Carrier phase tracking is a highly accurate means of positioning used in GNSS surveying applications.
The product of the period of the carrier frequency times the speed of light gives the wavelength, which is about 1900mm for the GPS L1 carrier. With a 1% of wavelength accuracy in detecting the leading edge, this component of pseudorange error can be as low as 2mm, compared with 3000mm for the C/A code and 300mm for GPS P code.
However, in order to employ carrier phase tracking and make use of this 2mm accuracy a specially equipped receiver is required that can measure the total phase—i.e., the total number of wavelengths plus the fractional wavelength.
At least two receivers are used to track carrier signals at the same time. The carrier phase is tracked at both receivers and the changes in tracked phase are recorded over time in both receivers. The phase difference changes in the two receivers are reduced using software to differences in three position dimensions between the reference station and the remote receiver.