The Global Positioning System (GPS) is used in a wide variety of both civil and military applications. These applications affect users in everyday activities, from communications and power generation to precisely determining one's location. The GPS is also increasingly being used in aircraft guidance. However, the operational GPS system was not designed to meet the stringent requirements for bringing aircraft safely within close proximity of other objects, and thus the GPS by itself does not sufficiently guarantee the required safety-of-life performance at all times. In particular, safety-of-life performance requires that any error in the reported position of an aircraft must be strictly limited at all times. Therefore, aviation authorities around the world, including the Federal Aviation Administration (FAA) in the United States, have defined augmentation systems to monitor the performance of the GPS and provide timely alerts to users when significant errors may be present.