GPS positions recorded at designated reference stations with the accurately known positions (determined through careful surveys) of those stations. DGPS stations broadcast any error factors they uncover to all GPS receivers within range, and the receivers use the data to correct their calculations, resulting in accuracy within a yard or two.
Using GPS Data
With the GPS providing position, altitude and time, today's miniaturized navigation devices compare the data against a stored geographic database (such as North America) and then calculate routes, provide directions, correct for instructions that users ignore or get wrong, and highlight nearby points of interest. This programming, along with the accompanying graphical displays, are marvels of compact technology that complete the illusion of magic.