Techniques to improve GPS accuracy

Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation process.

Examples of augmentation systems include the Wide Area Augmentation System, Differential GPS, and Inertial Navigation Systems.

The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.

The first is called Dual Frequency monitoring, and refers to systems that can compare two or more signals, such as the L1 frequency to the L2 frequency. Since these are two different frequencies, they are affected in different, yet predictable ways by the atmosphere and objects around the receiver. After monitoring these signals, it is possible to calculate and nullify that error.

Receivers that have the correct decryption key can relatively easily decode the P(Y)-code transmitted on both L1 and L2 to measure the error. Receivers that do not possess the key can still use a process called codeless to compare the encrypted information on L1 and L2 to gain much of the same error information. However, this technique is currently limited to specialized surveying equipment. In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see #GPS modernization, below). When these become operational, all users will be able to make the same comparison and directly measure some errors.

A second form of precise monitoring is called Carrier-Phase Enhancement (CPGPS). The error, which this corrects, arises because the pulse transition of the PRN is not instantaneous, and thus the correlation (satellite-receiver sequence matching) operation is imperfect. The CPGPS approach utilizes the L1 carrier wave, which has a period 1000 times smaller than that of the C/A bit period, to act as an additional clock signal and resolve the uncertainty. The phase difference error in the normal GPS amounts to between 2 and 3 meters (6 to 10 ft) of ambiguity. CPGPS working to within 1% of perfect transition reduces this error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS normally realizes between 20 and 30 centimeters (8 to 12 inches) of absolute accuracy.