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For GPS positioning, it is normally required that there be at least four GPS satellites in view. However, due to the frequent blockage of signals in an urban environment, it is difficult to meet that requirement; therefore, the operation of the GPS receiver may be interrupted. How to deal with this problem so that the service can be continuous is the main theme of this paper. The pseudorange predictor and the receiver clock bias predictor may be used to estimate the pseudorange and clock bias, respectively. The altitude-hold algorithm is developed to provide additional information under the assumption that the altitude of the vehicle is approximately a constant, which is deemed appropriate for urban applications. Furthermore, for land vehicles, it may be assumed that the speed of the vehicle is not significantly varying so that the constraint-filtering method with soft constraint may be applied. The integration of these methods yields a successful algorithm to manage the ill-conditioned positioning problem if the number of visible satellites drops to below 4. From both static and dynamic experimental results, it is shown that the proposed methodology indeed gives rise to an effective scheme that can sustain the service for a few minutes, even if there is no satellite in view at all.