GPS measurements to investigate the reason why GPS is less accurate in mountain areas

It has been pointed out that GPS results in mountain areas are usually less accurate than those in flat areas, especially in the vertical component, when the height difference among observation stations is large. It is thought that the decrease of accuracy is caused by insufficient corrections for tropospheric delay of the GPS microwave.In order to investigate the difference of tropospheric delays at sites located on high mountains and those at valleys, and furthermore to improve the accuracy of GPS results in mountain areas, we have repeated GPS measurements since September 1997 at Yamainudan (about 1400 m above sea level) near the top of Mt. Sobatsubu in the Southern Alps of Japan. Their data were processed with the data at four stations of surrounding valleys (300 – 400 m) by using GAMIT software, to investigate the relation of the coordinates obtained and zenith tropospheric delays. We found a negative correlation between the difference of zenith tropospheric delay and height difference between Yamainudan and a site at valley. These results scarcely changed when a mapping function used was changed.We also processed some GPS data obtained by the Geographical Survey Institute (GEONET data) in the mountain areas. A positive correlation between the difference of tropospheric zenith delay and height difference was found in the Northern Alps, and no significant correlation was found in the Central Alps of Japan.We think that these different correlations were caused by the difference of the shape of the valleys.