Abstract: Mountain-based GPS radio occultation sounding is an economic novel technique for monitoring temporal and spatial variations of regional lower atmospheric environments. The GPS receiver, which is on a mountain top with a downward looking perspective toward the earth's limb, can track any GPS satellite as it sets or rises behind the earth's limb, therefore it collects data at both negative and positive elevations to the receiver's local horizon. By combining both the negative and the positive elevations data, a high-resolution profile of refractivity below the height of the receiver and in some case extending to 1—2 km above the receiver can be obtained.An experiment of the mountain-based GPS radio occultation is conducted on the top of Mt Wuling during 1—29 August 2005. During the experiment, the radiosonde observation is taken at Xinglong County, which is about 30 km away to the southeast of the Mt Wuling. Mountain-based occultation data and radiosonde data are successfully obtained. The traditional Geometric OPtics (GOP) inversion method are described in details, and refractivity profiles below the GPS receiver are obtained from occultation data by this method. A novel Full Spectrum Inversion (FSI) is also applied for mountain-based occultation data, which can deal with the multi-path effects frequently occurring in the lower atmosphere, while the traditional method usually becomes inefficient here. FSI provides a simple and efficient tool for deriving the instantaneous frequencies of a signal composed of several narrow banded sub-signals. When certain criteria are fulfilled, FSI is capable of resolving the frequency variation of each signal component. Since FSI is based on the Fourier transform of the entire signal, the problem that different signals get to the GPS receiver at the same time in multi-path regions is thus solved. Refractivity profiles below the receiver are also obtained by FSI. The comparisons of the refractivity obtained from the same GPS radio occultation events indicate that with FSI method, the precision has been improved by 2%, and the standard deviation is less than 3%. The refractivity differences between FSI and radiosonde are also achieved in a time window of 1.5 h and azimuth angle window of 40°, the comparison results show that the mean refractivity relative deviation is-8.15% with a standard deviation of 1.4%. The results suggest that the FSI is an efficient inversion scheme for mountain-based GPS radio occultation data. In the future, further research will be carried out on how to improve the inversion precision, and use this method in space GPS radio occultation data.