Modified LAMBDA for absolute carrier phase positioning in the presence of biases

摘要

The least-squares integer ambiguity decorrelation adjustment (LAMBDA) method is adapted for absolute positioning with satellite-satellite single difference measurements. The integer decorrelation transformation of LAMBDA takes only the covariance matrix of float ambiguities into account which results in a substantial amplification of biases due to orbital errors, satellite clock offsets and multipath. Therefore, a partial integer decorrelation is suggested to achieve the optimum trade-off between variance reduction and bias amplification. The reliability of ambiguity resolution is further improved by using two geometrypreserving, ionosphere-free multi-frequency linear combinations: a code-carrier combination of maximum discrimination and a code-only combination of minimum noise. The optimized widelane and narrowlane linear combinations are presented for two, three and four Galileo frequencies. The achievable probabilities of wrong fixing have been computed for an exponential bias profile and are several orders of magnitude lower than in the case of complete integer decorrelation. Moreover, a partial fixing scheme is proposed for severe carrier phase multipath. The method bounds the biases by an exponential profile and determines the optimum fixing order by a sequential search with an azimuthal separation constraint between the satellites of two consecutive ambiguity fixings.