A review of current measurement reduction techniques for precise positioning using carrier phase observables from the Global Positioning System satellites is presented. The qualities of each are discussed. The main cause of data problems, cycle slips, is also given along with an example, and then a possible algorithm to correct slips over short time gaps when data from a single station are given. It is then argued that this algorithm will be useful for cycle slip detection in precise positioning of a moving platform as well as in a static relative positioning mode. Optimal estimation in the presence of unmodeled errors is discussed.