The improvement of the technology of the GPS-receivers within the last years led to sub-meter precision of the code measurements. This allowed a code-based differential kinematic positioning with accuracy of a few meters, which is sufficient for a variety of applications. However, for special applications, like the CAT III precision approach and landing of an aircraft, there is a need to enhance the accuracy of code- Differential GPS (DGPS). The paper describes a method, that improves the code-DGPS solution. The method is based on a realistic computation of the covariance matrix of the observations which is used in the least-squares adjustment. The proposed algorithm uses for the computation of the weighting matrix the signal-to-noise ratio (SNR) given by the receiver. In the first part of the paper the theoritical background of the ordinary and proposed weighting is given. In the second part a summary of extensive validation tests comparing the results of the two processing methods is given.