Algorithm and Flight Test Results to Exchange Code Noise and Multipath for Biases in Dual Frequency Differential GPS for Precision Approach

A modified Code Noise and Multipath (CNMP) algorithm is presented for dual-frequency differential GPS precision approach and landing. Pseudorange noise and multipath are exchanged for reducible biases in both reference station and aircraft ranging measurements. The second frequency is used only to correct the code-minus-carrier observable. Corrected pseudorange measurements are combined using a single-frequency carrier phase position domain smoothing (CPDS) algorithm. Flight test vertical and horizontal navigation system error is reduced from 0.80 m to 0.32 m and from 0.41 m to 0.33 m, respectively, as compared to Local Area Augmentation System single-frequency processing. For six included 150-s precision approaches, 95% error drops from 0.56 m to 0.26 m vertically and 0.28 m to 0.14 m horizontally. Composite protection levels (PLs) using CNMP-derived quantities are much smaller than modified LAAS PLs. Mean PL values are reduced from 5.90 m to 2.60 m vertically and from 3.24 m to 1.38 m horizontally.