The quest for greater and greater accuracy has spawned an assortment of variations on basic GPS technology. One technique, called "Differential GPS," involves the use of two ground-based receivers. One monitors variations in the GPS signal and communicates those variations to the other receiver. The second receiver can then correct its calculations for better accuracy.
Another technique called "Carrier-phase GPS" takes advantage of the GPS signal's carrier signal to improve accuracy. The carrier frequency is much higher than the GPS signal which means it can be used for more precise timing measurements.
The aviation industry is developing a type of GPS called "Augmented GPS" which involves the use of a geostationary satellite as a relay station for the transmission of differential corrections and GPS satellite status information. These corrections are necessary if GPS is to be used for instrument landings. The geostationary satellite would provide corrections across an entire continent.
All of these technologies are covered in more detail later in this tutorial.