Physical and Electrical Stabilty of GNSS Monumentation

Abstract:

We examine the stability of the relative positioning of GPS sites that are closely spaced (typically < 1km). There are deployments of GPS sites aimed at testing the stability of different types of monuments. One such experiment is being performed by UNAVCO as part of the Plate Boundary Observatory and seeks to compare drilled braced monuments will pillar type monuments. Other locations have had multiple monuments installed as part of the geodetic network around a collocation site. Tests have also been carried out with radomes being removed and replace specifically at sites where the radome can not be easily calibrated due to their geometry (i.e., the radome is attached to the top of a pillar or to the ground.) In this presentation we examine these data and assess the magnitudes of the variations seen in the position estimates and thus provide a lower bound on the stability of monuments and the effects of some radomes. We will also apply different analysis methods to these data to assess the impacts of processing methods. For radomes, site multipath effects and antenna problems, comparing dual frequency analyses (the method always needed for site separations of more than 1 km) with single frequency solutions can yield bounds on the frequency dependent errors that can arise from multipath and possibly radomes. In some cases, there can be up to 3 mm position differences between single and dual frequency results. The best of the short baseline results have root-mean-square (RMS) scatters of 100 microns horizontally and 200 microns vertically over a 12-month period but even with this precision annual signals with amplitudes of order 200 microns vertically and 70 microns horizontally can be seen. Some monument types show much greater variations. Radome effects can be several millimeters and in some cases the largest effect of removing radomes can be in the horizontal coordinates.