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.
Over 50 countries around the world have implemented similar systems.
Wide Area Augmentation System (WAAS)
The FAA’s WAAS is a differential technique accuracy improvement system which upload known inaccuracies to GPS satellites for broadcast over L band satellite transmissions. WAAS satellite is currently only available in North America providing extended coverage both inland and offshore compared to the land-based DGPS (differential GPS) system. WAAS does not require additional receiving equipment as with DGPS.
Similar systems are being developed by other governments with examples being the European Geostationary Navigation Overlay Service (EGNOS) and Japan’s Multi-Functional Satellite Augmentation System (MSAS). In time GPS the world over will have access to precise positioning data using these and other like systems.
Local Area Augmentation System (LAAS)
Although similar to WAAS with LAAS correction data is transmitted from a local source where accurate positioning information is required (eg. an airport). A typical range for LAAS stations is thirty to fifty kilometers. LAAS stations can provide vertical and horizontal accuracy to the order of a few tens of millimeters which for aircraft makes autopilot takeoffs and landings possible.
Real Time Kinematic
Real Time Kinematic (RTK) is a positioning technique based on the analysis of the carrier phase of GPS satellite signals rather than the usual pseudo random signal itself and is also referred to as Carrier-Phase Enhancement (CPGPS).
Normally, to determine its position a GPS receiver compares a satellite’s pseudo random signal to its own internal copy to determine the delay time and derive the distance from the signals broadcasting satellite. In RTK a receiver examines a satellite’s carrier signal rather than the pseudo random signal. The much higher frequency of the satellite’s carrier signal means that with the same signal matching accuracy the position is determined to a much smaller error radius.
A GPS C/A signal broadcast in the L1 signal has a frequency of 1.023 MHz yet, the L1 carrier has a frequency of 1575.42 MHz some one and a half thousand times higher. Using the L1 carrier can give an position accuracy of about 20 cm. Although easy to describe correct matching of the carrier with was not designed for easy matching as was the pseudo random signal is extremely difficult to achieve.
Commonly, RTK systems have a base station which re-broadcasts the carrier signal measured for a number of mobile units to compare their own measurements. The mobile units can calculate their position relative to the base unit within millimeters. Overall accuracy is dependent on the accuracy of the base stations position determination.
Gyroscopic and Speedometer Referencing
It is common for car GPS units to use gyroscopic and speedometer references to cover the situations where GPS signals are blocked by the surrounding terrain.