GPS was offi cially declared to have achieved full operational capability (FOC) on July 17, 1995, ensuring the availability of at least 24 operational, non-experimental, GPS satellites. To meet the future requirements, the GPS decision makers have studied several options to adequately modify the signal structure and system architecture of the future GPS constellation. The modernization program aims to, among other things, provide signal redundancy and improve positioning accuracy, signal availability, and system integrity (El- Rabbany 2006 ). Unfortunately, even with the modernized system, there exist situations where the GPS signal may be partially obstructed, e.g. in urban canyons, which in turn affect the availability and reliability of the PPP solution