There are several techniques to improve GPS accuracy. These include:
1. Differental GPS or DGPS: This can pare down a normal 4-20 meter GPS location to about 1-3 meters. The system uses a series of stationary GPS receivers and calculates the difference between their actual positions and the position calculated by these handsets as per the signal received. This difference is then sent as a local FM signal that the civilians use to get more accurate readings.
2. Wide Area Augmentation System or WAAS: In this system, there are a series of ground stations that calculate GPS errors and upload them to additional satellites that have been put up. These satellites then transmit these corrections to GPS receivers.
Since this system uses additional satellites and there are not many available, the method is not widely used. However, WAAS is seen to have tremendous potential and it is assumed that in the years to come the system can also be used reliably for critical applications like GPS-based instrument landing for planes. Variants of the WAAS system are being developed in Europe and in Japan.
3. Local Area Augmentation System or LAAS: This system is similar to WAAS, and uses correctional data. The only difference is that the data is broadcast from a local source, which can be an airport or any other place that requires accurate positioning. The data that is broadcast can only be used within 50 km of transmission.
4. CPGPS or Carrier-Phase Enhancement: This is a correctional technique that uses the 1.575 GHz L1 carrier wave and makes it act like a clock signal. This resolves the discrepancies to an astonishing 20-30 cm.
5. Wide Area GPS Enhancement (WAGE): This system tries to improve GPS accuracy by providing more accurate satellite clock and orbital data to specially-equipped receivers.
6. Relative Kinematic Positioning (RKP): This is yet another GPS-based positioning system. Here the range signal can be fixed with an accuracy of less than 10 cm. This is done by arriving at the number of cycles in which the signal is transmitted and received by the receiver.
Finally, at the most basic level, many automobile GPS systems combine GPS units with a gyroscope and a speedometer pick up. This allows the computers to maintain their own navigation when GPS signals are blocked.