The GPS atmospheric and ionospheric delays have been considered as an error source for a long time. In 1992 when the GPS became fully operational, Ware (1992) suggested limb sounding the Earth atmosphere using GPS atmospheric delay signals. In April 1995, the small research satellite of Microlab-1 was successfully put into a Low Earth Orbit (LEO) to validate the GPS radio occultation method (Feng and Herman, 1999). Since then, the GPS/Meteorology Mission (GPS/MET) has been widely used to produce accurate, all weather pressure, temperature, density profiles in the troposphere and the ionospheric total electron content (TEC) as well as electron density profiles (Rocken, 1997; Hajj and Romans, 1998; Syndergaard, 2000), to improve weather analysis and forecasting, monitor climate change, and monitor ionospheric events. While traditional observing instruments, e.g. water vapour radiometer (WVR), incoherent scatter radars (ISR), ionosonde, topside sounders onboard satellites, in situ rocket and satellite observations, are expensive and also partly restricted to either the bottomside ionosphere or the lower part of the topside ionosphere (usually lower than 800 km), such as ground based radar ionospheric measurements. While GPS satellites in high altitude orbits (~20,200 km) are capable of providing details on the structure of the entire atmosphere, even the plasma-sphere. Moreover, GPS is a low-cost, allweather, near real time, and high-temporal resolution (1~30s) technique. Therefore, GPS is a powerful tool to sound the atmosphere and ionosphere as well as their application in meteorology, climate and space weather.