Abstract: The function model and the stochastic model of short distance GPS mobile positioning are discussed in the OTF (on-the-flying) way; then the general normal equation to suit programming in computer is given in order that these four classes of observable may be selected in the way whether let the scale factors equal to zero. In addition, it is clarified that the OTF way is equivalent to the single epoch way in the case that only two classes of precise codes are selected in precise mobile positioning. Last, it is clarified by an example that the given mathematical model is accurate and effective.