To operate, guide and control vehicles in low visibility conditions, it is critical that the states of the vehicle are accurately estimated, which includes the three dimensiona position, velocity, and attitude. This can be accomplished by GPS (Global Positioning System) aided encoder or GPS aided inertial approaches. The overall positioning accuracy of either approach will be determined by the GPS performance. Real-time centimeter accuracy GPS positioning can be achieved using carrier phase measurements. This requires fast and reliable on-the-°y integer ambiguity resolution.In this dissertation, we focus on resolving GPS ambiguity problem, including both integer ambiguity estimation and integer ambiguity validation. For integer ambiguity esti- mation, a brief overview of pervious work on integer ambiguity resolution is ¯rst presented. Then, an improved integer ambiguity resolution method is proposed. Subsequently, simu-lations and real-world data are presented to demonstrate the e®ectiveness of the method. We also present integer ambiguity algorithms with auxiliary measurements and algorithms with multiple epoch measurements, both of which are useful in GPS challenging areas. For integer ambiguity validation, a brief overview is ¯rst presented, and then analytic discussion vind test results on several popular validations methods are studied. Finally we discuss GPS modernization and its e®ect on integer estimation and validation