This chapter presents the CRTK: cost-effective real-time kinematic positioning. It addresses challenges and factors influencing ambiguity resolution in smartphone-based RTK. The chapter begins by formulating the methods for estimating smartphone observation precisions, followed by a detailed explanation of the experimental setup and datasets used throughout the study. It then analyzes how smartphone brands, operating systems, and antenna attitudes impact the ambiguity integer property, data quality, and positioning performance. Two kinematic experiments are presented to demonstrate the ambiguity resolution and positioning performance for different smartphone models with both embedded and external antennas.