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APPENDIX C 165 Figure C-8 Differential GPS concept. (Courtesy of the Aerospace Corporation) Carrier Phase (Interferometric) GPS In addition to the use of C/A-code, Y-code, or both as measurements of pseudorange for obtaining a position solution, many GPS receivers also measure the L-band carrier phase itself. This enhancement technique can produce very high precision measurements, sometimes as good as 1 to 5 millimeters and, thus, is valuable for high- performance applications. The carrier phase data is used almost exclusively in an interferometric mode, where the phase data from two receivers are processed together to solve for the baseline between them. This eliminates atmospheric errors, and when combined with DGPS, can result in sub-centimeter positioning accuracies.