As is known to all, GPS has revolutionized the field of navigation, surveillance and communication. However, the standalone GPS cannot meet the accuracy, availability and integrity required in civil aviation, especially for precision approach (PA) landing phase. Hence it should be augmented for such special applications. As a means of augmenting GPS, Satellite Based Augmentation Systems (SBAS) are widely developed to improve the performance by many countries, such as the Wide Area Augmentation System (WAAS) for US and Puerto Rico, the European Geostationary Navigation Overlay Service (EGNOS) for Europe, the Multi-Functional Satellite Augmentation System (MSAS) for Japan and the GPS and GEO Augmented Navigation (GAGAN) for India. Also, China is developing his global satellite navigation system named BeiDou Navigation Satellite System (BDS) or Compass as well as its SBAS.
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