Less expensive, general purpose GPS receivers have an accuracy of about 10-20 meters ( 33 to 66 feet) which is too inaccurate for many GPS applications for example, vehicle GPS tracking systems. There are a number of techniques used to improve GPS accuracy.
Nationwide Differential Global Positioning System (NDGPS)
Nationwide Differential Global Positioning System (NDGPS) improves GPS navigational accuracy to a range of 1 to 3 meter (10 feet).
The concept underlying differential GPS is the use of a fixed reference station with a very accurately known position being used to calculate inaccuracies in satellite signals. The calculated inaccuracies for various GPS satellites is transmitted to mobile receivers which can use the established inaccuracies to more accurately determine their location. The accuracy possible by mobile receivers is dependent on distance from the NDGPS reference station and is approximately 1 m (3.3 feet) next to a station to 3 m (10 feet) at the extreme of a reference stations coverage area of 402 kilometers (km).
A more accurate future system, called High Accuracy-NDGPS, is being developed to achieve navigation accuracy to the centimeter level.
A secondary service performed by NDGPS facilities is monitoring GPS satellites for anomalous behavior. Warnings are broadcast to receivers of satellites which should not be used due to errors.
(read more..)
