This paper presents results from a study of elevation-angle-dependent systematic effects on estimates of relative site positions within the Swedish permanent Global Positioning System (GPS) network. Two months of data from 16 sites have been analyzed with three different elevation cutoff angles, namely, 10¿, 15¿, and 20¿. We present offsets between these solutions and demonstrate that estimates of the vertical component of several baselines strongly depend on the minimum elevation angle (elevation cutoff angle) of the data analyzed. Offsets of 22.3¿1.6 mm in the vertical component are evident when the elevation cutoff angle is changed from 10¿ to 20¿. We investigate these offsets and conclude that a significant part is due to differential phase errors caused by scattering from structures associated with the mounting of the antenna to the pillar and with the pillar itself. The horizontal components of baseline are less affected. We found, however, that the offsets in the horizontal components increase with baseline length. For the longest baselines (~1500 km) offsets of more than 5 mm are evident in the north component when the elevation cutoff angle is changed from 10¿ to 20¿. These offsets are most likely due to differential phase errors caused by nonuniform antenna phase patterns; an effect that presumably increases with baseline length and which also might increase because of scattering from the pillars and the antenna mounts. We identify the scattering structure and reduce associated errors in the vertical component of baseline to a significant degree on one of the sites by using microwave-absorbing material. The results presented are of importance for those analyzing data from existing networks and for those who intend to establish permanent GPS geodetic networks. ¿ American Geophysical Union 1996