Global Positioning System (GPS) radio occultation (RO) is a new technique for obtaining profiles of atmospheric properties, specifically: refractivity, temperature, pressure, water vapor, and electron density. Low-Earth orbit (LEO) satellites, equipped with a GPS receiver, track GPS radio signals as they set behind the Earth. The atmosphere refracts GPS signals traveling to the receiver and these data can be used to infer information about atmospheric refractivity. Because GPS RO will be utilized in an array of atmospheric models, attaining the highest level of accuracy is essential. Therefore, this research provides insight to the challenges for GPS RO within the planetary boundary layer (PBL), due to the phenomenon of superrefraction (SR). SR is caused by a sharp decrease in refractivity with an increase of altitude, normally present at the top of the PBL, and results in a loss of the radio signal acquired by a LEO satellite. Consequently, the retrieval technique produces a refractivity profile that is negatively biased compared to global models. By examining the negative refractivity biases (as compared to the European Center for Medium-Range Weather Forecast model) in different seasonal periods, geographic regions, and altitude regimes, this research determined that SR is most probable off the west coast of continents where dry air sinks from above and creates a sharp vertical gradient in water vapor near the top of the PBL. Identifying regions that have a high occurrence of SR was a first step in preventing the use of negatively-biased RO profile data for future missions.