Four papers are presented that introduce several applications of modeling and estimation with emphasis on the Global Positioning System (GPS). The main thrust of this work, contained in the first two papers, concerns modeling ionosphere-induced scintillation in GPS radio waves and modeling the effect of scintillation on GPS receiver carrier tracking loops. The third paper introduces a novel magnetometer-based attitude and rate estimation strategy for application to spacecraft with wire booms. The fourth paper investigates certain anomalous peaks in the power spectra of GPS-derived zenith neutral delay measurements and concludes that these peaks are caused by atmospheric pressure tides.