The last few years have shown an increased interest in the commercialization of the global navigation satellite system (GNSS), which is often referred to as the global positioning system (GPS) in various applications. A GPS system uses spread spectrum signals-binary phase-shift keying (BPSK)-emitted from satellites in space for position and time determinations. Until recently, the use of GPS was essentially reserved for military use. Now there is great interest in using GPS for navigation of commercial aircraft. The US Federal Aviation Administration implemented a wide area augmentation system for air navigation to cover the whole United States with one system. There are also applications in the automotive industry, surveying, and personal and recreational uses.