Relationship Between Terrestrial and Satellite Doppler Systems

Abstract

Classical horizontal geodetic networks are commonly combined with space observations, mostly satellite Doppler, in order to optimize the accuracy of geodetic control points and, thus, satisfy as many types of users as possible. Since satellite Doppler observations refer to a fully defined three-dimensional reference system and terrestrial observations, through the presence of Laplace stations (astronomical longitude and azimuth), contribute also to the pole and longitude orientations, it is imperative to ensure the highest possible degree of compatibility between the astronomical and satellite Doppler systems to maintain optimization of the accuracy of control points. Since gravity and geopotential (in the form of spherical harmonics) data are usually combined to evaluate geoid undulations and deflections of the vertical which are in turn used to reduce terrestrial angular and range observations, it is equally imperative to ensure that the satellite Doppler system and that of the geopotential solution are truly geocentric and thus compatible with the gravity data which should refer to a single equipotential surface. In order to estimate the degree of compatibility in terms of longitude orientation between satellite Doppler and geodetic astronomical systems as realized by current observations, astrogeodetic (based on CIO pole, BIH longitudes, and NWL9D satellite Doppler system) and gravimetric deflections of the vertical were compared at several hundred stations of the Canadian geodetic framework and U.S. transcontinental traverse. It was found that, when using the U.S. data subset only, incompatibility between the zero geodetic meridian plane of the NWL9D system and the zero astronomic meridian plane of the BIH was of the order of 0.78, which is in good agreement with previous results. However, inter-comparisons between various North American subsets revealed inconsistencies between areas of up to 0.78 (between Canadian and U.S. geodetic astronomical longitude observations). These results are based on the assumption that gravimetric deflections are bias free. The geocentricity of the NWL9D system with respect to other systems such as the Goddard Earth Models and SAO Standard Earths is also analyzed by comparing satellite Doppler derived geoid undulations with GEM and SAO SE undulations. An incompatibility of 4 m in Z (axis) exists between the origin of the NWL9D system and that of the other systems.