GNSS enhancement refers to techniques used to improve the accuracy of positioning information provided by the Global Positioning System or other global navigation satellite systems in general, a network of satellites used for navigation.
Enhancement 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 provides additional navigational or vehicle information to be integrated into the calculation process.
The network uses code-division multiple access (CDMA) to allow separate messages from the individual satellites to be distinguished.
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways.
After Selective Availability was turned off by the U.S. government, the largest error in GPS was usually the unpredictable delay through the ionosphere.
Without keys, it is still possible to use a codeless technique to compare the P(Y) codes on L1 and L2 to gain much of the same error information.
In the future, additional civilian codes are expected to be transmitted on the L2 and L5 frequencies (see GPS modernization).
Then all users can perform dual-frequency measurements and directly compute ionospheric-delay errors.
[1] 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 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.
This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information, and ambiguity resolution techniques via statistical tests, possibly with processing in real time.
With a 1% of wavelength accuracy in detecting the leading edge, this component of pseudo-range error might be as low as 2 millimeters.
We now describe a method that could potentially be used to estimate the position of receiver 2 given the position of receiver 1 using triple differencing followed by numerical root finding and a mathematical technique called least squares.
A detailed discussion of the errors is omitted in order to avoid detracting from the description of the methodology.
The satellite carrier's total phase can be measured with ambiguity regarding the number of cycles.
Thus the triple difference result has eliminated all or practically all clock bias errors and integer ambiguity.
This triple difference is Triple-difference results can be used to estimate unknown variables.
Triple difference results for three independent time pairs quite possibly will be sufficient to solve for the three components of the position of receiver 2.
This initial value could probably be provided by a position approximation based on the navigation message and the intersection of sphere surfaces.
Although multidimensional numerical root finding can have problems, this disadvantage may be overcome with this good initial estimate.
Greater accuracy may be obtained by processing triple-difference results for additional sets of three independent time pairs.
Other examples of GNSS enhancements include Inertial Navigation Systems and Assisted GPS.